AN EXPERIMENTAL INVESTIGATION OF THE DISSOCIATION OF H2O ON O(NX1)-AG(110) - FORMATION OF OH(1XM) ORDERED LAYERS

Water adsorption on the oxygen-covered Ag(110) surface has been studied by a combined analysis of structural properties (by helium diffraction) and of electronic states at the surface (by metastable deexcitation and angle-resolved ultraviolet photoemission spectroscopies). H2O adsorption on well ordered O(n X 1) phases (n even) leads to the observation of pure OH(1 x m) phases with m = n/2. Spectroscopic data indicate that OH is bonded to the surface via the O atom. The most distinctive spectral feature induced by OH(a) is the dispersion of electronic states in the 2-4 eV binding energy range, above Ag4d bands, along (1 $($) over bar$$ 10). These levels are interpreted as anti-bonding states originating from the covalent mixing between OH1 pi and Ag4d states. A donation of s-p metal charge to the adsorbate is suggested by work function and metastable deexcitation spectroscopy measurements. The band structure of the induced levels and the surface corrugation experienced by helium diffraction indicate, for all phases, that -Ag-OH- chains are formed along the (1 $($) over bar$$ 10) azimuthal direction of the substrate. A different local geometry of adsorption for OH in the (1 X 1) phase with respect to (1 X 2) and (1 X 3) cases is suggested. An adsorption site of C-2v symmetry is expected on the latter phases. Helium diffraction data on OH phases are incompatible with the missing-row reconstruction model, which is known to be effective on oxygen phases.