Influence of vibrational excitation and collision energy on the ion-molecule reaction NH+3(ν2)+ND3

Abstract

The influence of vibrational excitation and collision energy on the ion-molecule reaction NH+3(ν2)+ND3 has been investigated using a recently constructed quadrupole-octopole-quadrupole mass spectrometer. The NH+3 reagent ions are prepared state selectively with 0–7 quanta in the ν2 umbrella bending mode by (2+1) resonance enhanced multiphoton ionization through the B̃ or C̃′ Rydberg states of ammonia. Reactive collisions between the mass-filtered ion beam and a thermal distribution of neutral reagent molecules occur with controlled collision energies (0.5–10.0 eV center of mass) within the octopole ion guide, enabling product ions to be collected independent of scattering dynamics. The reaction of NH+3 with ND3 has three major product channels: (1) deuterium abstraction, (2) charge transfer, and (3) proton transfer. Each of these channels exhibits a strong dependence on ion vibrational excitation and collision energy. Product branching ratios and relative cross sections are reported and compared with previous results. Briefly, both deuterium abstraction and charge transfer are enhanced by vibrational excitation, whereas proton transfer is suppressed. As the collision energy increases, the branching fraction for charge transfer increases sharply, that for proton transfer decreases, and that for deuterium abstraction remains nearly unchanged. These results point to a short-lived collision complex in which vibration and translation play inequivalent roles.