The Al K-and L II/III -edge XANES of aluminium oxide are interpreted using empirical molecular orbital theory (EHMO) and ab initio self-consistent field real space multiple scattering calculations (FEFF8). Most features in the XANES at the K-and L II/III -edges are interpreted as shape resonances; although some fine structure, visible at both edges, arises from multiple scattering over the medium range (∼15 Å). The change in local symmetry between octahedral and tetrahedral Al explains the observed differences in the electronic structure. First, Al p-d hybridization is allowed only in tetrahedral symmetry, resulting in a lower absorption edge in tetrahedral Al than in the octahedral. Second, only in octahedral Al do the oxygen orbitals near the valence band maximum (the HOMOs) have the right symmetry to mix with the Al p orbitals just above the band gap (the LUMOs). This gives a more screened core hole in the octahedral case. Calculations on distorted octahedral Al sites reveal both p-d and s-d hybridizations; however, the latter is less prominent. The diffuse d orbitals, which hybridize with the p or s orbitals in tetrahedral or distorted octahedral symmetry, are primarily responsible for the fine structure in the near-edge region (0-15 eV) that is determined by medium-range scattering (up to ∼15 Å). The observed difference in the magnitude of this fine structure at the K-and L II/III -edges is caused by the different degrees of d orbital hybridization with the s and p orbitals.
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