Intrinsic Optical Properties of Alkali Halides

Dielectric constants for KI, KBr, and KCl are deduced from reflectance data between 1 and 25 eV. The data may be described unambiguously in terms of individual particle and collective excitations. Strong absorption peaks originating from both the anion and the deeper lying ${\mathrm{K}}^{+}$ $3p$ band are believed to be associated primarily with excitons. The presence of collective effects is indicated by a plasma resonance which corresponds roughly to an electron density of six per molecule. It occurs in a spectral region where damping effects due to individual particle excitations from both filled bands are weak. A qualitative argument, based on a treatment of the many electron problem in the Hartree-Fock approximation, is given to show why excitons should predominate over ordinary interband transitions in alkali halides. Photoelectric emission data are re-examined and shown to be compatible with this viewpoint.