SECONDARY-ION EMISSION FROM METAL-SURFACES

The formation of secondary ions during sputtering of metals is discuss ed. The emission from a clean surface can be rationalized in terms of the electron-tunneling model. A mechanism is proposed which explains w ithin the same framework the large observed enhancement of positive se condary ion emission from a metal surface covered by an electronegativ e species. The presence of an adsorbate on the surface induces an elec trostatic potential that causes a shift of the electronic level of the emitted atom. This increases the effective neutralization distance an d exponentially decreases the neutralization probability. It is shown that the bond breaking model describing the emission from an oxidized surface can be generalized to a clean metal surface.