Chemisorption of oxygen atoms on aluminum (100): A molecular-orbital cluster study

Abstract

Self-consistent-field $X\alpha$ scattered-wave molecular-orbital calculations have been performed for clusters of 5, 9, and 25 aluminum atoms representing the (100) surface in interaction with one, four, or five oxygen atoms at various positions above, in, and below the surface. Comparison of the results with recent ultraviolet and x-ray photoemission (UPS and XPS) results lend further support to the contention that at room temperature oxygen atoms are incorporated in or below the surface even at low exposure. Projected density of states curves for the 25-atom cluster with an oxygen atom centered in the first surface layer show three energy regions of significant oxygen character at about -9.5, -7, and -3 eV relative to the Fermi energy. Inspection of the wave functions for the corresponding levels shows that the -9.5 eV peak is due mainly to bonding combinations of in-plane oxygen $p$ orbitals with aluminum $s$ and $p$ orbitals. The -7-eV peak arises from both in-plane and out-of-plane oxygen $p$ orbitals, again in bonding combinations while the orbitals responsible for the -3-eV peak are best classified as nonbonding with respect to Al-O interactions. The two peaks at highest binding energy (-9.5 and -7 eV) have been observed in recent UPS experiments while at present there is only inconclusive evidence for the third and it is therefore suggested that a detailed angle and photon-energy dependent study be performed.