Picosecond studies of the vibrational predissociation of van der Waals complexes

by Semmes, David Haynes

The vibrational predissociation of several van der Waals complexes of aromatic molecules with rare gas atoms, isolated in a free-jet molecular beam expansion, has been studied using time-resolved spectroscopy. Direct measurements have been made, on a picosecond time scale, of the fluorescence intensity from the initially excited single vibrational level and from the various final vibrational states formed in the dissociation of the complexes. The time-resolved data is analyzed with the aid of different kinetic models and can distinguish between parallel decay paths, sequential paths with intramolecular vibrational redistribution (IVR) preceding vibrational predissociation, and sequential paths with reversible IVR preceding vibrational predissociation. The contribution of the van der Waals modes to the total vibrational state density induces dissipative IVR in the complexes at much lower energies than in the bare aromatic molecule. The measured predissociation rates are compared with rates calculated according to the statistical RRKM theory of unimolecular reactions. Some of the experimental results show mode-selective behavior, while other results suggest statistical behavior.