Theoretical studies of two-photon absorption processes. I. Molecular benzene

The theory of two-photon transitions is extended to include vibronic-coupling effects. The theory of symmetry-forbidden two-photon transitions is applied for the theoretical study of excitations in benzene. The introduction of vibronic-coupling effects implies that the transition probability should be temperature dependent. A definite assignment of the electronic states of benzene can be obtained from theoretical predictions of the intensity ratios, the vibrational structure, and the polarization dependence for two-photon absorption cross sections in this system.