Investigation of the roles of vibrational excitation and collision energy in the ion-molecule reaction NH 3+(v2) + ND 3

Abstract

The influence of vibrational excitation and collision energy on the reaction NH<SUB>3</SUB><SUP>+</SUP>((nu) <SUB>2</SUB>) + ND<SUB>3</SUB> has been investigated using a quadrupole-octopole- quadrupole mass spectrometer. The NH<SUB>3</SUB><SUP>+</SUP> reagent ions are prepared state- selectively with 0 - 7 quanta in the (nu) <SUB>2</SUB> umbrella bending mode by (2 + 1) resonance enhanced multiphoton ionization. The mass-filtered reagent ion beam interacts with a thermal distribution of neutral ND<SUB>3</SUB> molecules at controlled center-of-mass collision energies (0.5 - 10.0 eV) within the octopole ion guide, enabling product ions to be collected independent of scattering dynamics. The reaction of NH<SUB>3</SUB><SUP>+</SUP> with ND<SUB>3</SUB> has three major product channels: (1) deuterium abstraction, (2) charge transfer, and (3) proton transfer. The product branching ratios and relative cross sections for each of these channels exhibit strong dependences on ion vibrational excitation and collision energy. Briefly, both deuterium abstraction and charge transfer are enhanced by vibrational excitation, whereas proton transfer is suppressed. As the collision energy is increased, the branching fraction for charge transfer increases sharply while proton transfer decreases. The branching ratio for deuterium abstraction does not exhibit a significant dependence on collision energy. The influence of ion vibrational excitation is discussed in terms of its relationship to the reaction coordinates for the three product channels. The behavior of this reaction points to a short-lived collision complex in which vibration and translation play inequivalent roles.