PICOSECOND RESONANCE RAMAN STUDIES OF VIBRATIONAL COOLING OF ELECTRONICALLY EXCITED TRANS-STILBENE IN ALCOHOLS AND ALKANES

The vibrational cooling of electronically excited trans-stilbene has been studied in ethanol, hexanol, hexane, and decane by pump-probe resonance Raman (RR) spectroscopy employing pulses of 1.5-2.0 ps with 15 cm-1 spectral width. Excitation 3200 cm-1 above the electronic origin creates a transient rise in the stilbene vibrational temperature of approximately 150 K. The subsequent intermolecular transfer of this excess energy to the surrounding solvent leads to time-dependent changes in the peak position and bandwidth of the ethyleneic band at approximately 1565 cm-1. In agreement with previous studies, we find that the lineshape of this mode is nearly Lorentzian at all delay times and that the width decays exponentially with increasing delay time. The peak position, however, shifts to higher frequency only after a 3-5-ps induction period. The vibrational cooling kinetics, as measured by the dynamics of the bandwidth, do not depend on solvent, though the dephasing time of the ethylenic mode is faster in alcohols than in alkanes. We interpret these results as arising from coupling of the ethylenic motion to two or more low frequency modes that can exchange energy with the solvent via collisions. We discuss the role of solvent-solute interactions and solvent thermal conduction in the cooling process.