Anomalous enhancement of negative sputtered ion emission by oxygen

Enhancement of negative sputtered ion yields by oxygen (either O⁺₂ bombardment or O₂ gas with Ar⁺ bombardment) is demonstrated for Si^-, As^-, P^-, Ga^-, Cu^- and Au^-, sputtered from a variety of matrices. Because oxygen also enhances positive ion yields of the same species, this effect cannot be simply explained on the basis of existing sputtered ion emission models. To rationalize these phenomena, a surface polarization model is developed which invokes localized electron emissive or electron retentive sites associated with differently oriented surface dipoles in the oxygenated surface. Such sites are considered to dominate the emission of negative and positive ions respectively. The model is shown to correctly predict that Au⁺ and Au^- ion yields are much more strongly enhanced by oxygen in dilute Au-Al alloys than in pure gold.