VIBRATIONAL MODE-SELECTIVE EFFECTS IN THE PICOSECOND TIME-RESOLVED RESONANCE RAMAN-SPECTRUM OF SINGLET EXCITED TRANS-STILBENE

Picosecond time-resolved resonance Raman spectra of singlet excited tr ans-stilbene have been obtained using several photolysis (pump) and Ra man excitation (probe) wavelengths. Vibrational mode-selective changes in peak wavenumbers and bandwidths are dependent on the sample temper ature, the pump wavelength, and the solvent. The effects are greatest for the band at almost-equal-to 1570 cm-1, attributed to the olefinic bond stretching mode. The peak wavenumber of this band decreases and t he band broadens as the temperature is raised. Excess energy of excita tion results in an initial decrease of peak wavenumber and increase in width followed by an increase in peak wavenumber and decrease in widt h as the delay between pump and probe increases further, with a simila r time dependence for peak position as for width. Fits to the sum of t wo exponentials give time constants in the range 1 to 5 ps for the ini tial and 9 to 12 ps for the later stages of the evolution. There is a similarity between the temperature and excess energy dependence, but b andwidth is more ( x 2.9) sensitive than peak wavenumber to excess ene rgy. The mode-selective changes and dynamics are interpreted with a mo del involving intramolecular vibrational relaxation and reorientation of the solvent cage in response to microdielectric stabilisation force s and quasi-equilibration with bulk solvent. The role of vibrationally ''hot'' low-wavenumber modes is clarified.