This article appeared in a journal published by Elsevier. The attached
copy is furnished to the author for internal non-commercial research
and education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling or
licensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of the
article (e.g. in Word or Tex form) to their personal website or
institutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies are
encouraged to visit:
http://www.elsevier.com/copyright
Author's personal copy
Progress in Surface Science 85 (2010) 92–160
Contents lists available at ScienceDirect
Progress in Surface Science
journal homepage: www.elsevier.com/locate/progsurf
Review
Interaction of rotationally aligned and of oriented
molecules in gas phase and at surfaces
L. Vattuone a,b, L. Savio b, F. Pirani c, D. Cappelletti d, M. Okada e,f,g, M. Rocca a,b,*
a
Dipartimento di Fisica dell’Università di Genova, Via Dodecaneso 33, 16146 Genova, Italy
IMEM-CNR, U.O. di Genova, Via Dodecaneso 33, 16146 Genova, Italy
c
CNISM Unità di Perugia and Dipartimento di Chimica Università di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy
d
CNISM Unitàdi Perugia and Dipartimento di Ingegneria Civile ed Ambientale Universitàdi Perugia, Via G. Duranti, 93, 06125
Perugia, Italy
e
Renovation Center of Instruments for Science Education and Technology, Osaka University, 1-2 Machikaneyama-cho, Toyonaka,
Osaka 560-0043, Japan
f
Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan
g
PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
b
a r t i c l e
i n f o
Commissioning Editor P.M. Echenique
Keywords:
Stereodynamics
Chemical reactions
Gas-surface interactions
Heterogeneous catalysis
a b s t r a c t
Recent developments concerning the generation of molecular
beams containing oriented/aligned molecules will be reviewed and
applications of such tools to the study of elementary processes
occurring both in homogeneous and heterogeneous phases will be
presented. First we will discuss the case of symmetric top molecules
oriented by hexapoles. Here the molecular polarization is obtained
by the use of an external field and allows to control which end of
the molecular projectile is going to collide with the target. Then
we will review the so-called collisional alignment, a molecular polarization phenomenon occurring in supersonic expansions of gaseous
mixtures. The key feature, in this case, is the velocity dependence of
the alignment degree, which allows the use of mechanical devices to
filter out of the beam the molecules having either a random (statistical) or a preferential (non-statistical) spatial distribution of their
rotational angular momentum J with respect to the molecular beam
axis. The physical mechanisms underlying the collisional alignment
will be resumed and some relevant gas-phase experiments demonstrating its occurrence will be illustrated. Application of such methodologies to the investigation of the stereodynamics of elementary
* Corresponding author. Address: Dipartimento di Fisica dell’Università di Genova, Via Dodecaneso 33, 16146 Genova, Italy.
Tel.: +39 0103536392.
E-mail address: rocca@fisica.unige.it (M. Rocca).
0079-6816/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.progsurf.2009.12.001
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
93
processes occurring in gas-surface interaction will be presented and
discussed for both weakly and strongly interacting systems.
Ó 2009 Elsevier Ltd. All rights reserved.
Contents
1.
2.
3.
4.
5.
6.
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
Stereodynamic effects as deduced by applying detailed balance to desorption experiments . . . . . . . . 98
2.1.
H2/D2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
2.2.
Heavier non-polar molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
2.2.1.
A system without rotational rainbow: N2/W(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
2.2.2.
A system with rotational rainbow: N2/Ag(1 1 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
2.3.
Heavier polar molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
2.3.1.
NO/Ag(1 1 1) and NO/Pt(1 1 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
2.3.2.
An apparently opposite behaviour: CH3F and OCS on glass . . . . . . . . . . . . . . . . . . . . . 105
2.4.
Some considerations on the application of detailed balance . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
Experiments with oriented molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
3.1.
Experimental. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
3.1.1.
State-selected molecular beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
3.1.2.
State selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
3.1.3.
Orientation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
3.1.4.
Measurements of initial sticking probability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
3.2.
Steric effects at the strongly bound CH3Cl/Si(1 0 0) and NO/Si(1 1 1) and at the weakly bound
CH3Cl/HOPG and CH3Cl /Si(1 1 1) systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
3.2.1.
Steric effects appearing in the strongly bound system of polyatomic CH3Cl/Si(1 0 0)
[150,151] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
3.2.2.
The weakly-bound systems of polyatomic CH3Cl/HOPG. . . . . . . . . . . . . . . . . . . . . . . . 123
3.2.3.
Steric effects appearing in the strongly bound system of diatomic NO/Si(1 1 1) . . . . . 126
Preparation of aligned molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
4.1.
Collisional alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
4.2.
Experimental studies in the Perugia laboratory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
4.2.1.
The case of diatomic molecules: O2 and N2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
4.2.2.
Small hydrocarbons: the cases of C2H2 and C2H4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
Interaction of aligned molecules with surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
5.1.
Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
5.2.
Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
5.2.1.
Weakly interacting systems: hydrocarbon adsorption on noble metals . . . . . . . . . . . . 139
5.2.2.
Strongly interacting systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
5.3.
Role of theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
Perspectives and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
1. Introduction
Molecular beam techniques allow a detailed investigation of several aspects of molecular interactions and of molecular dynamics [1]. The dependence on spatial orientation of the involved molecules
of elastic, inelastic and reactive molecular collisions, occurring both in homogeneous and heterogeneous phases, is of particular interest in this context [2–6]. These events can be activated either thermally or by photo-absorption and the ‘‘geometrical” dependence is known as steric effect. First
attempts to control the molecular orientation have been carried out on polar molecules exploiting
an electrostatic quadrupolar field [7]. The method was successfully applied to characterise the anisotropy in some two-body gas-phase van der Waals interactions through the analysis of integral cross section changes measured for different molecular orientations [8,9]. However, the investigation of the
Author's personal copy
94
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Nomenclature
U
h
H
lel
ð2Þ
A0
AQ
Ag
Ang
CW
DFT
DMD
Ei
FT
FWHM
HE
HOPG
HREELS
KW
LEED
LIF
ML
Pi (Pf)
PES
PSMB
Q total
QMS
R
RT
S
S0
SLID
SMB
ST
t
T
Tdes
TOF
UHV
UPS
v
vp
XPD
XPS
flux in the supersonic molecular beam
angle of incidence of the molecules with respect to the surface normal
coverage
permanent dipole moment
quadrupole alignment parameter
relative cross section anisotropy
glory component of AQ
non-glory component of AQ
molecule in cartwheeling motion
density functional theory
dimethyldichlorosilane
translational energy of the molecules of a supersonic molecular beam
fast tail (of the supersonic molecular beam velocity distribution)
full width at half maximum
molecule in helicoptering motion
highly oriented pyrolytic graphite;
high resolution electron energy loss spectroscopy
retarded reflector method by King and Wells
low energy electron diffraction
laser induced fluorescence
monolayer
initial (final) partial pressure in KW experiments
potential energy surface
pulsed supersonic molecular beam
integral cross section
quadrupole mass spectrometer
stereo-asymmetry
room temperature
sticking coefficient
initial sticking coefficient
surface light-induced drift
supersonic molecular beam
slow tail (of the SMB velocity distribution)
time
sample temperature
desorption temperature
time of flight
ultra-high vacuum
ultra-violet photoemission spectroscopy
probed velocity of the SMB
peak velocity of the SMB
X-ray photoelectron diffraction
X-ray photoemission spectroscopy
stereodynamics of elementary processes [10–13] represents still one of the main themes of advanced
research in several areas of molecular sciences and is of interest also for applied research concerning,
for instance, the synthesis of special materials and the construction of ‘‘eco-consistent” devices.
The purpose of this review is to account for some aspects regarding the possibility of overcoming
steric hindrance in physical and chemical processes occurring at the gas-surface interface and to char-
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
95
acterise trapping, absorption, reaction and desorption phenomena. The main focus will be on recent
achievements in experimental techniques for controlling the spatial orientation of molecules impinging on and desorbing from a surface. We will also present recent advances in the understanding of
some of the important elementary processes involved.
Since it is well known that molecules trapped on a surface often adsorb assuming a well defined
orientation and that the surface itself can serve to orient the reactants [14,15], molecular beam-surface experiments can provide unique information on the stereodynamics of bimolecular chemical and
physical processes.
The sticking probability, S, of reactants coming from the gas phase is one of the key parameters
determining the reaction rate in catalytic processes in the heterogeneous phase. It was therefore characterised as accurately as possible for a large variety of conditions and its dependence on the properties of the gas phase species as well as of the surface was controlled by performing state resolved
experiments and theoretical simulations. From quantum mechanics the state of a gas phase species
is described by its momentum vector, containing the information on translational energy and polar
and azimuthal angles of impingement onto the surface, and by the quantum numbers related to electronic, vibrational and rotational degrees of freedom. At variance with the molecule, the surface is
characterised by a manifold of parameters which include surface temperature, T, surface coverage
of the reactant, H, and of other co-adsorbed species, surface crystallographic face, surface defectivity
and, in general, surface morphology.
From the experimental side, S can be best investigated in ultrahigh vacuum (UHV) conditions by
using supersonic molecular beams (SMB) [16–19], which allow to accurately define the momentum
vector of the molecules in the gas phase and thus to perform state resolved experiments.
UHV is needed to prepare the crystal surface in a well defined condition, minimising the probability
of surface contamination. The use of SMBs is, in this respect, useful since it allows to reach a local pressure on the sample of 107 mbar while the rest vacuum remains in the 1010 mbar range, thus reducing the load on the pumping system. UHV is also needed to employ electron based surface science
analytical techniques, such as Low Energy Electron Diffraction (LEED), X-ray and Ultra-Violet Photoemission Spectroscopy (XPS and UPS), and High Resolution Electron Energy Loss Spectroscopy
(HREELS), and to reach low temperatures. Even if a manifold of space resolved methods is nowadays
available for surface analysis, the electron based techniques necessarily complement the information
obtained by nanoscale resolved microscopy.
When adsorption is translationally activated, hyperthermal SMBs allow to simulate the fate of the
molecules populating the high energy tail of the Boltzmann distribution. The latter represent indeed
only a small fraction of the total statistical population, but are the only one which matter if all the others are prevented from adsorption and bounce back into the gas phase upon collision with the surface.
The flux of hyperthermal molecules obtained with a beam [20] is indeed comparable to the one hitting
the surface under atmospheric conditions. Even if recent experimental developments allow to perform
photoemission experiments at pressures close to the ambient one [21,22] SMB experiments remain
therefore the most straightforward way to obtain detailed information on the dependence of S on
molecular observables.
SMB experiments enabled also the control of the vibrational degrees of freedom in the gas phase
[23]. This was initially achieved simply by varying the nozzle temperature in order to populate the
first vibrational levels [24–27]. These experiments showed that the translational energy barrier to
adsorption may indeed depend on the vibrational state of the incoming molecule, being lower for vibrationally excited molecules [28,29]. A paradigmatic case is represented by systems for which the barrier in the potential energy surface (PES) is mainly along an intra-molecular coordinate, as it is often
the case for dissociative adsorption. However, only the lowest excited states are accessible in this way
and no mode selectivity is possible. Alternatively, valuable information on the dependence of S on
vibrational and even rotational quantum numbers was retrieved thanks to arguments based on detailed balance [30,31]. The latter links the sticking probability of a molecule in a certain quantum state
to the probability to find it in the same state when desorbing off the surface. Laser methods were employed to analyse the quantum state of the desorption products or of molecules having undergone
reactive scattering. The pivotal effect of vibrational energy in overcoming dissociation barriers could
finally be demonstrated thanks to the availability of powerful tunable lasers in the 1990s, which
Author's personal copy
96
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
enabled the researchers to prepare beams of molecules in well defined vibrational states and to probe
their interaction with surfaces directly [32–34].
Rotational quantum numbers received comparatively less attention because of the smaller energy
of rotational compared to vibrational quanta, except possibly for hydrogen molecules [35]. Fundamental information on their influence on S was indeed reported mainly for H2, by using detailed balance
arguments and by measuring with laser methods the H2 states after reactive scattering. Time of flight
spectra and detailed balance applied to D2/Cu(1 1 1) allowed to determine the state resolved initial
sticking probability S0(Ei, v, J), where Ei is the translational energy, hv the vibrational quantum number
and J the rotational quantum number. It came out that rotational excitation hinders dissociative
adsorption for J < 5, while it promotes it for J > 5 [35]. This non-monotonic effect was explained by
the competition of the decrease of the time during which the reactant maintains the favourable geometry for dissociation during the collision, (thus decreasing S), and the coupling of rotational motion to
the reaction coordinate which enables to transfer rotational into translational energy during the collision (thus increasing S). This trend was observed also for reactions in the gas-phase [36]. Moreover,
the same experiment [35] showed that the dependence of S0 on rotational angular momentum decreases with increasing v.
The paucity of experimental data regarding the effect of rotational energy for heavier molecules is
due to two reasons: (a) the experimental difficulty in preparing beams of molecules in (at least partially) defined rotational states; (b) the (wrong!) argument that, due to the relatively small magnitude
of rotational quanta, their effect on adsorption dynamics should be small, if present at all. Indeed some
of us demonstrated [37] that rotationally hot C2H4 has a much lower trapping probability on Ag(0 0 1)
than rotationally cold molecules.
Even less consideration was given to steric effects associated to the spatial direction of the angular
momentum in the gas phase. Steric effects are connected either with the orientation or with the alignment of the spatial distribution of the molecular axis W(cos c) defined, respectively, by the relations:
Z
1
Wðcos cÞdðcos cÞ ¼ 1;
ð1:1Þ
Wðcos cÞ cos c dðcos cÞ ðorientationÞ;
ð1:2Þ
Wðcos cÞ cos2 c dðcos cÞ ðalignmentÞ;
ð1:3Þ
1
Z
1
1
Z
1
1
where gamma indicates the angle between the molecular and the reference axes. Other more operative definitions will be provided in the next sections.
Eq. (1.1) corresponds to the normalization of the distribution. Eq. (1.2) informs about the importance of the difference between the head and tail orientations of the molecule for the collision and
reaction dynamics. Eq. (1.3) accounts for the importance of the difference between the long-side
and broad-side approaches of the molecule. Less formally, if we consider a molecule like NO, orientation accounts for the different outcome of the reaction if the surface is hit by molecules with either the
O or the N-end down. Alignment determines, on the other hand, what happens whenever the molecule
hits long-side (NO axis along the beam direction, i.e. molecules in cartwheeling motion (CW) when
impinging normally to the surface) or broadside (NO axis normal to the beam direction, i.e. molecules
in a helicoptering motion (HE)).
In the case of dipolar molecules the interest focuses mainly on the orientation since the two ends of
the molecule are chemically very different, while for homopolar molecules (such as for O2, N2) only
alignment may play a role. A similar argument holds for more complex molecules consisting of two
groups on the opposite sides of one molecular axis (e.g. C3H6, C2H4, C2H6, CH3Cl, etc.).
Stolte and Kleyn firstly demonstrated the existence of large steric effects connected to the orientation of NO impinging on Ag(1 1 1) [38] by employing an electrostatic technique to prepare a beam of
oriented NO (see Section 3). The method was then extended to more systematic investigations of
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
97
sticking [39,40], scattering [40–45] and reaction probability [46–49] of NO and CO on different surfaces and, afterwards, of more complex molecules [50].
Experimental information on the effect of alignment is definitely more scarce, being available almost only for H2 and D2. Some theoretical studies predicted that broadside (helicoptering) D2 should
have a higher sticking probability [51–53], which was indeed confirmed experimentally by detailed
balance [54–57]. Further studies [10,55] investigated the alignment of D2 formed in the reaction of
D atoms on Cu(1 1 1) as a function of the kinetic energy of the desorbing D2. They found that most
of it desorbs with the bond axis parallel to the surface for energies below 0.8 eV; detailed balance analysis indicates therefore that, at low energy, the dissociative sticking probability is larger when D2 impinges on the surface as a helicopter. Similar conclusions were reached theoretically for H2/Cu(1 0 0)
[58,59], for H2/Pd(1 0 0) [60,61] and for H2/Pd(1 1 1) [62] and confirmed experimentally for the last
system [56].
The effect of rotational alignment on surface reactions for systems other than H2 (D2)/metal was
retrieved by measuring the rotational state distribution of N2 [63] and NO [64] scattered off
W(1 1 0) and Pt(1 1 1), respectively. They found that for translational energies lower than 0.5 eV the
molecules in low J states rotate like helicopters (HE) while molecules in high J states rotate like cartwheels (CW). At higher translational energies only cartwheeling molecules were observed. These
experimental results indicate a large dependence of the interaction potential on the angle of impingement of the molecule, as theoretically predicted by Kara and DePristo [65,66] and by Holloway and
Jackson [67]. Rotational alignment of a wider variety of molecules (C2H4 [68,69], C3H6 [70] and O2
[71]) could be explored only recently thanks to the exploitation of the velocity dependent collisional
alignment occurring in supersonic expansion (see Section 5).
While several experimental techniques have been employed to prepare beams of polar molecules
with a high degree of control on their orientation (see e.g. the recent review of Aquilanti et al. [72]),
preparation of beams of non-polar molecules in defined alignment states proved to be a much more
difficult task, despite the effort dedicated to it (see e.g. the reviews by Stapelfeldt and Seideman [73]
and Herschbach [74]). Laser techniques were employed to produce beams of aligned molecules
[73,75], as demonstrated both for polar (LiH [75], N2O [76], CS2 [77]) and homopolar (I2; N2 [76–
78], H2, D2, O2 [76], hexane [77]) molecules. These modern and potentially powerful techniques suffer,
however, from the fact that alignment disappears when switching off the laser pulse, and are thus
inadequate to perform gas phase and surface chemistry experiments. Moreover, even if such a problem could be overcome, their application would be limited to systems that do not suffer for the presence of the concomitant (strong) electric field necessary for the alignment [79]. Surface chemistry
clearly does not belong to this realm.
This key issue forced researchers to look for field free alignment methods employing e.g. a laser
pulse with long turn on (during which molecules are adiabatically aligned) and fast turn off time (leaving molecules in a broad rotational wave packet). This method leads to an efficient alignment under
field free conditions, as demonstrated experimentally for N2 [80]. However, to the best of our knowledge, so far there are no experiments which exploited such laser techniques for gas and surface chemistry investigations. The main reason for this is the short time stability of the achieved degree of
alignment when the molecules enter into the field free region. A theoretical and numerical investigation of the achievable alignment as a function of electric field was performed by Seideman [81], who
discussed in details the adiabatic and short pulse alignment.
Recently alternative approaches have been developed:
(a) A SMB of polar molecules was aligned along one direction by laser pulses; the molecules were
then spatially dispersed by a strong inhomogeneous static electric field according to their quantum state. Molecules residing in the lowest-lying rotational states could be selected and used as
targets for further experiments. This method was successfully applied to the iodobenzene molecule [82]
(b) State selection of O2 was achieved by a magnetic hexapole. The inversion of population for the
O2 spin-rotational states was produced by a two-coil spin flipper [83].
(c) Preparation of molecules in low rotational states by supersonic expansion and subsequent rotational excitation due to collision with an intersecting Ar beam was demonstrated for H2O [84].
Author's personal copy
98
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
(d) Preparation of rotationally aligned molecules has been obtained by velocity selection of a
seeded supersonic molecular beam [85]. Alignment is produced by the collisions of the molecules with the inert carrier gas during the supersonic expansion. A new design allows for a high
transmittivity enabling gas-surface interaction studies [86].
These approaches seem promising for gas phase and surface chemistry experiments since they spatially split the beam according to its initial quantum state. Experiments with at least partially defined
quantum states should thus become soon feasible by stopping the undesired components.
The present review is organised as follows:
After this introduction, we briefly review in Section 2 the stereodynamical information on gas-surface interaction obtained by measuring the state of desorbing molecules and invoking detailed balance.
In Section 3 we report on experiments with oriented molecules. Since previous results (part of which
have been mentioned in this introduction) have already been summarized by Ref. [45]), we shall focus
here on the most recent achievements and on more complex molecules. Section 4 is devoted to the
exploitation of collisional alignment for the preparation of aligned beams, on how to measure the degree of alignment and on applications to gas phase scattering. In Section 5 we show finally experimental results about the interaction of collisionally aligned molecules with surfaces. The last section gives a
glance at future perspectives in this fertile field of gas-phase and surface chemistry.
Results obtained by laser methods are not included in the present review in order to keep a well
defined focus and limit it to a reasonable length. Readers interested in laser methods are therefore advised to consult the recent review by Ref. [87]).
2. Stereodynamic effects as deduced by applying detailed balance to desorption experiments
In this section we firstly address the cases on H2 and D2 which, at variance with heavier molecules,
are characterised by a relatively large energy separation between rotational quantum levels. Afterwards, we will focus on larger molecules; we will distinguish between polar and non-polar molecules,
since for the latter stereodynamical effects are due only to the direction of the angular momentum
while for the former also molecular orientation can play a role.
2.1. H2/D2
H2 (and D2) interaction with surfaces has been studied extensively both theoretically and experimentally. The work on H2/Cu(1 1 1) by Rettner et al. [30] is one of the most detailed experimental
determinations of the effect of rotational energy on S. These authors determined the state resolved
flux, I(E, h, T, v, J), by measuring the time of flight distribution of H2 molecules desorbing from
Cu(1 1 1) in different rotational states (J) and in the fundamental vibrational (t) state. Invoking detailed balance, I(E, h, T, v, J) is related, in the limit of small coverage, to the state resolved sticking probability, S0(E, h, T, v, J), via the relationship:
IðE; h; T; m; JÞ sin h dh d/ dE / S0 ðE; h; T; m; JÞNðm; J; TÞEeE=kT cos h sin h dh d/ dE;
ð2:1Þ
where T is the surface temperature, N(v, J, T) is the Boltzmann population of state (v, J) and EeE=kT dE is
the probability to have translational energy E in the interval between E and (E + dE).By describing the
sticking probability with the usual empiric form:
Aðm; JÞ
En E0 ðm; JÞ
S0 ðE; h; T; m; JÞ ¼
1 þ erf
½88
2
Wðm; JÞ
ð2:2Þ
where A(v, J) is the saturation value of S0 for molecules in state (v, J), it was possible to obtain the
height E0(v, J) and the width W(v, J) of the translational energy barrier for different sets of quantum
numbers (v, J). The result is shown in Fig. 2.1 [30].
It is apparent that E0 initially increases and then decreases with J for both the fundamental and the
first vibrationally excited state. The behaviour at low J is connected to the rotational cooling of the
desorbing molecules while the reduction of the barrier height at high J results in an overpopulation
Author's personal copy
99
Ó American Institute of Physics 1995
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Fig. 2.1. Dependence of the parameters E0 and W on the rotational quantum number for H2 (v = 0) and H2 (v = 1) desorbed from
Cu(1 1 1) at 925 K. The lines are quadratic fits to the points. Taken with permission from Ref. [30].
of high J states, in agreement with previous literature [89]. Similar results were found also for the twin
systems D2/Cu(1 1 1) [35] and for H2/Pd(1 1 1) [90]. The non-monotonic dependence of E0 on J was explained by the following argument. For low J rotational motion may hinder adsorption by reducing the
time spent by the molecule in a favourable orientation with respect to the surface. For example D2
with E = 0.6 eV will rotate by 40° when moving over the distance of 0.5 Å in which the interaction
with the surface is significant if J = 6 but only by 9° if J = 1. There are in principle two ways in which
the rotational motion can inhibit adsorption: (a) the fraction of trajectories leading to adsorption may
be limited to a narrow cone of acceptance or (b) the molecule can adsorb also with an unfavourable
orientation, but a larger translational energy is then required to overcome the barrier. In the former
case the limiting value of S (parametrized by A(v, J)) should depend on the rotational state while in
the latter A(v,J) should be independent of J. The experimentally observed independence of A(v, J) on
J and the strong dependence of E0 on J indicate that the second hypothesis is the right one, i.e. that
there is a distribution of effective energy barriers associated with the different orientations of the
molecular axis with respect to the surface. The decrease of the barrier E0 for high J values is explained,
on the contrary, by an energy effect: rotational motion is coupled to the reaction coordinate allowing
the rotational energy to be used to overcome the barrier. Even if there is no general agreement on
whether coupling occurs mainly with translational or with vibrational [91,92] degrees of freedom,
it seems, however, quite unlikely that the reduction of E0 at large J is due only to the (limited) increase
in the bond length due to rotational excitation [35].
Further investigations on the D2/Cu(1 1 1) [10] system and on D2/Pd(1 0 0) [56] allowed to determine experimentally the state of alignment of the desorbing molecules. A quantitative description of
ð2Þ
this quantity can be given in terms of the quadrupole alignment parameter A0 :
ð2Þ
A0 ¼ h3 cos2 h 1i;
ð2:3Þ
which is defined as twice the expectation value of the second Legendre polynomial of cos h, i.e. of the
angle between the D2 angular momentum vector J and the surface normal z.
Two limiting cases are commonly reported in the literature: if J is perfectly perpendicular to the
ð2Þ
surface, h = 0 or p, and A0 ¼ 2. As already mentioned in the introduction, such a situation, strictly possible only for classical motion, is colloquially indicated as ‘‘helicoptering or propeller motion” because
the molecules mimic the motion of propellers ‘‘flying” away from the Cu surface. The second classical
Author's personal copy
100
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
limit consist in the perfect alignment of J parallel to the surface, corresponding to h ¼ p=2 and
ð2Þ
A0 ¼ 1, and is colloquially referred to as ‘‘cartwheeling motion”.
The outcome of the experimental determination of the quadrupole alignment moment for D2
desorbing from Cu(1 1 1) is shown in Fig. 2.2 [10]. The data demonstrate that a preference for helicoptering motion is present at low translational energy Ei and disappears as soon as Ei exceeds the average
activation barrier for adsorption (see inset). Therefore at low Ei the dissociative adsorption probability
is higher for D2 molecules colliding with the molecular axis parallel to the surface. A similar result was
found for D2/Pd(1 0 0) [56] and it was confirmed nicely by ab initio 6D calculations for H2/Pd(1 0 0)
[60] (see Fig. 2.3). The overall picture indicates that molecules approaching as helicopters are facilitated for dissociation. Theoretical calculations [59] predict, however, that, at translational energies
ð2Þ
Ei < 0.4 eV for H2/Cu(1 0 0), negative values of A0 are expected. This would imply a preference for cartwheeling motion, as previously suggested by Dino et al. [93] although with a different motivation.
Unfortunately no experimental confirmation is available yet.
2.2. Heavier non-polar molecules
Ó American Chemical Society 1987
Although most investigations on stereodynamics dealt with H2/metal systems, literature also reports both theoretical and experimental studies for N2/W(1 1 0) and NO/Ag(1 1 1). These systems
are characterised by the presence and by the absence of the rotational rainbow, respectively. This phenomenon, firstly observed for NO/Ag(1 1 1) [94,95], consists in an excess population at high J after the
ð2Þ
Fig. 2.2. The rotational quadrupole alignment moment A0 as a function of the translational energy of the D2 molecules
desorbing in specific rovibrational quantum states from Cu(1 1 1). The solid line shows the results for D2(v = 0, J = 11), whereas
the dashed line is for D2(v = 1, J = 6), both obtained from the best fits to the time-of-flight spectra. The vertical error bars reflect
the uncertainties in the fits, noise in the data, and in the case of (0, 11), the difference between two independent measurements.
ð2Þ
The horizontal error bars reflect the energy resolution in the experiment. (Inset) A0 plotted as a function of the difference
between the translational energy Ei (marked as Etrans in the figure) and E(iso)(v, J), where E(iso)(v, J) is the translational energy
threshold for an unpolarized ensemble of molecules. The similarity between the two is even more striking when one notes that
ð2Þ
ð2Þ
the quantum limiting value for A0 (0, 11) is 1.75, whereas that for A0 (0, 6) it is 1.57. Taken with permission from Ref.
[10].
Author's personal copy
101
Ó Elsevier 1997
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Fig. 2.3. Rotational alignment of hydrogen molecules desorbing from a Pd(1 0 0) surface. Boxes: experimental results for D2
[56]. Circles: 6-D calculations for H2. Taken with permission from Ref. [60].
scattering, so that the distribution of rotational quantum numbers does not follow Boltzmann’s
distribution.
2.2.1. A system without rotational rainbow: N2/W(1 1 0)
Kara and DePristo predicted a dependence of the sticking probability on the initial alignment for
N2/W(1 1 0) [66]. Experiments followed some years later, when rotationally inelastic scattering of
N2 from W(1 1 0) was reported [63]. Besides a dominant direct and rotationally inelastic contribution,
the measurements of the angular momentum alignment showed that, for E < 0.5 eV, molecules in low J
states rotate like helicopters,those in high J states rotate like cartwheels. For Ei > 0.5 eV only cartwheels are present, as evident from Fig. 2.4.
It is instructive to compare the behaviour of N2/W(1 1 0) with the one of N2/Pt(1 1 1). Both systems
are basically chemically inert and have no rotational rainbows. They should thus show a comparable
amount of rotational excitation while scattering experiments reveal that W(1 1 0) is 50% more efficient than Pt(1 1 1) in causing rotational excitation of the scattered molecules. This effect is not due
to the dissociation of molecules in low J states impinging side-on, since in this case such a marked
rotational excitation should appear only for translational energies for which dissociation is significant
(E P 0.5 eV), contrary to experimental evidence. It was then concluded that the larger rotational excitation observed for N2/W(1 1 0) is due to the larger dependence of the potential on the molecular orientation [96].
2.2.2. A system with rotational rainbow: N2/Ag(1 1 1)
From a classical point of view, the rotational rainbow arises from a maximum in the excitation
function vs rotor orientation.
This effect is illustrated in Fig. 2.5 for the case of N2 on Ag(1 1 1) [97], which was thoroughly investigated by the group of Zare [97–101]. The figure shows indeed that the maximum of the excitation
probability for N2 molecules shifts to higher J values with increasing translational energy.
ð2Þ
Fig. 2.6 (left panels) shows the quadrupole moment A0 for N2 scattered from Ag(1 1 1) under subspecular (top), specular (middle) and superspecular (bottom) conditions. The data indicate in all cases
a preference for cartwheeling motion, which increases with increasing J. The right column shows the
dipole moment: a positive value of this quantity indicates that the J vector points in the same direction
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Ó American Vacuum society 1993
102
Fig. 2.4. Angular momentum alignment vs exit rotational state for: (a) E = 0.25 eV, (b) E = 0.5 eV and (c) E = 1.0 eV N2 scattered
from W(1 1 0) at normal incidence and detected along the normal direction. The error bars are the uncertainties in determining
ð2Þ
A0 for each J state. Taken with permission from Ref. [63].
of the laser propagation direction and its magnitude is a measure of the fractional difference in the
population of molecules in states with +J and J, i.e. of the direction of rotation.
There is a complicated dependence of the orientation parameters such as final scattering angle and
final rotational state. Indeed molecules do not simply develop some amount of top-spin on impact but,
in certain conditions, those which are excited to intermediate final rotational states exhibit a counterintuitive backspin sense of rotation. This behaviour is explained by an intuitive frictional hard-core–
hard-ellipsoid model in which the presence of friction does not imply truly dissipative tangential
forces on the real molecule but may simply mimic a corrugated gas-surface interaction potential. Truly
dissipative mechanisms can, however, be represented by the creation of tangentially directed phonons
or of electron–hole pairs, similarly described by an effective friction coefficient. In presence of in-plane
forces, as in case D of Fig. 2.6 the momentum imparted by forces perpendicular to the surface is negative and adds to the still negative but smaller contribution due to tangential friction; so an even higher negative total momentum is obtained, corresponding to topspin. On the contrary, in case E the
dominant impulsive forces produce a positive momentum (backspin), and the (absolutely smaller in
Author's personal copy
103
Ó American Institute of Physics 1988
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Fig. 2.5. Boltzmann population plots as a function of the incident beam energy of N2 on Ag(1 1 1) for hi = 15°, hf = 20° at a
surface temperature of 90 K. Taken with permission from Ref. [97].
modulus) negative momentum provided by frictional forces only decreases the modulus of the maximum momentum, which however remains positive. In conclusion, the presence of tangential friction
splits the rainbow (which is a singularity in the cross section for rotational excitation): the components corresponding to situation D) are blueshifted in energy, those corresponding to E) are redshifted. This causes a net backspin in the region of lower energy of the rainbow, as observed experimentally. This oversimplified picture is supported by more realistic stochastic trajectory calculations
[100]. Additionally, further investigations showed that the angular momentum orientation is strongly
affected by T, being high at 90 K and substantially smaller at 540 K.
2.3. Heavier polar molecules
2.3.1. NO/Ag(1 1 1) and NO/Pt(1 1 1)
The very first determination of alignment in molecular desorption was reported for NO/Ag(1 1 1)
[102] and, later on, for NO/Pt(1 1 1) [64]. In the former experiment laser induced fluorescence (LIF)
2
P, whose intensity for the Q branch is:
was employed [102] which detects NO via the transition 2R
2 b2
P 2 ðcos h0 Þ ;
Iðh0 Þ b0 1 þ
5 b0
ð2:4Þ
where h0 is the angle between the direction of the laser polarization and the surface normal, P2 is the
second Legendre polynomial and bn is the state to be populated. Fig. 2.7 reports the total population
(plotted as ln(b0/(J + 1)), panel a) and the polarization anisotropy (P = b2/b0, panel b) vs the final internal energy. Both quantities are obtained by measuring the intensity at the angle h0 = 57°, for which
P2(cos h0) = 0. By definition, 2.5 6 P 6 5. The lower and upper limits correspond, respectively, to perfect alignment of J perpendicular and parallel to the surface normal, i.e. to ideal CW and HE motion.
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Ó American Institute of Physics 1988
104
Fig. 2.6. Quadrupole (left) and dipole (right) moments of N2 scattered off Ag(1 1 1), for subspecular (emission angle hf = 25, top),
specular (hf = 35, medium) and superspecular (hf = 50, bottom) conditions. The scheme reported in panels (D) and (E) gives the
basic idea of the frictional cube model: A and B are the major and minor axes of the ellipse and d is the moment arm at the
instant of impact. Cases (D) and (E) indicate the impact angles leading to a maximal rotational excitation. In absence of in-plane
forces they lead to the same rotational excitation (invariance by reflection with respect to the plane perpendicular to the surface
and passing through the point of impact). If an in-plane force is present, this degeneracy is removed and different rotational
excitations for the two situations arise. Taken with permission from Ref. [99].
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
105
The population exhibits the typical non-Boltzmann behaviour at high J, explained by the rotational
rainbow [94]. Moreover, a preference for the cartwheeling configuration is deduced from the negative
P values.
Similar results were found for the NO/Pt(1 1 1) system [103], for which both scattering and desorption were investigated. Inspection of the rotational population evidences that such quantity decreases
with increasing J and that the scattering event populates rotational levels with energies exceeding the
total incident beam energy. The excess energy can only come from the surface, most likely from surface vibrations. Such behaviour can be related to the fact that NO has a deep chemisorption well,
which accelerates the molecule onto the surface and enhances the impulsiveness of the collision.
Moreover, the polar nature of the potential, favouring adsorption via the N atom down, induces a larger degree of rotational excitation. The values of the quadrupole moment for NO molecules scattered
inelastically or desorbed from Pt(1 1 1) are shown in Fig. 2.8 (top and bottom panels, respectively). For
ð2Þ
J < 12.5, A0 0, indicating that no rotational alignment is present both for scattering and for desorpð2Þ
tion. For higher J, on the contrary, A0 is positive (prevalence of HE) in desorption experiments and
negative (prevalence of CW) for scattering measurements. This fact may be analysed in the light of
the principle of detailed balance. Jacobs and Zare [103] performed a classical calculation treating
the molecule as a rigid rotor and using the potential obtained by Muhlhausen et al. [104] for scattering
and trapping. They reproduced nicely the main experimental findings and, particularly relevant in this
context, the effect of orientation. They found indeed that at low J the orientation of the angular
momentum does not matter since the impinging reactant contains an equal number of molecules with
the nitrogen atom down (favourable condition) and with the oxygen atom down (unfavoured). On the
contrary at high J a preferential alignment sets in due to a dynamical effect. Rotational angular
momentum is more efficiently converted into linear momentum in the direction normal to the surface
for molecules rotating as CW than for molecules rotating as HE. The trapping probability will thus be
lower for the former than for the latter, in good agreement with experimental data. The effect becomes
relevant only when the amount of rotational energy is significant compared to translational energy,
thus explaining why the system appears to be stereo insensitive at low J.
Ó American Physical Society 1982
2.3.2. An apparently opposite behaviour: CH3F and OCS on glass
In 1991 Broers et al. reported a Surface Light Induced Drift (SLID) study on the accommodation
coefficient of CH3F and OCS colliding with a glass surface [105–107]. In this technique a narrow band
laser is tuned within the Doppler broadened absorption profile of a molecular gas at low pressure
Fig. 2.7. Rotational state distribution ln[b0/(2J + 1)] (a) and rotational polarization P = b2/bo (b) for NO scattering from Ag(1 1 1).
Taken with permission from Ref. [102].
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Ó American Institute of Physics 1989
106
Fig. 2.8. Quadrupole moment A20 vs J for NO inelastically scattered by (top panel) and desorbing from (Tdes = 553 K – bottom
panel) Pt(1 1 1). Taken with permission from Ref. [64].
(Knudsen gas), thus producing a velocity dependent selective excitation from one specific rovibrational level to another. If the accommodation coefficient for momentum parallel to the surface is different for the two selected states, a drift of the gas will occur. This causes in turn a pressure difference
proportional to the change in the accommodation coefficients along a closed tube containing the gas.
We address the readers to the original papers for more information on the experimental details and
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
107
focus here on the physical results. The experiments showed indeed that, for molecules having rotational energy well below the thermal one, trapping (and due to detailed balance also desorption)
has a higher probability for cartwheels than for helicopters. This is in apparent contradiction with
the NO/Pt(1 1 1) results reported in the previous section, for which a preference for helicoptering motion for trapping/desorbing molecules was demonstrated. A more detailed inspection shows indeed
that the anisotropy observed for NO/Pt(1 1 1) is present only at high J values, while the data by Broers
et al. refer to molecules in low J states. The authors provide a convincing qualitative explanation of the
observed effect: on a highly corrugated surface and at low J the coupling between translations and
rotations is such that the molecules suffer a loss of their ‘‘normal energy” due to translational into
rotational energy conversion, thus enhancing trapping. This coupling can be achieved more efficiently
for cartwheels, since helicopters have their molecular axis parallel to the surface and cannot experience a torque able to change their rotational state. Thus CW can be more easily trapped in the low
J limit. We mention that attempts to observe this effect by preparing a SMB by collisional alignment
(see sect. IV) failed so far. For high J values, on the contrary, the excess rotational energy is counterproductive for the trapping of CW, since it can be used to bounce the molecules back into the gas
phase. In this condition the trapping/desorption probability is therefore higher for HE, in accord with
what experimentally observed for NO/Pt(1 1 1).
2.4. Some considerations on the application of detailed balance
The principle of detailed balance states that, for a system at equilibrium, the rates of all forward
reactions must be balanced exactly by the rates of all reverse reactions. Thus, by observing a given
reaction it is possible to infer information its inverse process.
As evident also from the brief overview presented above, detailed balance was largely employed to
retrieve information on the dependence of S on vibrational and rotational quantum numbers by examining the state of the desorbing and/or scattered molecules. Such experiments involved indeed SMBs
without any previous state preparations except for the rotational cooling occurring because of the
supersonic expansion. A final a comment is thus necessary.
Clausing [108] showed that, for the case of a gas interacting with a surface in thermal equilibrium,
the angular distribution of particles which depart from the surface must follow a cosine flux distribution when summed over all channels. As pointed out by Jacobs et al. [64], however, this statement was
never proved rigorously for a system far from equilibrium (such as a supersonic jet interacting with a
surface at arbitrary temperature), although experiments have shown little deviation from the predictions of detailed balance [109]. Moreover, detailed balance states that, for a system in thermal equilibrium, the flux distribution of angular momenta for particles leaving the surface is isotropic when
summed over all channels; it does not say anything about the distribution within the single channels,
even if practically it was applied separately to direct inelastic scattering and to trapping desorption
[110]. To the best of our knowledge, however, there are no surface science reactions escaping this
more compelling formulation, so that information obtained from detailed balance studies maintains
its entire validity. In spite of that, the possibility to prepare molecules in a defined aligned or oriented
state remains an attractive challenge, not only for knowing the dependence of sticking on internal state
variables (information available also via detailed balance) but, more importantly, for using this tool to
enhance a surface reaction or to select among different possible reaction paths.
3. Experiments with oriented molecules
This chapter is devoted to describe how molecular orientation can be selected and controlled.
According to a classical picture, the orientation angle c of the molecular axis with respect to the
surface normal and the degree of orientation clearly affect the collision dynamics occurring on the
picosecond time scale, i.e., the rotational excitation of a molecule [41,111]. On the other hand, in
the stationary state of a molecule adsorbed on the surface, another orientation-dependent situation
may appear, i.e., the dependence of the molecular geometry on the adsorption site. Bond-breaking
and -forming processes, such as dissociative adsorption, are also important because they are usually
Author's personal copy
108
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Ó Elsevier 1999
Fig. 3.1. Angular distributions for direct scattering of NO from (a) Ag(1 1 1) and (b) Pt(1 1 1). Red and blue lines correspond to
N-end and O-end collisions, respectively. Copyright by Nature Publishing Group and American Physical Society. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Taken
with permission from Refs. [38,116].
Fig. 3.2. Time-of-flight distributions for N2O and N2. The time zero coincides with the opening of the pulsed nozzle. Upper
diagram: scattered N2O beam. Lower diagram: scattered N2 abstraction products for O-end down (filled circles) and N-end
down (open circles) incidence of an N2O beam, respectively. The inset indicates the scattering geometry. Taken with permission
from Ref. [123].
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
109
Ó Elsevier 1996
followed by surface chemical reactions. Now, the important question arises: how does the incoming
molecular orientation contribute to the final destinations, i.e. the bond-breaking and forming processes at surfaces?
The anisotropy of the interaction potential can be deduced as a parameter from very accurate studies of the rotational energy transfer in gas-surface experiments for simple diatomic molecules such as
NO [104,112]. However, studies with molecules prepared in well-defined spatial orientations provide
direct answers to the question concerning the anisotropy, or the so-called steric dependence, of the
interaction potential and its related chemical reactions.
As illustrated in the previous sections, in the past many attention focussed on the behaviour of
adsorbing/desorbing NO, a polar open-shell diatom having the behaviour of a symmetric top molecule.
The state selected for NO is defined (according to the Hund case (a) [113]) by |J, X, Mi, where J represents the total angular momentum, X is the electronic component along its axis and M is the J projection along the orienting field direction.
Fig. 3.3. Upper panel: CO2 partial pressure vs time for both spatial orientations of an NO beam impinging on Rh(1 0 0) at 393 K.
Lower panel: corresponding CO2 reaction asymmetry, Ar, as a function of time. Taken with permission from Ref.
[48].
Author's personal copy
110
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
In the first report on molecular-orientation effects in surface scattering by Kleyn’s group, the analysis of the angular distribution of NO scattering from Ag(1 1 1) (see Fig. 3.1(a)) demonstrated that the
approach in the less attractive orientation (O end towards the surface) results in a higher adsorption
probability than for N-end incidence [38]. Since NO/Ag(1 1 1) is a weakly-bound system, these results
can be understood by a strongly anisotropic, or orientation-dependent, repulsion and by the resulting
preferential rotationally-mediated adsorption for the O-end incidence [41,111,114,115]. The steric effect depends strongly on the substrate, i.e., on the interaction potential. Indeed, at variance with the
example reported before, on the strongly bound NO/Pt(1 1 1) system [44,45,116] (see Fig. 3.1(b)) the
N-end approach is more attractive than the O-end and leads to a higher trapping probability. The
highly reactive nature of the N-end approach is also demonstrated for NO/Al(1 1 1) and it is well
understood by density functional theory (DFT) calculations [159]. The authors also demonstrated that
the adsorption of H atoms on the surface modifies the interaction potential and its related steric effects in NO scattering from H/Ru(0 0 0 1) [117].
The molecular-orientation effects in reactive systems were also investigated extensively by Heinzmann’s group through measurements of the sticking probability and of the gas-phase products
[39,40,46–49,118–123]. They found that the N-end down approach is more reactive for the strongly
bound NO/Ni(1 0 0) [118,119] and NO/Pt(1 0 0) [40] systems. The results are consistent with the scattering experiments of Kuipers et al. [116]. In the reactions of oriented N2O molecules with alkali-metals adsorbed on Pt(1 0 0) and Rh(1 0 0) [121–123], the harpooning reactions occur preferentially in the
O-end down approach and the resulting N2 production is more effective for this orientation (see
Fig. 3.2). The same authors measured the steric effects in the NO–CO reactions on Pt(1 0 0) and
Rh(1 0 0) [46,48,120]. Fig. 3.3 shows the CO2 production in the reaction of oriented NO with CO/
Rh(1 0 0); it demonstrates that also molecules impinging with N-end down can be active in molecule–molecule reactions at the surface. This result hints to the possibility of a steric control of the
chemical reactions.
As an example of complex polyatomic systems we mention Bernstein’s systematic studies of alkylhalide molecules scattering from Highly Oriented Pyrolytic Graphite (HOPG) [124–132]. The results
are summarized in Table 1. Bernstein and co-workers performed scattering experiments by fixing
the total scattering angle and by rotating the sample. They demonstrated steric effects in the weak
interaction of polyatomic molecules. The magnitude of such effects depends on the alkyl chains and
halogens of alkyl-halide molecules, while their sign (with respect to the direction of the dipole vector
relative to the surface normal) differs from case to case: sometimes the positive end of the molecule
sticks preferentially, sometimes the negative one does. Therefore, the origins of the steric effects are
still unclear.
Table 1
Experiments on steric effect in molecule-surface scattering.
Molecule (beam)
CH3F
CH3Cl
CHF3
CHCl3
(CH3)3CCl
CH3CCl3
CH2Cl2
CClF3
CFCl3
CH3NO2
CH3OH
CH3CN
(CH3)3N
a
b
Steric effect
Preferred orientation for sticking
Signa
Magnitudeb
+
+
+
+
+
M
M
S
W
M
W
W
W
W
M
M
W
W
+H3CF Gr
+H3CCl Gr
F3CH+ Gr
Cl3CH+ Gr
+(H3C)3CCl Gr
Cl3CCH3+ Gr
Cl2CH2+ Gr
F3CCl+ Gr
+Cl3CF Gr
O2NCH3+ Gr
+H3COH Gr
NCCH3+ Gr
N(CH3)3+ Gr
Uncorrected ‘‘raw data” results: +: positive effect; : negative effect.
S = strong; M = medium; W = weak. Data are taken from Ref. [124]; copyright by Elsevier.
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
111
In the following of this section we describe how one can select a specific rotational state for a molecule and orient the latter to investigate the steric dependence of its reaction with a surface (Section
3.1). Afterwards, (Section 3.2), we describe the steric effects in the strongly bound systems of polyatomic CH3Cl on Si(1 0 0) and diatomic NO on Si(1 1 1) and in the weakly-bound systems of polyatomic CH3Cl on HOPG and Si(1 1 1).
3.1. Experimental
3.1.1. State-selected molecular beam
Two experimental setups, available at Osaka University and at SPring-8 in Japan, are shown schematically in Fig. 3.4(a) and (b), respectively. Each apparatus consists mainly of three parts. The first
contains the nozzle and buffer chamber generating a pulsed supersonic molecular beam (PSMB).
The second contains the hexapole state selector with a beam stopper and guiding electrodes. The third
consists in the ultra high vacuum chamber for analyzing surface-reaction processes. The detailed
descriptions of those apparatuses are available elsewhere [133–135].
The permanent dipole moments lel of 6.24 1030 and 5.3 1031 Cm for CH3Cl and NO, respectively, give the possibility to distinguish the two ends of the molecule in an electric field. The Stark
effect of a simple molecule was studied in the 1950s [136] and its application to the achievement
of focusing and orientation started about 40 years ago [137–139]. The degree of orientation has been
studied quantitatively [139,140]. Recently, beam focusing and manipulation of a molecule using the
Fig. 3.4. (a) Schematic top view of the oriented-molecular-beam line and surface-reaction analysis chamber at Osaka
University: PV, pulsed valve; SK, skimmer; C, beam collimator; HP, hexapole device; BS, beam stopper; GE, guiding electrode;
OE, orientation electrode; HOPG is the KW shutter; SA, sample; QM, quadrupole mass spectrometer. (b) Schematic top view of
the oriented-molecular-beam line and surface-reaction analysis chamber at SPring-8: C1, beam collimator; C2, beam collimator
(orientation electrode).
Author's personal copy
112
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Stark effect have been applied in a sophisticated way to excited CO molecules, OH radicals and NH3
[141,142]. In this section, we review the principles of focusing and orientation by the electrostatic
hexapole and a uniform field in front of the crystal surface. Concerning the focusing of the NO molecule, an excellent review has already been published [143]. Thus, we concentrate here on the focusing
of CH3Cl. Finally, we also introduce our evaluation of the sticking probability.
3.1.2. State selection
The force exerted by the electric field E inside the electrostatic hexapole on molecules possessing a
permanent dipole, lel, is radial and follows the derivatives of the Stark energy Ustark [2]. For a symmetric top molecule such interaction energy depends on the total angular momentum J and on its projections on the molecular axis, K, and on the space-fixed axis defined by the electric field, M. Ustark is
given, including second-order effects, by [136]
U stark ¼ qlel E þ
q ¼ hcos ci ¼
(
f ðJ; K; MÞ ¼
l2el E2
2Bhc
f ðJ; K; MÞ;
ð3:1Þ
KM
;
JðJ þ 1Þ
ðJ 2 K 2 ÞðJ 2 M 2 Þ
J 3 ð2J 1Þð2J þ 1Þ
ð3:2Þ
ððJ þ 1Þ2 K 2 ÞððJ þ 1Þ2 M 2 Þ
ðJ þ 1Þ3 ð2J þ 1Þð2J þ 3Þ
)
:
ð3:3Þ
Here, J, K, and M are the quantum numbers corresponding to J, K, and M, respectively. B, h, and c are
the rotational constant, Planck constant and the velocity of light, respectively. The angle c corresponds
to that defined by lel and E. lel and E stand for |lel | and |E|, respectively.
The force due to the inhomogeneous field of the state selector equals the derivative of the interaction energy Ustark with respect to the radial distance r from the hexapole central axis:
Fr ¼ dU stark
KM dE
:
¼ lel
JðJ þ 1Þ dr
dr
ð3:4Þ
Ó The Japan Society of Applied Physics 2005
Only the first-order term in Eq. (3.3) is considered because the contribution of the second-order term
is small. The force remains radial because the azimuthal angular derivative dUstark/dU vanishes for a
linear Stark effect in a hexapole field. Considering the field strength of E = 3V0r2/R2 in the hexapole
state selector we obtain:
Fig. 3.5. TOF spectra for PSMB of CH3Cl at |JKMi = |1 1 1i (red dashed lines) together with TOF spectra with no hexapole voltage
(blue full lines). (a) CH3Cl (25%)/Ne, (b) CH3Cl (25%)/Ar, and (c) CH3Cl (25%)/Kr. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.) Taken with permission from Ref.
[134].
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Fr ¼ dU stark 6lel V 0 KM
r;
¼
dr
R3 JðJ þ 1Þ
113
ð3:5Þ
where V0 is the absolute value of the voltages applied to the hexapole. Negative and positive voltages
are applied to every other pole of the hexapole, respectively. R is the radius of the circle inscribed in
the rod surfaces. From Eq. (3.5) it directly follows that the states with a positive Stark effect (i.e., KM
< 0) yield Fr < 0 and thus are deflected towards a region with a low field strength on the axis of the
PSMB. This force shows a linear dependence on r, and therefore, the molecules focused using the hexapole deflector follow a sinusoidal trajectory. The states with a negative Stark effect (i.e., KM > 0) reduce
their potential energy in the high-field regions near the rods. They are pulled away from the axis and
defocused. Since the molecules move with an uniform velocity along the x-axis of the hexapole, the
focal length of the molecule can be controlled by adjusting V0. In addition to orientation experiments,
the state-selection technique using the hexapole can be applied to the state-selected chemistry of excited CO [144] and OH radicals [141] interacting with surfaces.
The state selection using the hexapole is primarily checked by measuring the QMS intensity IB produced by the SMB in the detection chamber as a function of the voltage on the rods of the hexapole. By
tuning the hexapole voltage, optimal focusing is easily achieved in the signal IB on the detector. Fig. 3.5
shows the typical TOF spectra of the |JKMi = |1 1 1i state for CH3Cl (25%) seeded in Ne, Ar and Kr, corresponding to kinetic energies Ei = 180, 120, and 65 meV, respectively (in the representation of |1 1 1i,
the signs were neglected for simplicity). Both TOF spectra corresponding to the turn-on and -off of the
hexapole voltage are shown for each Ei. These data suggest that the direct beam is completely blocked
by the beam stopper and only the state-selected part is well focused in the UHV chamber.
Fig. 3.6(a)–(c) shows the focusing curves for the PSMBs with, respectively, Ei = 180, 120, and
65 meV impinging directly upon the QMS. Here, the intensity IB is plotted as a function of the absolute
value of voltages applied to the rods of the hexapole. There are several peaks in the focusing curves
shown in Fig. 3.6. To assign them to the proper rotational states we carried out trajectory simulations
of CH3Cl molecules in the following manner [134,135]: the PSMB generated at the nozzle (x = 0) enters
the hexapole state selector and is subject to the force described by Eq. (3.5) due to the hexapole field.
Thus, the trajectory of a molecule can be written as
rðt þ DtÞ ¼ rðtÞ þ
ar ðtÞ 2
Dt þ tr ðtÞDt;
2
tr ðt þ DtÞ ¼ tr ðtÞ þ ar ðtÞDt;
ð3:6Þ
ð3:7Þ
Ó The Japan Society of
Applied Physics 2005
where ar(t) = Fr/m, vr(0) = r(0)v/x(0) and t = [x(t) x(0)]/v, with x(t) being the distance along the beam
central axis at time t, m the molecular mass, ar(t) the acceleration of the molecule at the radial displacement r(t), vr(t) the velocity in the radial direction at time t, and v the velocity along the beam axis.
The entrance of the hexapole field is set as the origin of time (t = 0). The outcome of the simulations
(full lines) are shown together with the experimental data in Fig. 3.6. From these best-fit simulated
results, the rotational temperatures reproducing the focusing curve are 25 K, 11 K, and 12 K for the
CH3Cl (25%)/Ne, CH3Cl (25%)/Ar and CH3Cl (25%)/Kr PSMBs, respectively. As a result of simulation,
Fig. 3.6. Focusing curves for (a) CH3Cl (25%)/Ne, (b) CH3Cl (25%)/Ar, and (c) CH3Cl (25%)/Kr. The open circles (red on-line) and
full lines (blue on-line) indicate the measured and simulated focusing curves, respectively. Peak A in each panel corresponds to
the |JKMi = |1 1 1i state and its related TOF spectrum is shown in Fig. 3.5. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.) Taken with permission from Ref. [134].
Author's personal copy
114
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
peak A in Fig. 3.6 is assigned to |JKMi = |1 1 1i. As mentioned above, the corresponding TOF spectra for
this state are shown in Fig. 3.5. The purity levels of the |JKMi = |1 1 1i state are more than 70% for
Ei = 65 and 120 meV and more than 50% for Ei = 180 meV.
Fig. 3.7 shows the focusing curve and its simulation for a 58 meV NO SMB, measured with the apparatus in Fig. 3.4(b). The state |JXMi= |0.5 0.5 0.5i (see introduction of Section 3) is focused as a welldefined sharp peak. The state purity of |JXMi = |0.5 0.5 0.5i is 90% even without beam stopper.
3.1.3. Orientation
Molecules in a single KM state become oriented in a uniform electric field. The latter is applied to
obtain a beam of preferentially oriented CH3Cl molecules. When a state-selected beam is introduced
adiabatically into a uniform electric field, the orientation distribution for a molecule in the |JKMi rotational state can be described in a series of Legendre polynomials Pn(cos c) [145]:
PJKM ðcos cÞ ¼
2J
2J þ 1 X
C n ðJKMÞPn ðcos cÞ:
2 n¼0
ð3:8Þ
Here, the coefficient Cn is given in the 3 j symbolic description by
C n ðJKMÞ ¼ ð2n þ 1Þð1Þ
MK
J
J
n
K
K
0
J
J
n
M
M
0
ð3:9Þ
Coefficients Cn have been tabulated for |JKMi values up to J = 4 [145]. Eq. (3.8) can also be expressed in
terms of the Legendre moments of the probability distribution function, as in Refs. [146,147]:
Pn hPn ðcos cÞi;
PJKM ðcos cÞ ¼
2J X
2n þ 1
n¼0
2
n Pn ðcos cÞ:
P
ð3:10Þ
Ó The Japan Society of Applied Physics 2008
n for symmetric tops have been tabulated for J 6 3 [147].
The Legendre moments P
In real experiments, it is not feasible to select just one single state of the molecules. Thus, the orientation distribution of the state-selected molecules can be described as the sum over the individual
states weighted by the population of each state. As a result, the orientation distribution function
Fig. 3.7. Focusing curve for NO(20%)/Ar. The contribution of the direct beam was subtracted. The open circles (red on-line) and
full lines (blue on-line) indicate the measured and simulated focusing curves, respectively. The dashed and dotted lines (blue
on-line) correspond to the components of |JXMi = |0.5 0.5 0.5i and |1.5 0.5 1.5i, in the simulated curve, respectively. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Taken
with permission from Ref. [135].
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
115
WV(cos c) at a given hexapole voltage, V0, can be obtained once we know the distribution WV(JKM) of
the |JKMi states at V0 [146]:
W V ðcos cÞ ¼
X
PJKM ðcos cÞW V ðJKMÞ;
ð3:11Þ
JKM
Ó The Japan Society of Applied Physics 2005
where PJKM(cos c) is given by Eq. (3.8), (3.10).
Based on the simulation, we can obtain the state distribution function WV(JKM) of the |JKMi states
at fixed V0, and thus the orientation distribution function WV(cos c) in Eq. (3.11). The obtained
WV(cos c) for the CH3Cl (25%)/Ne, CH3Cl (25%)/Ar and CH3Cl (25%)/Kr SMBs are shown in Fig. 3.8 in
a polar plot for the Cl-end distribution and for the random orientation distribution. The Legendre mo n hPn ðcos cÞi of W V ðcos cÞ for the j111i state at 120 meV is, taking the state purity into conment P
1 ¼ 0:47, and P
2 ¼ 0:06, higher terms being negligibly small. P
1 and P
2 correspond
0 ¼ 1, P
sideration: P
n of W V ðcos cÞ at Ei = 65 meV and 180 meV are
to the average orientation and alignment, respectively. P
P 1 ¼ 0:47 and 0.48, and P 2 ¼ 0:06 and 0.05, respectively. When we reverse the electric field, the distribution corresponds to the CH3-end distribution.
Fig. 3.9 reports a similar polar plot of the orientation distribution function for the
|JXMi = |0.5 0.5 0.5i state in a 58 meV NO SMB. The plot was determined from the focusing curve of
Ó The Japan Society of Applied Physics 2008
Fig. 3.8. Polar plots of estimated orientational distributions WV(cos c) (solid lines around the shaded area, red on-line) at peak A
in Fig. 3.6 for (a) CH3Cl (25%)/Ne, (b) CH3Cl (25%)/Ar, and (c) CH3Cl (25%)/Kr. The additional line in (b) (blue on-line) corresponds
to the |JKMi = |2 1 2i state. If the orientational electric field is applied as indicated in the figure, the Cl-end of CH3Cl will be
distributed as shown. The circles in the centres correspond to a random orientational distribution. Data are taken with
permission from Ref. [134].
Fig. 3.9. Polar plot of the estimated orientational distribution WV(cos c) for the NO (20%)/Ar SMB at a hexapole voltage of 4.6 kV
(solid line, red on-line), associated to the focusing curve reported in Fig. 3.7. The dotted line corresponds to a random
orientational distribution. The polar plots correspond to the N-end down distribution of NO. Only the contribution of stateselected beams is considered. (For interpretation of the references to colour in this figure legend, the reader is referred to the
web version of this article.) Taken with permission from Ref. [135].
Author's personal copy
116
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Fig. 3.7. The state of |JXMi = |0.5 0.5 0.5i can be used for the orientational experiments. The corre n hP n ðcos cÞi, taking the state purity into consideration, is:
sponding Legendre moment P
P0 ¼ 1; P1 ¼ 0:303 and P2 ¼ 0, the other higher-order terms being negligibly small. Here, c is the angle
between the orientational field and the NO axis, defined to point from O towards N, opposite to the
permanent dipole moment [148].
3.1.4. Measurements of initial sticking probability
The initial sticking probability of CH3Cl on clean Si(1 0 0) was measured by the retarded reflector
method of King and Wells (KW in the following) [149]. This technique, extremely useful for the measurement of gas-surface interaction dynamics as long as the sticking coefficient is larger than 2%, is
extensively employed also in the study of rotationally aligned molecules (see Section 5). Therefore, we
describe it in some details, making reference to Fig. 3.10.
Fig. 3.10. Schematic drawing representing the different steps of a KW experiment (left panel) and the corresponding KW traces
(right panel): (a) in absence of hexapole voltage no beam enters the surface-reaction analysis chamber since it is intercepted by
the beam stopper; (b) when the hexapole voltage is switched on and the HOPG shutter is on the line in sight of the beam the
pressure rises to a level determined by the incident flux and the pumping speed of the UHV system; (c) the hexapole voltage is
on and the HOPG shutter is removed: the beam is dosed on the Si surface and the pressure decreases due to the gettering action
of the sample; (d) the hexapole voltage is on and the HOPG shutter is moved back to the line of sight of the beam. The pressure
returns to the level it had for case (b); (e) when the hexapole voltage is switched off the beam is stopped and the pressure in the
surface-reaction analysis chamber recovers the value it had in (a).
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
117
The experimental set-up consists of a SMB, of two inert (HOPG or other) beam stoppers installed
in line of sight with it (in the SMB and in the analysis chamber, respectively) and of a QMS not in
line of sight with the beam (to record the partial pressure in the analysis chamber). In the case of a
hexapole-oriented CH3Cl beam, when no voltage is applied to the hexapole state selector
(Fig. 3.10(a)), the PSMB with only diverging trajectories is blocked by the first beam stopper and
it cannot enter the surface-reaction analysis chamber. The CH3Cl partial pressure corresponds
therefore to the background level. When the voltage corresponding to some rotational state is applied to the hexapole, the state-selected PSMB is focused into the surface-reaction analysis chamber, as witnessed by the sudden increase in the CH3Cl partial pressure to the equilibrium value Pi
(Fig. 3.10(b)). As the second HOPG flag is still on the SMB path, the latter does not hit the sample.
Since the background pressure in the UHV chamber remains low (109 mbar), the sample is still
clean. Next, we remove the beam stopper and allow the state-selected PSMB to hit the clean Si surface. As a result of the reaction of CH3Cl with Si, part of the molecules are removed from the gas
phase and their partial pressure decreases suddenly to the value Pf (Fig. 3.10(c)). We can evaluate
the initial sticking probability as:
S0 ¼
Pi Pf
:
Pi
ð3:12Þ
Ó The American Physical Society 2005
After a fixed exposure time the HOPG shutter is restored back to the original position and the initial Pi
value is reproduced (Fig. 3.10(d)). For systems in which metastable species accumulate on the sample
(e.g. physisorbed species) desorption is observed at this stage of the experiment due to the breaking
up of the equilibrium conditions (not shown in Fig. 3.10).
Finally, the beam is stopped by turning the high voltage applied to the hexapole state selector off,
in order to ensure that no changes occur in the background of the spectrum. We can measure the orientation dependence of the KW spectrum by varying the direction of the electrostatic orientation field
in front of the surface. KW experiments are generally affected by an error of ±0.015 in the determination of S. For velocity selected beams (see Section 5) it increases up to 0.07 due to the reduced beam
flux and to the consequently higher noise level. The method can therefore be applied only if S is great-
Fig. 3.11. Molecular orientation dependence of KW spectra for a state selected CH3Cl |1 1 1i beam with Ei = 120 meV impinging
on a Si(1 0 0) surface at 323 K. From left to right, the spectra correspond to the CH3-end, random orientation, and Cl-end
collision, respectively. The numbers in the left spectrum correspond the same steps as in Fig. 3.10. Dashed and dot-dashed lines
are guides for the eyes. Taken with permission from Ref. [150].
Author's personal copy
118
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
er than this value. Moreover it measures the total sticking coefficient, without information on the
chemical state of the adsorbed species. Alternative, complementary techniques as vibrational or photoemission spectroscopy must therefore be applied for a complete characterisation of the adsorbate–
metal system under investigation.
KW traces give a lot of information on the behaviour of S as a function of exposure and on the total
coverage on the sample. S(t) is deduced by subtraction of the trace from the level corresponding to the
equilibrium partial pressure with the beam on. Once the flux, U, is known, the total coverage (inclusive of all stable and metastable adsorbed species) can be calculated from integration of such curve
through the formula:
HðtÞ ¼ U
Z
t
Sðt 0 Þdt
0
ð3:13Þ
0
so that also the dependence of S vs. total coverage can be drawn.
For the NO/Si(1 1 1) system, we evaluated the reactive sticking probability from the slope of the
uptake curves of dissociated N and O atoms recorded by X-ray photoemission spectroscopy (XPS).
XPS measurements were performed with the apparatus reported in Fig. 3.4(b), as described in Ref.
[135]. This apparatus will be connected to the synchrotron light source in the future.
3.2. Steric effects at the strongly bound CH3Cl/Si(1 0 0) and NO/Si(1 1 1) and at the weakly bound CH3Cl/
HOPG and CH3Cl /Si(1 1 1) systems
3.2.1. Steric effects appearing in the strongly bound system of polyatomic CH3Cl/Si(1 0 0) [150,151]
The direct reaction of elemental silicon (Si) with methyl chloride (CH3Cl) [152], forming dimethyldichlorosilane (DMD) as a major product along with other methylchlorosilanes, plays a central
role in the silicon industry. DMD is a key intermediate in the manufacture of this technologically
important silicon polymer. It has been reported that DMD formation can readily take place by reaction
with elemental Si alone, in absence of a catalyst [153]. However, many other side reactions occur. In
order to understand the generally important silane-formation reaction, it is necessary to investigate
the dynamical aspects of the interaction of CH3Cl with Si in each elementary step of the reaction. Such
investigation is also important in the field of diamond and silicon carbide film growth [154–157] particularly because of the high reactivity of methyl halides compared to hydrocarbons.
The dissociative adsorption of CH3Cl on Si is the first elementary step of the silane-formation reaction. According to scanning tunneling microscopy (STM) studies [154,158], the ratio of adsorbed Cl to
CH3 is 2:1 for the dissociative adsorption of CH3Cl on Si(1 0 0) at room temperature. This anisotropy in
the dissociated products suggests a steric effect in the adsorption process and also the possibility of
controlling the final products by manipulating the orientation of the molecules colliding with the surface. CH3Cl adsorbs dissociatively on Si(1 0 0) via precursor states [156,157] that may scramble the effect of the incoming molecular orientation. Although steric effects of reactant have been reported in
the collision dynamics [38,41,44,45,111,114–116,124–132] and reactions [39,40,46–49,
118–123,159] of several systems, the role of the incoming molecular orientation for precursor-mediated adsorption on Si has not been understood yet, in spite of its possible importance for chemical
reactions of organic molecules with Si [154,160].
Fig. 3.11 shows KW spectra recorded for a CH3Cl SMB in the |1 1 1i state incident with Ei = 120 meV
on clean Si(1 0 0) at 323 K. The different traces refer to beams with different orientation; such a
parameter was varied by changing the direction of the electrostatic orientation field in front of the surface. Clearly, S0 is higher for the Cl-end down (SCl) than that for the CH3-end down ðSCH3 Þ, while S0 for
the random orientation (Srandom) has an intermediate value.
Fig. 3.12 reports the outcome of sticking probability measurements of non-oriented CH3Cl beams
with Ei = 120 meV on Si(1 0 0). Panel (a) shows the surface-temperature dependence of Srandom for
the |1 1 1i state (empty symbols) and a few data for the |2 1 2i state (full symbols). The strong T
dependence of Srandom suggests that the dissociative adsorption of CH3Cl is precursor-mediated
[156,157]. This hypothesis is also supported by the result of no obvious incidence-angle dependence
of Srandom. We exclude that the Srandom(T) dependence may be caused by recombinative desorption of
Author's personal copy
119
Ó The American Physical Society 2005
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Fig. 3.12. (a) Srandom(T) curve for the CH3Cl |1 1 1i (open circles, red on-line) and |2 1 2i (squares, blue on-line) beams with
Ei = 120 meV impinging on Si(1 0 0); (b) T dependence of SCl/Srandom and SCH3 /Srandom for the CH3Cl |1 1 1i (solid and open circles,
respectively, red on-line) and |2 1 2i beams (solid and open squares, respectively, blue on-line) with Ei = 120 meV impinging on
Si(1 0 0). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this
article.) Taken with permission from Ref. [150].
dissociatively chemisorbed species, since it occurs at much higher T [156]. Panel (b) reports the T
dependence of SCl/Srandom and SCH3 /Srandom for incidence in the |1 1 1i state (black open and solid circles). Unity corresponds to the absence of orientation effects. This is the case below 280 K, while above
that temperature an orientational effect appears, whose onset agrees well with the decrease of Srandom(T). It is quite interesting that, when the |2 1 2i state (corresponding to the additional line in
Fig. 3.8(b), blue on-line) is dosed on the surface, no orientational effects are observed (Fig. 3.12(b),
small full and empty squares, blue on-line).
Surprisingly, no evident orientational effects were observed over the same T range for the T dependence of SCl/Srandom and SCH3 /Srandom in the |1 1 1i state and with Ei = 180 meV and 65 meV (see
Fig. 3.13), although a similarly strong Srandom(T) dependence is present at these energies (see Ref
[150]).
The sticking probability S0(cos c) as a function of orientation can be expanded in the Legendre polynomials Pn in analogy to Eq. (3.8),
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Ó The American Physical Society 2005
120
Fig. 3.13. T dependence of SCl/Srandom (red solid circles) and SCH3 /Srandom (blue open circles) for CH3Cl |1 1 1i beams at Ei =(a) 65
and (b) 180 meV. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of
this article.) Taken with permission from Ref. [150].
2J
X
S0 ðcos cÞ ¼
S0;n Pn ðcos cÞ:
ð3:14Þ
n¼0
Thus, Soriented/Srandom (Soriented = SCH3 or SCl) can be written as
Soriented
¼
Srandom
Z
1
S0 ðcos cÞW oriented ðcos cÞdðcos cÞ:
ð3:15Þ
1
Here, Woriented(cos c) corresponds to WV(cos c) for the Cl-end or CH3-end collisions in Eq. (3.11). Moreover, the relation:
Z
1
S0 ðcos cÞW random ðcos cÞdðcos cÞ ¼ 1:
ð3:16Þ
1
is required for normalization (Wrandom(cos c) corresponds to WV(cos c) for the random orientation in
Eq. (3.11)).
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
121
For the |JKMi = |1 1 1i state of the symmetric top molecule, the Legendre moment for n = 2 corresponding to the alignment effect is smaller than those for n = 0 and 1, as discussed in Section 4.2.3.
Thus, we can neglect the contribution of the second Legendre moment and write the approximated
relation:
½SCl ðhcos ciÞ þ SCH3 ðhcos ciÞ=2 Srandom :
ð3:17Þ
Ó The American Chemical Society 2007
The orientation effect observed at Ei = 120 meV almost satisfies the relation in Eq. (3.17) (see
Fig. 3.12(b)). The lack of orientation effects at 65 and 180 meV in Fig. 3.13 suggests that in the present
system S0,1 in Eq. (3.14) depends on translational energy through dynamical effects.
Which mechanism could induce the strong steric effect appearing specifically for the j1 1 1i state at
Ei = 120 meV in the precursor-mediated adsorption? The correlation between the onset of the steric
effect and the decrease of Srandom in Fig. 3.12 suggests that the observed steric effect strongly couples
with the desorption from the potential well of the precursor state. If the desorption occurs from the
stabilized precursors, the memory of the initial incoming molecular orientation will be lost. Thus, it
is considered that desorption occurs from a non-equilibrium state, before the molecule is thermally
stabilized into a precursor state via energy dissipation processes of phonon and/or electron–hole-pair
excitations [161,162]. Such a desorption may occur, possibly by coupling with phonons, within the
time scale of thermalization of 100 ps [161], after the molecules are transiently trapped into the precursor potential well. A molecule occupying an upper-level (more weakly bound) in the potential well
desorbs more easily than one in a deeper (more strongly bound) one. The Cl-end collision is expected
to suffer more effective energy dissipation than the CH3-end collision because of the higher mass and
larger electron density. It has been reported that the transient trapping probability for the Cl end is
higher than the one for the CH3 end in scattering experiments off non-reactive graphite surfaces, as
will be discussed in the next section. Thus, the observed steric effect in Fig. 3.12(b) comes from the
molecular-orientation effect in the energy dissipation process of incoming molecules and from the
resulting trapping probability into the precursor state. During transient trapping, the successive chemical processes also contribute to the steric effect. For the CH3Cl/Si(1 0 0) system, the abstraction
reaction of CH3Cl (g) + Si = Si ? Si–Cl(s) + Si(s) + CH3(g) is expected to be thermodynamically and stereo-dynamically favourable for the Cl-end collision [154], in addition to the dissociative adsorption
Fig. 3.14. Molecular-orientation dependence of the KW spectra recorded upon exposure of a Si(1 0 0) surface at 80 K and 147 K
to a CH3Cl j1 1 1i beam with Ei = 120 meV. Continuous (red on-line) and dashed (blue on-line) lines correspond to the QMS
traces recorded for Cl-end and CH3-end incidence, respectively. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.) Taken with permission from Ref. [167].
Author's personal copy
122
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Ó The American Institute of Physics 1990
process. The direct access to the abstraction-reaction channel during the non-equilibrium energy dissipation process enhances the observed steric effect.
At Ei = 65 meV steering effects become dominant and smear out the initial memory of molecular
orientation in the energy dissipation process and in the resulting reaction paths. This can be rationalized considering: (i) the sticking probability close to unity for T < 320 K (see Fig. 3.12(a)) and (ii) the
relatively small kinetic energy compared to the expected potential well depth for the precursor state
(270 meV) [163]. At Ei = 180 meV an incoming molecule collides with a more corrugated repulsive potential barrier. As a result, effective rotational excitations and momentum conversions into the direction parallel to the surface are induced. This expectation was supported by the clear decrease of S0
with increasing Ei, compared to Ei = 65 and 120 meV [150]. The decrease suggests the rapid quenching
of the steering effects and the increase of the repulsive interaction [164]. Both dynamical processes
smear out the initial molecular-orientation effect, as shown in Fig. 3.13.
Theoretical calculations [163,165,166] proposed several geometries for the stable precursor state
into the dissociative adsorption. One of them is almost parallel to the surface with CH3 between the
two dimer rows and Cl close to the lower Si of a dimer. On the other hand, for the j2 1 2i state at
0 ¼ 1, P
1 ¼ 0:33, and P
2 ¼ 0:11. The contribution of P
2 relative
n are P
120 meV, the corresponding P
to P1 is larger for the j2 1 2i state than for the j1 1 1i state. The average orientation (P 1 ) is closer to
the surface parallel for the j2 1 2istate (hcos ci ¼ 0:33) than for the j1 1 1i state (hcos ci ¼ 0:47). More 1 , the rotational mo 2 ) relative to P
over, from the significant contribution of the average alignment (P
tion of the j2 1 2i state is also closer to the stable precursor geometry as shown in Fig. 3.8(b) and thus
the orientation effect is smeared out effectively by steering [151,164] in the attractive well of the precursor state.
Now, we will move back to the original question of how the initial molecular orientation contributes to the final products of dissociated species. As already mentioned, when Si(1 0 0) is exposed to
thermal CH3Cl gas at room temperature (Ei 25 meV), the ratio between adsorbed Cl atoms and
CH3 groups is 2, from which we expect a contribution of molecular orientation [154,158]. However,
at Ei = 65 meV, there were no molecular-orientation effects in S0, as shown in Fig. 3.13. Thus, the
anisotropy in the dissociation is not governed by the orientation-dependent trapping probability into
a precursor state but by the reaction paths from the precursor state to the dissociatively adsorbed
state. In one reaction path both CH3 and Cl are adsorbed on the surface (stoichiometric dissociative
adsorption), but another reaction path exists for which CH3 desorbs from the surface (abstraction).
Now, our finding of a clear steric effect opens the following scenario: the trapping process of the
j1 1 1i state at Ei = 120 meV keeps memory of the incoming molecular orientation. This process may
couple strongly with the bifurcation of orientation-dependent reaction paths, which is smeared out
due to dynamical effects at Ei = 65 and 180 meV.
Fig. 3.15. Angular dependence of the steric effect for the CH3Cl(16%)/He beam. The symbols correspond to experimental data
points and the dashed line to the best-fit curve from the theoretical component model. Taken with permission from Ref.
[127].
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
123
The reaction path selection was observed in the KW kinetics. Fig. 3.14 shows KW traces recorded
during exposure of Si(1 0 0) at 80 K and at 147 K to a CH3Cl j1 1 1i beam with Ei = 120 meV [167]. At
80 K (top traces) the Cl-end approach reached the saturation faster than the CH3-end approach. This
result can be understood considering the reaction-route selection (molecular vs dissociative adsorption) due to molecular orientation: the Cl-end approach selects the dissociative adsorption, while
the CH3-end approach selects the molecular adsorption. When we rise the surface temperature to
147 K (bottom traces), the KW kinetics does not show the molecular-orientation dependence since
at this T molecular adsorption is only a metastable precursor to dissociation and thus, the final fate
of the molecule is the same for both orientations.
3.2.2. The weakly-bound systems of polyatomic CH3Cl/HOPG
In Section 3.2.1, we proposed that transient trapping into the weakly bound precursor states plays
a key role in the observed steric effects. In order to elucidate the role of the weak interaction potential,
Fig. 3.16. (a) Typical TOF spectrum (solid, red on-line curve) for the 320 meV CH3Cl beam (randomly oriented) scattered from
HOPG at 300 K. The solid (blue on-line) and dashed (green on-line) curves correspond to the direct inelastic and trappingdesorption components, respectively. (b) Molecular-orientation dependence of the TOF spectra for the 320 meV CH3Cl |1 1 1i
beam scattered from HOPG at 300 K. The red and blue lines correspond to the CH3-end and Cl-end incidence, respectively. The
angle of incidence was 34° and the exit angle was 56°. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
Author's personal copy
124
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
we performed scattering experiments on CH3Cl/HOPG and CH3Cl/Si(1 1 1). The former is a weaklybound physisorbed system [168], while the latter is a weakly-bound physisorption/chemisorption system [169]. As we mentionned in Section 3.1, also Bernstein et al. studied extensively steric effects in
the interaction between alkyl-halide molecules and HOPG [124–132]. For the same CH3Cl/HOPG system presented here, they found orientation effects in the scattering yield and a higher trapping probability for the Cl-end approach than for the CH3-end approach. Fig. 3.15 reports the steric effect
measured vs the exit angle [127]. For this experiment the authors used a quite wide pulsed beam
Fig. 3.17. Angle dependence of the stereodynmical asymmetry for the 320 meV CH3Cl beam scattered from HOPG. See text for
details.
Fig. 3.18. Molecular-orientation dependence of the TOF spectra for the 320 meV CH3Cl |1 1 1i beam scattered from Si(1 1 1) at
300 K. The solid (red on-line) and dotted (blue on-line) lines correspond to the CH3-end down and Cl-end down cases,
respectively. The angle of incidence and of detection are 45°. (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
125
(1.5 ms width) and measured the TOF spectra without beam stopper and without differential pumping, with the result of having a large background. We have repeated the experiment improving the
experimental conditions, measuring the TOF spectra with a 0.2 ms pulsed beam and using a beam
stopper and differential pumping.
Fig. 3.16(a) shows the typical TOF spectrum for a 320 meV CH3Cl j1 1 1i beam impinging on HOPG
at the incident angle of 34° from the surface normal. We set the total scattering angle to 90° (it was 60°
for Bernstein and co-workers [127]), and thus the detection angle is 56° from the surface normal. We
can separate the spectrum into two components, based on an analysis similar to the one used by Sasaki and Yoshida [170]. One corresponds to direct inelastic scattering (solid curve, blue on-line) and the
other to trapping desorption (dashed curve, green on-line). Fig. 3.16(b) shows the typical orientation
dependence of the TOF spectra. It is evident that the steric effect appears in the direct inelastic scattering and the CH3-end approach is scattered more strongly than the Cl-end approach. We could thus
reproduce an orientation dependence similar to the one reported by Bernstein and co-workers [127].
From the TOF measurements, we could plot the stereo-asymmetric factor, defined as
R¼
ICH3 ICl
;
2Irandom
ð3:18Þ
where ICH3 , ICl and Irandom correspond to the intensities obtained integrating the TOF spectra for the direct inelastic scattering of CH3-end, Cl-end and random orientation, respectively (Fig. 3.17). Positive R
values were observed for all surface temperatures and scattering geometries, confirming that the trapping probability for Cl-end molecules is higher than for CH3-end molecules.
A similar trend was also observed in the scattering of 320 meV CH3Cl j1 1 1i from the Si(1 1 1) surface. This is also a weakly-bound system, in which no adsorption occurs at T above 140 K [169],
while molecular adsorption occurs below this T. Fig. 3.18 shows the typical orientation dependence
of the TOF spectra measured at 45° specular reflection. The steric effect appears again in the direct
inelastic scattering and the CH3-end approach is scattered more strongly than the Cl-end approach.
These experimental results, demonstrating the higher trapping probability for the Cl-end molecule
than for the CH3-end molecule, are consistent with the interpretation of the steric effects appearing for
the CH3Cl/Si(1 0 0) system described in Section 3.2.1.
Several possibilities should be considered for the steric effect to appear in the weakly-bound
system.
Ó The American Physical Society 2008
(1) Molecular-orientation dependence of the energy dissipation via phonon and/or electron–holepair excitations in the anisotropic interaction potential. It is considered that the dipole vs the
induced dipole interaction depends strongly on the molecular orientation [132]. It is expected
that the highly charged Cl-end is more attractive than the CH3-end and, as a result, the energy
dissipations is larger.
Fig. 3.19. Left panel: S0(Ei) for randomly-oriented NO impinging on the surface at T = 400 K. The dashed and full lines represents
the Baule-type and the Hard Cube models, respectively. Right panel: Schematic of the interaction potential. Taken with
permission from Ref. [171].
Author's personal copy
126
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
(2) Molecular-orientation dependence of the energy dissipation via rotational excitations. Due to
the similarity with t-butyl chloride, some rotational excitation is expected, although this contribution to the total energy dissipation might be small [130]. In this case, Cl-end collision excites
the rotational motion more efficiently. The rotational excitation will induce multiple collisions
which result in smearing out the steric effects in the trajectory, depending on incident energy,
scattering geometry and corrugation of the interaction potential.
Ó The American Physical Society 2008
3.2.3. Steric effects appearing in the strongly bound system of diatomic NO/Si(1 1 1)
The stereodynamics of NO interaction with a surface has been the target of several studies using
various experimental approaches [39,40,46–49,118–123,159]. In the scattering of NO from the inert
Ag(1 1 1) surface [38,41–45,111], the corresponding orientation-dependent interaction potential accounted for the observed steric effects and for the resultant rotational excitation of scattered NO. In
the reactive NO/Pt(1 1 1) system, the trapping probability is higher for N-end down than for O-end
down incidence [116]. N-end down molecules were demonstrated to be more reactive also by sticking
probability measurements on Ni(1 0 0) and Pt(1 0 0) [40,118,119]. In these experiments, little attention was given to the surface products. However, (see also the introduction) it should be noted that
an elucidation of the molecular-orientation effects in the reaction products on the surface is necessary
to completely understand the surface reactions; this is of fundamental importance especially when we
go further and try to apply the obtained knowledge for material fabrications and other practical
purposes.
The flux of the incident NO is divided into the streams of scattered and reacted molecules; the latter is, in turn, divided into the fluxes of gas-phase and surface products. Therefore, we expect that the
Fig. 3.20. (a) Uptake curves of HTotal = HO + HN and HO as a function of exposure time (t) for the incidence of a 58 meV
randomly-oriented NO beam on Si(1 1 1) at T = 330 (squares), 400 (circles) and 600 K (triangles). (b) HN/HO ratio, estimated
from (a), as a function of t. Taken with permission from Ref. [171].
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
127
sign of the steric effect due to molecular orientation found in scattered NO should be present also in
the surface products. Thus, we performed a direct analysis of the surface products generated by the
oriented NO beam. Fig. 3.19 shows the reactive sticking probability for NO on Si(1 1 1) [171]; S0
was obtained from the XPS-determined uptake curves of HTotal(t) = HO(t) + HN(t) for randomly oriented SMBs with Ei = 28, 58, 78, and 240 meV and T = 400 K. Here,
Sðt ¼ 0Þ ¼ S0 ¼ lim
t!0
HN ðtÞ þ HO ðtÞ
2Ut
ð3:19Þ
is approximated with the initial slope of the uptake curve (HTotal(t) 6 0.2 ML), with U and t being the
beam flux and the exposure time, respectively. HN/HO is 1 at T = 400 K independently of Ei, while S0
decreases with increasing Ei. Such behaviour can be reproduced both by the Baule-type model [110]
with the critical energy Ec = 72 meV and by the hard-cube (HC) model [172] (dashed and solid lines in
Fig. 3.19, respectively). In the HC model, a NO molecule interacts with 10 Si atoms for a 130 meV
physisorption well [173]. As a result, the precursor-mediated reaction via molecular physisorption
and/or chemisorption well (see Fig. 3.19, right panel) should be considered in this low energy region.
Fig. 3.20(a) shows the evolution of HTotal and HO as a function of exposure time (t) for a
NO|0.5 0.5 0.5i beam with Ei = 58 meV and random orientation, impinging on Si(1 1 1) at T = 330,
400 and 600 K. The T dependence of the uptake curve, i.e., the T dependence of S, suggests that the
precursor-mediated mechanism is dominant at 58 meV. No desorption of dissociatively adsorbed N
and O atoms occurs at T 6 600 K [174,175]. Interestingly, the HN/HO ratio is 1 at 330 and 400 K
and increases to 1.2 at 600 K, as shown in Fig. 3.20(b). A similar T dependence of HN/HO was observed upon NO exposure by backfilling [176,177]. At T = 600 K, the steric effect becomes much smaller; the HN/HO ratio does not depend on molecular orientation and increases at this T (vide infra,
Figs. 3.21 and 3.22). Thus, the initial molecular orientation of an incoming NO does not contribute
to the increase of HN/HO. The anisotropy may come from the direct reactions between stabilized
NO precursors formed on the adatom and the nearest rest atom [176,177].
Fig. 3.21 shows the orientation dependence of the O-1s and N-1s XPS spectra, measured at
HTotal 0.41 ML for the 58-meV oriented NO|0.5 0.5 0.5i beam at T = 400 K. It is clear that N-end collisions are more reactive than O-end ones.
Fig. 3.22(a) reports the dependence of the stereo-asymmetry R on HTotal and parametric in T
(T = 330, 400 and 600 K, corresponding to squares, circles and triangles, respectively), with R defined
as follows:
Fig. 3.21. Orientation dependence of the O-1s (left) and N-1s (right) XPS spectra after a 6 min exposure of 58 meV oriented NO
beams on Si(1 1 1) at T = 400 K. The N-end and O-end collisions are indicated by full (red on-line) and open (blue on-line) circles,
respectively. Spectra correspond to HTotal 0.41 ML. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
Author's personal copy
128
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Fig. 3.22. (a) Stereo-asymmetry R as a function of HTotal at T = 330 K (squares), 400 K (circles) and 600 K (triangles). HTotal
evaluated upon exposure of randomly-oriented NO is used for the plot of R. (b) Average reactive sticking probability Sav,i as a
function of the averaged coverage HAv ¼ HTotal ðt1 Þþ2 HTotal ðt2 Þ, obtained for the dose of a random-oriented NO beam. The N-end or Oend collisions are indicated as full and open symbols, respectively.
R¼
IN I O
;
0:5ðIN þ IO Þ
ð3:20Þ
where IN,O corresponds to the integrated intensity of the O-1s XPS peak recorded after the same exposure to the N-end and O-end beams, respectively. Thus,
Ii ðt 0 Þ ¼ U
Z
t0
Si ðtÞ dt;
ð3:21Þ
0
where Si(t), i = N or O, is the reactive sticking probability for the N- or O-end collision, respectively.
HTotal values are determined from an identical exposure with a randomly oriented beam. We can
easily observe the following: (1) R is positive, meaning that the reaction probability is higher for N-end
than for O-end collisions; (2) R depends on T, being largest at T = 400 K and smallest at T = 600 K; on
the contrary, HN/HO, revealing no steric effects, is largest at T = 600 K (see Fig. 3.20). Result (1) is also
supported by measurements of the N-1s region.
Fig. 3.22(b) shows the orientational dependence of the reactive sticking probability Sav,i averaged
with respect to HTotal, where i = N or O denotes the N- or O-end incidence, respectively. Sav,i is defined
as
Sav;i ¼
HTotal;i ðt2 Þ HTotal;i ðt 1 Þ
;
2Uðt2 t 1 Þ
t2 > t1 :
ð3:22Þ
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
129
Plotting Sav,i vs HAv ¼ HTotal ðt1 Þþ2 HTotal ðt2 Þ for the dose of a randomly-oriented NO SMB, molecular-orientation effects appear for HAv < 0.25 ML at T = 330 K and 400 K. This is the origin of the positive R in
Fig. 3.22(a). For HAv P 0.25 ML, molecular-orientation effects in Sav become smaller.
The strong T dependence of the uptake curves in Fig. 3.20(a) indicates that the precursor-mediated
reaction [178] is a dominant process in NO dissociative adsorption on Si(1 1 1). The smaller R at 600 K
suggests little contribution of direct dissociative adsorption to the steric effect. In the precursor-mediated reaction [178], two kinds of precursor potentials to dissociative adsorption should be considered: a physisorption well with typical depth of 130 meV [173] and a molecular chemisorption
well. The ‘‘atop” state (Si–N bonding), with a binding energy of 0.6–0.8 eV (estimated residence time
109 s at 400 K), is the most probable molecularly chemisorbed precursor to dissociation [180]. If only
trapping into the physisorption well contributed to the observed steric effect, we would expect a negative R due to the rotational excitation of NO [41]. In the case of the present study, the positive R suggests an additional contribution coming from the chemisorption well. On the other hand, a strongly
attractive chemisorption potential with a well depth of 0.6–0.8 eV, compared to the translational energy of 58 meV, would adiabatically reorient the NO molecule into a favourable configuration during
its approach to the surface [104]. This is expected to smear out the orientation effects.
We believe that the moderate orientation-dependent depth of the molecular chemisorption potential contributes mainly to the steric effect. The effective chemisorption well depth depends on where
and how NO approaches at the first encounter with the surface. As the distance from the reactive site
increases and the approaching geometry strays out of the stable tilted geometry [181], the potential
well depth may be shallow enough as to reduce the reorientation effect on the first bounce. However,
the steric dependence of the interaction potential still remains. The interaction is more attractive for
the N-end approach [181]. Thus, we expect a larger energy dissipation for the N-end approach, via
transient excitations of phonons and/or electron–hole pairs in the shallow well [40,182]. Therefore,
the trapping probability of NO is larger for the N-end collision and reveals the T dependence as a result
of desorption, via excitation of surface phonons, before being trapped into a further, deeper chemisorption well. This orientation dependence causes the observed steric asymmetry of S at T = 330 K
and 400 K.
Increasing T to 600 K enhances the probability of desorption from the transient weakly-bound
states via phonons, while the dissociative adsorption via the strong attractive interaction remains.
The dissociation process is thus or mediated by a strongly-bound-state. Both smear out the steric effects at low energy because of steering [104,164]. Thus, the orientation effects become smaller at high
T. Steric effects depending strongly on T and Ei appeared on the trapping into the transient precursor
state in CH3Cl/Si(1 0 0) as shown in Section 2.3.1. The largest R value at T = 400 K (see Fig. 3.22), despite monotonic decrease of S with increasing T, suggests a similar, strong T dependence of the steric
effect..
As shown in Fig. 3.22(b), the steric effect decreases with increasing HAv. Thus, the observed steric
effect appears via an intrinsic precursor well, i.e., via direct interaction with the substrate. The repulsive interaction of incident NO with the pre-adsorbed N and O atoms reduces the interaction length
and the strength of attractive interaction in the precursor well. As a result, the anisotropy of NO becomes smaller at high coverage.
4. Preparation of aligned molecules
4.1. Collisional alignment
As outlined in the previous sections, alignment and orientation of the rotational angular momentum J [179] are important cases of molecular polarization. They correspond respectively to the nonstatistical symmetric and asymmetric distribution of the J components with respect to a quantization
axis z defined by the quantum number MJ (see Fig. 4.1). Orientation is usually referred to as non-statistical distribution both in direction and sense, whereas we talk about alignment when only the direction matters (see also Ref. [72]).
Alignment and consequent molecular polarization can be induced by collisions in environments
with anisotropic velocity distributions or temperature gradients. The occurrence of collisional
Author's personal copy
130
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
alignment was firstly evidenced long ago in connection with the study of gaseous transport phenomena of paramagnetic molecules like O2 [183–185]. Subsequently, Beenakker et al. demonstrated the
phenomenon to be common to nearly all diatomic gases, including diamagnetic molecules [186].
Non-equilibrium alignment effects due to anisotropic velocity distributions were directly observed
Fig. 4.1. Spatial quantization of the rotational angular momentum J. MJ is the projection of J on the z quantization axis, which
coincides with the SMB axis. M = |MJ| is the helicity quantum number. Cases of alignment and polarization of J are schematized
below.
Fig. 4.2. Reference terminology is introduced for diatomic molecules in high J (classical) and low J (quantum) states and for a
polyatomic molecule (ethene) in low J states. Ethene is an asymmetric top molecule but, since it possesses two similar inertia
moments while the third one is much smaller, its behavior in a supersonic expansion is not much different from that of a
diatomic molecule (see Refs. [215,216]).
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
131
through flow birefringence [187,188], while those related to temperature gradients (Kagan polarization) were measured by optical spectroscopy [189]. The so-called Senftleben–Beenakker effect has
been extensively discussed in a paper by Beenakker and Mc Court [190] becoming an important tool
for describing transport properties of diatomic gases and polyatomic molecules when measured in
presence of magnetic and electric fields.
Collisional alignment in molecular beams, from supersonic nozzle sources, was anticipated by
Ramsey [191] and first reported by Steinfeld, Korving and co-workers [192,193] in the seventies. Many
aspects, experimental and theoretical, have been extensively investigated in the last thirty years (the
phenomenon having been studied also in drift tubes), discovering new features but also leaving open
issues and possible developments. Information presently available includes the effects of source
shape, source conditions (stagnation pressure, nozzle diameter, etc.) and specific details of the investigated systems [85,192–217].
In the following the molecular beam axis, is referred to as the z axis. A basic quantity is the molecular helicity M, which defines the absolute J projections with respect to the flight direction. (Fig. 4.1).
The most important region of the molecular beam is the supersonic expansion zone. Here, in
seeded conditions (a diluted mixtures of molecules in lighter carrier atoms), hundreds of elastic and
inelastic collisions occur among the particles in the forward direction of the gas expansion at a collision energy determined by the velocity slip (the relative velocity between seeded and carrying gases).
Briefly, such collisions change the translational energy and the internal state distribution of the molecules as they enter into the collision free zone, which typically occurs few millimeters away from the
nozzle. The fate of molecules depends on the collisional impact parameter and may change during the
expansion since both velocity slip and gas density decrease with the distance from the nozzle. Collisions at small and large impact parameters are responsible respectively for focusing in the forward
direction and for internal cooling, accompanied by acceleration, while collisions at impact parameters
of the order of molecular dimensions can bend (align) the molecular rotation plane through non-helicity conserving processed fostered by Coriolis couplings. For light molecules the process can be strongly
quantum–mechanical (for theory and application see Ref. [211]).
All the experimental findings suggest that molecules in zero helicity state, flying in edge-on mode
(see Fig. 4.2), tend to increase their population downstream: as already explained in previous sections,
such molecules behave like cartwheels when impinging at normal incidence onto a surface; moreover,
molecules in high helicity states, flying in the broad-side configuration, tend to disappear in the formed
supersonic molecular beam (SMB) and behave like helicopters with respect to the same surface.
The experimental studies performed since the early days have also pointed out the role of source
geometry and operative conditions and of mass ratio between the seeded molecule and the carrier
species [211]. More recently clear dependences on the rotational states and on the final velocity
Fig. 4.3. The PG experimental apparatus. The SMB angular divergence is better than 0.06 degree, corresponding to a transversal
velocity of molecules at the scattering box of less than 1 m s1.
Author's personal copy
132
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
reached by the molecules after the expansion have been discovered and the role of the probed angular
cone around the SMB axis was also found to be relevant [214–217].
4.2. Experimental studies in the Perugia laboratory
The experimental apparatus in Perugia (PG) is composed by a molecular beam source, a sequence
of molecular beam collimators, a mechanical velocity selector, a scattering chamber, a magnetic analyzer and a mass spectrometric detector. In the experiments described in this section, the velocity
selector used is 26 cm long, is made up by a set of eight rotating disks, and operates with a resolution
of 5% (FWHM). The apparatus has been extensively described before [201,203,208,210]. A sketch is
reported in Fig. 4.3 in order to point out its key features: high angular and velocity resolution, necessary to measure small deflections in Stern–Gerlach type of experiments, quantum integral cross sections and anisotropies in zero angle scattering. Therefore, two different experiments can be performed
to diagnose the SMB conditions well downstream from the expansion zone. In both measurements the
SMB is well collimated by a sequence of skimmers, delimited by slits while the molecular velocity is
analyzed by a high resolution mechanical selector. This leads to probe molecules flying in the SMB at
defined longitudinal velocity and with a nearly zero transversal component.
The first experiment, limited to paramagnetic molecules such as O2, involves the measurement of
the molecular beam paramagnetism, related to the Zeeman sublevel distribution, which is expected to
be non-statistical in presence of molecular alignment.
The second type of experiment is of more general applicability and concerns the investigation of
the SMB attenuation caused by the passage through a scattering chamber filled with a target gas.
The scattering box is placed along the molecular beam path, and thus the scattering process has the
same cylindrical symmetry of the expansion process. This zero-angle observable, related to the total
integral cross section, Q, varies with the molecular beam velocity, v, and, more effectively, with the
strength of the projectile-target interaction potential, V. Specifically, the latter experimental quantity
is expected to be different for projectile molecules approaching the target with different orientation,
such as in the edge-on and broad-side cases described above. At thermal collision energies Q(v) depends on collisional events at large impact parameters, being mostly affected by the long range
Fig. 4.4. Relative cross section anisotropy, A, as a function of the selected SMB velocity v for scattering of O2 (top) and N2
(bottom) molecules by a Xe target. For unpolarized molecular beams A = 0 within the experimental uncertainty for all v.
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
133
attractive component of V. These events determine the smooth component of the cross section. Additional contributions can derive from collisions at intermediate impact parameters. Such collisions,
controlled by V in the region of the potential well, determine an oscillatory pattern in Q(v) associated
with the glory quantum interference. In order to probe the molecular alignment one aims to measure
changes in the Q value, related to modifications in the helicity states of the molecules: these effects
can originate only if V varies with the geometry of the collisional complex, i.e. the potential energy
surface (PES) of the system is significantly anisotropic. Therefore, measurable changes in Q depend
on the variation of the molecular alignment degree weighted on the anisotropy of the interaction.
The analysis of the experimental data, based on the knowledge of the PES, can be approached by
exploiting helicity decoupling schemes, developed within the centrifugal sudden framework
[201,203,208]. In such schemes the collision is described as driven by effective potentials, adiabatically correlating the free rotor states of the molecule (defined by M) with the hindered rotor and libration states in the proximity of the target. The Coriolis coupling is considered to take non-adiabatic
transitions into account, allowed among helicity states with the same parity quantum number and
occurring for a specific range of intermolecular distances [201,203,208,218].
4.2.1. The case of diatomic molecules: O2 and N2
Oxygen is an open-shell molecule confined in the Hund case (b), with an electronic spin angular
momentum S (S = 1) coupled to the rotational angular momentum, here designed as K, to give the total
angular momentum J to which spin-rotational states are associated (at variance with the general case
of diamagnetic molecules where J is the rotational quantum number) [219]. Only odd rotational states
are allowed for the most abundant isotope, 16O2).
Paramagnetism measurements [85,201] probing the spin-rotational state distribution indicated
that, after the supersonic expansion, most of the O2 molecules (>95%) seeded in a lighter carrier such
as He or Ne relax to the ground rotational state K = 1 and show a propensity to fly in the aligned edgeon mode. Moreover, some of us reported the first experimental evidence [85,201] of the strong dependence of the alignment on the final molecular speed, while a much less effective dependence was
found changing the carrier gas from He to Ne to H2. A careful analysis of a large experimental data
basis, including the dependence on source pressure, lead us to conveniently represent all observations
in a unifying picture by using the v/vp ratio as scaling factor. Herein, vp is the peak velocity of the SMB
and v is the probed velocity. In particular, the molecular alignment degree, related to the variation of
edge-on (cartwheel) and broadside (helicopter) fractions with respect to the statistically expected values, was found to strongly depend on v/vp. This evidence suggested a possible indirect way to control
the molecular alignment by exploiting a high resolution velocity selection of the SMB. Specifically, the
use of different seeding gases allows to produce SMB with very different vp, but by selecting the same
v/vp it is possible to filter out molecules with a similar alignment degree but different translational
energy.
This possibility has been demonstrated by Q(v) measurements performed with velocity selected
seeded SMBs of O2, scattered by rare gas atom targets [202,208–210]. The experiments involved the
selection of molecules at a fixed v/vp ratio, i.e. with a defined degree of molecular alignment, as a function of v. This has been obtained for each selected v varying vp by fine tuning the carrier gas mixture
composition in order to achieve the chosen v/vp ratio. Cross section measurements have been carried
out in a wide v range and for four different v/vp ratios.
At each v a cross section variation has been observed which depended on the selected v/vp ratio.
These results [202] provided thus the first correlation between the changes in a scattering observable
and the variation of the alignment degree in the molecular beam and are fully consistent with the
measurements of paramagnetism.
The same methodology was applied to N2 SMBs [203], containing closed-shell diamagnetic molecules, both in the ortho and para form, relaxed mainly in J = 0, 2 and J = 1 rotational states, respectively; we observed variations in the cross sections similar to those seen for the O2 case. Obviously,
Q(v) values are expected to be independent on the v/vp ratio in cases of a statistical population of
the helicity states within the same velocity distribution. We recall that in our experimental conditions,
changes in J level population should be limited and that collision theory suggests that they should generate minor effects on the Q(v) values. Therefore, the scattering anisotropies of N2 molecules, in line
Author's personal copy
134
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
with those observed with O2, suggest that the velocity dependence of the alignment is not peculiar of
the paramagnetic oxygen molecule but a rather general phenomenon, at least for molecules with mass
comparable with O2 and N2. Moreover, depolarization effects due to the coupling of the rotational
angular momentum with spin components may reduce the degree of the observed polarization. Nevertheless, the experiments described so far provide observations of the collisional alignment far from
the expansion region, specifically at distances from the nozzle relevant for gas phase and surface scattering experiments.
The relative cross section anisotropy, AQ, can be obtained from the experimental observations as a
function of the collision energy and a further interesting comparison can be drawn from O2 and N2
scattering against Xe. AQ can be defined at each velocity v as
AQ ¼
Q v =v p Q 0
;
Q0
0.10
0.05
0.00
1.0
(b)
0.8
0.6
0.4
0.2
160
180
200
f (Hz)
220
0.20
(c)
0.15
0.10
0.05
0.00
1.0
(d)
0.8
FT
0.72
0.67
0.6
ST
0.4
0.2
100
0.42
120
140
f (Hz)
160
180
Ó Wiley VCH 2006
(a)
0.15
Normalised intensity
0.20
Cartwheel fraction
Cartwheel fraction
Normalised Intensity
where Q0 is the cross section of molecules sampled at v/vp = 0.95 (the lowest), for which the molecular
alignment has been found to be negligible [202,203,208] and is therefore used as reference. Herein, we
exploited the calculated fitting cross sections, as reported in Refs. [203,208]. The results, reported in
Fig. 4.4, refer to molecules selected in the fast front of the velocity distribution (v/vp = 1.1), showing
the highest degree of molecular alignment and at the most probable velocity (v/vp = 1.0), where it is
smaller but still evident. The cross section anisotropy exhibits an oscillatory behavior superimposed
to an average component suggesting both a glory, Ag, and a non-glory, Ang, contribution to AQ
[202,203,208]. Ag arises from a quantum interference effect at zero scattering angle and depends on
the interaction anisotropy of the projectile-target system in proximity of the potential well; Ang, affected by scattering at small angle, depends on the long range attraction anisotropy. Specifically,
the observed cross section anisotropy for O2 shows a more pronounced Ang contribution because of
Fig. 4.5. The case of O2 SMBs seeded in He: summary of experimental results (circles and crosses) and Monte Carlo simulations
(dashed and continuous lines). Panel (A): Typical velocity distribution measured in PG and results of simulation. Panel (B):
Cartwheel fraction measured in PG as a function of the v/vP ratio as obtained from paramagnetism experiments (full circles) and
by independent scattering experiments (open circles). Simulations have been obtained assuming two ideal velocity
dependencies of the cartwheel fraction and by averaging them over the geometrical features of the PG apparatus. Panel (C):
measured and simulated SMB intensity in the GE apparatus. Panel (D): Modulation of the cartwheel fraction as obtained by
convoluting the PG results over the angular and velocity resolution of the GE apparatus. Taken with permission from Ref.
[71].
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
135
the larger polarizability anisotropy with respect to N2 [202,203]. In both cases A reduces when sampling molecules at a lower v/vp ratio.
Another interesting comparison is also possible with literature data on the scattering of oriented
NO by Xe, as reported time ago by Reuss and co-workers [220,221]. These authors controlled the orientation of NO by means of the external electric fields of a hexapole. Such a comparison is meaningful
considering that NO and N2 show a very similar van der Waals interaction energy with a given rare gas
atom. A close correspondence can be observed between the anisotropies in the NO and N2 systems,
measured with completely different techniques. Specifically, results in Refs. [220,221] exploit the
molecular alignment as due to the symmetric top behavior of the open-shell NO molecule in an external field, while N2 has been collisionally aligned in a supersonic nozzle source and velocity selected by
a mechanical device. This analysis confirms also that the use of molecular scattering, combined with a
detailed velocity selection of the SMB, can provide information on the molecular alignment.
Fig. 4.5 summarizes for O2 seeded SMBs the results on the dependence of the CW fraction versus v/
vp ratio, as obtained by combining the paramagnetism and the scattering information. These results
are important to evaluate changes in the molecular alignment degree under different experimental
conditions (see below).
4.2.2. Small hydrocarbons: the cases of C2H2 and C2H4
Following the ideas introduced above, some of us recently [216] measured Q(v) for the scattering of
rotationally relaxed acetylene molecules by Ar atoms. In particular, we recorded Q(v) values at three
velocity ratios, namely v/vP = 0.93, 1.0, 1.07, in order to compare the behavior of molecules flying in
the slow tail, around the peak velocity and in the fast front of the velocity distribution. In this experiment a pronounced Ang and a small Ag was observed and attributed to the large polarizability anisotropy of C2H2. Moreover, in a complementary experiment, performed at fixed velocity v and with an
Fig. 4.6. (Top) collision cross section plotted as Q(v)v2/5 to emphasize the glory interference effect, and (bottom) cross section
anisotropy as a function of v/vP for C2H2–Ar (see text).
Author's personal copy
136
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
increasing v/vp ratio, a gradual increase of the cross section value was measured. The result is reported
in Fig. 4.6. The cartwheel fraction scale in the right-hand axis of the bottom panel such figure has been
obtained from the analysis of the experimental data and refers to J = 1, the most populated level [216]
(the population ratio between J = 1 and J = 2 levels is approximately 3:1).
It must be also noted that the analysis of such data required a parallel effort addressed to the characterisation of the full anisotropic interaction of the C2H2–Ar system, including the effective adiabatic
potentials controlling the collisions of molecules in defined helicity states [216,222]. To this aim, a
new semiempirical method has been used and predictions have been tested on scattering data from
other laboratories, on pressure broadening coefficients and on some spectroscopic features of the system. The so obtained PES made it possible to perform the analysis of total cross sections and AQ values
measured in our laboratory, providing information on the change in the helicity state population and,
thus, on the average variation of the molecular alignment as a function of the molecular beam velocity
also for C2H2. Similar experiments have been carried out also for C2H4 SMBs [215] obtaining comparable results. Q(v) and AQ data, of interest for the interpretation of surface scattering experiments, are
reported in Fig. 4.7 (for more details see Refs. [215–217]). The cartwheel fraction scale in the righthand axis of the bottom panel of Fig. 4.7 has been obtained from the analysis of the experimental data
and refers to J = 1. J = 1 and J = 2 are representative of the few J states populated in the C2H4 SMB
[215,217], in which 4/5th of the total population is in J 6 2). These data have been used also to obtain
the degree of alignment in the Genoa setup (see Section 5).
A series of complementary spectroscopic experiments have been performed in Trento (TN), providing further information on the ethene and acetylene collisional alignment. In the TN apparatus the
SMB crosses perpendicularly an IR laser beam with photons polarized parallel or perpendicular to
the SMB propagation direction. Selective laser absorption, AL, gives rise to a signal at the superconducting bolometer used as detector. A collimating slit in front of it allows to vary the angular resolution of
the experiment. The AL signal for parallel and perpendicular laser polarizations is expected to be dif-
Fig. 4.7. As in Fig. 4.6, but for C2H4–Ar.
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
137
ferent in presence of molecular alignment. Such an apparatus permits to investigate the dependence of
this quantity on the rotational states and also to assess the effect of the angular divergence of the SMB.
Results for acetylene and ethylene SMBs, produced by a nozzle source in the same conditions as in
the PG experiment, showed [215–217] an evident anisotropy in the absorption of polarized IR laser
light. Moreover, an increase of anisotropy effects was recorded reducing the acceptance angle of the
detector, i.e. increasing the angular resolution of the experiment. Fig. 4.8 reports the TOF spectra measured by probing ethene molecules in J = 1, the most populated rotational level. In the TN experiments
it was not possible to resolve the velocity dependence of the molecular alignment so far, for reasons
discussed in Refs. [214–216]. Nevertheless, the experimental results have clarified two important issues: the presence of a pronounced molecular alignment for low J states and the critical role of the
angular resolution on the experimental observations.
The estimate of the dependence of the alignment degree under different angular and velocity resolution conditions has been crucial to transfer information from PG to the surface scattering experiments carried out in GE and at the Elettra Synchrotron Light Source in Trieste. In these apparatuses
the SMB polarization cannot be measured directly and the velocity selection is carried out with a
two disks mechanical velocity selector. This device has a much lower resolution than the one operating in the PG laboratory (longer and made up by eight slotted disks). A Montecarlo simulation of the
velocity selection procedure, taking into account the geometrical features of the apparatuses, number,
dimension and distance of the disks of the mechanical velocity selectors and SMB angular divergence,
has been used to calculate the modulation of the SMB intensity as a function of the rotational frequency f of the mechanical velocity selector. The results are compared in Fig. 4.5 with values measured
in the PG and GE apparatuses. Being grounded to the PG results, the simulation allowed to transfer
information on the molecular alignment degree of seeded O2 and C2H4 SMBs under the experimental
conditions of the GE and Trieste apparatuses [86].
5. Interaction of aligned molecules with surfaces
As already pointed out in the introduction, the interaction of rotationally aligned SMBs with well
defined surfaces can give precious information to clarify new and so far neglected aspects of gas-surface interaction. In the following we will briefly review the effect of molecular alignment on several
systems investigated in the GE laboratory and at the Elettra Synchrotron Light Source in Trieste. After
a short experimental notice, we shall focus, firstly, on weakly interacting systems, leading to non dissociative adsorption, and on the effect of C-contamination on the stereodynamics of the interaction of
Fig. 4.8. Polarized laser light absorption intensity AL as a function of time of flight for two rotational states of ethene. The inset
shows the ideal behavior of a fully aligned SMB, predicted by using a phenomenological model (dashed and continuous lines)
[216,217] and the Fano–Macek theory (dotted and continuous lines) [200,216] for J = 1.
Author's personal copy
138
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
ethene molecules with Cu; secondly, we will analyse strongly interacting systems, which lead either to
molecular or to dissociative adsorption.
5.1. Experimental methods
QMS signal (arb. units)
The eight disk velocity selector described in Section 4.2 and employed for the gas-phase experiments in PG does not allow for the high molecular fluxes required for sticking and reactivity measurements in gas-surface experiments. In order to generate sufficiently intense SMBs we exploited the
performances of a two-disk velocity selector [86], which still allows to differentiate between helicoptering or cartwheeling molecular projectiles by selecting either the slow or the fast tail of the supersonic velocity distribution. With respect to the more complex selector employed for gas-phase
experiments, this simpler and more compact device is thus characterised by a limited velocity resolution but by an order of magnitude higher flux (at most ten times lower than the non-selected SMB);
the signal-to noise ratio when detecting the SMB is therefore good enough to perform the desired uptake experiments, although the error on the sticking probability is necessarily 3–5 times higher than
for non-selected SMBs. In a typical experiment using ethene or oxygen seeded in Ne or He, the flux of
an aligned SMB at the sample is between 0.005 and 0.02 ML/s, depending on parameters like the employed seeding gas, the nozzle pressure, etc.
The experimental setup available in Genova consists of a SMB, on which the two-disk velocity
selector is installed, and of a UHV apparatus optimised for investigations of surface reactivity. However it does not allow to measure the degree of alignment in situ. The latter quantity has then to be
deduced from the data acquired in PG, where the high resolution velocity selector is available. Once
the degree of alignment as a function of velocity is known with sufficient accuracy, the polarization
of the SMB can be computed numerically by integrating the degree of alignment vs. velocity over
the transmitted velocity distribution. The latter is evaluated by a detailed Montecarlo simulation of
the transmittance of a given selector under realistic conditions, i.e. taking into account the finite size
of the selector slits, their width and, above all, the finite angular acceptance of the apparatus. A typical
result is shown in Fig. 4.5 for an O2/He SMB: symbols in panels A and B represent the measured trans-
(b)t0
(a)
ST
FT
X4
0
40
120
(c)
X1
0
X1
40
80
120
0
40
80
120
t (s)
1.0
0.8
80
t1
ΘO =0.03 ML
(d)
2
Ei=0.12 eV
2
ΘO =0.35 ML
(f)
2
Ei=0.36 eV
Θ(Smax)
Ei=0.36 eV
S
0.6
ΘO =0.03 ML
(e)
0.4
0.2
0.0
0.00
0.05
0.10
0.00
0.05
0.10
ΘC H (ML)
2
0.00
0.05
0.10
4
Fig. 5.1. KW traces (top panels) and S(H) curves (bottom panels) showing the outcome of ethene uptake experiments on O2
pre-covered Ag(1 0 0). Different O2 pre-coverages and C2H4 translational energies are reported.
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
139
mitted intensity and the concentration of cartwheels as a function of the rotational frequency of the
eight-disks velocity selector. The continuous lines give the outcome of the corresponding simulations
and underline the substantial agreement between experimental data and simulated results. The measured and calculated intensity for the two-disk velocity selector are compared in panel C, while panel
D shows the corresponding fraction of cartwheels as evaluated by a weighted average over the transmitted distribution. We mention that similar curves were found for other systems, like C2H4 in He
[68]. As it can be inferred from Fig. 4.5D, the concentration of CW estimated for the two-disks velocity
selector operating in Genova is (0.42 ± 0.03) for the slow tail (ST) of the velocity distribution and
(0.67 ± 0.05) for the fast tail (FT). Uncertainties correspond to the propagation through the simulation
of the errors of the Perugia experiment. The cartwheel fraction is attenuated with respect to that of the
eight-disks device; this is due to the different transmission function of the two-disk velocity selector,
and to the different source conditions and angular divergence of the molecular beam.
The dynamics of gas-surface interaction is then determined by measuring the sticking coefficient S
as a function of exposure by the retarded reflector method of King and Wells [149], already described
in Section 3.1.4.
5.2. Results and discussion
5.2.1. Weakly interacting systems: hydrocarbon adsorption on noble metals
In this section we show the effect of molecular alignment on the sticking probability of the following, weakly interacting systems:
(a) C2H4/O2/Ag(0 0 1)
(b) C3H6/Ag(0 0 1)
(c) C2H4 on C-contaminated Cu(4 1 0)
For sake of clarity, in all figures the continuous (red on-line) and dotted (blue on-line) curves correspond to traces referring to the slow tail (ST) and to the fast tail (FT) of the SMB velocity distribution,
respectively.
Figs. 5.1–5.3 show the outcome of uptake experiments performed exposing different surfaces to a
rotationally aligned hydrocarbon SMB. Fig. 5.1 refers to C2H4 on O2/Ag(0 0 1) [68]; different O2 precoverages and different translational energies of the C2H4 molecules were investigated. The top panels
show the KW traces, the bottom ones the SðHC2 H4 Þ curves. Fig. 5.2 reports the corresponding data for
the C3H6/Ag(0 0 1) system [70]. Finally, Fig. 5.3 shows the interaction of rotationally aligned C2H4
SMBs with Ei = 0.10 and 0.36 eV with a C-contamined Cu(4 1 0) surface [223]. Since Cu(4 1 0) is a
stepped surface characterised by 3-atom-row wide (1 0 0) terraces and monoatomic (1 1 0)-like steps,
two angles of impingement are investigated, corresponding to grazing incidence (h = 60°) and incidence at 15° on the step heights (h = +45°) (see inset of Fig. 5.3C). We note that, in all cases, the behaviour of the S(H) curves does not imply a difference in the saturation coverage, since the latter cannot
be monitored if it is reached with sticking probabilities lower than the sensitivity of the KW method as
in the present cases.
The relevant experimental outcomes are:
(1) Interestingly, the initial sticking probability is always the same for ST and FT, and independent
of the HE or CW nature of the incoming molecules. We remind that under our experimental conditions
(high seeding ratio of the C2H4 beam and nozzle at room temperature) the SMB is rotationally cold.
Therefore the finding implies that the interaction with the bare surface is not affected by the
alignment, at least for rotationally cold SMBs. This result is not obvious. Comparison of the different
investigated systems indicates that the additional surface corrugation introduced by the presence of a
pre-adsorbed species or by a regular array of steps is not enough to cause a visible effect of rotational
alignment on the initial sticking probability. Possibly, despite the weakness of the bond, steering
forces are still strong enough to suppress rotational effects. This argument, definitely valid for cold
SMBs, is no longer true for rotationally hot molecules. Although no experiments with hot rotationally
aligned beams have been performed so far, some indirect information can be found comparing adsorption of non-selected rotationally hot and cold SMBs. For example, for C2H4/Ag(1 0 0) it was demon-
Author's personal copy
140
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
95 Hz PV
85 Hz ST
t 0 t1
0
50
100
150
Time (sec)
1.0
0.8
t2 (a)
C3H6-Ag(001)
Θ (Smax)
Θ (Smax)
200
(b)
S
0.6
0.4
0.2
0.0
0.00
PV
(95 Hz)
ST
(85 Hz)
FT
(106 Hz)
0.04
0.08
ΘC3H6 (ML)
Ó The American Chemical Society 2005
QMS signal (arb. units)
106 Hz FT
0.12
Fig. 5.2. (a) KW experiments recorded during C3H6 exposure on Ag(1 0 0) with ST, FT and the central portion of the velocity
distribution (PV). The traces are vertically shifted for sake of clarity. (b) S(H) curves calculated from the traces of panel a. Taken
with permission from Ref. [70].
strated that rotations inhibit adsorption since S0 was significantly lower for a rotationally hot than for
a cold SMB of the same translational energy [37]. On Ag(4 1 0), a vicinal surface of Ag(1 0 0), this effect
is strongly reduced due to the additional steering introduced by the steps. We ascribe the different
behaviour of hot and cold SMBs mainly to the CW component because of its higher rotational into
translational energy conversion efficiency. These experimental data indirectly support some – still debated – theoretical simulations predicting, indeed, a suppression of the adsorption probability for CW
due to rotational to translational energy conversion [224]. We further note that our measurements
were performed at T = 110 K. It is possible that stereodynamical effects favouring the initial sticking
probability of rotationally cold helicopters exist for more weakly-bound systems (i.e. physisorbed layers, such as C2H6/Ag). However, at the moment, our laboratory in Genova is still the only one equipped
to perform this kind of measurements and this experimental setup does not allow to reach lower crystal temperatures. The question remains therefore still open.
(2) A steric anisotropy is present in all cases at intermediate hydrocarbon coverage. As reported in
literature [225], hydrocarbon adsorption is mediated by an adsorbate assisted mechanism. When a
hyperthermal molecule collides with an adsorbate of similar mass (of the same or of a different species) instead than against a much heavier metal atom, the energy dissipation is more efficient and the
molecule has a larger probability of being adsorbed. As a consequence, the sticking probability initially
increases with coverage. In the data presented here, this effects is modulated by rotational alignment.
In fact S increases at intermediate coverage but in different amounts for ST and FT. In particular, for the
C2H4/O2/Ag(1 0 0), C3H6/Ag(0 0 1) and C2H4/C/Cu(4 1 0) systems the increase is always larger for ST.
This observation can be explained considering that the cross section for collision with pre-adsorbed
molecules (lying flat or slightly tilted on the surface) is larger for HE than for CW. Moreover, the average classical turning point position (corresponding to the centre of mass) reached by helicopters is closer to the surface. The magnitude of the stereodynamical effect at relatively low hydrocarbon coverage
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Ei=0.36 eV
Ei=0.10 eV
1.0
141
(A)
(B)
2
ST
FT
-8
QMS current (x 10 A)
0.5
0.0
θ=+45°
1.0
(C)
θ=31o
θ=+45°
0
(D)
θ=-60o
2
0.5
0.0
θ=-60°
0
50
θ=-60°
0
100 150 200 250 0
Time (s)
20
40 60
Time (s)
80 100
Fig. 5.3. Uptake curves for rotationally aligned ethene on C-contamined Cu(4 1 0). SMBs with different translational energy
(0.10 eV, A and C; 0.36 eV, B and D) impinge on the surface either step-up (h = +45°, A and B) or step down (h = 60°, C and D).
The ST and FT traces are clearly different for the high translational energy as it is most evident in panel D (underlined by the
arrow).
leads to the further conclusion that the stereodynamical anisotropy cannot be explained by direct collision with pre-adsorbed molecules. In this case, in fact, for relatively small molecules like ethene and
a pre-coverage of 0.20 ML, e.g., we should expect an increase 620%. On the contrary, the C2H4 molecules that are not scattered immediately after the first hit with the surface are temporarily trapped as
a mobile and non-thermalized precursor with a memory of their initial alignment; in this condition,
CWs have a higher chance of being scattered back into the gas phase than HEs when colliding with
already chemisorbed flat-lying C2H4 molecules. We further note that the observed stereodynamical
effect is due only to the interaction with pre-adsorbed molecules of the same species, since it would
otherwise show up as an initial effect in case of ethene interaction with O2 or C pre-covered surfaces.
Evidently, what matters is the anisotropy of the ethene–ethene scattering cross section.
(3) For C2H4/O2/Ag(1 0 0) the effect of rotational alignment on S is comparable for all translational
energies (see Fig. 5.1d and e), while for C2H4/C/Cu(4 1 0) it is larger at higher Ei. This observation further supports the hypothesis that steering plays a critical role in the adsorption dynamics: at a given
temperature, if the system is weakly interacting, as in the case of C2H4/O2/Ag(1 0 0), steering is efficient enough at all Ei. If the system it more strongly bound, as in the case of the stepped Cu surface,
steering is certainly more effective when the translational energy is lower. A possible alternative
explanation takes into account the higher rotational-to-translational energy conversion efficiency of
CW with respect to HE. When Ei is low enough, such conversion cannot supply enough energy to
CW molecules to backscatter them into the gas phase, thus resulting in similar values of the sticking
for CW and HE. The stereodynamical effect might eventually appear again for much more weaklybound systems, since the critical parameter is indeed the ratio between the incoming energy and
the depth of the adsorption well.
(4) Oxygen pre-coverage on Ag(1 0 0) produces an increase of the C2H4 sticking probability (compare Fig. 5.1b and c) so that, at fixed temperature and exposure, a larger coverage is attained for a larger oxygen pre-coverage. This effect implies an adsorption mechanism assisted by an adsorbate of a
different species. It increases the overall sticking coefficient with respect to the clean surface, but it
is not sensitive to the helicity state of the hydrocarbon molecules, as discussed in previous point
(2). The stereodynamical effect appears indeed only after a sufficient amount of ethene is present onto
Author's personal copy
142
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
the surface. We further note that different pre-adsorbed species lead to a different degree of stabilization and hence to different adsorption dynamics. E.g. O adatoms (not shown) are less efficient than O2
admolecules in stabilizing ethene, so that the C2H4 coverage remains low and no stereodynamical
anisotropy is observed under experimental conditions similar to those of Fig. 5.1.
(5) For the case of C- contamined Cu(4 1 0) the stereodynamical anisotropy due to molecules in
high and low helicity states is superimposed to a further anisotropy related to the stepped nature
of the surface. Indeed ST and FT behave similarly when the C2H4 SMB impinges step-up (h = + 45°),
while for h = 60° (ethene moving step down) the sticking probability is initially the same but it becomes larger for ST than for FT after the critical coverage H 0.05 ML is reached. The difference is
largest at H 0.1 ML and reduces approaching saturation; the effect is stronger for Ei = 0.36 eV, but
it is already present for Ei = 0.10 eV. Comparison with identical experiments performed on a C-clean
Cu(4 1 0) sample shows that the stereodynamical effect is not present for this system and that the
phenomenon is therefore C-related. The same contamination has the additional effects, observed also
for non-selected SMBs, of increasing the initial sticking coefficient and, as expected, of reducing the
saturation coverage. C-contamination derives from thermal dissociation of previously adsorbed ethene. Since it has been proved that ethene dissociates at Cu(4 1 0) steps [226], we expect the pollutant
to be in the form of C or C@C units located at the steps. C-decoration might slightly modify the geometry or the corrugation of the steps, which become thus more reactive, and this change could be more
favourable to helicopters than to cartwheels because of the larger cross section of the former. The increased S0 value and the reduced saturation coverage can be explained by initial adsorption assisted
by carbon units (see also point (4)) and by the poisoning of adsorption sites, respectively [223]. On the
C-contamined sample the stereodynamical effect is largest for a molecular beam impinging at –60°.
The hot precursor needs thus to have a translational component oriented step down to give an effect,
which links the angular anisotropy of the C2H4 uptake to the crystallographic anisotropy of the
Cu(4 1 0) surface. This demonstrates that the maximum uptake is due to the chemistry occurring at
the step edge. Since h = 60° corresponds to the condition in which the step rises are completely in
shadow with respect to the SMB, the uptake under discussion must occur on the upper edge of the
steps, more precisely either at the atomic row at the very edge of the terrace or, possibly, on the second row, as it is the case for ethene Ag(n 1 0) [227].
Ó Institute of Physics 2006
5.2.2. Strongly interacting systems
If the gas-surface interaction is strong, it may lead either to molecular or to dissociative chemisorption. In the present review we consider both cases through the following model systems:
Fig. 5.4. Sticking coefficient versus coverage for C2H4 adsorption on a bare Pd(1 0 0) surface. Data are reported for the
interaction of randomly oriented (mostly HE; continuous line) and aligned (mostly CW; dotted line) ethylene beams. The ethene
SMB energy is Ei = 0.36 eV. Taken with permission from Ref. [229].
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
(a)
(b)
(c)
(d)
143
C2H4/Pd(0 0 1);
O2/Ag(1 0 0);
O2/Pd(1 0 0);
O2/CO/Pd(1 0 0).
QMS signal (arb. units)
CO2
O2
The first is an example of molecular adsorption; in the second case chemisorption may be either
molecular or dissociative, depending on experimental conditions; the last two systems are characterised by purely dissociative adsorption.
C2H4 adsorption on bare and O covered Pd(0 0 1) was thoroughly investigated by Stuve and Madix
by thermal desorption and vibrational spectroscopies [228]. At 80 K both the di-r and p-bonded
forms of C2H4 are stable on the clean surface. The p-bonded form desorbs above 100 K, while the
di-r-bonded one undergoes dehydrogenation to form a vinyl species (HCCH2) and ultimately methylidyne groups (CH) at 300 K. Heating to 500 K causes further dehydrogenation of the CH groups and the
formation of surface carbon. Atomic oxygen is found to inhibit C2H4 adsorption in the di-r-bonded
form and hence dissociation. p-bonded ethene is, on the contrary, stable up to oxygen coverages of
0.5 ML. Uptake experiments performed with rotationally aligned ethene SMBs [229] demonstrate that
a small stereodynamical effect at non zero coverage is present for the p-bonded species (see Fig. 5.4);
the effect is similar to the one observed on O2/Ag(1 0 0) and C/Cu(4 1 0) [68,223], although of smaller
entity. No stereo-anisotropy is observed, on the contrary, for di-r-bonded C2H4. This finding confirms
the important role of steering, already discussed in the previous section, and supports the general conclusion that significant stereodynamical effects are expected in case of weakly-bound systems.
The comparison between O2 adsorption at Ag(1 0 0) and at Pd(1 0 0) offers an overview of the possible destiny of the same molecule on substrates characterised by different reactivity. On Ag oxygen
adsorbs molecularly below 150 K in a negatively charged chemisorbed precursor (O2 in a peroxide
state). Above this T dissociation occurs. On the chemically more active Pd(1 0 0) substrate, on the con-
ΘCO=0.17 ML
ST
FT
ΘCO=0 ML
0
20
40
60
80
100
120
Time (s)
Fig. 5.5. O2 partial pressure in the crystal chamber, measured by a QMS not in line of sight with the SMB, as a function of time
for the ST and FT tails of oxygen SMBs interacting with the bare surface (bottom traces) and with Pd(1 0 0) pre-covered with
0.17 ML of CO (middle traces). The top traces report the CO2 production during exposure.
Author's personal copy
144
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
trary, dissociation occurs also at low T and always through a direct mechanism. In both cases, however, no stereodynamical anisotropy was observed for S, neither as an initial effect nor in its coverage
dependence. This is demonstrated in Fig. 5.5 (bottom traces) by the perfect superposistion of the KW
traces recorded upon ST and FT O2 exposure on clean Pd(1 0 0). Both for adsorption into a peroxide
state and for direct dissociation, steering is therefore strong enough to mask any possible stereodynamical dependence. This outcome is not surprising if we consider that the energy typical of rotational
excitations is much lower than the O2 translational energy (Ei = 0.40 eV).
At variance with the case of O2 on clean Pd(1 0 0), O2 adsorption on CO pre-covered Pd(1 0 0) is the
first example of a system showing significant stereodynamical effects both for the initial sticking probability and for the overall reactivity [71]. Fig. 5.5 reports a comparison between uptake experiments
performed with rotationally selected O2 beams on bare Pd(1 0 0) (bottom traces) and on the same substrate pre-covered with 0.17 ML of CO (middle traces). As already remarked, in the former case the KW
traces of ST and FT are almost identical. In presence of CO, on the contrary, we have that: (a) S0 is larger
for FT than for ST; (b) ST and FT show a markedly different behaviour of the sticking probability vs
exposure; (c) the reactivity of oxygen adatoms produced by CW or by HE is different, as witnessed
by the different CO2 production rate measured during the O2 exposure by ST and FT (top traces in
Fig. 5.5). In particular the oxidation rate is higher for FT, coherently with the larger S0 value measured
for this tail, but the total amount of CO2 produced is larger when dosing with ST.
Oxygen adsorption is a direct and non-activated process also in presence of CO pre-coverage [230],
a precursor mechanism being excluded by the dependence of S on oxygen coverage in the absence of
CO (see Fig. 5.5) and by the independence of S0 on T between 273 K and 733 K (data not shown). The
evolution of the uptake curve with O2 exposure is complicated by the formation and desorption of CO2,
which makes new free sites available. However, such process does not affect S0. The investigation was
extended to various crystal temperatures in the range 250 K < T < 400 K and to a lower Ei (0.26 eV, obtained by seeding O2 in a He/Ne mixture), finding no dependence on T and a weak dependence on the
translational energy for ST only [231]. All these findings are in agreement with a direct and non-activated nature of the chemisorption process. Therefore, most of the difference between the results observed upon exposure with ST and FT arises from the different adsorption probability for molecules
moving as helicopters and cartwheels.
To better clarify this point we investigated the dependence of S0 on CO pre-coverage (see Fig. 5.6).
Data were recorded during exposure of the CO/Pd(1 0 0) sample at two different temperatures to an O2
SMB with Ei = 0.40 eV. The results of a Monte Carlo simulation of the investigated system (discussed
1.0
ST
313 K
394 K
MC
0.8
FT
313 K
394 K
MC
S0(O2)
O2/CO/Pd(100)
0.6
0.4
0.2
0.0
0.0
0.1
0.2
0.3
0.4
ΘCO (ML)
Fig. 5.6. Initial sticking probability of O2 as a function of CO pre-coverage HCO for ST and FT and with the sample at two
different temperatures. Lines represent the result of a Monte Carlo model calculation of S0 as described in the text.
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
145
below) are also shown. The different behaviour of ST and FT is further remarked: for a CO pre-coverage
of 0.17 ML, e.g., the value of S0 is about 60% of the clean surface value for FT (from 0.75 to 0.45), while
it drops to 25% of that value for ST (from 0.75 to 0.20). The data of Fig. 5.6 confirm that the initial
sticking coefficient is always higher for CW than for HE. The magnitude of this effect indicates, moreover, that it cannot be due only to mere site blocking by pre-adsorbed CO molecules. Finally, we note
that the trend is opposite to what observed for weakly-bound systems, for which ST presented the
higher adsorption probability.
p
p
On Pd(1 0 0) CO molecules sit at bridge sites, in an incomplete c(2 2 2) structure due to mutual repulsion [232] (see Fig. 5.7A). We explained, therefore, the different behaviour of the two tails
considering the larger number of free sites required to allow for a molecule with helicopter-like motion to land between CO admolecules. A Monte Carlo simulation of the CO/Pd(1 0 0) system was performed using the same code and parameters for the interaction energy between adsorbed CO already
employed to reproduce the coverage dependence of the heat of adsorption of CO on Pd(1 0 0) [233]. A
strong repulsive interaction between molecules was imposed to prevent occupation of neighbouring
sites (x2 = 25 kJ/mol, x3 = 2.5 kJ/mol), which experimentally does not occur below 0.5 ML. The CO–Pd
bond was set to the experimental value of 174 kJ/mol, corresponding to the heat of adsorption of CO in
the low coverage limit. The probability (PMC) to find the configurations reported in Fig. 5.7B was then
calculated; such arrangements correspond to four free (bridge) sites in a square (I), six free sites (II),
and a more complicated set of 20 free sites (III). Configurations I and II are, of course, the most probable when the CO coverage increases. They may be adequate for the landing of a molecule in (or close
to) an upright position, i.e. with the O–O bond (nearly) perpendicular to the surface, provided that CO
can be displaced during O2 dissociation to allow the resulting oxygen adatoms to occupy next-neighbour sites. This assumption is reasonable since the adsorption energy of oxygen atoms is much larger
than that of carbon monoxide [233]. The much more stringent configuration (III) corresponds to an
eight (fourfold hollow) sites requirement for dissociation of O2 on oxygen-precovered Pd(1 0 0)
[234] if it is supposed that all bridge sites belonging to the seven first neighbours of the two oxygen
adatoms are empty. This array is certainly sufficient for the landing and dissociation of oxygen molecules moving with a helicopter-like motion and does not require CO to move away. We have finally
considered that FT and ST are not pure states, but that the fraction of CW is 0.67 and 0.42 in the two
SMB tails, respectively. The value of S0 for the bare surface is thus multiplied by the probability to find
a set as in (I) for the cartwheeling molecules and a set as in (III) for molecules with helicopter-like motion. The thick lines in Fig. 5.6 show the good agreement between MC simulation and experimental
data and support thus the validity of the proposed model.
I
II
(A) (B)
ω2
III
Ó Wiley VCH 2006
ω3
p
p
Fig. 5.7. (A) Illustration of the interaction between first (x2) and second (x3) neighbour sites occupied in the c(2 2 2)R45°
structure corresponding to a CO coverage of 0.5 ML. (B) Schematization of the configurations I–III, corresponding to the squared
array of four bridge sites (I, pink on-line), the rectangular shaped set of six free bridge sites (II; red on-line) and the array of 20
free sites (III; blue on-line) obtained by eight fourfold hollow sites blocked by two oxygen atoms (green on-line). (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Taken
with permission from Ref. [71].
Author's personal copy
146
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Fig. 5.6 shows also a clear difference in the reactivity of ST and FT with respect to CO oxidation.
Since adsorption is totally dissociative and CO2 production occurs via a Langmuir–Hinshelwood mechanism, the CO2 desorption curves are not expected to be qualitatively different if O adatoms generated
by CW or HE dissociation populate the same adsorption sites. According to previous literature, in the
low coverage limit they should occupy fourfold hollow sites forming a disordered layer or a p(2 2)
structure [235]. The observed difference in the CO2 yield is, on the contrary, indicative that adatoms
produced by O2 molecules impinging with helicoptering or cartwheeling motion populate non equivalent sites. A more refined look at the experiments shows, moreover, that the CO2 production starts
only after a critical coverage of O adatoms produced by ST is reached. Such pre-coverage of
(0.03 ± 0.01) ML corresponds, reasonably, to saturation of surface defects.
To better clarify this issue we investigated the dissociation of rotationally aligned O2 molecules
on the bare Pd(1 0 0) surface by high resolution X-ray photoemission spectroscopy (XPS) and by Xray Photoelectron Diffraction (XPD) [236]. Experiments were performed at the SuperEsca beamline
of the Elettra Synchrotron Light Source in Trieste, which may be equipped with a velocity selected
SMB. The degree of alignment of this experimental apparatus is slightly different from the one estimated for the Genoa SMB due to the different angular divergence. The concentration of CW is thus
estimated to be (77 ± 5)% and (36 ± 5)% in FT and ST, respectively. The photoemission information
was retrieved from the O-2s peak because of the overlap between the O 1s and Pd 3p3/2 lines. Additional O-2s intensity due to adsorbed CO is shifted by 6.5 eV and is therefore clearly distinguishable,
if present. Fig. 5.8 shows the O-2s photoemission spectra recorded at T = 220 K (at which T the removal of O by background CO is thermally hindered even on the time scale of several hours) with a
photon energy ht = 150 eV and for two different photoemission polar angles (h) along the [110]
direction. The spectra correspond to the bare surface (black) and to oxygen layers obtained after
dosing 0.10 ML O2 at normal incidence with ST and FT, respectively. We recall that the uptake curves
recorded during KW experiments on the clean Pd(1 0 0) surface are identical for the two tails (see
Fig. 5.5), so that the estimated atomic O coverage is 0.12 ML in both cases. In spite of that, the photoemission intensities are markedly different, confirming that most O atoms originated from ST and
FT end up into different sites. The inset of Fig. 5.8 reports the XPD intensity curves for ST and FT
plotted vs. photoemission angle h. The difference in the photoemission patterns for O generated
by ST and FT is evident, confirming our conclusion about the different composition of the so generated O layers.
However, the observed anisotropy between the two tails is too large to be due only to the different
degree of rotational alignment. Apart from this parameter, ST and FT differ in translational energy and
may slightly differ in the population of the rotational levels. These differences can play only a minor
role for the outcome of the described experiments since: (a) the translational energy is only 0.04 eV
higher in FT than in ST and it is thus irrelevant for the sticking probability of a non-activated adsorption system and unlikely to affect the reactivity; (b) at a nozzle temperature of 300 K, due to the low
rotational temperature of the SMB (5–10 K), the global population of the next rotational state (K = 3)
does not exceed 10%. The observed effect is then mainly due to the difference between the rotational
alignment of FT and ST. The amplification of the difference implies that part of the HE in FT ends up in
the same final configuration as O generated by CW either because of the above mentioned differences
in the molecular beam or because of an active role of the surface.
To analyse the XPD data the intensity curves recorded for FT and ST exposure were deconvoluted
according to the rotational polarization to extract the CW and HE contributions. Deconvoluted curves
are reported in Fig. 5.9A and B as a function of h (dashes) and compared to the result of multiple scattering calculations (continuous lines) performed with a proper atomic cluster model and potential
[237,238]. For HE the best agreement is found for oxygen adatoms in the fourfold hollow site (Ofour),
(0.82 ± 0.02) Å above the Pd surface plane. This site agrees with previous experimental and theoretical
studies [239–241]. The CW data, on the contrary, can be reproduced only by considering oxygen atoms
in the octahedral interstitial sites (Oocta), sitting (1.60 ± 0.10) Å below the surface plane and inducing a
deformation of the surface atomic geometry to attain a Pd–O distance of about (2.0 ± 0.10) Å. The errors are related to the clear-cut minima in the R-factors (see insets) which, in turn, are due to the high
sensitivity to the atomic geometry for the scattering of the photoelectrons in the present conditions.
We note that the occupation of hollow and octahedral sites is similar to what observed for O–Pd(1 1 1)
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
147
24
Ó Wiley VCH 2009
[242], for which both the fcc hollow site, predicted by DFT calculations, and the octahedral one, produced by filling a subsurface defect, were reported. Moreover, subsurface site occupation at low coverage was suggested to account for the temperature dependence of the oxygen Auger signal for the O/
Pd(0 0 1) system [243].
The reactivity/passivity of HE/CW generated oxygen atoms towards CO is a natural consequence of
their location above/below the surface. A subsurface location of most of the CW generated O may appear surprising and is indeed not supported by DFT calculations [239,240]. This discrepancy might be
Fig. 5.8. XPS spectra of the O-2s level recorded upon O2 adsorption on Pd(1 0 0) at T = 220 K. The dotted (black on-line) curve
corresponds to the clean surface, while the continuous (red on-line) and dotted (blue on-line) ones are recorded after dosing
0.10 ML of O2 by ST or FT, respectively. The different count rates at different angles and the low signal for FT at +15° are evident.
The inset reports the O-2s photoemission intensity vs. polar angle along the [110] direction as evaluated from the complete set
of measurements. Data are normalized to the background value. Taken with permission from Ref. [236].
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Ó Wiley VCH 2009
148
Fig. 5.9. Deconvoluted experimental XPD intensities of the O-2s level vs. electron emission angle (dot-dashed lines) for HE (a)
and CW (b), compared with the theoretical fit with the model described in the text (continuous line). The photon energy is
150 eV and the detection direction is along the [110] azimuth. The insets report the R-factors vs. adsorption height. Taken with
permission from Ref. [236].
indicative of the occurrence of non-equilibrium conditions, which may elude a static DFT investigation. Such configurations may arise because of the release of energy (some eV) in the chemisorption
process which ends up in the generated O atoms. Their velocity is mainly parallel to the surface for
HE and at some angle with it for CW, causing implantation into the surface in the latter case. Last
but not least, the O2 molecules in the present experiment have a translational energy of 0.40 eV
and, thanks to the absence of an activation barrier for adsorption, part of it could be employed to overcome the barrier between on-surface and subsurface sites. Coherently with this picture, no stereosensitivity is observed in the reactivity when dosing O2 at T = 730 K.
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
149
5.3. Role of theory
The theoretical investigation of the topics treated in this review allowed to gain insight and suggested further developments. The challenge [244] is thereby firstly to explain the experimental findings qualitatively and secondly to reproduce the experimental data quantitatively. For H2 the latter
aim became affordable, at least in principle, only when fully quantum calculations incorporating all
six degrees of freedom of the molecule became feasible. Generally speaking, when tackling the issue
of stereodynamics at surfaces theoretically, one has to face the following steps:
(1) Assume (in model calculations) or calculate (in ab initio approaches) a Potential Energy Surface
which includes the rotational degrees of freedom.
(2) Determine the evolution of the state of the system for the above determined potential.
(3) Compare the theoretical results with the available experimental data (often obtained measuring
the sticking probability with SMBs for different intial states or by considering the quantum state
of the scattered or of the desorbing molecules and invoking detailed balance arguments) and
formulate hypotheses about the reasons for possible discrepancies..
Each of these steps involves non-obvious choices. Since this is not the place for a comprehensive
review of the theoretical attempts (for which we refer to Refs. [61,244–246]) we shall briefly resume
only some systems. Most studies dealt with strongly reactive systems such as H2/Pd(1 1 1)
[90,247,248], and H2/Pd(1 0 0) [56,61]. Other investigations addressed intermediately reactive systems such as H2/Pt(1 1 1) [249,250] or little reactive systems such as H2/Cu(1 0 0) [58,251,252] and
H2/Cu(1 1 1) [10,253].
When the molecule approaches the surface, several different phenomena can occur. We expect
intuitively that at sufficiently low translational energy the forces experienced by the molecule when
approaching the surface may both deviate it from its original trajectory and cause a modification of its
rotational state by steering it into the most favoured position and/or orientation with respect to the
surface. At higher translational energy steering becomes less effective and the initial rotational state
may still play an observable role.
When looking for a qualitative or semiquantitative understanding of the process a classical approach may provide useful insight into the phenomena. A general question which cannot be addressed
directly experimentally is whether dissociation is determined by molecular orientation or by the collision parameters of the molecules at impact. We mention as an example, the investigation by Kara
and DePristo [66], who considered the position in the unit cell and the azimuthal and polar angle distribution of the molecules at the time of dissociation for H2/Ni(1 0 0) and for N2/W(1 1 0).
With reference to Fig. 5.10(a), for H2/Ni(1 0 0) the dissociating molecules point with their axis
towards 0° and 90°, rather than towards the atop site (i.e. U = 45°), in accord with the site sensitivity of this system. For N2/W(1 1 0) (see Fig. 5.10(b)), on the contrary, the flat dependence on the
polar angle indicates insensitivity to U, corresponding in fact to no site selectivity for dissociation.
However the sharp peak near 90° for the azimuthal angle implies that dissociation is favoured
strongly for molecules oriented nearly parallel to the surface. Particularly interesting for the interpretation of the experimental data is the suggestion that when molecular orientation dominates
over site selectivity, S0 remains lower than unitary also at high translational energies, while it will
approach unity when the opposite holds true. These authors found also that high symmetry sites
are not necessarily dominant in the dissociation process, implying that in order to obtain a significant comparison with experiment simulations must sample all the unit cell in ab initio DFT calculations of the PES.
The results mentioned above were obtained by a classical stochastic molecular dynamics simulation, while the present state of the art involves 6D PES and wavepacket calculations. Such more time
consuming and complicated approach is unavoidable if one aims at a quantitative comparison between theory and experiment, as we showed e.g. in Fig. 2.3.
As a further example we mention here the very detailed calculations performed by McCormack
et al. for H2/Cu(1 0 0) [244], who computed the collision energy dependence of the probability of vib-
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Ó American Institute of Physics 1990
150
Fig. 5.10. (a) Azimuthal angle distribution for H2/Ni(1 0 0), Ei = 0.14 eV and rigid surface; (b) polar and azimuthal angle
distributions for N2/W(1 1 0) at Ei = 0.8 eV and Ts = 800 K at the time of dissociation. The inset shows the definition of the
azimuthal angle relative to the surface atoms. Taken with permission from Ref. [66].
rationally elastic scattering into different rotational states. As shown in Fig. 5.11, such probability
came out to depend in a complex way on the translational energy and on the initial (helicoptering
or cartwheeling) rotational state.
We finally underline that only comparison with experiments can test the results of such sophisticated models and possibly justify, a posteriori, the choice of a simplified PES or the use of a classical or
of a semi-classical approach.
Author's personal copy
151
Ó Royal Society of Chemistry 1998
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Fig. 5.11. Collision energy dependence of the probability of vibrationally elastic scattering into rotational (j0 , m0 ) states. P(v0, j0,
m0) ? (v0 = 0, j0 , m0 ). Each plot is labelled by (j0 , m0 ). Panels to the left are for (v0 = 0, j0 = 4, m0 = 0) and those to the right for
(v0 = 0, j0 = 4, m0 = 4). The upper panels are for rotationally elastic scattering, the central ones, for the remaining j0 = 4 states, and
the lower ones for j0 = 0 and j0 = 2. Taken with permission from Ref. [244]
6. Perspectives and conclusions
In spite of the growing number of recent investigations, both theoretical and experimental, the
influence of stereodynamics on gas-surface interaction and on simple catalytic reactions is still a
young topic which is likely to reach significant new achievements in the near future. New frontiers
can be explored using newly developed experimental techniques, which allow to prepare molecules
in defined quantum states [254] under field free conditions, and increasing computing power, enabling to perform fully multidimentional ab initio calculations of the PES and of the wavepacket motion. When talking about the perspectives of the field we should distinguish between investigations
aiming at the understanding of the stereodynamics at surfaces and investigations aiming at using
these tools in catalysis or for the growth of ordered layers. Regarding the former, the present review
has shown that the existing investigations have opened a series of questions. Among them we like to
point out at the following issues:
(1) No experimental evidence was found so far for an effect of rotational alignment on the initial
sticking probability of non-polar molecules interacting with bare surfaces. The phenomenon
was predicted for H2/Pt [255], a molecule for which collisional rotational alignment is, however,
experimentally not feasible. For the investigated weakly-bound systems steering is probably
Author's personal copy
152
(2)
(3)
(4)
strong enough to mix up the different alignments. The phenomenon might, however, be observable at high enough translational energies, if the extra available energy, resulting from the conversion of rotational into translational motion, increases the probability for CW to be
backscattered into the gas phase. Good candidates for the observation of this effect are linear
alkanes (i.e. ethane) on metal surfaces for which the adsorption energy is significantly weaker
than for alkenes (implying desorption lower than at liquid nitrogen temperature) and for which
collisional alignment is feasible.
The effect of the population of high J states (achievable by heating the nozzle) has not been
investigated yet. This is a topic of major interest since such information might reduce the gap
separating collisional alignment and laser techniques in preparing beams of molecules in
defined quantum states.
Laser methods will possibly achieve the goal of selectively populating a desired rovibrational
state for molecules heavier than hydrogen or deuterium for a time long enough to enable surface science experiments. Even more challenging is the possibility to use SMBs of oriented or
aligned molecules to perform a given surface reaction, possibly with increased selectivity
towards a desired product, or to grow a layer with pre-defined electronic or mechanical
properties.
Molecular alignment may be exploited to selectively perform desired surface reactions. Indeed
our recent studies on the interaction of rotationally aligned O2 SMBs on Pd(1 0 0) (see Section 5.2.2) demonstrate that stereodynamical effects may control the final state reached by
the dissociation fragments. A possible development in this direction could be to exploit the stereochemistry for the catalytic oxidation of benzene. Quantum chemical calculations [256] have,
indeed, shown that the path followed in the reaction between benzene and oxygen depends on
the steric arrangements of the reacting system. When the O2 molecule approaches benzene
with the O–O bond parallel to the C–C bond a by-product (o-dihydroxybenzene) is formed,
while total oxidation takes place when the O-O bond is perpendicular to the benzene ring.
Some years ago Kasai et al. [257] suggested that the ortho (o-H2) to para (p-H2) hydrogen conversion [258] due to the interaction with a metal or metal–oxide catalyst is stereo dependent,
occurring more efficient for CW than for HE. The suggested mechanism is shown in Fig. 6.1. The
Ó Elsevier 2003
(5)
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
Fig. 6.1. The suggested method to enhance the o–p H2 conversion consists of two steps. The first involves the so-called
dynamical quantum filtering and the second relies on the stereoselectivity of the o–p conversion. Taken with permission from
Ref. [257].
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
153
basic idea is that, after permeation of H through a metal, the desorbing H2 will be mostly cartwheeling or helicoptering depending on the final kinetic energy, the HE motion corresponding
to the fastest molecules. One could then take advantage of the larger o-h conversion rate for
cartwheeling H2 by velocity selecting the desorbing molecules. To the best of our knowledge
no experimental confirmation of this effect exists yet.
(6) Application of collisional alignment to the larger size molecules of interest for opto-electronic
and molecular electronic applications (e.g. phtalocyanines and benzenic molecules like tetracene and pentacene [259,260]) will possibly allow to exploit collisional alignment to grow more
ordered and functional organic films.
(7) Performing studies on the interaction of aligned molecules on stepped [261] or on otherwise
nanostructured substrates may give insight on the role of stereodynamics at non normal incidence and for more complex surface structures.
In conclusion we have reported an overview of the recent developments in the field of stereodynamics in gas-surface interaction focussing on experiments exploiting the orientation of polar molecules achieved by hexapole devices and the rotational alignment of homopolar molecules obtained by
collisions in supersonic expansions. We hope to have offered with this review a flavour for the many
exciting possibilities of this field for the fundamental understanding of catalytic phenomena and for
the knowledge based design of new catalysts.
References
[1] G. Scoles (Ed.), Atomic and Molecular Beam Methods, Oxford University Press, 1988.
[2] K.H. Kramer, R.B. Bernstein, Focusing and orientation of symmetric-top molecules with the electric six-pole field, J. Chem.
Phys. 42 (1965) 767–770.
[3] P.R. Brook, Reactions of oriented molecules, Science 193 (1976) 1116.
[4] S. Stolte, Reactive studies on oriented molecules, Ber. Bunsen Ges. Phys. Chem. 86 (1982) 413–421.
[5] R.N. Zare, Optical preparation of aligned molecules, Ber. Bunsen Ges. Phys. Chem. 86 (1982) 422–425.
[6] B. Friedrick, D.R. Herschbach, Spatial orientation of molecules in strong electric fields and evidence for pendular states,
Nature 353 (1991) 412–414.
[7] H.G. Bemewitz, W. Paul, Ch. Schlier, Fokussierung polarer Moleküle, Z. Phys. 141 (1955) 6–15.
[8] H.G. Bemewitz, K.H. Kramer, W. Paul, J.P. Toennies, Messung der Anisotropie des van der Waals-Potentials durch Strenung
von Molekülen in definiertem Quantenzustand, Z. Phys. 177 (1964) 84–110.
[9] J.P. Toennies, Molekularstrahlmessungen von Stobquerschnitten für Übergänge zwischen definierten Rotationszuständen
zwei-atomiger Moleküle, Z. Phys. 182 (1965) 257–277.
[10] R.B. Bernstein, D.R. Herschbach, R.D. Levine, Dynamical aspects of stereochemistry, J. Phys. Chem. 91 (1987) 5363–5377.
[11] H.J. Loesch, J. Bulthuis, S. Stolte, A. Durand, J.C. Loison, J. Vigué, Molecules oriented by brute forces, Europhys. News 27
(1996) 12–15.
[12] P.R. Brooks, Orientation effects in electron transfer collisions, Int. Rev. Phys. Chem. 14 (1995) 327–354.
[13] H. Hou, S.J. Goulding, C.T. Rettner, A.M. Wodtke, D.J. Auerbach, The stereodynamics of a gas-surface reaction, Science 277
(1997) 80–82.
[14] E.B.D. Bourdon, P. Das, I. Harrison, J.C. Polanyi, J. Segner, C.D. Stanners, R.J. Williams, P.A. Young, Photodissociation,
photoreaction and photodesorption of adsorbed species. Part 2. CH3Br and H2S on LiF(0 0 1), Faraday Discuss. Chem. Soc.
82 (1986) 343–358.
[15] St.J. Dixon-Warren, E.T. Jensen, J.C. Polanyi, G.-Q. Xu, S.H. Yang, H.C. Zeng, Photochemistry of adsorbed molecules. Part 10.
Harpooning a fixed target: charge transfer from Ag or K substrates to halide adsorbates, Faraday Discuss. Chem. Soc 91
(1991) 451–463.
[16] C.R. Arumaynagam, J.R. Madix, Molecular beam studies of gas-surface collision dynamics, Prog. Surf. Sci. 38 (1991) 1–102.
[17] W.C. Simpson, J.A. Yarnoff, Fundamental studies of halogen reactions with III–V semiconductor surfaces, Ann. Rev. Phys.
Chem. 47 (1996) 527–554.
[18] J.H. Larsen, I. Chorkendorff, From fundamental studies of reactivity on single crystals to the design of catalysts, Surf. Sci.
Rep. 35 (1999) 163–222.
[19] G. Boato, G.G. Volpi, Experiments on the dynamics of molecular processes: a chronicle of fifty years, Ann. Rev. Phys. Chem.
50 (1999) 23–50.
[20] C.T. Rettner, H.E. Pfnur, D.J. Auerbach, Dissociative chemisorption of CH4 on W(1 1 0): dramatic activation by initial
kinetic energy, Phys. Rev. Lett. 54 (1985) 2716–2719.
[21] D.F. Ogletree, H. Bluhm, G. Lebedev, C.S. Fadley, Z. Hussain, M. Salmeron, A differentially pumped electrostatic lens system
for photoemission studies in the millibar range, Rev. Sci. Instrum. 73 (2002) 3872–3877.
[22] G. Ketteler, D.F. Ogletree, H. Bluhm, H. Liu, E.L.D. Hebenstreit, M. Salmeron, In situ spectroscopic study of the oxidation
and reduction of Pd(1 1 1), J. Am. Chem. Soc 127 (2005) 18269–18273.
[23] X.M. Yang, A.M. Wodtke, Int. Rev. Phys. Chem. 12 (1993) 123–147.
[24] C.T. Rettner, D.J. Auerbach, H.A. Michelsen, Role of vibrational and translational energy in the activated dissociative
adsorption of D2 on Cu(1 1 1), Phys. Rev. Lett. 68 (1992) 1164–1167.
Author's personal copy
154
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
[25] H.F. Berger, M. Leisch, A. Winkler, K.D. Rendulic, A search for vibrational contributions to the activated adsorption of H2 on
copper, Chem. Phys. Lett. 175 (1990) 425–428.
[26] R.W. Verhoef, D. Kelly, C.B. Mullins, W.H. Weiberg, Direct dissociative chemisorption of methane and ethane on Ir(1 1 0):
isotope effects and vibrationally assisted chemisorption, Surf. Sci. 311 (1994) 196–213.
[27] R.W. Verhoef, D. Kelly, C.B. Mullins, W.H. Weiberg, Angular and vibrational effects in the direct dissociative chemisorption
of deuterated methane and ethane on Ir(1 1 0), Surf. Sci. 325 (1995) 93–101.
[28] A. Hodgson, J. Moryl, P. Traversaro, H. Zhao, Energy transfer and vibrational effects in the dissociation and scattering of D2
from Cu (1 1 1), Nature 356 (1992) 501–504.
[29] B.E. Hayden, C.L.A. Lamont, Coupled translational–vibrational activation in dissociative hydrogen adsorption on Cu(1 1 0),
Phys. Rev. Lett. 63 (1989) 823–1825.
[30] C.T. Rettner, H.A. Michelsen, D.J. Auerbach, Quantum-state-specific dynamics of the dissociative adsorption and
associative desorption of H2 at a Cu(1 1 1) surface, J. Chem. Phys. 102 (1995) 4625–4641.
[31] A. Hodgson, State resolved desorption measurements as a probe of surface reactions, Prog. Surf. Sci. 63 (2000) 1–61.
[32] L.B.F. Juurlink, P.R. McCabe, R.R. Smith, C.L. Di Cologero, A.L. Utz, Eigenstate-resolved studies of gas-surface reactivity: CH4
(v 3) dissociation on Ni(1 0 0), Phys. Rev. Lett. 83 (1999) 868–871.
[33] H. Hou, Y. Huang, S.J. Gulding, C.T. Rettner, D.J. Auerbach, A.M. Wodtke, Enhanced reactivity of highly vibrationally excited
molecules on metal surfaces, Science 284 (1999) 1647–1650.
[34] R.D. Beck, P. Maroni, D.C. Papageorgopoulos, T.T. Dang, M.P. Schmid, T.R. Rizzo, Vibrational mode-specific reaction of
methane on a nickel surface, Science 302 (2003) 98–100.
[35] H.A. Michelsen, C.T. Rettner, D.J. Auerbach, R.N. Zare, Effect of rotation on the translational and vibrational energy
dependence of the dissociative adsorption of D2 on Cu(1 1 1), J. Chem. Phys. 98 (1993) 8294–8307.
[36] C.K. Man, R.C. Estler, The dynamics of the reaction Sr + HF(v = 1) ! SrF + H: effect of rotation, J. Chem. Phys. 75 (1981)
2779–2785.
[37] L. Vattuone, M. Rocca, U. Valbusa, Influence of rotational energy on adsorption probability for a physisorbed system: C2 H4
on Ag(0 0 1), Phys. Rev. Lett. 82 (1999) 4878–4881.
[38] E.W. Kuipers, M.G. Tenner, A.W. Kleyn, S. Stolte, Observation of steric effects in gas–surface scattering, Nature 334 (1998)
420–422.
[39] H. Muller, B. Dierk, F. Hamza, G. Zagatta, G.H. Fecher, N. Bowering, U. Heinzmann, Collision of oriented NO with Ni(1 0 0)
and with oriented CO on Ni(1 0 0), Surf. Sci. 269–270 (1992) 207–212.
[40] M. Brandt, H. Muller, G. Zagatta, O. Wehmeyer, N. Bowering, U. Heinzmann, Sticking and scattering of gas-phase oriented
NO at Pt(1 0 0) as a function of the translational energy and of molecular orientation, Surf. Sci. 331–333 (1995) 30–34.
[41] M.G. Tenner, E.W. Kuipers, A.W. Kleyn, S. Stolte, Classical trajectory study of the interaction of oriented NO and Ag(1 1 1),
Surf. Sci. 242 (1991) 376–385.
[42] M.G. Tenner, E.W. Kuipers, A.W. Kleyn, S. Stolte, Direct inelastic scattering of oriented NO from Ag(1 1 1) and Pt(1 1 1), J.
Chem. Phys. 94 (1991) 5197–5207.
[43] F.H. Geuzebroek, A.E. Wiskerke, A.W. Kleyn, Velocity distribution of oriented NO scattered from Ag(1 1 1), Nucl. Instrum.
Methods B 58 (1991) 354–359.
[44] R.J.W.E. Lahaye, S. Stolte, S. Holloway, A.W. Kleyn, Orientation and energy dependence of NO scattering from Pt(1 1 1), J.
Chem. Phys. 104 (1996) 8301–8311.
[45] A.W. Kleyn, Non reactive orientations of molecules at surfaces, Prog. Surf. Sci. 54 (1997) 407–420.
[46] H. Muller, G. Zagatta, M. Brandt, O. Wehmeyer, N. Bowering, U. Heinzmann, Orientation dependent CO2 production by
exposing a CO pre-covered Pt(1 0 0) surface to gas-phase oriented NO, Surf. Sci. 307–309 (1994) 159–164.
[47] H. Muller, B. Dierks, G.H. Fecher, N. Bowering, U. Heinzmann, Influence of orientation on co-adsorption dynamics CO
displacement from a c(2X2) precovered Ni(1 0 0) surface by free oriented NO, J. Chem. Phys. 101 (1994) 7154–7160.
[48] M. Brandt, H. Muller, G. Zagatta, N. Bowering, U. Heinzmann, Reaction of NO and CO on a Rh(1 0 0) surface studied with
gas-phase oriented NO, Surf. Sci. 352 (1996) 290–294. 1996.
[49] M. Brandt, G. Zagatta, N. Bowering, U. Heinzmann, Reaction of NO and CO on a Pt(1 0 0) surface as a function of the NO
translational energy and of molecular orientation, Surf. Sci. 385 (1997) 346–356.
[50] M.A. Weibel, T.D. Hain, T.J. Curtiss, Hexapole-selected supersonic beams of reactive radicals: CF3, SiF3, SH, CH, and C2H, J.
Chem. Phys. 108 (1998) 3134–3141.
[51] G. Darling, S. Holloway, Rotational motion and the dissociation of H2 on Cu(1 1 1), J. Chem. Phys. 101 (1994) 3268–3281.
[52] T. Brunner, W. Brenig, Rotational cooling and heating in activated adsorption/desorption for D2/Cu(1 1 1), Surf. Sci. 317
(1994) 303–308.
[53] J. Dai, J. Sheng, J.Z.H. Zhang, Symmetry and rotational orientation effects in dissociative adsorption of diatomic molecules
on metals: H2 and HD on Cu(1 1 1), J. Chem. Phys. 101 (1994) 1555–1563.
[54] D. Wetzig, M. Rutkoski, R. David, H. Zacharias, Rotational corrugation in associative desorption of D2 from Cu(1 1 1),
Europhys. Lett. 34 (1996) 31–36.
[55] S.J. Gulding, A.M. Wodtke, H. Hou, C.T. Rettner, H.A. Michelsen, D. Auerbach, Alignment of D-2(v,J) desorbed from
Cu(1 1 1): low sensitivity of activated dissociative chemisorption to approach geometry, J. Chem. Phys. 105 (1996) 9702–
9705.
[56] D. Wetzig, R. Dopheide, M. Rutkowski, R. David, H. Zacharias, Rotational alignment in associative desorption of D-2(v00 = 0
and 1) from Pd(1 0 0), Phys. Rev. Lett. 76 (1996) 463–466.
[57] W.A. Dino, H. Kasai, A. Okiji, Rotational alignment in the associative desorption dynamics of hydrogen molecules from
metal surfaces, J. Phys. Soc. Japan 67 (1998) 1517–1520.
[58] D.A. Mc Kormack, G.J. Kroes, R.A. Olsen, E.J. Baerends, R.C. Mowrey, Rotational effects in six-dimensional quantum
dynamics for reaction of H-2 on Cu(1 0 0), J. Chem. Phys. 110 (1999) 7008–7020.
[59] G.J. Kroes, Six-dimensional quantum dynamics of dissociative chemisorption of H2 on metal surfaces, Prog. Surf. Sci. 60
(1999) 1–85.
[60] A. Gross, M. Scheffler, Steering and ro-vibrational effects on dissociative adsorption and associative desorption of H2/
Pd(1 0 0), Prog. Surf. Sci. 53 (1997) 187–196.
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
155
[61] A. Gross, S. Wilke, M. Scheffler, 6 dimensional quantum dynamics of adsorption and desorption of H2 at Pd(1 0 0): steering
and steric effects, Phys. Rev. Lett. 75 (1995) 2718–2721.
[62] H.F. Busnengo, E. Pijper, G.J. Kroes, A. Salin, Rotational effects in dissociation of H-2 on Pd(1 1 1): quantum and classical
study, J. Chem. Phys. 119 (2003) 12553–12562.
[63] T.F. Hanisco, A.C. Kummel, Rotationally inelastic scattering of N2 from W(1 1 0), J. Vac. Sci. Technol. A 11 (1993) 1907–
1913.
[64] D.C. Jacobs, K.W. Kolasinski, S.F. Shane, R.N. Zare, Rotational population and alignment distributions for inelastic
scattering and trapping/desorption of NO on Pt(1 1 1), J. Chem. Phys. 91 (1989) 3182–3195.
[65] A.K. Kara, A.E. DePristo, Dynamics of dissociative chemisorption: N2/W(1 1 0), Surf. Sci. 193 (1988) 437–454.
[66] A.K. Kara, A.E. DePristo, On the concept and distribution of reactive sites in dissociative chemisorption, J. Chem. Phys. 92
(1990) 5653–5660.
[67] S. Holloway, B. Jackson, Rotational state distributions as a probe of dissociative adsorption, Chem. Phys. Lett. 172 (1990)
40–42.
[68] L. Vattuone, A. Gerbi, M. Rocca, U. Valbusa, F. Pirani, F. Vecchiocattivi, D. Cappelletti, Stereodynamic effects in the
adsorption of ethylene onto a metal surface, Angew. Chem. Int. Edit. 43 (2004) 5200–5203.
[69] A. Gerbi, L. Vattuone, M. Rocca, U. Valbusa, F. Pirani, D. Cappelletti, F. Vecchiocattivi, New insights on the stereodynamics
of ethylene adsorption on an oxygen-precovered silver surface, J. Chem. Phys. 123 (2005) 224709.
[70] A. Gerbi, L. Vattuone, M. Rocca, F. Pirani, U. Valbusa, D. Cappelletti, F. Vecchiocattivi, Stereodynamic effects in the
adsorption of propylene molecules on Ag(0 0 1), J. Phys. Chem. B. 109 (2005) 22884–22889.
[71] A. Gerbi, L. Savio, L. Vattuone, F. Pirani, D. Cappelletti, M. Rocca, Role of rotational alignment in dissociative chemisorption
and oxidation: O2 on bare and CO-precovered Pd(1 0 0), Angew. Chem. Int. Edit. 45 (2006) 6655–6658.
[72] V. Aquilanti, M. Bartolomei, F. Pirani, D. Cappelletti, F. Vecchiocattivi, Y. Shimizu, T. Kasai, Orienting and aligning
molecules for stereochemistry and photodynamics, PCCP 7 (2005) 291–300.
[73] H. Staplelfeldt, T. Seideman, Colloquium: aligning molecules with strong laser pulses, Rev. Mod. Phys. 75 (2003) 543–557.
[74] D. Herschbach, Chemical stereodynamics: retrospect and prospect, Eur. Phys. J. D 38 (2006) 3–13.
[75] T. Seideman, Rotational excitation and molecular alignment in intense laser fields, J. Chem. Phys. 103 (1995) 7887–7896.
[76] Y.H. Chen, S. Varma, A. York, H.M. Milchberg, Single-shot, space- and time-resolved measurement of rotational
wavepacket revivals in H2, D2, N2, O2, and N2O, Opt. Exp. 15 (2007) 11341–11357.
[77] R. Velotta, N. Hay, M.B. Mason, M. Castillejo, J.P. Marangos, High-order harmonic generation in aligned molecules, Phys.
Rev. Lett. 87 (2001) 183901–183904.
[78] T. Suzuki, Y. Sugawara, S. Minemoto, H. Saai, Optimal control of nonadiabatic alignment of rotationally cold N2 molecules
with the feedback of degree of alignment, Phys. Rev. Lett. 100 (2008) 033603.
[79] A. Rouzée, S. Guérin, V. Boudon, B. Lavorel, O. Faucher, Field-free one-dimensional alignment of ethylene molecule, Phys.
Rev. A 73 (2006) 033418.
[80] J.G. Underwood, M. Spanner, M.Y. Ivanov, J. Mottershead, B.J. Sussmann, A. Stolow, Switched wave packets: a route to
nonperturbative quantum control, Phys. Rev. Lett. 90 (2003) 223001.
[81] T. Seideman, On the dynamics of rotationally broad, spatially aligned wave packets, J. Chem. Phys. 115 (2001) 5965–5973.
[82] L. Holmegaard, J.H. Nielsen, I. Nevo, H. Stapelfeldt, F. Filsinger, J. Kupper, G. Meijer, Laser-induced alignment and
orientation of quantum-state-selected large molecules, Phys. Rev. Lett. 102 (2009) 023001.
P
[83] M. Kurahashi, Y. Yamauchi, State selection of molecular oxygen (3 g ) by a hexapole magnet and the inversion of
population for the spin-rotational states, Phys. Rev. A 78 (2008) 022708.
[84] W.B. Chapman, A. Kulcke, B.W. Blackmon, D.J. Nesbitt, Rotationally inelastic scattering of jet cooled H2 O with Ar: stateto-state cross sections and rotational alignment effects, J. Chem. Phys. 110 (1999) 8543–8554.
[85] V. Aquilanti, D. Ascenzi, D. Cappelletti, F. Pirani, Velocity dependence of collisional alignment of oxygen molecules in
gaseous expansions, Nature 371 (1994) 399–401.
[86] F. Pirani, D. Cappelletti, F. Vecchiocattivi, L. Vattuone, A. Gerbi, M. Rocca, U. Valbusa, A simple and compact mechanical
velocity selector of use to analyze and select molecular alignment in supersonic seeded beams, Rev. Sci. Instrum. 75
(2004) 349–354.
[87] G.O. Sitz, Gas surface interactions studied with state-prepared
Rep. Prog. Phys. 65 (2002) 1165–1193.
R x molecules,
2
[88] The analytical form of the error function erf is: erf ðxÞ ¼ p2ffiffipffi 0 et dt.
[89] G.D. Kubiak, G.O. Sitz, R.N. Zare, Recombinative desorption dynamics: molecular hydrogen from Cu(1 1 0) and Cu(1 1 1), J.
Chem. Phys. 83 (1985) 2538–2552.
[90] M. Gostein, G.O. Sitz, Rotational state-resolved sticking coefficients for H2 on Pd(1 1 1): testing dynamical steering in
dissociative adsorption, J. Chem. Phys. 106 (1997) 7378–7390.
[91] K. Yang, T.S. Rahman, Rotational cooling in associative desorption, J. Chem. Phys. 9 (1990) 6834–6840.
[92] X.Y. Chang, S. Holloway, Probing the transition state by time-dependent quantum dynamics, Surf. Sci. 251–252 (1991)
935–941.
[93] W.A. Dino, H. Kasai, A. Okiji, Rotational alignment in the associative desorption dynamics of hydrogen molecules from
metal surfaces, J. Phys. Soc. Jpn 67 (1998) 1517–1520.
[94] A.W. Kleyn, A.C. Luntz, D.J. Auerbach, Rotational energy transfer in direct inelastic surface scattering: NO on Ag(1 1 1),
Phys. Rev. Lett. 47 (1981) 1169–1172.
[95] G.D. Kubiak, J.E. Hurst, H.G. Rennagel, G.M. McClelland, R.N. Zare, Direct inelastic scattering of nitric oxide from clean
Ag(1 1 1): Rotational and fine structure distributions, J. Chem. Phys. 79 (1983) 5163–5178.
[96] C.N.R. Rao, G.R. Rao, Nature of nitrogen adsorbed on transition metal surfaces as revealed by electron spectroscopy and
cognate techniques, Surf. Sci. Rep. 13 (1991) 223–263.
[97] G.O. Sitz, A.C. Kummel, R.N. Zare, Direct inelastic scattering of N2 from Ag(1 1 1). I. Rotational populations and alignment,
J. Chem. Phys. 89 (1988) 2558–2571.
[98] G.O. Sitz, A.C. Kummel, R.N. Zare, Alignment and orientation of N2 scattered from Ag(1 1 1), J. Chem. Phys. 87 (1987)
3247–3248.
Author's personal copy
156
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
[99] G.O. Sitz, A.C. Kummel, R.N. Zare, Direct inelastic scattering of N2 from Ag(1 1 1). II. Orientation, J. Chem. Phys. 89 (1988)
2572–2582.
[100] A.C. Kummel, G.O. Sitz, R.N. Zare, J.C. Tully, Direct inelastic scattering of N2 from Ag(1 1 1). III. Normal incident N2, J. Chem.
Phys. 89 (1989) 6947–6955.
[101] A.C. Kummel, G.O. Sitz, R.N. Zare, J.C. Tully, Direct inelastic scattering of N2 from Ag(1 1 1). IV. Scattering from high
temperature surface, J. Chem. Phys. 91 (1989) 5793–5801.
[102] A.C. Luntz, A.W. Kleyn, D.J. Auerbach, Observation of rotational polarization produced in molecule-surface collisions,
Phys. Rev. B 25 (1982) 4273–4275.
[103] D.C. Jacobs, R.N. Zare, Simplified trajectory method for modeling gas–surface scattering: the NO/Pt(1 1 1) system, J. Chem.
Phys. 91 (1989) 3196–3207.
[104] C.W. Muhlhausen, L.R. Williams, J.C. Tully, Dynamics of gas–surface interactions: scattering and desorption of NO from
Ag(1 1 1) and Pt(1 1 1), J. Chem. Phys. 83 (1985) 2594–2606.
[105] B. Broers, G.J. van der Meer, R.W.M. Hoogeveen, L.J.F. Hermans, Role of rotational alignment in molecule–surface
interaction for CH3 F and OCS, J. Chem. Phys. 95 (1991) 648–655.
[106] A.V. Ghiner, M.I. Stockman, M.A. Vaksman, Phys. Lett. A 96 (1983) 79–82.
[107] R.W.M. Hoogevenn, R.J.C. Spreeuw, L.J.F. Hermans, Observation of surface light-induced drift, Phys. Rev. Lett. 59 (1987)
447–449.
[108] P. Clausing, Ann. Phys. 48 (1930) 533.
[109] R.L. Palmer, J.N. Smith Jr., H. Saltsburg, D.R. O’Keefe, Measurements of the reflection, adsorption, and desorption of gases
from smooth metal surfaces, J. Chem. Phys. 53 (1970) 1666–1676.
[110] C.T. Rettner, E.K. Schweizer, C.B. Mullins, Desorption and trapping of argon at a 2H–W(1 0 0) surface and a test of the
applicability of detailed balance to a nonequilibrium system, J. Chem. Phys. 90 (1989) 3800–3813.
[111] F.H. Geuzebroek, A.E. Wiskerke, M.G. Tenner, A.W. Kleyn, S. Stolte, A. Namiki, Rotational excitation of oriented molecules
as a probe of molecule–surface interaction, J. Phys. Chem. 95 (1991) 8409–8421.
[112] H. Voges, R. Schinke, A double rainbow interpretation of rotational energy transfer in energetic NO/Ag(1 1 1) collisions,
Chem. Phys. Lett. 100 (1983) 245–250.
[113] G. Herzberg, Spectra of Diatomic Molecules, D. van Nostrand Company Inc., 1950. pp. 219–221.
[114] A.W. Kleyn, E.W. Kuipers, M.G. Tenner, S. Stolte, Steric effects in scattering and adsorption of NO at Ag(1 1 1), J. Chem. Soc.
Faraday Trans. 285 (1989) 1337–1345.
[115] D.C. Jacobs, I. Pois, E.W. Kuipers, A.W. Kleyn, L. Schröter, D.H. Zacharias, G. Fecher, M. Volkmer, N. Böwering, B. Pawlitzky,
U. Heinzmann, R.D. Levine, K.J. Snowdon, R.N. Dixon, General discussion, J. Chem. Soc. Faraday Trans. 285 (1989) 1357–
1376.
[116] E.W. Kuipers, M.G. Tenner, A.W. Kleyn, S. Stolte, Steric effects for NO/Pt(1 1 1) adsorption and scattering, Phys. Rev. Lett.
62 (1989) 2152–2155.
[117] B. Berenbak, B. Riedmüller, S. Stolte, A.W. Kleyn, Impact site-dependent molecular anisotropy: NO scattering from
Ru(0 0 0 1)–(1 1)H, Chem. Phys. 301 (2004) 309–313.
[118] G.H. Fecher, N. Böwering, M. Volkmer, B. Pawlitzky, U. Heinzmann, Dependence of the sticking probability on initial
molecular orientation: NO on Ni(1 0 0), Surf. Sci. 230 (1990) L169–L172.
[119] H. Müller, B. Dierks, F. Hamza, G. Zagatta, G.H. Fecher, N. Böwering, U. Heinzmann, Collision of oriented NO with Ni(1 0 0)
and with oriented CO on Ni(1 0 0), Surf. Sci. 269–270 (1992) 207–212.
[120] M. Brandt, G. Zagatta, N. Böwering, U. Heinzmann, Reaction of NO and CO on a Pt(1 0 0) surface as a function of the NO
translational energy and of molecular orientation, Surf. Sci. 385 (1997) 346–356.
[121] M. Brandt, T. Gerber, N. Böwering, U. Heinzmann, The Role of Molecular State and Orientation in Harpooning Reactions:
N2 O on Cs/Pt(1 1 1), Phys. Rev. Lett. 81 (1998) 2376–2379.
[122] M. Brandt, T. Gerber, F. Kuhlmann, N. Böwering, U. Heinzmann, State- and orientation-dependent N2 emission in the
N2O + Cs reaction, Surf. Sci. 402–404 (1998) 160–164.
[123] M. Brandt, F. Kuhlmann, T. Gerber, N. Böwering, U. Heinzmann, Interaction of gas-phase oriented N2 O with lithium
metal: evidence for an Eley–Rideal mechanism, Surf. Sci. 439 (1999) 49–58.
[124] T.J. Curtiss, R.B. Bernstein, Steric effect in the scattering of oriented CH3 F molecules by graphite (0 0 0 1), Chem. Phys.
Lett. 161 (1989) 212–218.
[125] R.S. Mackay, T.J. Curtiss, R.B. Bernstein, Determination of preferred orientation for sticking of polar molecules in beams
incident on a graphite (0 0 0 1) surface, Chem. Phys. Lett. 164 (1989) 341–344.
[126] R.S. Mackay, T.J. Curtiss, R.B. Bernstein, Strong orientation dependence of the scattering of fluoroform by graphite
(0 0 0 1), J. Chem. Phys. 92 (1990) 801–802.
[127] T.J. Curtiss, R.S. Mackay, R.B. Bernstein, Steric effect in the scattering of hexapole-oriented beams of symmetric-top
molecules by graphite(0 0 0 1), J. Chem. Phys. 93 (1990) 7387–7405.
[128] S.I. Ionov, M.E. Lavilla, R.S. Mackay, R.B. Bernstein, Surface temperature dependence of the steric effect in the scattering of
oriented tert-butyl chloride and fluoroform molecules by graphite(0 0 0 1), J. Chem. Phys. 93 (1990) 7406–7415.
[129] S.I. Ionov, M.E. Lavilla, R.B. Bernstein, Orientational dependence of the translational energy transfer in the scattering of
oriented fluoroform and tert-butyl chloride molecules by a graphite(0 0 0 1) surface, J. Chem. Phys. 93 (1990) 7416–7426.
[130] S.I. Ionov, R.B. Bernstein, Hard-cube analysis of the steric effect in molecule–surface scattering, J. Chem. Phys. 94 (1991)
1564–1571.
[131] D.B. Whitehouse, A.D. Buckingham, R.B. Bernstein, V.A. Cho, R.D. Levine, Orientation dependence of the induction
contribution in molecule-graphite interactions, J. Phys. Chem. 95 (1991) 8175–8177.
[132] I.V. Ionova, S.I. Ionov, R.B. Bernstein, An image charge model for the classical trajectory simulations of molecule-surface
scattering: steric effects in the scattering of trifluoromethane on graphite (0 0 0 1), J. Phys. Chem. 95 (1991) 8371–8376.
[133] K. Moritani, M. Okada, S. Goto, T. Kasai, New design and development of an oriented-molecular-beam machine
compatible with ultra-high-vacuum, J. Vac. Soc. Jpn. 46 (2003) 692–697.
[134] M. Okada, K. Moritani, S. Goto, T. Kasai, New development of ultrahigh-vacuum oriented-molecular-beam machine and
its application to chemical reactions on silicon surface, Jpn. J. Appl. Phys. 44 (2005) 8580–8589.
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
157
[135] M. Okada, M. Hashinokuchi, K. Moritani, T. Kasai, Y. Teraoka, New oriented-molecular-beam machine for surface
stereochemistry with X-ray photoemission spectroscopy, Jpn. J. Appl. Phys. 47 (2008) 3686–3691.
[136] C.H. Townes, A.L. Schawlow, Microwave Spectroscopy, McGraw-Hill, New York, 1955.
[137] P.R. Brooks, Reactions of oriented molecules, Science 193 (1976) 11–16.
[138] S. Stolte, J. Reuss, H.L. Schwartz, Orientational anisotropy in the total collision cross section of state selected no molecules,
Physica 66 (1973) 211–216.
[139] D.H. Parker, R.B. Bernstein, Oriented molecule beams via the electrostatic hexapole: preparation, characterization, and
reactive scattering, Ann. Rev. Phys. Chem. 40 (1989) 561–595.
[140] T. Kasai, K. Kuwata, Steric effects in small radical formation, in: K. Liu, A. Wagner (Eds.), Advanced Series in Physical
Chemistry, vol. 6, World Scientific, Singapore, 1995, p. 842. Chapter 19.
[141] K.M. Backstrand, M.A. Weibel, R.M. Moision, T.J. Curtiss, Temperature programmed desorption studies of OD coadsorbed
with H2 on Pt(1 1 1), J. Chem. Phys. 112 (2000) 7209–7218.
[142] H.L. Bethlem, G. Berden, F.M.H. Crompvoets, R.T. Jongma, A.J. A van Roij, G. Meijer, Electrostatic trapping of ammonia
molecules, Nature 406 (2000) 491–494.
[143] M.G. Tenner, E.W. Kuipers, W.Y. Langhout, A.W. Kleyn, G. Nicolasen, S. Stolte, Molecular beam apparatus to study
interactions of oriented NO and surfaces, Surf. Sci. 236 (1990) 151–168.
[144] R.T. Jongma, G. Berden, D. van der Zande, T. Rasing, H. Zacharias, G. Meijer, State-to-state scattering of metastable CO
molecules from a LiF(1 0 0) surface, Phys. Rev. Lett. 78 (1997) 1375–1378.
[145] S.E. Choi, R.B. Bernstein, Theory of oriented symmetric-top molecule beams: precession, degree of orientation, and
photofragmentation of rotationally state-selected molecules, J. Chem. Phys. 85 (1986) 150–161.
[146] S.E. Choi, R.B. Bernstein, Orientational opacity function for CH3I + Rb reactive backscattering: the steric cone of
nonreaction, J. Chem. Phys. 83 (1985) 4463–4469.
[147] R.B. Bernstein, S.E. Choi, S. Stolte, Determination of molecular orientation and alignment from polarized laser
photofragmentation measurements. Oriented CH3I molecular beams, J. Chem. Soc. Faraday Trans. 2 85 (1989) 1097–1113.
[148] A. Gijsbertsen, W. Siu, M.F. Kling, P. Johnsson, P. Jansen, S. Stolte, M.J.J. Vrakking, Direct determination of the signof the NO
dipole moment, Phys. Rev. Lett. 99 (2007) 213403.
[149] D.A. King, M.G. Wells, Molecular beam investigation of adsorption kinetics on bulk metal targets: nitrogen on tungsten,
Surf. Sci. 29 (1972) 454–482.
[150] M. Okada, S. Goto, T. Kasai, Dynamical steric effect in the decomposition of methyl chloride on a silicon surface, Phys. Rev.
Lett. 95 (2005) 176103.
[151] M. Okada, S. Goto, T. Kasai, Steric effects in dissociative adsorption of low-energy CH3Cl on Si(1 0 0): orientation and
steering effects, J. Phys. Chem. C 112 (2008) 19612–19615.
[152] E.G. Rochow, The direct synthesis of organosilicon compounds, J. Am. Chem. Soc. 67 (1945) 963–965.
[153] T.C. Frank, J.L. Falconer, Silane formation on silicon: reaction kinetics and surface analysis, Langmuir 1 (1985) 104–110.
[154] M.J. Bronikowski, R.J. Hamers, Atomically resolved scanning tunneling microscopy study of the adsorption and
dissociation of methylchloride on Si(0 0 1), J. Vac. Sci. Technol. A 13 (1995) 777–781.
[155] M.L. Colaianni, P.J. Chen, H. Gutleben, J.T. Yates Jr., Vibrational studies of CH3I on Si(1 0 0)-(2 1): adsorption and
decomposition of the methyl species, Chem. Phys. Lett. 191 (1992) 561–568.
[156] K.A. Brown, W. Ho, The interaction of methyl chloride and Si(1 0 0) 2 1, Surf. Sci. 338 (1995) 111–116.
[157] J.Y. Lee, S. Kim, Adsorption mechanism of CH3Cl on Si(1 0 0)-2 1, Surf. Sci. 482-485 (2001) 196–200.
[158] A. Woelke, S. Imanaka, S. Watanabe, S. Goto, M. Hashinokuchi, M. Okada, T. Kasai, Dissociative adsorption of methyl
chloride on Si(0 0 1) studied by scanning tunneling microscopy, J. Electron Microscopy 54 (Supplement 1) (2005) i21–i24.
[159] A.J. Komorowski, H. Ternow, R. Razaznejad, B. Berenbak, J.Z. Sexton, I. Zoric, B. Kasemo, B.I. Lundqvist, S. Stolte, A.W.
Kleyn, A.C. Kummel, Dissociative adsorption of NO upon Al(1 1 1): orientation dependent charge transfer and
chemisorption reaction dynamics, J. Chem. Phys. 117 (2002) 8185–8189.
[160] M. Nagao, H. Umeyama, K. Mukai, Y. Yamashita, J. Yoshinobu, K. Akagi, S. Tsuneyuki, Precursor mediated cycloaddition
reaction of ethylene to the Si(1 0 0)c(4 2) surface, J. Am. Chem. Soc. 126 (2004) 9922–9923.
[161] W. Mönch, Semiconductor Surfaces and Interfaces, Springer, Berlin, 1993.
[162] A. Raukema, A.W. Kleyn, Transient trapping desorption of molecules at surfaces, Phys. Rev. Lett. 74 (1995) 4333–4336.
[163] A.H. Romero, C. Sbraccia, P.L. Silvestrelli, F. Ancilotto, Adsorption of methylchloride on Si(1 0 0) from first principles, J.
Chem. Phys. 119 (2003) 1085–1092.
[164] A. Groß, Theoretical Surface Science, Springer, Berlin, 2002.
[165] M. Preuss, W.G. Schmidt, F. Bechstedt, Methyl chloride adsorption on Si(0 0 1)-electronic structure, J. Phys. Chem. B 108
(2004) 7809–7813.
[166] M. Preuss, W.G. Schmidt, K. Seino, F. Bechstedt, Methylchloride adsorbed on Si(0 0 1): an ab initio study, Appl. Surf. Sci.
234 (2004) 155–161.
[167] M. Okada, S. Goto, T. Kasai, Reaction-path selection with molecular orientation of CH3Cl on Si{1 0 0}, J. Am. Chem. Soc. 129
(2007) 10052–10053.
[168] J. Wilkes, C.L.A. Lamont, L. Siller, J.M. Coquel, R.E. Palmer, VUV photochemistry of oriented molecules: methylchloride on
highly oriented pyrolytic graphite, Surf. Sci. 390 (1997) 237–242.
[169] R. Küster, K. Chritmann, The interaction of methyl chloride with a silicon(1 1 1) surface, Ber. Bunsenges. Phys. Chem. 101
(1997) 1799–1810.
[170] M. Sasaki, S. Yoshida, Stoichiometry-dependent scattering of trimethylgallium from GaAs(1 0 0) surface, Surf. Sci. 315
(1994) L964–L968.
[171] M. Hashinokuchi, M. Okada, H. Ito, T. Kasai, K. Moritani, Y. Teraoka, Stereodynamics in dissociative adsorption of NO on
Si(1 1 1), Phys. Rev. Lett. 100 (2008) 256104(1–4).
[172] E.W. Kuipers, M.G. Tenner, M.E.M. Spruit, A.W. Kleyn, Differential trapping probabilities and desorption of physisorbed
molecules: application to NO/Ag(1 1 1), Surf. Sci. 205 (1988) 241–268.
[173] C.E. Brown, P.G. Hall, Physical adsorption of nitric oxide on graphite and silica and adsorption of gases on nitric oxide
preadsorbed on carbon, J. Colloid Interface Sci. 42 (1973) 334–341.
Author's personal copy
158
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
[174] P. Gupta, C.H. Mak, P.A. Coon, S.M. George, Oxidation kinetics of Si(1 1 1) 7 7 in the submonolayer regime, Phys. Rev. B.
40 (1989) 7739–7749.
[175] A.G. Schrott, S.C. Fain Jr., Nitridation of Si(1 1 1) by nitrogen atoms, Surf. Sci. 111 (1981) 39–52.
[176] M. Hashinokuchi, H. Ito, Y. Teraoka, K. Moritani, M. Okada, T. Kasai, Dissociative adsorption of nitric oxide on Si(1 1 1)(7 7) surface, Jpn. J. Appl. Phys. 47 (2008) 1672–1676.
[177] M.D. Wiggins, R.J. Baird, P. Wynblatt, Thermal nitridation of Si(1 1 1) by nitric oxide, J. Vac. Sci. Technol. 18 (1981) 965–
970.
[178] W.H. Weinberg, Kinetics of surface reactions, in: M. Grunze, H.J. Kreuzer (Eds.), Kinetics of Interface Reactions, Springer,
New York, 1987, p. 94.
[179] R. Zare, Angular Momentum: Understanding Spatial Aspects in Chemistry and Physics, second ed., Wiley-Interscience,
NY, 1988.
[180] Z.C. Ying, W. Ho, Thermo- and photo-induced reactions of NO on Si(1 1 1)7 7. I. Adsorption and chemical reactions, J.
Chem. Phys. 91 (1989) 2689–2705.
[181] Z. Su, X. Lu, Q.N. Zhang, The chemisorption of NO on Si(1 1 1)-7 7 surface. A DFT study, Chem. Phys. Lett. 375 (2003)
106–112.
[182] H. Müller, G. Zagatta, N. Bowering, U. Heinzmann, Orientation dependence of NO sticking and scattering at Pt (1 0 0),
Chem. Phys. Lett. 223 (1994) 197–201.
[183] H. Senftleben, Effects of magnetic fields on paramagnetic gas heat conductivity, Phys. Z. 31 (1930) 822.
[184] H. Senftleben, Further studies concerning the effects of magnetic fields on paramagnetic gas heat conductivity, Phys. Z. 31
(1930) 961–963.
[185] C.J. Gorter, Interpretation of the senftleben effect, Naturwissenschaften 26 (1938) 140.
[186] J.J.M. Beenakker, G. Scoles, H.F.P. Knaab, J.M. Jonkman, The influence of magnetic field on transport properties of diatomic
molecules in gaseous state, Phys. Lett. 2 (1962) 5–6.
[187] S. Hess, Flow birefringence of polyatomic gases, Phys. Lett. 30A (1969) 239–240.
[188] F. Baas, Streaming birefringence in CO2 and N2, Phys. Lett. 36A (1971) 107–108.
[189] R.J. van den Oord, M.C. de Lignie, J.J.M. Beenakker, J. Korving, Optical observation of angular momentum alignment in a
heat-conducting gas, Phys. Rev. Lett. 59 (1987) 2907–2910.
[190] J.J.M. Beenakker, F.R. Mc Court, Magnetic and electric effects on transport properties, Ann. Rev. Phys. Chem. 21 (1970) 47–
72.
[191] N.F. Ramsey, Collision alignment of molecules, atoms, and nuclei, Phys. Rev. 98 (1955) 1853–1854.
[192] M.P. Sinha, C.D. Caldwell, R.N. Zare, Alignment of molecules in gaseous transport: alkali dimers in supersonic nozzle
beams, J. Chem. Phys. 61 (1974) 491–503.
[193] A.G. Visser, J.P. Bekooy, L.K. van der Meij, C. de Vreugd, J. Korving, Angular momentum polarization in molecular beams of
I2 and Na2, Chem. Phys. 20 (1977) 391–408.
[194] W.R. Sanders, J.B. Anderson, Alignment of I2 rotation in a seeded molecular beam, J. Phys. Chem. 88 (1984) 4479–4484.
[195] U. Hefter, G. Ziegler, A. Mattheus, A. Fischer, K. Bergmann, Preparation and detection of alignment with high m selectivity
by saturated laser optical pumping in molecular beams, J. Chem. Phys. 85 (1986) 286–302.
[196] H.G. Rubahn, J.P. Toennies, A molecular beam study of the potential anisotropy of laser vibrationally excited Li2(v = 0, 20)
scattered from Kr, J. Chem. Phys. 89 (1988) 287–294.
[197] D.P. Pullmann, B. Friedrich, D.R. Herschbach, Facile alignment of molecular rotation in supersonic beams, J. Chem. Phys.
93 (1990) 3224–3236.
[198] B. Friedrich, D.P. Pullmann, D.R. Herschbach, Alignment and orientation of rotationally cool molecules, J. Phys. Chem. 95
(1991) 8118–8129.
[199] A.J. Saleh, A.J. McCaffery, Alignment of diatomic molecules in a free-jet expansion, J. Chem. Soc., Faraday Trans. 89 (1993)
3217–3222.
[200] M.J. Weida, D.J. Nesbitt, Collisional alignment of CO2 rotational angular momentum states in a supersonic expansion, J.
Chem. Phys. 100 (1994) 6372–6385.
[201] V. Aquilanti, D. Ascenzi, D. Cappelletti, F. Pirani, Rotational alignment in supersonic seeded beams of molecular oxygen, J.
Phys. Chem. 99 (1995) 13620–13626.
[202] V. Aquilanti, D. Ascenzi, D. Cappelletti, S. Franceschini, F. Pirani, Scattering of rotationally aligned oxygen molecules and
the measurement of anisotropies of van der Waals forces, Phys. Rev. Lett. 74 (1995) 2929–2932.
[203] V. Aquilanti, D. Ascenzi, D. Cappelletti, R. Fedeli, F. Pirani, Molecular beam scattering of nitrogen molecules in supersonic
seeded beams: a probe of rotational alignment, J. Phys. Chem. A 101 (1997) 7648–7656.
[204] E.B. Anthony, W. Schade, M.J. Bastian, V.M. Bierbaum, S.R. Leone, Laser probing of velocity-subgroup dependent rotational
alignment of Nþ
2 drifted in He, J. Chem. Phys. 106 (1997) 5413–5422.
[205] E.B. Anthony, Laser Probing of Transport Properties and Rotational Alignment of Nþ
2 Drifted in He, Ph.D. Thesis, University
of Colorado, Boulder, Colorado, 1998.
[206] S. Harich, A.M. Wodtke, Anisotropic translational cooling: velocity dependence of collisional alignment in a seeded
supersonic expansion, J. Chem. Phys. 107 (1997) 5983–5986.
[207] D.P. Pullmann, B. Friedrich, D.R. Herschbach, Collisional alignment of molecular rotation: simple model and trajectory
analysis, J. Phys. Chem. A 99 (1995) 7407–7415.
[208] V. Aquilanti, D. Ascenzi, D. Cappelletti, M. De Castro-Vitores, F. Pirani, Scattering of aligned molecules. The potential
energy surfaces for the Kr–O2 and Xe–O2 systems, J. Chem. Phys. 109 (1998) 3898–3910.
[209] V. Aquilanti, D. Ascenzi, M. Bartolomei, D. Cappelletti, S. Cavalli, M. De Castro-Vıtores, F. Pirani, Quantum interference
scattering of aligned molecules: bonding in O4 and role of spin coupling, Phys. Rev. Lett. 82 (1999) 69–72.
[210] V. Aquilanti, D. Ascenzi, M. Bartolomei, D. Cappelletti, S. Cavalli, M. De Castro-Vıtores, F. Pirani, Molecular beam scattering
of aligned oxygen molecules. The nature of the bond in the O2–O2 dimer, J. Am. Chem. Soc. 121 (1999) 10794–10802.
[211] V. Aquilanti, D. Ascenzi, M. de Castro Vıtores, F. Pirani, D. Cappelletti, A quantum mechanical view of molecular alignment
and cooling in seeded supersonic expansions, J. Chem. Phys. 111 (1999) 2620–2632.
Author's personal copy
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
159
[212] X. Chen, R. Araghi, R. Baranowsky, M. Thachuk, Collision-induced alignment of NO+ drifting in argon: calculated
distribution functions and microscopic quadrupole alignment parameters, J. Chem. Phys. 116 (2002) 6605–6613.
[213] T.-M. Su, I. Chao, Rotamer dynamics of substituted simple alkanes. 1. A classical trajectory study of collisional orientation
and alignment of 1-bromo-2-chloroethane with argon, J. Phys. Chem. A 108 (2004) 2567–2575.
[214] F. Pirani, M. Bartolomei, V. Aquilanti, M. Scotoni, M. Vescovi, D. Ascenzi, D. Bassi, D. Cappelletti, Collisional orientation of
benzene molecular plane in supersonic seeded expansions probed by infrared polarized laser absorption spectroscopy
and by molecular beam scattering, J. Chem. Phys. 119 (2003) 265–276.
[215] D. Cappelletti, M. Bartolomei, V. Aquilanti, F. Pirani, G. De Marchi, D. Bassi, S. Iannotta, M. Scotoni, Alignment of ethene
molecules in supersonic seeded expansions probed by infrared polarized laser absorption and by molecular beam
scattering, Chem. Phys. Lett. 420 (2006) 47–53.
[216] F. Pirani, D. Cappelletti, M. Bartolomei, V. Aquilanti, G. De Marchi, P. Tosi, M. Scotoni, The collisional alignment of
acetylene molecules in supersonic seeded expansions probed by infrared absorption and molecular beam scattering,
Chem. Phys. Lett. 437 (2007) 176–182.
[217] D. Cappelletti, F. Pirani, M. Scotoni, G. De Marchi, L. Vattuone, A. Gerbi, M. Rocca, Cooling and alignment of ethene
molecules in supersonic seeded expansions: diagnostic and application to gas phase and surface scattering experiments,
Eur. Phys. J. D 38 (2006) 121–127.
[218] V. Aquilanti, L. Beneventi, G. Grossi, F. Vecchiocattivi, Coupling schemes for atom-diatom interactions and adiabatic
couplings treatment of rotational temperature effects on the glory scattering, J. Chem. Phys. 89 (1988) 751–761.
[219] G. Herzberg, Spectra of Diatomic Molecules, D. van Nostrand Company Inc., 1950. pp. 221–224.
[220] J. Reuss, Scattering from oriented molecules, Adv. Chem. Phys. 30 (1975) 389–415.
[221] H. Thuis, S. Stolte, J. Reuss, Investigation of the angle dependent part of the intermolecular potential of NO-inert gas
systems using crossed molecular beams, Chem. Phys. 43 (1979) 351–364.
[222] D. Cappelletti, M. Bartolomei, M. Sabido, G. Blanquet, J. Walrand, J.P. Bouanich, F. Thibault, Collision cross sections,
pressure broadening coefficients and second virial coefficients for the acetylene-argon complex: experiments and
calculations on a new potential energy surface, J. Phys. Chem. A 109 (2005) 8471–8480.
[223] V. Venugopal, L. Vattuone, T. Kravchuk, M. Smerieri, L. Savio, J. Jupille, M. Rocca, Dynamics of ethene adsorption on clean
and C-contaminated Cu(4 1 0), J. Phys. Chem. C. 113 (2009) 20875–20880.
[224] J.A. Stinnett, R. Madix, J.C. Tully, Stochastic simulation of the trapping of ethane on Pt(1 1 1) from a realistic potential: the
roles of energy transfer processes and surface corrugation, J. Chem. Phys. 104 (1996) 3134–3142.
[225] L. Savio, L. Vattuone, M. Rocca, Coverage dependence of the sticking probability of ethylene on Ag(4 1 0), Surf. Sci. 587
(2005) 110–120.
[226] T. Kravchuk, L. Vattuone, L. Burkholder, W.T. Tysoe, M. Rocca, Ethylene decomposition at under coordinated sites on
Cu(4 1 0), J. Am. Chem. Soc. 130 (2008) 12552–12553.
[227] A. Kokalj, A. Dal Corso, S. Gironcoli, S. Baroni, Adsorption of ethylene on stepped Ag(n 1 0) surfaces, Surf. Sci. 566–568
(2004) 1018–1023.
[228] E.M. Stuve, R.J. Madix, C.R. Brundle, The adsorption and reaction of ethylene on clean and oxygen covered Pd(1 0 0), Surf.
Sci. 152/153 (1985) 532–542.
[229] D. Cappelletti, A. Gerbi, F. Pirani, M. Rocca, M. Scotoni, L. Vattuone, U. Valbusa, Collisionally aligned molecular beams: a
tool for stereodynamical studies in the gas phase and at surfaces, Phys. Scripta 73 (2006) C20–C24.
[230] G.W. Simmons, Y.N. Wang, J. Marcos, K. Klier, Oxygen adsorption on Pd(1 0 0) surface: phase transformations and surface
reconstruction, J. Phys. Chem. 95 (1991) 4522–4528.
[231] When decreasing Ei from 0.40 eV (O2 in He) to 0.26 eV (O2 in He/Ne mixture) S0 remains 0.42 eV for FT while it increases
from 0.20 to 0.34 for ST.
[232] R.J. Behm, K. Christmann, G. Ertl, M.A. Van Hove, Adsorption of CO on Pd(1 0 0), J. Chem. Phys. 73 (1980) 2984–2995.
[233] Y.Y. Yeo, L. Vattuone, D.A. King, Calorimetric investigation of NO and CO adsorption on Pd{1 0 0} and the influence of
preadsorbed carbon, J. Chem. Phys. 106 (1997) 1990–1996.
[234] J.W. Evans, Nonequilibrium percolative c(2 2) ordering: oxygen on Pd(1 0 0), J. Chem. Phys. 87 (1987) 3038–3048.
[235] K.H. Rieder, W.A. Stocker, A helium diffraction study of the p(2 2) phase of oxygen on Pd(1 0 0), Surf. Sci. 150 (1985)
L66–L70.
[236] L. Vattuone, A. Gerbi, D. Cappelletti, F. Pirani, R. Gunnella, L. Savio, M. Rocca, Selective production of reactive and
nonreactive oxygen atoms on Pd(0 0 1) by rotationally aligned oxygen molecules, Angew. Chem. Int. Ed. 48 (2009) 4845–
4848.
[237] R. Gunnella, F. Solal, D. Sebilleau, C.R. Natoli, MSPHD: a full multiple scattering code for low energy photoelectron
diffraction, Comput. Phys. Commun. 132 (2000) 251–266.
[238] D. Sebilleau, R. Gunnella, Z.Y. Wu, S. Di Matteo, C.R. Natoli, Multiple-scattering approach with complex potential in the
interpretation of electron and photon spectroscopies, J. Phys. Condens. Matter 18 (2006) R175–R230.
[239] E. Lundgren, J. Gustafson, A. Mikkelsen, J.N. Andersen, A. Stierle, H. Dosch, M. Todorova, J. Rogal, K. Reuter, M. Scheffler,
Kinetic hindrance during the initial oxidation of Pd(1 0 0) at ambient pressures, Phys. Rev. Lett. 92 (2004) 046101.
[240] J. Belilev, K. Reuter, Private communication: a more recent DFT investigation also finds subsurface location of oxygen to
be unstable.
[241] D. Kolthoff, D. Jurgens, C. Schwennicke, H. Pfnuer, The local geometry of chalcogen atoms on Pd(1 0 0): the low coverage
phases of O and S, Surf. Sci. 365 (1996) 374–382.
[242] R. Rose, A. Borg, T. Mitsui, D.F. Ogletree, M. Salmeron, Subsurface impurities in Pd(1 1 1) studied by scanning tunneling
microscopy, J. Chem. Phys. 115 (2001) 10927–10936.
[243] S.L. Chang, P.A. Thiel, Oxygen on Pd(1 0 0): order, reconstruction, and desorption, J. Chem. Phys. 88 (1988) 2071–2082.
[244] D.A. McCormack, G.-J. Kroes, E.-J. Baerends, R.C. Mowrey, Six dimensional quantum dynamics of dissociation of
rotationally excited H2 on Cu(1 0 0), Faraday Disc. 110 (1998) 267–285.
[245] R.A. Olsen, G.J. Kroes, E.J. Baerends, The influence of molecular rotation on the direct subsurface absorption of H2 on
Pd(1 1 1), J. Chem. Phys. 109 (1998) 2450–2459.
Author's personal copy
160
L. Vattuone et al. / Progress in Surface Science 85 (2010) 92–160
[246] P. Rivière, A. Salin, F. Martin, The role of molecular rotation in activated dissociative adsopriton on metal surfaces, J.
Chem. Phys. 124 (2006) 064706.
[247] T. Mitsui, M.K. Rose, E. Formin, D.F. Ogletree, M. Salmeron, Dissociative hydrogen adsorption on palladium requires
aggregates of three or more vacancies, Nature 422 (2003) 705–707.
[248] M. Beutl, M. Riedler, K.D. Rendulic, Strong rotational effects in the adsorption dynamics of H2/Pd(1 1 1): evidence for
dynamical steering, Chem. Phys. Lett. 247 (1995) 249–252.
[249] J.K. Vincent, R.A. Olsen, G.-J. Kroes, E.J. Baerends, Dissociative chemisorption of H-2 on Pt(1 1 1): isotope effect and effects
of the rotational distribution and energy dispersion, Surf. Sci. 573 (2004) 433–445.
[250] N.B. Arboleda, H. Kasai, W.A. Dino, H. Nakanishi, Quantum dynamics study on the interaction of H-2 on a Pt(1 1 1) surface,
Thin Solid Films 509 (2006) 227–229.
[251] J.J. Xie, K.M. Zhang, The dissociative adsorption of H2 on Cu(1 0 0): orientation dependence and impurity effects, J. Phys.
Condensed Matter 6 (1994) 7217–7226.
[252] J.A. White, D.M. Bird, Dissociative adsorption of H2 on Cu(1 0 0), Chem. Phys. Lett. 213 (1993) 422–426.
[253] A. Gross, B. Hammer, M. Scheffler, W. Brenig, High-dimensional quantum dynamics of adsorption and desorption of H2 at
Cu(1 1 1), Phys. Rev. Lett. 73 (1994) 3121.
[254] S. De, I. Znakovskaya, D. Ray, F. Anis, Nora G. Johnson, I.A. Bocharova, M. Magrakvelidze, B.D. Esry, C.L. Cocke, I.V.
Litvinyuk, M.F. Kling, Field-free orientation of CO molecules by femtosecond two-color laser fields, Phys. Rev. Lett. 103
(2009) 153002.
[255] M. Luppi, D.A. McCormack, R.A. Olsen, E.J. Baerends, Rotational effects in the dissociative adsorption of H2 on the Pt(2 1 1)
stepped surface, J. Chem. Phys. 123 (2005) 164702.
[256] M. Witko, E. Broclawikand, J. Haber, Stereochemistry of the catalytic oxidation of benzene as revealed by quantum
chemical calculations, J. Mol. Catal. 35 (1986) 179–189.
[257] H. Kasai, W. Agerico Dino, R. Muhida, Surface science-based reaction design increasing the ortho-para conversion yield
ovia molecular orientation, a case study, Prog. Surf. Sci. 72 (2003) 53–86.
[258] K. Fukutani, K. Ioshida, M. Wilde, W.A. Diño, M. Matsumoto, T. Okano, Photostimulated desorption and ortho-para
conversion of H2 on Ag surfaces, Phys. Rev. Lett. 90 (2003) 096103.
[259] Y. Wu, T. Toccoli, N. Koch, E. Iacob, A. Pallaoro, P. Rudolf, S. Iannotta, Controlling the early stages of pentacene growth by
supersonic molecular beam deposition, Phys. Rev. Lett. 98 (2007) 076601.
[260] L. Casalis, M.F. Danisman, B. Nickel, G. Bracco, T. Toccoli, S. Iannotta, G. Scoles, Hyperthermal molecular beam deposition
of highly ordered organic thin films, Phys. Rev. Lett. 90 (2003) 206101.
[261] L. Vattuone, L. Savio, M. Rocca, Bridging the structure gap: surface chemistry at well defined defects, Surf. Sci. Reports 63
(2008) 101–168.