Vacuum ultraviolet resonance Raman studies of the valence excited electronic states of benzene and benzene-D₆: The E_1U state and a putative A_2U state

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 ¹E_1U (ππ*) state results'in Raman spectra dominated by progressions in the totally symmetric ring breathing vibration (v₁₀) both alone and in combination with the overtone of the e _1g CH bending vibration (V₁₀) and the overtone of the b_2g ring deformation vibration (v₄). Much weaker progressions are observed involving v₁ 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 ¹E_1u electronic state. In addition, the observation of the v₁₀ fundamental transition and the 2v₁₀, 2v ₄, and 2v₈ 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.