Structure of the Jahn-Teller-inducedBabsorption band ofTI+-type centers in alkali halides

The $B$- as well as $A$- and $C$-band shapes due to the electron-lattice interaction have been calculated systematically by using the Franck-Condon approximation. The linear electron-lattice interaction within the $a_{1g}{t}_{1u}$ excited states has been taken into consideration; it is written in terms of a 12 × 12 matrix by using the six interaction mode coordinates which belong to the ${\alpha }_{1g}$, ${\epsilon }_g$, and ${\tau }_{2g}$ irreducible representations of $O_h$ group. The matrix has been diagonalized by the Givens-Householder method and the integration has been performed by the Monte Carlo method. The results can explain most of the characteristic features so far observed, indicating the validity of the Franck-Condon approximation and of the interaction mode coordinates even in the case of the Jahn-Teller-induced absorption band. An exception is the small peaks sometimes found in the middle part of the $B$ band of ${\mathrm{Sn}}^{2+}$ center at very low temperatures (e.g., 4.2 K) which cannot be reproduced in the present calculation; the origin of these small peaks has been discussed. The effect of a magnetic field on the band shape (the magnetic circular dichroism shape) has also been calculated preliminarily.