Electronic relaxation and vibrational predissociation of benzene-acetylene van der Waals complexes

Fluorescence excitation spectra and dispersed fluorescence spectra of van der Waals complexes formed between benzene and acetylene, B . A(n) with n=1,2, were measured in the vicinity of the benzene S-1 <--S(0)6(0)(1)1(0)(n) transitions. Based on spectral shifts, fluorescence lifetimes and an acetylene concentration study, the complex exhibiting the largest blue shift (+ 136 cm(-1)) is assigned to the B . A dimer complex. Two other complex peaks, blue shifted by + 125 and + 116 cm(-1), respectively, are assigned to two different B . A(2) isomers. Vibrational predissociation is observed for all the clusters above various 6(1)1(n) levels in the excited state. Upper bounds of binding energies of both the electronic ground and excited states are deduced. Furthermore, analysis of the dispersed fluorescence spectra from product benzene after predissociation shows that extensive vibrational energy redistribution occurs prior to predissociation, leading to a complete loss of the nu(1) mode excitation. A lengthening of the fluorescence lifetimes of nondissociating B . A(2) levels is observed. Suppression of rotation-induced intramolecular relaxation caused by complexation is suggested for understanding this phenomenon. (C) 1999 American Institute of Physics. [S0021-9606(99)01317-3].