Abstract
There have been suggestions that the radiation defect pair (F-H centers) in alkali halide crystals can be generated from higher excited states of exciton. Recent experimental works indicate production of fast F center before the self-trapped exciton (STE) is formed. In this paper the relaxation of an exciton in KBr crystal while the hole is in an excited state is presented. The study is based on a recently developed molecular dynamics (MD) simulation program. A cluster of about 150 atoms, containing a Frenkel-type free exciton embedded in an infinite lattice, is allowed to relax at 10 K. The hole is kept at low-lying excited states (of molecular orbitals g and u ) while the electron is localized at an anion site. The principal results are as follows: (1) Generally, the hole jumps about over the Br ions until it forms a molecule bond Br 2 when the excess kinetic energy (converted from the electronic excitation energy) has suitably dissipated. (2) Depending on the rate of energy dissipation, the molecule-type excited hole center appears as early as after 0.5 pico-second away from the F center. (3) The formed hole center, however, is a V k -center occupying two anion sites and is not a H-center occupying one anion site. These results are discussed with the recent observation of early F center creation.
Keywords: