Photon stimulated desorption from alkali halide surfaces at near threshold energies

Desorption processes induced in alkali halides by photons of synchrotron radiation (of energy 5-20 eV) have been investigated by means of quadrupole mass spectroscopy. The relative yields and angular characteristics of alkali and halogen atoms emitted from (100) surfaces of KI and RbI crystals were measured as a function of the photon energy and the sample temperature. For comparison, analogous measurements were performed with a 1 keV electron beam. In the latter case the relevant desorption processes are already better understood. We have found that both the fundamental exciton and the fundamental gap excitations result in alkali and halogen desorption with the same angular distributions. This leads to the conclusion that dynamic desorption processes, pronounced for electron excitation, are inefficient during desorption stimulated by photons with near threshold energies. It is argued that such a difference can be understood by taking into account different selection rules, and differences in excitation depth and excitation density distribution for photon- and electron-induced processes.