Inelastic processes in ion/surface collisions: Direct recoil ion fractions as a function of kinetic energy

Time-of-flight (TOF) spectra of the scattered and recoiled particles resulting from 1--10 keV Ar+ ions impingent on surfaces of MgO, Mg(OH)2, graphite, Si, and SiO2 have been obtained. Measurements of directly recoiled (DR) neutrals plus ions and neutrals only are used to calculate positive and negative ion fractions Y/sub +,-/ from DR events. These positive and negative ion yields observed for DR of H, C, O, and Si have distinctly different behavior as a function of ion kinetic energy. The Y/sub +/ values exhibit a ''threshold-type'' behavior with a steep rise followed by a slowly rising or plateau region at higher energy. The Y/sub -/ values exhibit a maximum in the low energy region followed by a decreasing yield as energy increases. The Y/sub -//Y/sub +/ ratio for C and O is very sensitive to the amount of hydrogen present, with the Y/sub +/ yields dropping as hydrogen concentration increases. The recently developed model for electronic transitions in keV ion/surface collisions which considers Auger and resonant transitions along the ion trajectory and electron promotions in the quasidiatomic molecule of the close atomic encounter is extended to include DR events. Analytical expressions for Y/sub +,-/ are derived for the case of surface atoms in positive, neutral, and negative bonding environments. These model expressions are fitted to the experimental data, allowing determination of the probabilities of ionization in the close atomic encounter and of electron capture along the outgoing trajectory