Momentum-transfer dependence of x-ray Raman scattering at the Be K-edge

Inelastic x-ray scattering spectra have been measured for energy losses around the Be K-edge in order to perform a systematic study of the dependence of the corresponding dynamic structure factor on both the absolute value and the direction of the momentum transfer. The measured x-ray Raman spectra show clear differences between spectra of different momentum transfer which are related to deviations from the dipole approximation of the x-ray Raman scattering cross section with increasing momentum transfer. In particular, the directional differences between x-ray Raman spectra for momentum transfer parallel and perpendicular to the c axis of single-crystal Be vanish completely at high momentum transfer. These results are attributed to monopole transitions of the excited electrons to s-type final states according to the results of a first-principles calculation which takes the particle-hole interaction into account. This theoretical approach is a valuable tool for modeling x-ray Raman spectra.