Core Transitions from the Al 2p Level in Amorphous and Crystalline Al2O3

The soft-X-ray spectra of aluminium oxide in the polycrystalline γ and amorphous forms have been investigated in detail with improved resolution using the synchrotron radiation of the Frascati electron synchrotron. In contrast to a previous measurement, it is found that the L2,3 threshold starts at almost the same energy in both forms. The sharp peaks observed at the absorption edge are interpreted as excitons. This picture fits well both with ESCA results and with the optical spectra at the fundamental edge in the ultraviolet. It is suggested that metastable excitons are responsible for the intense structure observed at (82.6 ± 0.3) eV in the ordered phase and lacking in the amorphous phase. The spectral features of crystalline alumina are analyzed on the basis of the band structure. The modifications induced by the lack of long-range order in the amorphous samples are discussed thoroughly.