Time-resolved impulsive stimulated Raman scattering from excited-state polyatomic molecules in solution

Time-domain Raman measurement of the excited state of a polyatomic molecule was demonstrated for the first time. Time-resolved impulsive stimulated Raman scattering TR-ISRS) measurements were carried out for trans-stilbene in solution, and resonantly enhanced signals due to the S-1 state were observed under the resonance condition with the S-n <-- S-1 absorption. The observed signal consisted of a spike-like feature around the time origin, an oscillatory component with a period of similar to0.12 ps, and a slowly decaying traditional transient-grating (TG) signal. A Fourier transform analysis clarified that the oscillatory ISRS component was attributed to an in-plane bending vibration of S-i trans-stilbene (nu(24), 285 cm(-1)). The origin of the TG signals was examined by three-pulse absorption measurements, and it was concluded that the transient grating was created reflecting two relaxation processes following the S-n <-- S-1 excitation: the vibrational cooling process of S-1 trans-stilbene and the loss of the S-1 population. The present study demonstrated that time-resolved time-domain Raman spectroscopy can provide spectral information about low-frequency tetrahertz motions of the excited-state, which cannot be accessed by ordinary time-resolved frequency-domain Raman spectroscopy.