Excitonic mechanism of athermal halogen atom desorption from NaBr, KBr and RbBr

Abstract

Energetic halogen atom desorption from the (100) surface of NaBr, KBr and RbBr crystals under electronic excitation is studied from the point of view of excitonic instability. It is shown that the triplet self-trapped exciton (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 eV. From the third layer, only in KBr and RbBr such ejection seems possible. The trajectory of the ejected Br atom which is originally along the halogen row in the 〈110〉 axis direction undergoes a deflection resulting in a trajectory at the end which is found to deviate from the normal 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. Excited alkali atom desorption preceded by that of a halogen atom is also briefly studied, with the indication that it is energetically possible if the F centre is in an excited state.