VACUUM ULTRAVIOLET RESONANCE RAMAN STUDIES OF THE VALENCE EXCITED ELECTRONIC STATES OF BENZENE AND BENZENE-D6 - THE E1U STATE AND A PUTATIVE A2U 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 1E1u (pi-pi*) state results in Raman spectra dominated by progressions in the totally symmetric ring breathing vibration (v1) both alone and in combination with the overtone of the e1g CH bending vibration (v10) and the overtone of the b2g ring deformation vibration (v4). Much weaker progressions are observed involving v1 and fundamental and overtone transitions of the Jahn-Teller active e2g vibrations. The observation of intensity in e2g fundamental vibrations indicates that the E1u 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 1E1u electronic state. In addition, the observation of the v10 fundamental transition and the 2v10, 2v4, 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 A2u symmetry in the high energy tail of the strong pi-pi* transition.