Ultraviolet resonance Raman spectroscopy of benzene vapor with 220–184 nm excitation

Abstract

Raman scattering spectra of benzene vapor with excitation in the 220–184 nm region reveal strong enhancement of the binary overtones and combinations of the e2g vibrations ν8 and ν9, supporting the conclusions of a previous study of benzene with 213 nm radiation [Ziegler and Hudson, J. Chem. Phys. 74, 982 (1981)]. The fact that e2g vibrations are vibronically active with all excitation wavelengths confirms the assignment of the electronic state in this region as one of B1u symmetry. The dominance of the spectra at all excitation wavelengths by transitions involving ν8 and ν9 demonstrates 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 the a2u mode ν11 in the 213 and 204 nm spectra provides tentative evidence for Raman activity of the E1g symmetry state in this 6 eV region. The characteristic symmetric ring expansion mode ν1 at 992 cm−1 becomes 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 [Ziegler and Hudson, J. Chem. Phys. 79, 1134 (1983)] where progressive methyl substitution shifts the 1La band to lower energy. This intensity variation is shown to be due to destructive interference between the vibronic C-type contribution to the intensity and A-type contributions from higher energy allowed transitions. The 184 nm spectrum shows very strong bands due to even overtones and combinations of the b2g mode ν4 and combinations of these bands with strong ν1 progression. This demonstrates that either there is a vibronically active electronic state with E2u symmetry in this region or that the E1u state is strongly modified relative to the ground state for displacement along this b2g coordinate.