Two-photon spectroscopy in the gas phase: Assignments of molecular transitions in benzene

Ro-vibronic resolved two-photon excitation spectra of benzene, C6H6, and C6D6 have been measured in the region of the S1 ←S0 transition which is parity and symmetry forbidden in two-photon absorption but can be induced by suitable ungerade vibrations. Rotational envelopes for the vibronic transitions are calculated for circular and linear polarized light excitation and are in good agreement with experiment. The polarization behavior is shown to change strongly across even a single band contour of a totally symmetric vibronic two-photon transition. The polarization analysis even for a randomly rotating gas phase molecule provides a severe constraint on the possible assignments and hence is an important tool for the assignment of new transitions. About 85% of the observed two-photon excitation spectrum of benzene can thus be assigned. The appearance of combination bands shows that anharmonic mixing plays an important role in the excited state.