Initial stages of oxidation of (100) and (110) surfaces of iron caused by water

The initial stages of the oxidation of the (100) and (110) surfaces of iron caused by the adsorption of water have been investiagted using ab initio local-spin-density functional calculations including generalized gradient corrections. It is shown that at low coverages water dissociates spontaneously into hydrogen atoms and hydroxyl groups. Further adsorption of water molecules in the presence of preadsorbed H and OH does not lead to spontaneous dissociation. On both surfaces dissociation is a weakly activated process. This explains the presence of molecular water as observed experimentally. Especially on the (110) surface ${\mathrm{H}}_2\mathrm{O}$ dissociation in the presence of preadsorbed H and OH is found to be a complex process involving the formation of an intermediate molecular species.