Core Exciton Absorption in Sodium Halides

Abstract

The profiles of Na⁺L₂₃ absorption spectra of sodium halides have been measured in the vicinity of the threshold at room temperature and liquid nitrogen temperature. Apparent depression of the main bands caused by scattered light and second order light was diminished by minimizing the effect of scattered light and correcting the effect of second order light. Sharp doublets at the threshold, which are ascribed to creation of core excitons with holes in the 2p level of sodium, have large oscillator strengths as compared with other absorption bands in the spectra. The doublet can be decomposed approximately into two Gaussians at room temperature and two Lorentzians at liquid nitrogen temperature. These properties along with the nature of exchange energy between the electron and the hole of the core exciton are interpreted in terms of the electron-lattice and the electron-electron interactions and of the localized nature of the optical excitation involved.