Vibrational relaxation dynamics of trans-stilbene in the S1 state immediately after
photoexcitation is studied by picosecond time-resolved anti-Stokes Raman spectroscopy
with several pump and probe wavelengths. Pump-wavelength dependence of the anti-
Stokes spectrum indicates that, when pump photons with high excess energy
(≈5200cm-1) are used, the anti-Stokes Raman bands at 0 ps delay time arise from
vibrationally excited transients with excess vibrational energy not thermally distributed
in the molecule. Probe-wavelength dependence suggests that the vibrationally excited
transients at 0 ps are mostly on the lowest excited vibrational levels, as far as the olefinic
C═C stretching and the C–Ph stretching modes are concerned. The vibrational
relaxation process of S1trans-stilbene is discussed on the basis of the observed results.