Resonant Exchange Scattering in Dipole-Forbiddend-dExcitations in NiO(100)

Electron-exchange processes in dipole-forbidden d-d excitations in NiO(100) have been examined by ``complete'' spin-polarized electron energy-loss spectroscopy over a wide primary energy range and for different scattering geometries. We observe unexpected strong exchange scattering at energies two orders of magnitude higher than the excitation energy. The angular dependence of exchange scattering was found to be correlated to the possibility of resonant excitations. At primary energies corresponding to resonant excitations, it differs dramatically from that observed off resonance.