Picosecond photochemistry in molecular clusters

Abstract

Rates of chemical reactions in molecular clusters were measured by picosecond spectroscopy in a molecular beam mass spectrometer. We report on two classes of reactions. First, we consider acid-base proton transfer from an aromatic species to a basic solvent cluster . The chemistry involving the aromatic acids aniline cation and S phenol depended strongly on the solvent type and solvent cluster size. For example phenol*(NH3)n exhibited proton transfer for n 5. The rate constant for dissociation was ka (60 10 ps)' for n = 5 - 7. A geminate recombination rate of k_a (350 lOOps)1 occurs for n = 5. No proton transfer occurred in (CH3OH) solvent cluster up to n = 11. In another class of reactions, free radical chemistry in clusters was initiated by A state excitation in (CH3I) clusters. Extensive demethylation and formation of 12 and I2(CH3I)_2 fragments occured on the order of 10 ps. A mechanism is proposed for a chain of reactions driven in part by the energy of intracluster recombination of two CH3 radicals and the elimination of ethane.