THE COADSORPTION OF NA AND H2O ON NI(S)(111)

The coadsorption of Na alkali atoms and H2O molecules on a stepped Ni( s)(111) surface with nominal (775) orientation was studied with therma l desorption spectroscopy (TDS), work function change measurements (DE LTAPHI), low electron energy diffraction (LEED) and Auger electron spe ctroscopy (AES). On the clean Ni(775) surface five H2O desorption stat es denoted as A (T = 155-160 K), B (T = 175 K), C (T = 225 K), D (T = 260 K) and E (T = 335 K) can be found. State A is due to the adsorptio n of ice multilayers, state B to H2O bilayer clusters adsorbed at the terraces while C, D and E are step induced states. Covering the surfac e with Na atoms leads to an additional Na induced H2O state denoted as alpha, with a chemisorption strength which is almost the same as for H2O molecules adsorbed at steps. With increasing Na precoverage the st ep states C, D and E and the terrace state B disappear one after anoth er until only the Na induced state alpha remains. The disappearance of the step states is explained by a preferred adsorption of Na atoms at the steps, connected with a distortion of the electrostatic field. Th e remaining of only one Na induced H2O desorption state for higher Na precoverages suggests that the H2o adsorption behaviour no longer refl ects the step character of the Na/Ni(775) surface. Measurements of the work function change induced by H2O molecules as a function of Na cov erage point to a reorientation of H2O molecules adsorbed in the neighb ourhood of Na. Further measurements of DELTAPHI at 300 K yield a criti cal Na coverage for dissociation of 0.06 which does agree well with th e value of 0.07 from TDS experiments.