Absorption Band Shapes for theFCenter and Thallium in KCl

The shape of the optical absorption band has been measured for the $F$ center in KCl and for ${\mathrm{Tl}}^{+}$ in KCl over a temperature range from 4°K to room temperature and over a range of absorption coefficients extending to ${10}^{-4\mathrm{}}$ of the peak value. No evidence is found that Urbach's rule applies to the absorption bands of these centers. For the $F$ center in KCl it is possible to describe the long-wavelength side of the absorption band over the full range of absorption using configuration coordinate diagrams in which one of the approximations usually employed has been eliminated. It then becomes possible to determine the configuration coordinate diagram from absorption measurements alone. The configuration coordinate diagram determined in this way, however, fails to predict the $F$-center emission spectrum correctly. Several possible reasons for this failure are discussed.