Dynamical structure factor of LiF for all wave vector transfers: New results and insights

We report a theoretical-experimental investigation of the dynamical structure factor of LiF. The calculations are done within the TDLDA of time-dependent density functional theory; the measurements correspond to non-resonant inelastic x-rays scattering (NIXS) cross sections, obtained in absolute units. The TDLDA spectra contain one adjustable parameter: a ``scissors-operator'' shift of the conduction bands. This parameter is determined in view of the NIXS line shape for $q$= 6{\AA}$^{-1}$ ($q //$(111)). The TDLDA spectra display a non-trivial semi-quantitative agreement with the NIXS data, for all wave vectors (directed along the three high-symmetry directions); indeed, the line shape changes drastically from the coherent-response small-$q$ regime, to the incoherent-response large-$q$ regime. The picture of the excitations which emerges offers an alternative view relative to a seminal investigation involving an approximate solution of the Bethe-Salpeter equation; W. A. Caliebe et al. Phys. Rev. Lett. 84, 3907 (2000).