Ultraviolet Absorption of Insulators. III. fcc Alkali Halides

Abstract

The optical spectra of the alkali halides are divided into interband scattering and exciton structure. The scattering or direct-transition structure is shown to be similar to that of the zincblende crystals studied in earlier papers of this series. On this basis the band structure $E(k)$ is deduced empirically from the spectra for [100] and [111] axes in $k$ space. The results for the valence bands agree well with Howland's calculations for KCl. The exciton structure is shown to contain extra peaks which overlap the scattering continuum. These are called metastable excitons in contrast to the conventional halogen doublet. It is shown that there is an intimate connection between the scattering and exciton spectra; the latter can only be explained in terms of the former. It is conjectured that the extra excitons are metastable because of self-trapping.