Calculation of core-hole excitonic features on Al L(23)-edge x-ray-absorption spectra of alpha-Al2O3

We carry out first-principles molecular-orbital calculations for model clusters composed of 21 to 41 atoms with and without inclusion of a core hole. The strongest peak that appears near the Al L(23)-edge x-ray-absorption spectrum and electron energy-loss spectrum of alpha-Al2O3 is found to originate from the presence of a core hole. Such an effect is less significant in MgO and alpha-quartz (SiO2). The cation-cation overlap population in the lowest unoccupied molecular orbital (LUMO) is found to be exceptionally strong at one of the Al-Al bonds in alpha-Al2O3 because of its short Al-Al bond length. The LUMO strongly localizes when the core hole is introduced.