Enhanced excitonic effects in the energy loss spectra of LiF and Ar at large momentum transfer

It is demonstrated that the bootstrap kernel for finite values of q crucially depends upon the matrix character of the kernel and gives results of the same good quality as in the q → 0 limit. The bootstrap kernel is further used to study the electron loss as well as the dielectric tensor for Si, LiF and Ar for various values of q. The results show that the excitonic effects in LiF and Ar are enhanced for values of q away from the Γ-point. The reason for this enhancement is the interaction between the exciton and high-energy inter-band electron-hole transitions. This fact is validated by calculating the absorption spectra under the influence of an external electric field. The electron energy loss spectra are shown to change dramatically as a function of q. (paper)