A structural study of the interaction of SO2 with Cu(111)

The technique of chemical-shift normal-incidence X-ray standing waves (CS-NIXSW) has been applied to a study of the interaction of SO2 with Cu(111), yielding quantitative information on the local adsorption geometry of adsorbed SO2 at low temperature and coadsorbed atomic sulfur with an SOx species, identified on the basis of near-edge X-ray absorption fine structure (NEXAFS) as SO3. Atomic sulfur appears to occupy a mixture of face-centred cubic and hexagonal close-packed hollow sites, while the SO3 species adsorbs with its C3v axis perpendicular to the surface atop a surface copper atom with the SO bonds out of plane such that the oxygen atoms are closer to the surface; there appears to be some local distortion of the outermost copper layers around this species. While SO2 is found to adsorb with its molecular plane essentially perpendicular to the surface, and the data are most readily interpreted in terms of a bridging geometry bonding through the oxygen atoms, there are marked inconsistencies between these results and those of an earlier surface extended XAFS (SEXAFS) investigation of this species on Cu(111), and alternative interpretations are discussed.