A theoretical study of desorption induced by electronic transitions in alkali halides

We have studied desorption following the excitation of core- or valence-electrons using quantum chemical calculations of the potential energy surfaces. For core excitation, we simulate the state of two holes localised on a halogen ion using an extended semi-empirical molecular method (CNDO) to calculate the energy change associated with the displacement of the halogen ion or a neighbouring alkali ion. No instability was found for a bulk halogen ion with two localised holes, in agreement with recent experiments. However, also consistent with recent experiments, desorptive potential surfaces were obtained for both a (100) surface halogen ion with two localised holes and for a (100) surface alkali ion when two holes are localised on the nearest subsurface (second layer) halogen ion. We use the results of our earlier CNDO calculations to evaluate the energetics of desorption induced by valence-electron excitation and indicate the possible origin of atomic desorption. The difference between the experimental features of the desorption induced by core- and by valence-electron excitation are explained in terms of the differences in the energy surfaces and hence the coupling of the electronic excited states with the lattice.