High-resolution high-energy (e,2e) spectroscopy of metal oxides

We perform electron momentum spectroscopy studies of a Li₂O film using the (e, 2e) spectrometer of the Australian National University. This technique provides a direct observation of the electron motion through measurement of the energy-momentum resolved density. The spectrometer has a symmetric geometry with Incident and outgoing electron energies of 50 keV and 25 keV, respectively. These relatively high energies minimise the effect of multiple scattering on the spectral momentum density. The energy and momentum resolutions are about 1 eV and 0.1 a.u., respectively. We compare the results with calculations performed within the linear muffin-tin orbital approximation. The measured bandwidths of 1.8 ± 0.1 eV and 0.4 ± 0.1 eV for the upper (UVB) and lower (LVB) valence bands, respectively, are In good agreement with the calculation, but the momentum distribution for the UVB deviates strongly from theory. Similarly, the theory predicts correctly the bandwidths in the alkaline-earth metal oxides, BeO, MgO and CaO, except for the UVB in CaO. However, the measured intervalence gap for Li₂O is as much as 2.3 eV wider than that of theory. Similar results are also observed for the three other oxides.