Surface extended-x-ray-absorption-fine-structure study of oxygen interaction with Al(111) surfaces

We have investigated the oxidation of Al(111) surfaces by means of polarization-dependent surface extended-x-ray-absorption-fine-structure (EXAFS) measurements. The experiments were performed above the oxygen $K$ absorption edge (∼535 eV) using the total electron-yield detection technique. The oxidation range from monolayer chemisorption corresponding to an ordered oxygen (1 × 1) overlayer to the formation of a thick surface layer of amorphous oxide has been studied. For the chemisorbed monolayer (100-150-L ${\mathrm{O}}_2$) we find a strong polarization dependence of the EXAFS signal which allows the determination of the O-O intraoverlayer separation 2.90 ± 0.05 Å. The O-Al chemisorption bond length has been determined to be 1.79 ± 0.05 Å. Within experimental error we find the same O-Al distance for the initial oxide-like phase which accompanies the chemisorbed phase. At higher exposures (1000-L ${\mathrm{O}}_2$) the vanishing low-energy-electron-diffraction pattern is accompanied by an increase in the O-Al bond length (1.88 ± 0.03 Å). This value is also observed for heavily oxidized surfaces. The O $K$ edge shows the formation of a strong "white-line"-like threshold resonance with increasing oxygen exposure. This resonance is attributed to transitions to localized final states in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$-like molecular units. It is pointed out that detailed analysis of the absorption threshold may by itself provide information on the local structure of surface complexes. Models for the progressive oxidation process of Al(111) surfaces implied by our data are discussed.