Optical Absorption and Photoconductivity in theKBand of Some Colored Alkali Halides

The optical absorption of various alkali halides colored by different methods was measured in the $K$-band region at liquid-nitrogen and liquid-helium temperatures. Particular attention was devoted to RbCl, in which the $K$ band is well resolved at liquid-helium temperature. It was found that the $K$ band in RbCl is neither symmetric nor Gaussian and that the band has a relatively long high-energy tail. Photoconductivity studies on RbCl demonstrated that there is appreciable photoconductivity only in this high-energy tail. It is concluded that these data support the Mott-Gurney model of the $K$ band which attributes the main part of the $K$ absorption to allowed transitions of the $F$ center to a series of bound excited states beneath the conduction band.