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Abstract

The adsorption of water on ordered epitaxial FeO(111) and Fe3O4(111) films was investigated by
thermal desorption spectroscopy (TDS) and photoelectron spectroscopy (UPS, XPS) under adsorption-
desorption equilibrium conditions. On the purely oxygen-terminated FeO(111) surface water monomers
get physisorbed first, followed by the formation of a hydrogen-bonded bilayer with an ice-like structure
and condensation of ice multilayers as the coverage is increased. On the Fe3O4(111) surface exposing
both iron and oxygen atoms water dissociates resulting in adsorbed hydroxyl groups, followed by
coadsorption of water monomers and condensation of ice multilayers. A quantitative comparison
between the hydroxyl saturation coverage and the defect concentrations deduced from LEED and STM
measurements rules out a purely defect related dissociation of water. It is proposed that OH- groups
are bound to iron cations and the H+ species to oxygen anions exposed in the topmost layer of the
regular Fe3O4(111) surface. The comparison between the FeO(111) and Fe3O4(111) surface
chemistry demonstrates that the chemical reactivity of metal oxides is related to surface metal sites.
The saturation coverages, isosteric heats of adsorption, preexponential frequency factors and initial
dipole moments of the different species were determined quantitatively. Based on these data structural
models for the adsorbed phases on the iron oxide surfaces are propose