The role of highly localized excitons in the photoabsorption spectra of ionic crystals

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Abstract
The discrete structure associated with photoexcitation of the core levels of the positive ion in ionic crystals is interpreted in terms of highly localized excitons. These excitons are believed to be confined within the first layer of negative ions owing to a net repulsion between the photoelectron and the occupied levels of the negative ions. By adopting a quasimolecular representation for these final states, expressions are derived for the splittings and relative intensities of the exciton peaks. The relevant parameters involved are the electron-hole interaction, the crystal field interaction between the exciton and the surrounding ions, and the spin-orbit interaction of the hole. This scheme is applied to the photoabsorption by the sodium 2p level in sodium halide crystals. The relation between this quasimolecular point of view and band theory is briefly discussed.