| Research Abstract |
Radiative recombination processes of self-trapped excitons (STE) in alkali halide crystals were studied systematically in various kinds of mixed crystals. It was found that the pi emission bands in nine alkali halides, NaCl, KCl, RbCl, NaBr, KBr, RbBr, NaI, KI and RbI, should be classified into three distinct types (I, II, III). It has also been shown that the sigma emission bands which are recognized only in five crystals, NaCl, KBr, RbBr, KI and RbI, are essentially the same as the so-called "pi emission" bands in NaBr and NaI (Type I). These findings bring a pleasing orderliness to the appearance, peak energies and dipole strengths of singlet sigma and triplet pi transitions in alkali halides. In this new scheme, the sigma bands generally, and the corresponding pi bands in NaBr and NaI, are suggested to originate from an on-center STE configuration. The low energy pi bands, and the Ex band in RbI, are suggested to originate from two separate off-center STE configurations, all on the same adiabatic potential surface of the lowest energy.
Thus, the present study provides an unifying recognition for the mechanism of exciton self-trapping in terms of on-center and/or off-center distortion.
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