Vibrational and Translational Energy Effects in the Reaction of Ammonia Ions with Water Molecules

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The reaction of vibrationally state-selected ammonia ions with deuterated water is investigated with a quadrupole−octopole−quadrupole apparatus. Relative integral reaction cross sections for all ionic products are measured for collisional energies from 0.5 to 10.0 eV (center of mass) and ammonia ion vibrational states 1020-10 and 1122. The predominant charged products have masses of 18 and 21 amu and are shown to be NH2D+ (formal isotope exchange) and D2OH+ (proton transfer), respectively. Small amounts of product are also observed at masses 19, 20, and 22 amu. The cross section for proton transfer decreases with increasing collisional energy, but it increases with increasing internal energy at all collisional energies studied. The cross section for formal isotope exchange increases with increasing collisional energy and increases with increasing internal energy only at low collisional energies. Comparing the reactivity of NH3+(1025) to that of NH3+(1122) (internal energies of 0.60 and 0.63 eV, respectively) shows whether the system is sensitive to the specific vibrational motion, or only to the total amount of internal energy. The present system is shown not to be vibrational-mode selectivethe two internal preparations having comparable reactive cross sections at all collisional energies studied. The projection of the product velocity onto the ion-beam axis is obtained by transforming the experimentally measured product time-of-flight profiles. These velocity profiles contain information regarding energy released into product recoil. The isotope exchange products scatter tightly (0.6 eV, hwhm) about the center of mass and display forward−backward symmetry. The proton-transfer products yield a bimodal velocity distribution with one peak in the backward direction and another on the center of mass. The data observed for the isotope-exchange product indicate a reaction mechanism in which the first step is collision-induced dissociation of the ammonia ion (or water molecule), resulting in a hydrogen (deuterium) atom and an [NH2D2O]+ ([NH3OD]+) complex. This complex may decay to form the products NH2D+ and OD (OH). The majority of the D2OH+ product is formed via direct proton transfer.