A condensed phase study of the benzene B2u←A1g three-photon transition

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Abstract
Although group theoretically allowed, the benzene B2u←A1g three-photon transition appears to derive intensity exclusively through a vibronic coupling route. The ν6 (e2g) vibration is dominant in providing e1u vibronic symmetry, just as it is in the one-photon transition. Several experiments are presented that provide further insight into the lack of allowed vibronic b2u activity in the three-photon spectrum. Pseudoparity, applicable to even alternate hydrocarbons in the PPP formalism, and offering selection rules beyond those based on symmetry alone, is examined as a possible explanation by looking for effects from the inductive perturbation in a fluorine substituted benzene. The possibility of nonresonant third harmonic generation in the condensed phase sample, which may lead to a viable one-photon absorption route to the excited B2u state, is also explored in both a three-photon photoselection study as well as in an experiment comparing the relative three-photon to one-photon cross sections for benzene and biphenyl. The results obtained for the benzene B2u transition and the corresponding A′2 transition in the symmetrically substituted trifluorobenzene appear to confirm the importance of pseudoparity selection rules and suggest that the third harmonic generation pathway is of minor importance.