The vibrational cooling of electronically excited trans-stilbene has been studied in ethanol, hexanol, hexane, and decane by pump-probe resonance Raman ( R R ) spectroscopy employing pulses of 1.5-2.0 ps with 15 cm-I spectral width. Excitation 3200 cm-' 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 a t -1565 cm-1. In agreement with previous studies, we find that the lineshape of this mode is nearly Lorentzian a t 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, d o 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.W e discuss the role of solvent-solute interactions and solvent thermal conduction in the cooling process.
of two prototype structures that are illustrated by those observed in phenoxyl and p-benzosemiquinone anion radicals. One observes a progression from the phenoxyl to the semiquinone anion structure as the substituents become more capable of donating electrons to the phenyl system. This change is characterized by increased enhancement of the v8a and v9a vibrations in the more semiquinone anion like structures. The resonance Raman features of non-oxy radicals can also be described in terms of these prototype structures. For example, the aniline radical cation8 is structurally a phenoxyl-like radical, while the p-phenylenediamine cation30 is a semiquinone anion like radical.
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