Multilayer adsorption of water on NiO(100) at 120 and 143K

Multilayers of water adsorbed on thin epitactic NiO(100) films (Ag(100) substrate) were investigated by ultraviolet photoelectron spectroscopy (UPS). It could be demonstrated that multilayers growing at temperatures below 140 K, i.e. below the onset of the multilayer thermal desorption spectroscopy (TPD) peak, show a 1.4 eV higher work function than those grown above 140 K at increased water pressure. It is supposed that different species of ice are formed at 120 K and above 140 K. These might be geometrically different species as it is reported [P.V. Hobbs, Ice Physics, Carendon Press, Oxford (1974)] that amorphous ice is formed at low temperatures, which will be transformed in a crystalline structure. On the other hand, the shape of UPS spectra for both ice species are identical. Values in the literature for activation energy for the desorption of H2O multilayers vary between 45 kJ/mol [P.A. Thiel, T.E. Madey, 1987, Surf. Sci. Rep. 7 (1987) 211] measured by non-isothermal methods such as TPD, and 39 kJ/mol [J.-W. He, P.R. Norton, Surf. Sci. 238 (1990) 95; W. Kuch, W. Schurnberger, M. Schulze, K. Bolwin, J. Chem. Phys. 101 (1994) 1687] recorded with isothermal measurements. This may be induced by the different ice structures caused by the different temperatures for the preparation of the ice layer.