Structure of theABand in Thallium-Doped KCl and Its Temperature Dependence

The $A$ band in KCl: Tl has been resolved into its components using a computer. The observed absorption band is fitted quite accurately by the sum of two Gaussians ($A_1$ and $A_2$) throughout the temperature range 4-700 °K. Absorption cross sections (${\sigma }_{01},{\sigma }_{02}$), half-widths ($H_1,H_2$), peak positions ($E_{01},E_{02}$) for $A_1$ and $A_2$, and their separation ($\Delta E$) are given as functions of temperature. The parameters were found to be independent of the Tl concentration. The area under the $A$ band decreased smoothly by about 10% on warming from 4 to 700 °K, while the areas under $A_1$ and $A_2$ reached a maximum and minimum, respectively, near 160 °K. At 4 °K, the peak positions were $E_{01}=5.014\mathrm{}$ eV (2473 ÅA), $E_{02}=5.062\mathrm{}$ eV (2449 ÅA) and the half-widths were $H_1=0.060\mathrm{}$ and $H_2=0.067\mathrm{}$ eV. On warming, $H_1$ increases as ${\left[coth\right(\frac{\theta }{T}\left)\right]}^{\frac{1}{2}}$ with ${\theta }_{\perp }=27\mathrm{}$ °K. ${\mathrm{H}}_2$ remains constant up to about 160 °K. At higher temperatures, it can be approximated by the above law with ${\theta }_2=100\mathrm{}$ °K. $\Delta E$ follows the same law with ${\theta }_3=100\mathrm{}$ °K. This behavior supports the theory in which the structure is due to the dynamical Jahn-Teller effect.