Raman scattering spectra of benzene vapor with excitation in the 220ndash;184 nm region reveal strong enhancement of the binary overtones and combinations of thee2gvibrations ngr;8and ngr;9, supporting the conclusions of a previous study of benzene with 213 nm radiation lsqb;Ziegler and Hudson, J. Chem. Phys.74, 982 (1981)rsqb;. The fact thate2gvibrations are vibronically active with all excitation wavelengths confirms the assignment of the electronic state in this region as one ofB1usymmetry. The dominance of the spectra at all excitation wavelengths by transitions involving ngr;8and ngr;9demonstrates that these are the particular motions with the greatest vibronic activity. Progressively higher excitation energies result in increased relative intensity in higher order combinations and overtones. The appearance of weak bands corresponding to two quanta of thea2umode ngr;11in the 213 and 204 nm spectra provides tentative evidence for Raman activity of theE1gsymmetry state in this 6 eV region. The characteristic symmetric ring expansion mode ngr;1at 992 cmminus;1becomes very weak in the 204 and 200 nm spectra but reappears as a very strong progression in the 184 nm spectrum. This behavior is reminiscent of that observed for methyl benzenes with 213 nm excitation lsqb;Ziegler and Hudson, J. Chem. Phys.79, 1134 (1983)rsqb; where progressive methyl substitution shifts the1Laband to lower energy. This intensity variation is shown to be due to destructive interference between the vibronicChyphen;type contribution to the intensity andAhyphen;type contributions from higher energy allowed transitions. The 184 nm spectrum shows very strong bands due to even overtones and combinations of theb2gmode ngr;4and combinations of these bands with strong ngr;1progression. This demonstrates that either there is a vibronically active electronic state withE2usymmetry in this region or that theE1ustate is strongly modified relative to the ground state for displacement along thisb2gcoordinate.
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