Neutralization of energetic He ions scattered from clean and Cs-covered Si(100)

We report on a comprehensive study of the neutralization of (75-180)-keV He ions scattered from the UHV-cleaned and Cs-covered Si(100) surface, using surface-sensitive channeling techniques. It is shown that ions are neutralized exclusively at the solid surface on the ion's outward path. Angular depth and work-function dependence results are discussed. A model is proposed which includes resonant transitions to a broadened He $n=2$ quantum level, and is compared with experimental data.