Quasi-Jahn-Teller Optical Absorption Line Shape due to Transition from A_1g to (A_1g+T_1u)×t_1u System

Abstract

Optical absorption line shape due to the transition from the ground state of A_1g symmetry to the quasi-Jahn-Teller system of accidentally degenerate electronic states of A_1g and T_1u symmetry coupled through t_1u-mode phonon is calculated by three methods, that is, the semi-classical approximation, the independent ordering approximation and the exact method by numerical diagonalization of the vibronic Hamiltonian. In the strong coupling and/or high temperature limit, the line shape is composed of a strong delta function-like central peak and a pair of weak wing part. It is found that the independent ordering approximation does not reproduce the exact line shape for intermediate region so well as in the case of E×e system. The vibronic spectrum for non-degenerate case is also discussed.