Scattering of Low-Energy Noble Gas Ions from Metal Surfaces

Abstract

The energy distributions of positive ions ejected from polycrystalline Mo and Ni by He+, Ne+, and Ar+ primary ions in the energy range of 0.5 to 3.0 keV have been found to consist of a low-energy distribution characteristic of sputtered ions and a sharp peak at higher energy. These high-energy ions are shown to be primary ions which have experienced single two-body collisions with the surface atoms. Multiply-charged ions were not detected. It was found that, approximately, a monolayer of adsorbed gas was sufficient to prevent the detection of incident ions which have scattered from the metal target atoms. He+ primary ions have been used to identify CO as an adsorbed gas on the Ni surface. Furthermore, the data suggest that the CO molecule forms a linear bond with the Ni surface through the C atom, as has been previously observed by other techniques. In contrast, when more reactive gas ions were used as primaries, nonsharp or continuous scattered ion energy distributions were observed, suggesting that lower neutralization probabilities associated with these ions allow the detection of ions which have penetrated deeply into the surface and emerged with varying energy losses.