Excitonic instability and athermal halogen atom desorption from NaBr, KBr and RbBr

Abstract

Abstract Energetic halogen atom desorption observed from certain alkali halide crystals under electron or photon stimulation is being investigated. The relaxed structure of the localized one-center (Frenkel-type free exciton) and two-center (self-trapped exciton: STE) electronic excitations are studied. It is shown that the triplet STE state undergoes an instability on and near the surface (the first and second layer below the surface) similar to that in the bulk and results in the expulsion of neutral Br atom in all three bromides studied. The relaxation energy which could be converted to kinetic energy of the ejecta is estimated to be about 0.5–1.0 eV. From the third layer, only in KBr and RbBr such ejection seems possible. The trajectory of the ejected Br atom is originally along the halogen row in <110> axes. However, as the ejecta approaches the (100) surface, it undergoes a deflection resulting in a trajectory outside the crystal which is found to deviate from the vertical by only about ≌20° at a distance of twice the lattice parameter. It is also observed that a localized hole on the surface and below is efficiently converted into a Vk center without leading to a halogen atom desorption. These results are presented within a broader context of the STE relaxation in the alkali halide crystals.