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 studie d by a combined analysis of structural properties (by helium diffracti on) and of electronic states at the surface (by metastable deexcitatio n and angle-resolved ultraviolet photoemission spectroscopies). H2O ad sorption on well ordered O(n X 1) phases (n even) leads to the observa tion of pure OH(1 x m) phases with m = n/2. Spectroscopic data indicat e that OH is bonded to the surface via the O atom. The most distinctiv e spectral feature induced by OH(a) is the dispersion of electronic st ates in the 2-4 eV binding energy range, above Ag4d bands, along (1 $( $) over bar$$ 10). These levels are interpreted as anti-bonding states originating from the covalent mixing between OH1 pi and Ag4d states. A donation of s-p metal charge to the adsorbate is suggested by work f unction and metastable deexcitation spectroscopy measurements. The ban d structure of the induced levels and the surface corrugation experien ced by helium diffraction indicate, for all phases, that -Ag-OH- chain s are formed along the (1 $($) over bar$$ 10) azimuthal direction of t he 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. A n adsorption site of C-2v symmetry is expected on the latter phases. H elium diffraction data on OH phases are incompatible with the missing- row reconstruction model, which is known to be effective on oxygen pha ses.