Low energy ion–molecule reaction dynamics: Complex and direct collisions of O− with NH3

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Abstract
Reactive and nonreactive collisions of O− with NH3 are studied at relative collision energies of 0.65 and 1.24 eV. We observed a significant contribution to the collision dynamics from nonreactive encounters between the reagents. In addition to elastic scattering, we observed a direct contribution to this nonreactive scattering with a very strong dependence of energy transfer on scattering angle. A third contribution to nonreactive scattering arose from O−⋅NH3 collision complexes that regenerate the reactants. In these collisions, ∼80% of the incident translational energy is transformed into vibrational–rotational excitation of the NH3 reagent. The kinetic energy distribution is in reasonable agreement with statistical phase space theory calculations. We also observed reactive collisions. The hydrogen atom transfer process to yield OH− is exothermic by 0.11 eV and exhibits direct dynamics at all collision energies. Proton transfer to form NH−2, endothermic by 0.9 eV, was studied as its deuterium analog and was observed only at the higher collision energy, and took place with very small cross section. The product kinetic energy distributions for the hydrogen atom transfer reaction approach a Gaussian form at the higher collision energy, and we ascribe that behavior to the impulsive nature of reactive collisions in which the ground state vibrational wave function of the N–H bond to be broken is reflected onto product translational energy states through the ‘‘corner’’ of the potential energy surface. Such a Franck–Condon picture of the reaction is a consequence of the highly skewed potential energy surface associated with the heavy–light–heavy mass combination.