Charge exchange between low energy Si ions and Cs adatoms

Unexpectedly large yields of positive and negative ions are produced when 2 and 5 keV Si+ is singly scattered from Cs adatoms on Al(1 0 0) and Si(1 1 1). This is in contrast with Li+, in which case the ions are almost completely neutralized. The Si+ ions likely result from valence electron resonant charge transfer (RCT) enhanced by promotion of the ionization level as it interacts with the Cs 5p level, but incomplete resonance neutralization of the incoming Si+ cannot be absolutely excluded. The experimental data are quantitatively compared to the model and values of the microscopic parameters are estimated. Negative Si− ions are produced when the surface work function is very small, presumably by direct RCT to the projectile affinity level as it is bent downward by the image potential and by the dipole formed by the adsorbed Cs.