Hopping motion of self-trapped excitons in NaI

Abstract

Intensities and decay times of self-trapped exciton (STE) and Tl + luminescence in pure and Tl doped NaI have been measured as a function of temperature. It is found that the quenching temperature of the STE luminescence (4.2 eV band) depends on sample purity and method of excitation (one-photon or two-photon absorption process). In pure NaI, the temperature measured in two-photon process is higher than that in one-photon process. In Tl doped NaI, the quenching of the STE luminescence is compensated by the enhancement of the Tl + luminescence. From these results, STE in NaI is concluded to be mobile above 50 K, and the quenching of the STE luminescence in pure NaI is attributed to migration of STE to crystal surface where STE decays nonradiatively. In Tl doped NaI, the quenching of the STE luminescence is attributed to capture of STE by Tl + before reaching crystal surface. The activation energy of the STE hopping motion in NaI is decided to be 70 meV.