PROBE-WAVELENGTH DEPENDENCY OF PICOSECOND ANTI-STOKES-RAMAN SPECTRA OF TRANS-STILBENE IN THE S-1 STATE

The picosecond anti-Stokes Raman spectra of trans-stilbene in the S-1 state have been observed by using several probe wavelengths (580-670 n m) and a fixed pump wavelength giving a sufficient excess vibrational energy (approximate to 5200 cm(-1)). The probe-wavelength dependency o f the ratio of the anti-Stokes Raman-band intensity at 1180 cm(-1) due to the C-Ph stretching (v(15)) to that at 1570 cm(-1) due to the olef inic C=C stretching (v(7)) is analyzed by assuming a model based on th e Franck-Condon mechanism. It has been shown from the anti-Stokes inte nsity analysis that vibrationally excited transients at 0 ps delay tim e are mostly on the lowest excited vibrational levels of the S-1 state as far as the v(15) and v(7) modes are concerned, and the excess ener gy is not statistically distributed to other modes. A possible mechani sm that accounts for this observation is discussed.