The vibrational relaxation process of trans-stilbene in the lowest electronically excited singlet state after photoexcitation has been studied by picosecond time-resolved anti-Stokes Raman spectroscopy using several pump and probe wavelengths. Measurements of pump wavelength dependency have shown that, when the molecules are excited by pump photons with a high excess vibrational energy (∼5200 cm-1), part of the excess vibrational energy remains localized in the olefinic C=C stretching (v7) and the C-Ph stretching (v15) mode for a period within the time resolution of the present measurements (∼5 ps) after photoexcitation. Analysis of the probe wavelength dependency has indicated that the vibrationally excited transients observed in the picosecond anti-Stokes Raman spectra are for the most part in the lowest excited vibrational levels (n =1), as fa as the v7 and v15 modes are concerned. From these results it is suggested that the intramolecular vibrational relaxation process proceeds in roughly two steps. The optically pumped molecules on highly excited vibrational levels of the olefinic stretching modes first relax very rapidly (probably in the femto to subpicosecond time range) to the lowest vibrationally excited state. Then, slower relaxation occurs in several picoseconds to achieve an intramolecular (quasi) thermal equilibrium.
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