Vibronic interaction in the lower electronic states of benzene

In the present paper the pseudo Jahn-Teller interaction between the four lower electronic states of benzene is considered, with special reference to the mixing of the 3 B 1u and 3 E 1u states through the e 2g carbon-carbon stretching mode. First a review is given of the pioneering work of Liehr on the static problem, where the ‘electronic Hamiltonian’ is solved as a function of fixed nuclear coordinates. Results of a specific calculation are presented. Subsequently, the equations for the vibronic problem are set up by generalizing the methods proposed by Moffitt et al. and Longuet-Higgins et al. for the dynamic Jahn-Teller effect. These equations are then solved in the first (‘pseudo-cylindrical’) approximation in which only the strong coupling linear in the displacements between the B 1u and E 1u states is considered. The results are discussed with reference to known properties of the phosphorescent state of benzene.