Atomic and molecular ejection from ion-bombarded reacted single-crystal surfaces. Oxygen on copper(100)

Abstract

The trajectories of atomic and molecular species ejected from an ion-bombarded reacted single-crystal surface have been calculated using classical dynamics. As a model system, oxygen has been adsorbed in various coverages and site geometries on the (100) face of a copper surface, which is then bombarded by 600-eV ${\mathrm{Ar}}^{+}$ ions at normal incidence. The oxygen atoms have been placed at near zero (single-atom adsorption), $p(2\mathrm{}\times 2)$ and $c(2\mathrm{}\times 2)$ coverages in an $A$-top site, a fourfold bridge site, and a twofold bridge site. From the calculated positions and momenta of the ejected adsorbate and substrate atoms, we have identified the important ejection mechanisms, determined relative yields, and determined the factors that influence multimer formation. Of mechanistic interest is that oxygen is most often ejected by collisions with an adjacent copper atom rather than by collisions with the copper atom directly beneath it. The calculations show that multimers of the types ${\mathrm{Cu}}_2$, CuO, ${\mathrm{O}}_2$, ${\mathrm{Cu}}_3$, Cu${\mathrm{O}}_2$, ${\mathrm{O}}_3$, and several tetramers and pentamers can be expected to form. These multimers establish their identity over the surface and do not directly eject as a molecular entity. The influence of site geometry on multimer yields is discussed in detail. In general, the bridge sites have higher multimer yields than the $A$-top site. The surface coverage also exerts a systematic influence on the types of clusters that are observed. For example, molecules like ${\mathrm{O}}_2$ and Cu${\mathrm{O}}_2$ are not likely to be ejected from a $p(2\mathrm{}\times 2)$ surface due to a large O-O separation distance.