Ii. Solid State. Surface PHYSICS.THEORY of Core Excitons in Solids

The present understanding of electron-hole interaction effects on (bulk) inner-shell photoabsorption spectra of non-metallic solids is reviewed. In crystals with low electronic polarizability, such as the alkali halides, solid rare gases, etc..., this interaction is stronger than any solidstate effects, so that a close similarity exists with the corresponding atomic or ionic absorption spectra. Recent results on the controversial Li+ K-spectrum of LiF will be discussed, which seem to rule out the presence of forbidden transitions at photon energies as low as 54 eV. In semiconductors, valence electron screening of the inner hole considerably reduces the effective electron-hole interaction. Here the theoretical analogy for the description of the final state is to donor impurity states. This picture appears to be basically correct, although more experimental and theoretical work is called for to give a consistent interpretation of recent photoemission and modulation spectroscopy experiments. The effect of free carriers on core spectra is also discussed, in connection with recent experiments and with theories of X-ray edges in metals.