DEVIATIONS FROM LINDEMANN BEHAVIOR - PHOTOISOMERIZATION DYNAMICS OF TRANS-STILBENE UNDER COLLISIONAL GAS-PHASE CONDITIONS

Systematic fluorescence lifetime measurements of trans-stilbene in bat h gases nitrogen, methane, ethane and propane at pressures between 1 m bar and 10 bar were analysed by comparison with master equation simula tions. The results show that the low-pressure regime of the unimolecul ar photoisomerization reaction is located well below I bar. The increa se in the isomerization rate coefficient observed with further increas ing pressure contradicts the basic assumption of the Lindemann model a nd is discussed in terms of the solvent-induced changes of the potenti al energy surface. The role of restricted intramolecular vibrational e nergy redistribution (IVR) in determining the observed photoisomerizat ion dynamics is clearly of minor importance. (C) 1997 Elsevier Science S.A.