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Chemical Kinetics I Elementary Reactions I Surfaces I Water Adsorption I Work FunctionThe interaction of water vapor with clean and oxidized surfaces of polycrystalline Nickel was studied at temperatures from 300 K to 430 K. -The changes of the work function A@ were recorded as a function of the interaction time of water with these surfaces. Immediately after starting water exposure a steep decrease of the work function was observed at all temperatures studied. The amount of this initial decrease depends on the H 2 0 partial pressure. The subsequently observed increase of the work function was found to be mainly temperature dependent with a typical time constant of a few minutes. -Determining the relative stability of the H 2 0 molecules and their dissociation products the influence of the substrate temperature must be emphasized. Size and sign of A@ in the equilibrium state change due to variation of the substrate temperature from 320 K to 435 K. -The observed time dependence of the work function was attributed to a two step dissociation process. It's slight increase was related to the dissociation of OHad and to the beginning of oxide layer formation. This work function increase is a thermally activated process with an activation energy of 61.8 kJ/mol and preexponential factor of 6.3 . lo6 s-I.