The picosecond anti-Stokes Raman spectra of trans-stilbene in the S1 state have been observed by using several probe wavelengths (580-670 nm) and a fixed pump wavelength giving a sufficient excess vibrational energy (≈5200 cm-1). The probe-wavelength dependency of the ratio of the anti-Stokes Raman-band intensity at 1180 cm-1 due to the C-Ph stretching (ν15) to that at 1570 cm-1 due to the olefinic C = C stretching (ν7) is analyzed by assuming a model based on the Franck-Condon mechanism. It has been shown from the anti-Stokes intensity analysis that vibrationally excited transients at 0 ps delay time are mostly on the lowest excited vibrational levels of the S1 state as far as the ν15 and ν7 modes are concerned, and the excess energy is not statistically distributed to other modes. A possible mechanism that accounts for this observation is discussed.
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