Core-Level Photoemission and LEED Studies of Adsorption at Fe Surfaces: Comparison Between CO and O2

Carbon monoxide and oxygen interactions with αFe(100) and polycrystalline surfaces have been studied by x-ray photoemission (XPS or ESCA) and low energy electron diffraction (LEED) at temperatures between 123 K and 473 K. For CO, the XPS results demonstrate the existence of four electronically distinct CO adsorption states; one is dissociative and three are molecular. The binding energy analyses are consistent with one of the latter molecular adsorption states, formed on the polycrystalline surface, having a stretched CO bond compared with the equivalent state on the Fe(100) surface. For oxygen, only dissociative chemisorption is observed, even at 123 K. Assuming monolayer coverage at saturation allows calibration of the coverage for all other situations of CO and O
2
adsorption. It is demonstrated that at coverages of just greater than a monolayer (293 K adsorption), Fe oxide species are already present and that Fe
^III
dominates. Studies at grazing angles, designed to enhance the surface sensitivity of the core-level measurements, fail to reveal recognizable Fe
^II
or Fe
^III
species much below monolayer coverage even though LEED studies indicate that an FeO-like geometric structure has developed at coverages much lower than this.