PICOSECOND STRUCTURAL RELAXATION OF S1 TRANS-STILBENE IN SOLUTION AS REVEALED BY TIME-RESOLVED RAMAN-SPECTROSCOPY

Picosecond time-resolved Raman spectra of S1 trans-stilbene (S1 tSB) in heptane have been obtained with transform-limited (3 ps and 2 cm-1) picosecond pulses from a high repetition-rate amplified synchronously-pumped dye laser system. Both the peak positions and the bandwidths of two olefinic bands, the C=C stretch around 1565 cm-1 and the C-Ph stretch around 1180 cm-1, change with time. The changes in the bandwidths precede those of the peak positions and start immediately after the photoexcitation. The peak positions remain unaltered during the first few picoseconds of the photoexcitation and then start to change. This suggests that the changes of the peak positions are not solely due to an intramolecular process but reflect certain structural relaxation of S1 tSB that involves the motion of the surrounding solvent molecules.