A pre-impact model for ionic and atomic desorption induced by fast multicharged projectiles

Abstract

Desorption induced by highly charged particles is described through a model based on direct Coulomb interaction between the projectile and adsorbed molecules. It is assumed that the emission of ionized and neutral species from metals or insulators is triggered by the electric field of the projectile before its impact on the surface. The projectile interaction with the adsorbate layer induces ionizations or electronic excitations which lead to subsequent desorption through processes such as Coulomb explosion or molecular dissociation, causing the secondary emission of ions or neutral species. The pre-impact desorption (PID) model proposed here is based on a potential-ejection model normally used for projectile velocities below the Bohr velocity. It is considered that surface ionization occurs in a well defined period of time, when over-the-barrier electron emission (or electron promotion) is possible. The model predicts a q 3 cosθ behavior of the desorption yield, q being the projectile charge and θ the angle of incidence. The q 3-dependence has been observed for H + emission induced by keV and MeV energy beams. It also predicts an increase of the induced desorption yield, as the ionization potential of the adsorbed layer decreases. With regard to the yield dependence on the projectile velocity, the pre-impact model predicts a maximum around the Bohr velocity; the detailed description of the projectile-surface interaction may affect the position of this maximum but not the general trend of the function.