Coherence in UV resonance Raman spectroscopy of liquid benzene and toluene, but not ice

We have measured UV resonance Raman scattering at and near the resonance absorption lines of liquid benzene and toluene. Resonance occurs for excitation on the symmetry-forbidden but strongly phonon coupled states in the 1 B 2u band, ~230-270 nm, resulting in enhancements corresponding to the vapor phase absorptions rather than those of the liquid phase. This effect is related to the coherence forced by the internal molecular resonance required to absorb light at this energy. The resonance gains (~1000x) are larger than expected due to the narrower vapor phase lines. Several multiplet and overtone modes are enhanced along with the strongly coupled ring-breathing mode. A contrasting case of resonance Raman of ice is also discussed; in this case resonance is observed for excitation energy corresponding to absorptions that depend upon the final state shielding by the neighbors, and corresponds with the solid phase absorption. This typifies the more common, slow, time dependence of the resonance Raman process.