Alpha and Gamma Bands in Alkali Halides

This is a calculation of the effect produced by a nearby vacancy on the energy of the transition of an electron from a negative ion in an alkali halide crystal to a neighboring positive ion ("charge transfer" model of the exciton). Some, but not all, of the results agree satisfactorily with experimental observations of the $\alpha$ and $\gamma$ bands. The evaluation of certain "dipole" lattice sums of interest in other problems as well as this one is reported in the Appendix.