Oxygen chemisorption on Al: Unoccupied extrinsic surface resonances and site-structure determination by surface soft-x-ray absorption

Abstract

Surface soft-x-ray absorption (SSXA) spectroscopy using synchrotron radiation has been applied to study oxygen chemisorption on aluminum (100), (110), and (111) single-crystal surfaces. Evidence is presented of oxygen-related extrinsic surface resonances above the Fermi level on all faces. From analysis of the SSXA spectra over a large energy range, local structural informatin has been obtained. The Al (111) surface exhibits a SSXA spectrum dominated by the core transitions to the antibonding states of the Al-O complex in the chemisorption phase. Evidence of a chemisorption phase at oxygen exposures below 50 L has also been obtained for the Al (100) surface. Different charge transfer from Al to O occurs on the two faces, and we suggest the formation of an Al-O complex on the Al(100) with a small charge transfer from Al to O. The transition to the formation of ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ has been characterized by the appearance of the oxide core exciton.