FLUORESCENCE LIFETIMES OF JET-COOLED TRANS-STILBENE AND ITS (1 1)-VAN-DER-WAALS COMPLEXES WITH N-HEXANE - CLUSTER EFFECTS IN PHOTOISOMERIZATION/

The excess energy dependence of the fluorescence decay of jet-cooled t rans-stilbene in the vicinity of the photoisomerisation threshold is c ompared with k(E, J) as calculated from an optimized RRKM model. Van d er Waals 1 : 1-complexes between trans-stilbene and n-hexane are gener ated in a supersonic jet and identified by rotational coherence spectr oscopy. A comparison of the fluorescence lifetimes of isolated trans-s tilbene and the 1:1-complex in the excess energy range 0<E(ex)<2500 cm (-1) provides evidence for a competition between the isomerisation of stilbene in the bound complex and fragmentation of the complex accompa nied by internal energy loss in trans-stilbene. The threshold energy f or isomerisation in the 1:1-complex is found to be approximately 300 c m(-1) lower than for the isolated stilbene molecule.