Development of femtosecond time-resolved Raman spectroscopy using stimulated Raman scattering

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New femtosecond Raman spectroscopy has been developed to investigate ultrafast photoinduced structural changes of materials. Vibrational modes in the transient species are measured by stimulated Raman scattering using a Raman pump pulse with narrow bandwidth and a femtosecond supercontinuum probe pulse. The Raman signal can be measured without slowing the temporal response and broadening the spectrum, because the temporal and spectral resolutions of the new method can be improved independently without restriction of Fourier transform limit. This method can apply to any stimulated Raman scattering, i.e., Raman gain, Raman loss, CARS and CSRS and may also apply to other nonlinear optical spectroscopy. The temporal resolution of 250 fs and the spectral resolution of 25 cmィイD1-1ィエD1 has been obtained simultaneously in this study.Validity of the new system has been demonstrated by the application to the photoisomerization of organic dye. Time-resolved Raman signals in ultrafast trans-cis photoisomerization processes of DCM (4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran) solutions has been investigated. The observed Raman signals are separated to two groups from the temporal dependence and assigned to an intermediate state between trans- and cis-forms and a charge-transfer state in DCM.