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Viscosity and solvent dependence of lowbarrier processes: Photoisomerization of cisstilbene in compressed liquid solventsThe photoisom~rization .of stilbene has been studied in low viscosity liquid alkanes and in the gas ph~e. The bamer cross1Og process shows no evidence of a low-friction or energy controlled regton e~en at the lowest liquid visc~siti~s studied. We present evidence that the barrier crossing may be 10 the energy controlled regton 10 the thermal vapor. We discuss the influence of intramolecular.vibrati~nal energ! transfe~ on the observed dynamics and note that entropy effects should be conSIdered 10 companng expenmental data with theoretical models.

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An amplified colliding pulse modelocked laser system is described which is capable of subpicosecond resolution flash-photolysis studies. As an application of this system we used the anisotropic absorption technique (polarization spectroscopy) to measure the rotational reorientation of trans-stilbene. These measurements give a more accurate estimate of the microscopic friction involved in the isomerization dynamics of this molecule. When the friction is assumed proportional to the reorientation times, the isomerization rates in alkane solvents are adequately described by activated barrier-crossing theory.