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An experimental and theoretical study of electronic excitations in MgB2 covering the domain of large energy and momentum transfers is reported. Energy-loss spectra for several values of momentum transfers were measured in a polycrystalline sample by means of inelastic X-ray scattering spectroscopy. Ab initio calculations of the dielectric function as well as the energy-loss function were performed in the frame of the time-dependent local density approximation with inclusion of crystal local-field effects. We obtained very good agreement between the experimental and the theoretical energy dispersion of the peak maximum of the loss function. We found that crystal local-field effects are responsible for this agreement at large momenta. Fine structure observed in the measured spectra was interpreted in terms of strong interband transitions predicted by the calculations in the and directions. The theoretical dispersion of these features is in good accordance with the experimental data. Further spectral features in the measured spectra due to Mg 2s and 2p core electron excitations are also discussed.

An experimental and theoretical study of collective electronic excitations in Be is presented. The plasmon energy and linewidth were measured by means of inelastic x-ray scattering spectroscopy. Measurements were performed on a polycrystalline sample and in a broad range of momentum transfers within the plasmon excitation regime. Theoretical plasmon dispersion and its linewidth in the whole Brillouin zone were derived from ab initio evaluations of the electron density response function. The calculations were performed with full inclusion of the electron band structure within the random-phase approximation. Good agreement of experimental plasmon energy and linewidth dispersions with direction-averaged theoretical results in all investigated q-range is obtained. We conclude that, in Be, the band structure effects alone can account for the observed finite plasmon lifetime at q = 0, as well as for the linewidth dispersion in the long-wavelength domain.