Direct observation of the resonant-non-resonant cross-term contribution to coherent Raman scattering of quasi-continuous-wave noisy light in molecular liquids

Abstract

The resonant–non-resonant cross-term contribution to the coherent Raman scattering (CRS) signal produced by broadband non-transform limited quasi-cw (noisy) light is directly observed in the form of a spectrogram. The technique is a recently developed time–frequency domain version of CRS using noisy light. Vibrational modes of three (neat) molecular liquids were examined: the C–C stretching mode of acetone, the C–Cl stretching mode of methylene chloride and the ‘ring breathing’ mode of cyclohexane. The resonant–non-resonant cross-term is revealed as a striking asymmetry in the observed time–frequency domain spectrogram. This is the analogue of the dispersive resonant–non-resonant cross-term in conventional CRS spectroscopy. However, unlike conventional CRS, noisy light interferometry yields a CRS spectrum that is uncoupled from the linewidth which instead governs the interferometric decay. The ratio of the resonant to non-resonant cross-term contribution can be quantified and, as is well known, a reversible link between absolute Raman cross-sections and absolute non-resonant susceptibility of any chosen molecular liquid can be established. © 1997 John Wiley & Sons, Ltd.