Abstract
Luminescence of free excitons (FEs) and self-trapped excitons (STEs) in alkali iodide crystals are investigated under excitation in the n =1 exciton absorption region using a narrow-band UV laser. Experimental results support the identification that the prominent peaks of the FE luminescence spectra of KI and RbI are triplet emission and that of NaI is singlet emission. It is clarified that most singlet FEs in KI and RbI are scattered to the triplet state during the relaxation in the n =1 exciton band. The accumulation of FEs on the triplet state induces the decrease in the intensity of singlet STE luminescence. On the other hand, the scattering rate of the singlet FE to the triplet FE state in NaI is found to be small. The relation between scattering rate of FE and the magnitude of the exchange energy relative to the LO phonon energy is discussed.
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