Separability of Intra- and Intermolecular Vibrational Relaxation Processes in the S1 State of trans-Stilbene

Detailed time-resolved measurements of anti-Stokes intensities of several high-frequency modes of electronically excited trans -stilbene in hexane and methanol are reported. The most striking changes are observed in the ethylenic band at 1570 cm -1 . Nonequilibrium population in the v = 1 level of this mode is found to exhibit a biphasic decay, which is interpreted as a rapid energy redistribution followed by a slightly slower process that reflects the transfer of excess energy to the solvent. Comparison of the time-dependent intensity changes of several high-frequency bands with those from a model that assumes instantaneous thermalization of the excited state suggests that internal energy randomization is incomplete on the time scale of solute−solvent thermal equilibration process. No evidence of mode specificity in the energy redistribution process is observed. The results show us to place a value of ∼2 ps on the lifetime of the v = 1 level of the ethylenic stretching mode of trans -stilbene in both hexane and methanol. Conclusions from this work are discussed in the context of other recent time-resolved Raman measurements of vibrational relaxation in this system in both the gas phase and condensed phase.