Fluorescence lifetimes of jet-cooled trans-stilbene and its (1:1)-van der Waals complexes with n-hexane: Cluster effects in photoisomerisation

The excess energy dependence of the fluorescence decay of jet-cooled trans-stilbene in the vicinity of the photoisomerisation threshold is compared with k(E, J) as calculated from an optimized RRKM model. Van der Waals 1 : 1-complexes between trans-stilbene and n-hexane are generated in a supersonic jet and identified by rotational coherence spectroscopy. A comparison of the fluorescence lifetimes of isolated trans-stilbene 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 accompanied by internal energy loss in trans-stilbene. The threshold energy for isomerisation in the 1:1-complex is found to be approximately 300 cm(-1) lower than for the isolated stilbene molecule.