OxygenKnear-edge fine structure: An electron-energy-loss investigation with comparisons to new theory for selected3dTransition-metal oxides

Abstract

Excitations of the oxygen $1s$ subshell in selected $3d$ transition-metal oxides have been studied by inelastic scattering of 75-keV electrons. Striking variations in the near-edge fine structure are reported and an interpretation is developed based on an empirical molecular orbital energy-level model. We compare our observed fine structure with that evinced in the metal $K$ and $L_3$ edges in these same oxides. While the molecular-orbital model seems adequate for interpreting the spectra of Ti${\mathrm{O}}_2$, it fails for at least some of the oxides studied. For example, in the case of NiO, a self-consistent Hartree-Fock computation for the oxygen $1s$ excitation spectrum gives results showing that the near-edge structure is not adequately described by the unoccupied density of states of the solid before core-hole excitation. Instead, the initial spectral peaks are shown to be core excitons. However, for Ti${\mathrm{O}}_2$, a tight-binding extended Hückel calculation neglecting the core hole yields a density of states that displays peaks in good agreement with the experimental data. Speculations on the origin of the difference between the spectra of NiO and Ti${\mathrm{O}}_2$ are offered.