Electronic energy levels ofα−Al2O3fromL-edge photoabsorption of aluminum and small-cluster CNDO calculations

Abstract

Absorption spectra measured with synchrotron radiation on well-characterized self-supported polycrystalline thin films of $\alpha -{\mathrm{Al}}_2{\mathrm{O}}_3$ above the $L_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ 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 $L_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ 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.