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<INF>1</INF> state were observed under the resonance condition with the S<INF>n</INF><INF></INF> ← S<INF>1</INF> absorption. The observed signal consisted of a spike-like feature around the time origin, an oscillatory component with a period of ~0.12 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<INF>1</INF> trans-stilbene (ν<INF>24</INF>, 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<INF>n</INF><INF></INF> ← S<INF>1</INF> excitation:  the vibrational cooling process of S<INF>1</INF> trans-stilbene and the loss of the S<INF>1</INF> 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.