Gas-phase ion reaction dynamics

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Internal energy plays an important role in determining the outcome of the dynamics of chemical reactions.  Over the last seven years my group has been involved in studying the mechanisms and competing kinetics of unimolecular reactions of gas-phase ions - notably electrostatically bound complexes.  Due to the nature of the mass spectrometric experiments performed on metastable complexes the internal energy, and hence kinetics, of the unimolecular reactions are quite well defined.  This breaks down, however, when collision-based experiments are performed.  In these studies, which attempt to replicate particle bombardment in the upper atmosphere, the internal energy of the resulting excited ion is ill-defined.  Our research has thus expanded into learning about internal energy deposition in gas-phase ions in both high (keV) and low (eV) energy collision events by examining their dissociation characteristics and their spectroscopy.  A recently completed apparatus allows us to explore the fluorescence from collisions between gas-phase ions and neutral target gases from 190 nm - 1000 nm in our mass spectrometer as a function of collision energy (laboratory frame) from 8 keV to 100 eV (keeping in mind that the internal energy deposited into the species at these energies is considerably less than the collision energy).  Parallel investigations into the dissociation of ions from threshold to > 30 eV by threshold photoelectron-photoion coincidence spectroscopy (TPEPICO) allow us to compare our mass spectrometric and fluorescence observations with reliable energy thresholds for the appearance of fragment ions - thus permitting the quantitation of internal energy deposition in collision-based experiments.