Anisotropy of the oxygen interaction with Ag surfaces

A review of the most recent experimental progress in the investigation of the dynamics of the interaction of dioxygen with Ag single crystal surfaces is given. In such studies the dioxygen molecules are dosed by a supersonic molecular beam, which allows for angle and energy selectivity. Moreover the background pressure remains very low during dosing thus minimizing surface contamination. Energy and angle dependence of the initial sticking probability was investigated for the different low Miller index surfaces. We find that non dissociative adsorption occurs below 150 K only on Ag(110) and (001) with the same activation barrier. The dependence of the sticking probability on the angle of incidence is anisotropic with respect to surface azimuth for Ag(110) because of the anisotropic surface corrugation. Dissociation is always promoted by chemisorbed O-2 and, when heating the crystal, it prevails over desorption for Ag(110), while the contrary holds for Ag(001). For the latter case we find also evidence for subsurface diffusion at molecular dissociation. The coverage dependence of the sticking coefficient as well as of the TDS spectra are indicative of repulsive interactions between the admolecules, which are particularly strong for Ag(001).