TIME-RESOLVED FLUORESCENCE SPECTROSCOPY ON SOLIDS

1. Istituto di Fisica Applicata “Nello Carrara” IFAC – CNR, Firenze http://www.ifac.cnr.it JAHN-TELLER EFFECT INTERPRETATION OF THE EMISSION SPECTRA OF THE SCINTILLATOR PbWO4 The structured emission band has been interpreted on the basis of an excited state consisting of two closely lying triplet levels (3T1, 3T2). Ligand field, spin-orbit, Jahn-Teller and pseudo Jahn-Teller interactions were included in the Hamiltonian. The temperature dependence of the band shape (solid line: 4 K; long dashed line: 300 K) is well accounted for by considering a quantum distribution function on the excited states. 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 A vibronic degeneracy substitutes the original electronic degeneracy The above theorem plays an important role not only in determining the crystal structure but also in affecting the spectroscopic properties of the involved physical systems. 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. 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.