Istituto di Fisica Applicata “Nello Carrara” IFAC – CNR,
Firenze
http://www.ifac.cnr.it
TIME-RESOLVED FLUORESCENCE SPECTROSCOPY ON SOLIDS
M. Bacci, P. Fabeni, D. Mugnai, G.P. Pazzi, A. Ranfagni, C. Susini
In several field of human activity (medicine, industry, control systems, high-energy physics experiments, etc.) faster and more efficient scintillators are required to
improve the characteristics of the employed apparatuses. The suppression of the slow components is one of the more important features to be searched, in order to
speed up the operations and consequently to reduce the dose to the patient, to the operators, compatibly with light yield maximization and reduction of high
radiation damage of the crystal. The technique developed at IFAC-CNR of Firenze for photoluminescence decay kinetics measurements in the uv/visible range is
particularly suitable for simultaneous observation of fast (up to 1 ns) and slow (down to 1 s) decay components, with an intensity variation of several orders of
magnitude. Decay-kinetics measurements can be carried out on solid samples which show uv/visible photoluminescence by excitation with the lines of an
excimer laser (XeCl at 308 nm and KrF at 249 nm) or nitrogen laser (at 337 nm). The emission is detected by a fast photomultiplier (PMT) and recorded by a
digital sampling oscilloscope. Time-resolved spectra (with a gate of 20 ns) or steady state spectra can be obtained through an OMA (Optical Multichannel
Analyser) in the range 200-800 nm.
Emission band shapes are interpreted on the basis of the Jahn-Teller effect and pseudo-Jahn-Teller effect acting on the emitting levels.
Many of the above activities are carried out in cooperation with the Physics Institute of Academy of Sciences of Prague - Czech Republic ( M. Nikl), the Physics
Institute of Tartu – Estonia (S. Zazubovich), Dipartimento di Scienza dei Materiali – Università di Milano-Bicocca (A. Vedda), ENEA – Casaccia (S.Baccaro) and
Dipartimento di Fisica – Università di Roma 3 (F.Somma).
Investigated materials:
Crystals:
Lead Tungstate (PbWO4)
Perovskites (YAlO3:Ce3+, LuxY1-xAlO3:Ce3+)
Garnets (Y3Al5O12 :Ce3+, Lu3Al5O12 :Ce3+)
to be employed as scintillators in:
high-energy physics (CERN),
medicine (PET), industry (quality control) and
security (airport).
Doped alkali halides (KCl:Tl- type)
Quantum Dots in alkali halides (e.g. CsPbBr3 in CsBr:Pb2+)
Amorphous materials:
Phosphate glasses (e.g. 50% NaPO3 /40% GdPO4 /10% CePO4)
to be employed as scintillators.
Photo-luminescence decay
of YAP 7765/8 (50 ppm Ce),
exc. = 337 nm (N -laser),emiss.= 480 nm.
PL decay of phosphate glasses 4/1 (Na52%,Gd45%, Ce3%),
308 nm excitation (XeCl-laser), 350 nm emission.
1
2
10
1
laser
decay
2exp.fit
1
0.1
emission intensity (a. u.)
emission intensity
t1 = 16.5 ns
t2 = 2450 ns
0.001
0.0001
10
10
-6
100
1000
10
4
10
5
time (ns)
10
6
0.1
19 K
80 K
0.01
200 K
300 K
0.001
0.0001
10
10
Mo160
laser
0.1
0.01
laser
RT
emission intensity (a.u.)
RT
-5
PL decay kinetics of PWO:Mo,Ce or Y-codoped,
308 nm excitation (XeCl-laser), 500 nm emission.
10
Mo160Ce40
0.01
0.001
0.0001
slow components
increase
-5
10
-6
-5
10
10
-7
10
Mo160Y80
-6
10
-5
0.0001
-8
10
-7
-6
10
time (s)
10
-5
time (s)
Decay kinetics of KCl:Tl (100 ppm),
excitation by pulsed Xe-lamp (50 W) at 252 nm,
475 nm emission at 50K.
Deconvolution by the sum of three exponentials:
interpretation of the anomalous slow-components decay
by the tunneling effect in a Jahn-Teller model.
JAHN-TELLER EFFECT
INTERPRETATION OF THE EMISSION SPECTRA OF
THE SCINTILLATOR PbWO4
emission intensity (a. u.)
The structured emission band
has been interpreted on the
1
basis of an excited state
t (ms)
A
50K
4.6
0.84
consisting of two closely lying
50K-fit3
185
0.028
0.1
lamp
triplet levels (3T1, 3T2). Ligand
10940
0.00043
field, spin-orbit, Jahn-Teller
A
vibronic
degeneracy
substitutes
the
original
0.01
and
pseudo
Jahn-Teller
electronic degeneracy
interactions were included in
the
Hamiltonian.
The
0.001
The above theorem plays an important role not
temperature dependence of the
only in determining the crystal structure but also
band shape (solid line: 4 K;
0.0001
in affecting the spectroscopic properties of the
long dashed line: 300 K) is
involved physical systems.
well
accounted
for
by
-5
10
considering
a
quantum
0
200
400
600
800
1000
1200
1400
distribution function on the
time (ms)
excited states.
Selected references: 1) M. Nikl, K. Nitsch, E. Mihokova, N. Solovieva, J.A. Mares, P. Fabeni, G.P. Pazzi, M. Martini, A.. Vedda , S. Baccaro, Appl. Phys. Letters, 77, 2159-2161 (2000); 2) M. Nikl, P. Bohácek, E.
Mihóková, N. Solovieva, A. Vedda, M. Martini, G. P. Pazzi, P. Fabeni, M. Kobayashi, J. Appl.Phys., 91, 2791-2797 (2002); 3) A. Ranfagni, D. Mugnai, P. Fabeni, G.P. Pazzi, Phys.Rev. B 66, 184107/1-5 (2002); 4) M.
Bacci, E. Mihokova, and L. S. Schulman, Phys. Rev. B, 66 (2002) 132301.
If a non-linear molecule (or polyatomic ion)
has a degenerate electronic level (apart from
Kramers degeneracy) it is unstable with
respect to displacements of the atoms