H2O ADSORPTION ON NI(S) (111) SURFACES - EVIDENCE FOR A STEP INDUCED INFLUENCE ON THE ADSORPTION GEOMETRY

The adsorption of H2O On stepped Ni(s) (111) surfaces with nominal (221) and (665) orientation was studied with thermal desorption spectroscopy (TDS), low-energy electron diffraction (LEED), and work function change measurements (DELTA-PHI). LEED from both clean surfaces confirmed a (1 X 1) structure attributed to the presence of (111) terraces and (111BAR) steps [Ni(221) = 3(111) terraces + 1(111BAR) steps, Ni(665) = 11 (111) terraces + 1(111BAR) steps]. In TDS experiments on both surfaces desorption states are detected which can be attributed either to terrace or to step sites. The step induced states, which are missing on "flat" Ni(111), are found at higher desorption temperatures compared to the terrace state (denoted as B with T(m) = 175-178 K) or to the ice multilayer state (denoted as A with T(m) = 155-160 K). The step induced states are related to the desorption of H2O monomers adsorbed at step sites (denoted as C, T(m) = 225 K) or to the recombination of H2O dissociation fragments (state D and E at T(m) = 260 and 325 K, respectively). The DELTA-PHI measurements demonstrate a significant increase of the effective H2O dipole moment at step sites, an opposite behavior is found at terrace sites. These data demonstrate that at low H2O coverages the steps are first decorated by oriented H2O monomers. At higer coverages a bilayer is expanding from the oriented H2O monomers at step sites onto the (111) terraces Due to the presence of "flip-up" and "flop-down" H2O dipole orientations the effective dipole moment of the H2O bilayer terrace clusters is lowered.