INTERFERENCE BETWEEN 3D -] 3D ELECTRON-EXCHANGE TRANSITIONS AND INTERBAND EXCITATIONS IN MNO (100)

We report the observation of strong optically forbidden 3d --> 3d transitions in the electron-energy-loss spectra of vacuum-cleaved MnO (100). A striking sensitivity of the relative amplitudes of the various 3d --> 3d multiplet transitions to the interference from other excitation channels is observed for the first time in the transition-metal oxides. A model reflecting the electron exchange nature of the 3d --> 3d transition in MnO, similar to that used to describe the 4f --> 4f transitions in rare-earth metals, is proposed to explain the large amplitudes of the transitions. However, unlike the case of the rare-earth metals, the energy dependence of the amplitude of the transitions cannot be explained simply in terms of a resonant electron emission process.