RELAXATION OF EXCITONS IN IONIC HALIDES: MOLECULAR DYNAMICS STUDY

Abstract

We report here the preliminary results of molecular dynamics simulation of exciton and hole relaxation in KBr and NaBr. The previously used semi-classical program has been modified to implement the solutions of Newton's equations with a time step of 0.50 femtosecond. Hole self-trapping process is studied at 80K. In both materials, there is a rapid bond-length oscillation. The oscillation, however, is damped faster is NaBr than in KBr. The relaxation of a Frenkel-type exciton (localized on a single site) is studied at 10 and 30K. It is found that the localization of the excited electron at an anion site drives the relaxation process, resulting in the formation of H and F centers simultaneously. There is a conspicuous absence of molecular bond oscillation which was observed in the Vk -center relaxation. In KBr, the FH-pair created is about 10 A apart (third neighbor). In NaBr, the separation is about 3A. The maximum relaxation is achieved in about 1-2 picosecond, depending on the temperature. These results are discussed in comparison with recent experimental works.