Electronic energy levels of alfa-Al2O3 from L-edge photoabsrption of aluminum and small-cluster CNDO calculations

Absorption spectra measured with synchrotron radiation on well-characterized self-supported polycrystalline thin films of α-Al2O3 above the LII,III edge of aluminum are presented. The results are interpreted on the basis of the electron energy levels computed with a new complete-neglect-of-differential-overlap parametrization for a small cluster of atoms cut out from a perfect crystal of aluminum oxide. For the valence states, good agreement is found with the experimental bandwidths and positions and with recent band-structure and density-of-states calculations. The experimental structures up to 10 eV from Al LII,III absorption edge are interpreted as due to transitions to empty levels of the aluminum ion of predominantly s,p, and d character. The interpretation of the outer-most spectral features requires a larger basis set of aluminum orbitals and a bigger atomic cluster.