Synchrotron radiation photoemission studies of the adsorption of oxygen on magnesium and aluminium

Abstract

The sorption of oxygen on polycrystalline films of Mg and Al has been studied by photoemission using synchrotron radiation in the photon energy range 70–150 eV. The 2p levels, situated 50.0 eV (Mg) and 72.8 eV (Al) below the Fermi level in the clean metals, shift to higher binding energy by 1.3 (Mg) and 2.6 eV (Al) on heavy oxidation. There is some evidence of a smaller shift, corresponding to a chemisorbed state, coexistent with the oxide peak. For oxygen on magnesium, an interatomic Auger transition, as previously reported from electron-stimulated Auger electron measurements, was observed for a wide range of exposures. An O 2p valence-band resonance is seen 6.1 (Mg) and 7.1 eV (Al) below the Fermi level, with accompanying shoulders 2.6 eV deeper in both systems. Oxidation also produces a marked increase in the total 2p emission from the metallic species, which we attribute to a longer electron mean free path in the oxide than in the metal. The ratio of the mean free path in oxide and metal is typically a factor of 2 for both Mg and Al, and this ratio is energy dependent.