SOFT-X-RAY ABSORPTION-SPECTRA AND CORE-THRESHOLD BEHAVIOR IN THE ALKALI-METALS

We have calculated the nonlocal dielectric function of the alkali metals in the region of the p-core excitation threshold. The theory is at the level of the random-phase approximation and includes the self-consistent response of both valence and core electrons. We find that the peaking of threshold absorption is due to valence-electron-hole excitations which are induced by the dynamic dipole field of the core electrons. Our results are in good agreement with experiment and offer an alternative explanation of threshold behavior to that provided by the Mahan-Nozieres-de Dominicis theory.