Translational energy dependence of Ar++XY→ArX++Y (XY=H2,D2,HD) from thermal to 30 eV c.m.

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Abstract
Cross sections for the reactions of Ar+ with H2, D2, and HD to form ArH+ and ArD+ are measured using a new guided ion beam tandem mass spectrometer which affords an experimental energy range from 0.05 to 500 eV laboratory. The apparatus and experimental techniques are described in detail. Cross sections for H2 and D2 are found to be nearly identical over this entire energy range when compared at the same barycentric energy. The total HD cross section is the same as H2 and D2 at low energies, but differs significantly above 4 eV c.m., where product dissociation becomes important. The intramolecular isotope effect for reaction with HD exhibits a reversal at low energy, favoring the deuteride product below ∼0.14 eV c.m., and surprising nonmonotonic behavior at energies above 5 eV c.m. In all these systems, a new feature at higher energies is observed. This is interpreted as the onset of a product channel having an energy barrier of 8±1 eV. The room temperature rate constant derived from the data for the reaction with H2 is (9.5±2)×10−10 cm3 s−1, in good agreement with the literature. Analysis of the data indicates an activation energy of between 2 and 15 meV at room temperature. The results are compared to previous experimental determinations and to theoretical reaction models.