KBand in Colored Alkali Halide Crystals

Abstract

Recently the results of a number of experiments relating to the $K$ band in colored alkali halides have been reported and several new speculations have been put forth concerning this absorption. A comparison of the known properties of the band with the predictions of various theories leads to the conclusion that the available experimental evidence supports the original suggestion of Mott and Gurney that the $K$ band is an absorption of the $F$ center to a series of higher excited states. To provide a semiquantitative theoretical test of this explanation the Simpson model for the $F$ center was investigated in detail. The model problem was solved for the most important higher bound states, and transition energies and oscillator strengths for transitions to these states were calculated. It was found that for reasonable choices of parameters the model predicts an optical absorption spectrum that is strikingly similar to the composite $F$- and $K$-band absorption. Several experimental tests of the Mott-Gurney model are suggested and the possibility of $K$-band-like absorptions associated with other centers is discussed.