Chemisorption of oxygen atoms on aluminum (111): A molecular-orbital cluster study

Self-consistent-field $X\alpha$ scattered-wave molecular-orbital calculations have been performed for clusters containing up to 22 aluminum atoms representing the (111) surface of aluminum in interaction with an oxygen atom at various positions above, in, and below the surface. Reasonable agreement with experimental photoemission results for the valence and Al-$2p$ core regions of the spectrum was obtained for two distinct positions of the oxygen atom. The first of these has the oxygen atom between 1 and 2 bohrs above an unrelaxed surface, while the second has the oxygen atom in the first surface layer which has been relaxed by enlarging the threefold site. Both of these models are consistent with available experimental data for some coverage between zero and one monolayer. Additional, as yet unobserved, oxygen-related structure is predicted in the valence region between -5 eV and the Fermi level similar to our previous predictions for Al(100) + 0, which have subsequently been born out experimentally.