CLUSTER AND BARRIER EFFECTS IN THE TEMPERATURE AND PRESSURE-DEPENDENCE OF THE PHOTOISOMERIZATION OF TRANS-STILBENE

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, C 4H10, Xe, Co2, SF6, and CHF 3. 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. © 1990 American Institute of Physics.