Momentum-transfer resolved electron energy loss spectroscopy of solids: problems, solutions and applications

The critical issues in experimental momentum-transfer (q) resolved EELS in TEM are outlined. It is shown that despite the presence of various multiple scattering events, meaningful information about the symmetry of electronic states can be obtained. Such q-resolved electron energy loss spectroscopy of the valence band transitions in BaBiO3 has revealed that the dispersion of the optical gap (∼ 2 eV) with q is anisotropic and that an optically forbidden transition is present at 4.5 eV. The oscillator strength of the forbidden transition is stronger for q along [100], but weaker along [110]. A molecular orbital model is proposed and the forbidden transition is assigned to be the excitation from the O 2pσ non-bonding states to the empty Bi (6s) state. The model is in good agreement with the physical properties of the transition.