Intrinsic Luminescences and Exciton Diffusion in NaI Crystals

In addition to the well known intrinsic luminescence at 4.22 eV, two new luminescence at 3.67 and 2.78 eV are found under the excitation in the fundamental absorption bands at low temperatures in undoped NaI crystals. The 2.78 eV luminescence appears under the excitations in the band-to-band transition region and the first exciton absorption band, but the 3.67 eV luminescence appears only under the excitation in the exciton band. Three distinctive exponential decay time vary with temperature. The intrinsic character of these two luminescences are discussed. In Tl + -doped NaI, it is concluded that the energy transfer from free excitons to Tl + ions takes place at low temperatures.