Modelling of the slow emission decay of Pb2+, Tl+ centres

The observed nonexponential decay of the slow luminescence component of Pb2 + and Tl+ centres in alkali halide crystals has been modelled. The model is based on the assumption that there is lattice relaxation on the same time scale as the slow-component decay time. This relaxation is taken to be a re sponse to the Jahn-Teller local distortion of the Tl+ and Pb2+ luminescence centres. We successfully fit experimental data for the slow component deca y of KX: Pb2+ and KX:Tl+ (X = Cl, Br, I). An essential feature of the model is that the lattice relaxation causes a time-dependent coupling of the Jah n-Teller levels and it is found that the coupling strength systematically i ncreases as the anion size increases.