Cluster and barrier effects in the temperature and pressure dependence of the photoisomerization of t r a n s-stilbene

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Abstract
The pressure and temperature dependence of the photoisomerization rate coefficient of trans‐stilbene in the S1 state have been measured in the solvents C2H6, C3H8, C4H10, Xe, Co2, SF6, and CHF3. At constant temperature, the pressure dependences up to 6 kbar can be well represented by the Kramers–Smoluchowski model. The comparison of results in different solvents clearly indicates the importance of reactant–solvent cluster formation modifying the height and imaginary frequency of the barrier. The change of the temperature dependence with pressure points towards a multidimensional barrier of nonseparable character. Multidimensional barrier effects manifest themselves most clearly via the temperature dependence of the rate coefficient in the Kramers–Smoluchowski limit.