Environmental dependence of preresonance Raman cross-section dispersions: Benzene vapor-phase excitation profiles

The vapor-phase Raman cross-section excitation profiles of the two totally symmetric modes of benzene have been measured between 514 and 220 nm. At wavelengths longer than 280 nm, the 992 cm−1 ν1 ring breathing-mode cross-section dispersion is not indicative of preresonance enhancement by the dipole-allowed E1u electronic transition near 185 nm. With excitation wavelengths shorter than 240 nm, however, the ν1 mode dispersion is dominated by enhancement from the B1u/E1u state. The ν2 3060 cm−1 carbon–hydrogen stretching vibration shows no enhancement by the E1u state in the 514–220 nm spectral region. The cross-section dispersion is indicative of enhancement by states centered near 100 000 cm−1. We compare the gas- and condensed-phase Raman cross-section dispersions of benzene, cyclohexane, and acetonitrile. Surprisingly, we do not observe the ‘‘extra’’ dispersion predicted to occur in the condensed phase which derives from the dispersion in the local electromagnetic field strength.