Momentum dependence of fluorine K-edge core exciton in LiF

Nonresonant inelastic x-ray scattering spectra have been measured with an energy loss close to the fluorine K edge of lithium fluoride. The extracted dynamic structure factor is well reproduced by a first-principles computational scheme that takes the electron-hole interaction into account. A systematic study of the momentum-transfer dependence with high energy resolution in the close vicinity of the K edge shows clear deviations from the dipole approximation. With the help of the calculations, it is shown that the observed deviations are because of an s-type exciton.