Time-Resolved Impulsive Stimulated Raman Scattering from Excited-State Polyatomic Molecules in Solution

Abstract

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 S1 state were observed under the resonance condition with the Sn ← S1 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 S1 trans-stilbene (ν24, 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 Sn ← S1 excitation: the vibrational cooling process of S1 trans-stilbene and the loss of the S1 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.