MOLECULAR-BEAM STUDIES OF THE INTERACTION OF OXYGEN WITH SILVER SURFACES

The results of an extensive molecular beam study of the interaction dy namics of oxygen with the silver (111) and (110) surfaces are presente d. Both scattering and adsorption experiments have been carried out as a function of the incidence beam energy and angle, the surface temper ature 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 we ll. At incidence energies above 0.2 eV activated access to the molecul ar chemisorption potential becomes possible, leading to both molecular chemisorption and, indirectly, to dissociation. However, most molecul es are only transiently trapped at the surface. At incidence energies above 1.0 eV a thermally assisted direct dissociation channel dominate s the adsorption dynamics. On the Ag(110) surface the molecular chemis orption probability is much higher and the indirect channel is the onl y detectable dissociation mechanism across the entire energy range stu died. We propose that the transient state seen on Ag(111) is due to th e metastable nature of molecular oxygen on the unreconstructed silver surface. Molecular sticking is limited by the ability of the two diffe rent surfaces to relax during adsorption. Strong effects due to the ox ygen induced reconstructions of the silver surfaces are also seen in t he coverage dependences of the sticking probabilities.