Vacuum ultraviolet resonance Raman studies of the valence excited electronic states of benzene and benzene-d6: The E1u state and a putative A2u state

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Abstract
A resonance Raman study of benzene has been performed with the use of excitation wavelengths ranging from 184 to 160 nm. Excitation resonant with the 1 1 E 1u (ππ*) state results in Raman spectra dominated by progressions in the totally symmetric ring breathing vibration (ν1) both alone and in combination with the overtone of the e 1g CH bending vibration (ν1 0) and the overtone of the b 2g ring deformation vibration (ν4). Much weaker progressions are observed involving ν1 and fundamental and overtone transitions of the Jahn–Teller active e 2g vibrations. The observation of intensity in e 2g fundamental vibrations indicates that the E 1u electronic state is, at least slightly, Jahn–Teller active. More significant however, is the observation of considerable intensity in many of the out‐of‐plane vibrations indicating that benzene is distorted from planarity in the 1 1 E 1u electronic state. In addition, the observation of the ν1 0 fundamental transition and the 2ν1 0, 2ν4, and 2ν8 overtone transitions in the Raman spectra obtained using 171 to 160 nm excitation is indicative of the presence of one or more valence states of A 2u symmetry in the high energy tail of the strong ππ* transition.