X-ray-absorption near-edge structure at the fluorineKedge inCaF2andBaF2

A band-structure method has been developed for modeling the near-edge structure in fluorine K-edge x-ray absorption in ${\mathrm{CaF}}_2$ and ${\mathrm{BaF}}_2$. The core-hole potential is included with a supercell technique. The model describes the main features of the experimental absorption spectra up to about 15 eV above the absorption threshold. The excitonic peak at the absorption threshold is followed by a series of peaks whose spacing changes in going from ${\mathrm{CaF}}_2$ to ${\mathrm{BaF}}_2$ by an amount consistent with electron diffraction from crystal-lattice planes. The interpretation of the higher-energy part of the absorption spectrum is complicated by possible multielectron excitations.