Molecular beam studies of the interaction of oxygen with silver surfaces

The results of an extensive molecular beam study of the interaction dynamics of oxygen with the silver (111) and (110) surfaces are presented. Both scattering and adsorption experiments have been carried out as a function of the incidence beam energy and angle, the surface temperature and surface oxygen coverage. On the Ag(111) surface a number of processes are observed with increasing incidence energy. At the lowest energies the molecules trap and desorb from the weak physisorption well. At incidence energies above 0.2 eV activated access to the molecular chemisorption potential becomes possible, leading to both molecular chemisorption and, indirectly, to dissociation. However, most molecules are only transiently trapped at the surface. At incidence energies above 1.0 eV a thermally assisted direct dissociation channel dominates the adsorption dynamics. On the Ag(110) surface the molecular chemisorption probability is much higher and the indirect channel is the only detectable dissociation mechanism across the entire energy range studied. We propose that the transient state seen on Ag(111) is due to the metastable nature of molecular oxygen on the unreconstructed silver surface. Molecular sticking is limited by the ability of the two different surfaces to relax during adsorption. Strong effects due to the oxygen induced reconstructions of the silver surfaces are also seen in the coverage dependences of the sticking probabilities.