Molecular beam measurements of small specific rate constants for unimolecular reactions

The investigation of unimolecular reactions with small rate constants is difficult owing to competing processes (inelastic collisions and bimolecular reactions) and the diffusion of reactant and product molecules out of the detection volume. For this reason, a new experimental approach for the measurement of specific rate constants in a molecular beam experiment has been exploited; instead of monitoring the temporal change of intensity as in a cell experiment, we monitor the spatial change along the molecular beam axis after laser excitation. For a given particle velocity the flight path between excitation and detection region defines the reaction time. By varying the distance the specific rate constant can be determined directly both from the decrease in the number density of reactant molecules as well as from the increase in product molecules. As a model system, the laser-induced (λ = 193 nm) photodissociation of mesitylene (trimethylbenzene) is studied. Previous experiments on the specific rate constant of mesitylene at this excitation energy differ between each other by about a factor of ten. By combining the new results with measurements at higher excitation energies, rate constants over a range of two orders of magnitude are now available for this reaction. The differences between the various experimental results are discussed within the framework of a statistical theory.