EQUILIBRIUM DETERMINATION OF H2O DESORPTION KINETIC-PARAMETERS OF H2OK/NI(111)/

Equilibrium measurements in a dynamic equilibrium of adsorption and de sorption were performed to obtain coverage-dependent kinetic parameter s of water desorption from K/Ni(lll) with K coverages in the submonola yer regime. A H2O particle beam was employed which enabled beam fluxes up to 8X20(19) m(-2) s(-1) while the water equilibrium coverage in th e temperature range from 155 to 175 K was determined using x-ray photo electron spectroscopy (XPS). On the clean sample water desorbs with tw o 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 10(16+/-1) s(-1) were found, whereas for higher coverages values of E(y)=(38.6+/-0.3) kJ/mol and v(y)=8X10(12+/ -0.1) s(-1), characteristic for desorption from ice multilayers, were determined. The effect of preadsorbed K manifested itself in a dissoci ation of one H2O molecule per K atom to OH and H and in the presence o f a more strongly bound molecular water species. No K induced changes of the desorption kinetics of the further adsorbed water could be obse rved for Theta(K) less than or equal to 0.15 ML, and the water coverag e 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 t han 0.15 ML.