Optical Absorption by the Alkali Halides

The relations are investigated between three types of theory for explaining absorption of light by ionic crystals: absorption by independent atoms or ions, absorption according to Frenkel's excitation waves, and absorption according to energy bands and lattice functions, as used in the theory of metals. It is shown that each method leads to a set of unperturbed wave functions, which are related to each other by linear combinations, and that no one set is correct at the actual distance of separation in the crystal. Instead, combinations must be used, leading both to continuous absorption as in the energy bands, and to one or two discrete lines in connection with each continuum, as in absorption by independent atoms or in excitation waves. These conclusions are compared with experiment, explaining in a general way the sharp structure observed both in the near and far ultraviolet absorption spectra of these crystals. The discussion deals throughout with the undistorted crystal, leaving out of account the new absorption bands which are known to appear in these crystals after considerable illumination, resulting from the deposition of free alkali in the crystal.