MOMENTUM-TRANSFER RESOLVED ELECTRON-ENERGY-LOSS SPECTROSCOPY OF SOLIDS - PROBLEMS, SOLUTIONS AND APPLICATIONS

The critical issues in experimental momentum-transfer (q) resolved EEL S in TEM are outlined. It is shown that despite the presence of variou s multiple scattering events, meaningful information about the symmetr y of electronic states can be obtained. Such q-resolved electron energ y loss spectroscopy of the valence band transitions in BaBiO3 has reve aled that the dispersion of the optical gap (similar to 2 eV) with q i s anisotropic and that an optically forbidden transition is present at 4.5 eV. The oscillator strength of the forbidden transition is strong er for q along [100], but weaker along [110]. A molecular orbital mode l is proposed and the forbidden transition is assigned to be the excit ation from the O 2p sigma non-bonding states to the empty Bi (6s) stat e. The model is in good agreement with the physical properties of the transition.