THERMALLY ASSISTED DESORPTION PROCESSES IN ELECTRON-BOMBARDED ALKALI-HALIDES

The desorption of alkali and halogen atoms induced by the interaction of energetic electrons with surfaces of alkali halide crystals has bee n studied by means of an angular-resolved and mass-selected time-of-fl ight spectroscopy. It has been found that a considerable fraction of h alogen atoms was ejected with hyperthermal energies of the order of 0. 1 eV. However, alkali atoms and the remaining part of halogen emission had thermal (Maxwellian) spectra of kinetic energies. In this paper w e will report on systematic investigations of these thermal desorption processes for single crystal (100) NaCl, KCl, KBr, RbBr, and Kl surfa ces. The relative yield of the thermal component has been measured as a function of electron beam energy and beam current density at various sample temperatures it will be shown that thermal halogen emission ca n be explained by thermally assisted diffusion of interstitial halogen aroma produced in the bulk of the crystal from decaying self-trapped excitons. The origin of the alkali atom component will be described as due to neutralization and subsequent thermal evaporation of excess al kali atoms from the halogen deficient surface.