PHOTON-STIMULATED DESORPTION FROM ALKALI-HALIDE SURFACES AT NEAR-THRESHOLD ENERGIES

Desorption processes induced in alkali halides by photons of synchrotr on radiation (of energy 5-20 eV) have been investigated by means of qu adrupole mass spectroscopy. The relative yields and angular characteri stics of alkali and halogen atoms emitted from (100) surfaces of KI an d RbI crystals were measured as a function of the photon energy and th e sample temperature. For comparison, analogous measurements were perf ormed with a 1 keV electron beam. In the latter case the relevant deso rption processes are already better understood. We have found that bot h the fundamental exciton and the fundamental gap excitations result i n alkali and halogen desorption with the same angular distributions. T his leads to the conclusion that dynamic desorption processes, pronoun ced for electron excitation, are inefficient during desorption stimula ted by photons with near threshold energies. It is argued that such a difference can be understood by taking into account different selectio n rules, and differences in excitation depth and excitation density di stribution for photon- and electron-induced processes.