Desorption and surface topography changes induced by He+ ion bombardment of alkali halides

Desorption fluxes from alkali halide (RbI, KBr) crystals under 5 keV He+ bombardment and the corresponding surface topography evolution were investigated by means of quadruple mass spectroscopy (QMS) and dynamic force microscopy (DFM) in UHV. Although, for the energy range of He+ ions used, desorption of alkali halides occurs predominantly via electronic processes, there are significant differences between the ion-stimulated desorption (ISD) and the electron-stimulated desorption (ESD). The latter proceeds in a layer-by-layer mode and, as a result, oscillating desorption fluxes are observed. For ISD the DFM images show that the surface erosion does not follow the layer-by-layer scheme and no oscillation in desorption fluxes are found but they exhibit monotonic decrease to some saturation level. We suggest that these differences between electron- and ion-stimulated desorption are caused by ballistic processes disturbing the perfect lattice and introducing defects in the bulk and at the surface. Such lattice defects can act as recombination centres for diffusing Frenkel defects and inhibit the desorption process. Additionally, projectiles colliding violently with top-layer ions introduce additional nucleation centres for vacancies on the surface which lead to the surface erosion in the multi-layer mode. (C) 2004 Elsevier B.V. All rights reserved.