Deviations from Lindemann behaviour: Photoisomerization dynamics of trans-stilbene under collisional gas phase conditions

Systematic fluorescence lifetime measurements of trans-stilbene in bath gases nitrogen, methane, ethane and propane at pressures between 1 mbar and 10 bar were analysed by comparison with master equation simulations. The results show that the low-pressure regime of the unimolecular photoisomerization reaction is located well below I bar. The increase in the isomerization rate coefficient observed with further increasing pressure contradicts the basic assumption of the Lindemann model and is discussed in terms of the solvent-induced changes of the potential energy surface. The role of restricted intramolecular vibrational energy redistribution (IVR) in determining the observed photoisomerization dynamics is clearly of minor importance. (C) 1997 Elsevier Science S.A.