The reactions of N+and C+with H2, HD, and D2have been studied as a function of translational energy and hydrogen temperature in a guided ion beam mass spectrometer. For the N+systems, the cross sections for these slightly endothermic reactions depend significantly upon the temperature of the hydrogen reagent below ape;0.3 eV. The effect increases with increasing endothermicity for these reactions, with the maximum effect being a factor of 5 change in cross section for formation of NH+in the reaction with HD at a kinetic energy of 0.02 eV. The branching ratio for formation of NH+and ND+in the reaction with HD at 0.02 eV is 1:3 at 305 K and 1:13 at 105 K. These effects are consistent with rotational energy driving the reaction. The moderately endothermic reactions of C+with H2and D2have also been studied as a function of translational energy and hydrogen temperature. Cooling the hydrogen results in a sharper threshold for reaction because of reduced Doppler broadening and reduced hydrogen rotational energy. Results for these systems can be modeled by using phase space theory after accounting for Doppler broadening and internal energy effects.
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