Observation of multimode vibronic interference effects in ²E_1g benzene

This paper presents the first experimental demonstration of multimode vibronic interference effects arising from the interaction of topologically inequivalent vibrational modes with a two-level electronic degeneracy. The system is that of the E_1g molecular ion core of benzene Rydberg states. The interacting modes are the Jahn-Teller active e_2g, ring bend, v₆, and the Renner, out-of-plane e_2u ring bend, v₁₆. The nature of the interaction is seen as a coupling of the angular momenta of the two modes individually to the electronic angular momentum, producing new quantization conditions, within which interference interactions can be approximated perturbatively. Such a treatment explains qualitatively all splitting patterns observed in experimental spectra. Small quantitative deviations point the way to higher order couplings.