Equilibrium determination of H2O desorption kinetic parameters of H2O/K/Ni(111)

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Abstract
Equilibrium measurements in a dynamic equilibrium of adsorption and desorption were performed to obtain coverage-dependent kinetic parameters of water desorption from K/Ni(111) with K coverages in the submonolayer regime. A H2O particle beam was employed which enabled beam fluxes up to 8×1019 m−2 s−1 while the water equilibrium coverage in the temperature range from 155 to 175 K was determined using x-ray photoelectron spectroscopy (XPS). On the clean sample water desorbs with two different sets of desorption kinetic parameters, for water coverages below 0.42 monolayers (ML) a desorption energy of (52±3) kJ/mol and a pre-exponential factor of 1016±1 s−1 were found, whereas for higher coverages values of Eγ=(38.6±0.3) kJ/mol and νγ=8×1012±0.1 s−1, characteristic for desorption from ice multilayers, were determined. The effect of preadsorbed K manifested itself in a dissociation of one H2O molecule per K atom to OH and H and in the presence of a more strongly bound molecular water species. No K induced changes of the desorption kinetics of the further adsorbed water could be observed for ΘK≤0.15 ML, and the water coverage of 0.42 monolayers, at which the icelike state is initially observed, also remains unchanged. This implies that the structure of the first layer of adsorbed water is similar for potassium precoverages lower than 0.15 ML.