Time-resolved Raman scattering with incoherent light

In time-resolved Raman scattering (as well as in degenerate four-wave mixing) the time resolution is in principle determined by the coherence time of the laser pulses involved rather than the pulse widths. In this paper the theoretical time dependence of three different time-resolved Raman-scattering techniques, Raman-fringe decay (RFD), coherent Stokes Raman scattering, and time-resolved stimulated Raman gain, is calculated under various coherence conditions of the fields. Taking RFD generated by temporally incoherent light as the example, it is shown that due to complicated coherence effects the material relaxation time cannot be extracted from the experimental data without careful consideration of the stochastic properties of the fields and the different relaxation parameters of the system. The RFD technique has been experimentally applied to the 524-${\mathrm{cm}}^{\mathrm{-}1}$ mode of ${\mathrm{CH}}_3$I and the 656-${\mathrm{cm}}^{\mathrm{-}1}$ mode of ${\mathrm{CS}}_2$ and coherence effects are indeed observed. The relaxation times are found to be in agreement with previous experimental studies with coherent light.