Surface-embedded oxygen: Electronic structure of Ag(111) and Cu(poly) oxidized at atmospheric pressure

We investigated the surface electronic structure of an oxidized Ag(111) single crystal and a polycrystalline Cu foil prepared under conditions of pressure and temperature (1000 mbar and 900 K) far beyond those used in usual UHV studies. It has recently been shown that a strongly bound oxygen species (O-gamma) apart from oxygen atoms chemisorbed on the surface (O-alpha) or oxygen atoms dissolved in the bulk (O-beta) can be prepared on on silver surfaces. Using core-level photoemission we show these three atomic oxygen species, O-alpha, O-beta and O-gamma, exhibit characteristic spectral properties. X-ray absorption spectroscopy at the O K-edge indicates that the formation of the strongly bound O-gamma species is accompanied by a strong hybridization between O2p and Ag4d states and that also the Ag5sp states are important for the metal-to-oxygen covalent interaction. The reaction with methanol was used to selectively remove the surface-embedded atomic species and to study the effect of the strongly bound oxygen O-gamma species on the valence electronic structure of silver. This separation of the oxygen species did not work for the more reactive copper-oxygen system. The chemical bonding interaction of the novel O-gamma species with silver is discussed in terms of the ligand-field theory emphasizing the covalent character of the interaction. For copper, the spectroscopic analysis is complicated by the existence of a stable oxide Cu2O with similar bonding properties of the oxygen than postulated for the surface-embedded species in silver.