UV resonance Raman signatures of phonon-allowed absorptions and phonon-driven bubble formation

摘要

We have measured UV resonance Raman near and at the resonance phonon-allowed absorption lines of several liquid species. Resonance absorption with excitation on the symmetry-forbidden but strongly phonon coupled bands in the 230-290 nm spectral band present enhancement 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. Large resonance gains (～3500x) reflect the narrower vapor phase lines. At the low laser fluence used, bubble formation is observed when the excitation energy corresponds to the maximum in Raman signal generation, not at the wavelength of maximum absorption in the liquid sample, which is several nanometers away.