Theory of Optical Absorption and Magnetic Circular Dichroism for the Vibration-Induced Band (B-Band) of Tl⁺-Like Centers in Alkali Halides

Abstract

The line shape functions for the B-band with and without the effect of magnetic field are studied theoretically by use of the perturbation theory and the classical Franck-Condon approximation. The sum of the linear electron-lattice interaction and the Zeeman energy is treated as the perturbing Hamiltonian through which the mixing of the A- and C-states into the B-state occurs. Because of its vibration-dependent transition moments, the B-band is found to have the following characteristics. Although the T_2g mode is responsible for the structure of the absorption spectrum, the E_g mode gives the similar shape of the magnetic circular dichroism (MCD) spectrum to that due to the T_2g mode. In comparison with the A- and C-bands, the structure of the absorption spectrum is indistinct, while the shape of the MCD spectrum is complicated and very strange. The calculated spectra explain satisfactorily the experimental ones.