ROTATIONAL TEMPERATURE-DEPENDENCE OF THE REACTIONS OF N+ AND C+ WITH H2, HD, AND D2

The reactions of N+ and C+ with H-2, HD, and D2 have 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 fo r these slightly endothermic reactions depend significantly upon the t emperature of the hydrogen reagent below almost-equal-to 0.3 eV The ef fect increases with increasing endothermicity for these reactions, wit h the maximum effect being a factor of 5 change in cross section for f ormation 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 co nsistent with rotational energy driving the reaction. The moderately e ndothermic reactions of C+ with H-2 and D2 have also been studied as a function of translational energy and hydrogen temperature. Cooling th e hydrogen results in a sharper threshold for reaction because of redu ced Doppler broadening and reduced hydrogen rotational energy. Results for these systems can be modeled by using phase space theory after ac counting for Doppler broadening and internal energy effects.