Adsorption behavior of H 2O on clean and oxygen precovered Ni(s)(111)

Abstract

Photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS), isotope exchange experiments, work function change (δφ) and LEED were used to study the adsorption and dissociation behavior of H 2O on a clean and oxygen precovered stepped Ni(s)[12(111) × (111)] surface. On the clean Ni(111) terraces fractional monolayers of H 2O are adsorbed weakly in a single adsorption state with a desorption peak temperature of 180 K, just above that of the ice multilayer desorption peak ( T m = 155 K). In the angular resolved UPS spectra three H 2O induced emission maxima at 6.2, 8.5 and 12.3 eV below E F were found for θ ≈ 0.5. Angular and polarization dependent UPS measurements show that the C 2v symmetry of the H 2O gas-phase molecule is not conserved for H 2O(ad) on Ni(s)(111). Although the Δφ suggest a bonding of H 2O to Ni via the negative end of the H 2O dipole, the O atom, no hints for a preferred orientation of the H 2O molecular axes were found in the UPS, neither for the existence of water dimers nor for a long range ordered H 2O bilayer. These results give evidence that the molecular H 2O axis is more or less inclined with respect to the surface normal with an azimuthally random distribution. H 2O adsorption at step sites of the Ni(s)(111) surface leads in TDS to a desorption maximum at T m = 225 K; the binding energy of H 2O to Ni is enhanced by about 30% compared to H 2O adsorbed on the terraces. Oxygen precoverage causes a significant increase of the H 2O desorption energy from the Ni(111) terraces by about 50%, suggesting a strong interaction between H 2O and O(ad). Work function measurements for H 2O+O demonstrate an increase of the effective H 2O dipole moment which suggests a reorientation of the H 2O dipole in the presence of O(ad), from inclined to a more perpendicular position. Although TDS and Δφ suggest a significant lateral interaction between H 2O+O(ad), no changes in the molecular binding energies in UPS and no “isotope exchange” between 18O(ad) and H 2 16O(ad) could be observed. Also, dissociation of H 2O could neither be detected on the oxygen precovered Ni(s)(111) nor on the clean terraces.