Resonance Raman spectroscopy of the B_1u region of benzene: Analysis in terms of pseudo-Jahn-Teller distortion

Raman spectra of benzene and benzene-d₆ vapor obtained with excitation in the range from 208 to 188 nm are presented and discussed in terms of vibronic coupling of the resonant B_1u state and the nearby E _1u electronic state. The Raman spectra show strong activity in the binary overtones and combinations of the e_2g modes ν_g, ν₉, and ν₆ with the ν₈ activity being dominant. (The Wilson numbering scheme for the modes of vibration is used throughout.) These bands, plus a progression in ν₁ alone or in combination transitions involving two quanta of e_2g modes, constitute the major intensity bands in the spectra. A simple first-order vibronic coupling model can be constructed which accounts adequately for most of the intensity in the Raman spectra observed in resonance with the B_1u state, but this model results in a calculated absorption spectrum that is qualitatively different from that observed. The nature of the discrepancy indicates a significant Duschinsky mode rotation in the B_1u state relative to the ground state. An analysis of the vibronic coupling of the B _1u state with the nearby E_1u state is performed using semiempirical calculation methods to provide excited state geometries, vibrational frequencies, and vibronic coupling parameters. This analysis confirms that there is strong vibronic coupling between these states resulting in a pseudo-Jahn-Teller effect. This results in a highly distorted potential surface for the B_1u state with three D_2h symmetry antiquinoidal minima lower in energy by ∼ 1000 cm^-1 than the D_6h symmetry geometry. The three minima of the threefold symmetric potential surface are separated by local maxima corresponding to the quinoidal geometry with a height of ∼ 300 cm^-1. Three of the four e _2g modes are calculated to have significant vibronic activity with mode 8 much stronger than modes 6 or 9. A small Jahn-Teller coupling is calculated. A strong Duschinsky rotation results from the vibronic activity of more than one mode. The absorption spectrum and the resonance Raman spectra calculated from this model are in good agreement with the experimental results. This analysis provides the first evidence for the expected pseudo-Jahn-Teller instability of the isolated ¹B_1u state of benzene.