Abstract

UV resonance Raman studies of benzene have demonstrated that the Raman intensities are dominated by vacuum UV transitions. Because of the weak oscillator strengths and the significant homogeneous linewidths in the condensed phase, little Raman enhancement iscontributed by the ~240-260-nm B_2u transition. In the gas phase, however, the smaller homogeneous linewidth results in resonance enhancement. We show experimentally the distinction between resonance Raman scattering and single vibrational level fluorescence. Raman studies of substituted benzene derivatives illustrate that the resonance enhanced vibrational modes are those which distort the nuclear framework and electron density and the bond lengths in a fashion characteristic of the transition moment to the resonant excited state. Raman excitation profile studies of polycyclic aromatic hydrocarbons such as pyrene clearly show the underlying Franck Condon substructure of the diffuse absorption bands. The strong resonance enhancements permit studies of polycyclic aromatic hydro-carbons at ppb concentrations. The analytical utility of UV Raman spectroscopy is illustrated by studies of polycyclic aromatic hydrocarbons in coal liquid samples and in biological matrices.

© 1986 Optical Society of America