Temperature Dependence of F-Center Absorption in KBr and NaBr

The absorption due to $F$ centers in x‐irradiated KBr and NaBr has been measured over a wide range of temperatures from 4 to 370°K. In NaBr (which is exceedingly difficult to color) there is also formed a well‐isolated $M$ band without the usual difficulty of overlapping $R$ bands and so the temperature dependence of the shape of this $M$ band could also be determined accurately. The difficulty of the overlapping $K$ band has been dealt with in two ways: by Gaussian plots of the untreated data (after background subtraction) and by a computer resolution of the $F$ and $K$ bands. The latter calculations confirm the asymmetry of the $K$ band. The half‐widths of the $F$ and $M$ bands obey the usual formula $\text{H(T)\hspace{0.17em}=\hspace{0.17em}H(0)[}\coth {\mathrm{\hslash \omega }}_g{\text{\hspace{0.17em}/\hspace{0.17em}2kT]}}^{\text{1/2}}$ and from the temperature dependence of the half‐widths, values of the effective ground‐state frequency ${\nu }_g$ were obtained. To what extent this represents a genuine local mode remains unresolved. There is some evidence for structure in the $K$ band at low temperatures.