Properties of Exciton States in NaI. II. Excitonic Energy Transfer to Q and Tl Centers

As reported in Part I, upon excitation in the region of the first excition absorption band the efficiency of the intrinsic luminescence of NaI decreases with decreasing temperature below 60 °K. This part of our article yields information about the fate of the excition generated at low temperatures: 1. Only a very small fraction of the excitons contribute to thermoluminescence. 2. The exciton state is not the excited state of an emission with a small Stokes shift ΔE ≧ 0.1 eV. 3. However, an excitonic energy transfer to Q and Tl centers takes place. The lifetime of the exciton state in a crystal doped with 1 ppm of each Q and Tl centers is τ < 5 ns at LHeT.