Core-hole effects on theoretical electron-energy-loss near-edge structure and near edge x-ray absorption fine structure of MgO

First-principles molecular orbital calculations using model clusters are ma de in order to reproduce and interpret experimental electron-energy-loss ne ar-edge structure and near-edge x-ray absorption fine structure of MgO at M g K, L-2,L-3 and O K edges. Ground-state calculations using a model cluster composed of 125 atoms and by a band-structure method are in good agreement , but they do not reproduce the experimental spectra satisfactory. They are well reproduced only by the cluster calculations for the Slater transition state, where a half-electron is removed from a core orbital and placed int o the lowest unoccupied molecular orbital. The core-hole effect is therefor e essential for theoretical reproduction of the spectral shapes. A large su percell is required to reproduce the experimental spectra when one uses a h and-structure method. The origin of peaks appearing in the experimental spe ctra is interpreted in terms of orbital interactions using overlap-populati on diagrams. Some features of the spectra at different edges are pointed ou t to have common origins. Experimental spectra are aligned accordingly. The transition energies and qualitative features of experimental spectra are f ound to be reproduced even using a smaller cluster composed of 27 atoms, al though some of fine structure is missing.