The coadsorption of Na and H 2O on Ni(s)(111)

Abstract

The coadsorption of Na alkali atoms and H 2O molecules on a stepped Ni(sX111) surface with nominal (775) orientation was studied with thermal desorption spectroscopy (TDS), work function change measurements (Δφ), low electron energy diffraction (LEED) and Auger electron spectroscopy (AES). On the clean Ni(775) surface five H,0 desorption states 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 adsorption of ice multilayers, state B to H 2O bilaycr clusters adsorbed at the terraces while C, D and E are step induced states. Covering the surface with Na atoms leads to an additional Na induced H 2O state denoted as α, with a chemisorption strength which is almost the same as for H 2O molecules adsorbed at steps. With increasing Na precoverage the step states C, D and E and the terrace state B disappear one after another until only the Na induced state α 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. The remaining of only one Na induced H 2O desorption state for higher Na prccovcrages suggests that the H 2O adsorption behaviour no longer reflects the step character of the Na Ni(775) surface. Measurements of the work function change induced by H 2O molecules as a function of Na coverage point to a reorientation of H 2O molecules adsorbed in the neighbourhood of Na. Further measurements of Δφ at 300 K yield a critical Na coverage for dissociation of 0.06 which docs agree well with the value of 0.07 from TDS experiments.