Current studies of the dynamics of formation of self-trapped excitons and Frenkel pairs during the band-gap excitation of alkali halides were reviewed. Femtosecond time-resolved spectroscopic studies had revealed several interesting features. Special attention was paid to dynamic features of the relaxation of electron-hole pairs during their relaxation. It was noted that the relaxation of the pairs could be divided into 2 distinct stages, with differing natures. In the first stage, within a few ps of excitation, the interaction of electrons and holes during relaxation to a stable 2-center type of configuration played a crucial role. Frenkel pairs formed at this stage, due to the dynamic instability. The second stage essentially involved off-center relaxation of the on-center self-trapped exciton into F-H pairs and off-center self-trapped excitons. The occurrence of a dynamic instability which was induced during relaxation of the localized electron-hole pair was suggested to be one of the most important results. It was proposed that the instability was associated with a spontaneous symmetry lowering of holes from 1-center to 2-center configurations of the localized hole. This instability had been proved to be common to all alkali halide crystals of a certain class. However, it was pointed out that this dynamic instability was not limited only to these crystals, and was more general.

K.Tanimura, H.Fujiwara: Materials Science Forum, 1997, 239-241, 549-54