Kinetics of Self-Trapped Holes in Alkali-Halide Crystals: Experiments in NaI(Tl) and KI(Tl)

Abstract

A previous model calculation of the kinetics of hole diffusion in thallium-activated alkali iodides led to predictions concerning the role of binary electron-hole diffusion in the energy transport associated with the scintillation process. In this paper we report the resuls of time-resolved emission and absorption measurements on KI(Tl) and NaI(Tl) at various temperatures and compare these results with the model calculation. By absorption spectroscopy we recorded the growth and decay of the ${\mathrm{Tl}}^{++}$ absorption band following excitation by pulse of electrons. The characteristic diffusion time of the self-trapped hole was obtained from the growth curve, and recombination kinetics (due to electrons thermally excited from ${\mathrm{Tl}}^0$) were obtained from the decay curve. The luminescence intensity was also recorded as a function of time and this function was correlated with the diffusion and recombination processes. Experimental results are in accord with the principal conclusions of the diffusion calculation, and permit definitive statements regarding the mechanism of energy transport. In particular, it is concluded that nearly all of the energy transport in KI(Tl) occurs by electron-hole diffusion.