The three-photon spectrum of the 1B2u←1A1g transition in benzene: Analysis of vibronic and rotational structure

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Abstract
A vibrational and rotational analysis of the three‐photon fluorescence excitation spectrum of the 1B2u←1A1g transition in gas‐phase benzene is presented. The group theoretically allowed origin and any associated vibrational structure are absent. In addition, there is no evidence for any rotational structure based on a rank‐three transition tensor. Coherent loss mechanisms are carefully considered and then rejected as possible interferences in the three‐photon fluorescence spectrum. Polarization measurements and observed vibronic structure indicate a dominant rank one (vibronic) contribution to the three‐photon cross section. The lack of three‐photon allowed structure in this 1B2u←1A1g transition of benzene is consistent with the pseudoparity selection rules of alternant hydrocarbon theory, yet such strict adherence to them would seem surprising. The appearance of the e2g mode ν8 in the three‐photon spectrum marks the first experimental determination of its frequency in the 1B2u state.