INFLUENCE OF STEPS ON THE H2O ADSORPTION ON NI(S)(111)

To expand previous investigations about the adsorption of H2O on a ste pped Ni(s)(111) surface with nominal (221) orientation, the adsorption of H2O on a stepped Ni(s)(111) with nominal (11 119) orientation was studied in the present paper. The surfaces exhibit the same (111)-terr ace and (111BAR)-step orientation but they differ in the terrace width , which is about three times greater for Ni(11 119) than for Ni(221). The adsorption was studied with thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED) as well as work function chang e measurements (DELTAphi). The LEED data from the clean Ni(s)(111) sur face are consistent with the atomic arrangement expected for the ''ide al'' non-reconstructed (11 11 9) surface. In TDS experiments five deso rption states denoted as A (T(M) = 155-160 K), B (T(M) = 174-177 K), C (T(M) = 225 K), D and E (T(M) = 260 and 325 K) were found. A and B ex ist at similar temperatures on flat Ni(111) as well as Ni(221) and are assigned to adsorption of H2O clusters at terrace sites (B) or adsorp tion of ice multilayers (A). The step-induced states C, D and E, which were found at higher temperatures, are related to the adsorption of H 2O monomers at steps (C) and the recombination of dissociation product s (D and E). As expected, the amount of H2O molecules adsorbed at step sites as well as the contribution by step molecules to the work funct ion change is about three times smaller than for Ni(221), correspondin g to the lower step density of Ni(11 119). At higher coverages terrace sites are occupied by H2O Molecules developing a modified bilayer str ucture. Between 120 and 150 K a (square-root 3 x square-root 3)R30-deg rees structure, which has also been found on other flat surfaces with hexagonal symmetry, could be observed. Upon heating to 160 K the LEED pattern changed into a (2 x 2) structure. This behaviour is believed t o be due to a temperature- and coverage-induced reordering of state B.