Oxygen adsorption on the clean and O-precovered Fe (110) and (100) surfaces

The chemisorption of atomic oxygen on clean and oxygen-precovered Fe( 100) and Fe( 110) surfaces has been studied using ab initio density-functional techniques. It is demonstrated that although on both surfaces the adsorption of oxygen atoms remains an unactivated process up to full monolayer coverage, important differences are to be expected on exposure to molecular oxygen. The reason is that while on Fe( 100) the differential heat of adsorption is almost independent of the O-coverage, the differential heat of adsorption on Fe( 110) decreases strongly with increasing coverage. For coverages of 0.5 ML O or higher, the energy gain by adsorbing an additional O atom is comparable or lower than the energy ( per atom) required to dissociate an O-2 molecule. The consequences on the formation of thin-film oxides are discussed.