Resonant electron-exchange excitations in transition-metal oxides

Electron-exchange processes in dipole-forbidden d-d excitations of the $3d$ transition-metal oxides NiO, CoO, and MnO have been investigated by spin-polarized electron-energy-loss spectroscopy over a wide primary energy range and for different scattering geometries. At primary energies corresponding to certain excitation thresholds, the d-d excitations in CoO and MnO show resonant enhancement, similar to the resonances previously found for NiO. The resonance around 38 eV, which is assigned to simultaneous excitations within the d-multiplet and oxygen excitations, occurs at nearly identical primary energy, due to identical energetic positions of the oxygen levels in the three oxides. The metal 3s-3d resonance is shifted toward lower energy in CoO and MnO, according to the lower binding energies of the $3s$ levels. In resonance, the d-d excitations are highly exchange dominated. Off resonance, a strong superposition with direct dipole-scattering processes has been found for the dominant multiplicity-conserving transitions in NiO and CoO. Excitation by direct dipole scattering is completely missing in the spin-forbidden, multiplicity-changing transitions in MnO.