Oscillator Strengths for theα- andβ-Bands in Alkali Halide Crystals

The oscillator strengths for the $\alpha$- and $\beta$-absorption bands are calculated for NaCl. These bands, investigated experimentally by Delbecq, Pringsheim, and Yuster in KI, lie on the long-wavelength tail of the first fundamental absorption band and are interpreted by use of a model according to which valence electrons of the adjacent halide ions are raised to bound states in the fields of negative ion vacancies ($\alpha$-transitions) and $F$-centers ($\beta$-transitions). The oscillator strengths turn out to be only slowly varying with the assumed wave functions and are about 1.9 and 0.6 for the $\alpha$- and $\beta$-bands, respectively.