Semiclassical Theory of the Near-Infrared Spectrum of KCl:Tl0

The splitting of the ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ term of ${\mathrm{Tl}}^0$ in KCl is discussed semiquantitatively in terms of a configuration coordinate representing an odd-parity vibrational mode. The gross features of the data of Delbecq, Ghosh, and Yuster (preceding paper), and in particular the temperature dependence of the splitting of the term, may be understood on this model. It is proposed that this $T$-dependent splitting may occur rather generally and independently of any explicit dynamical Jahn-Teller effect in forbidden transitions at impurity centers.