A guided-ion beam study of the reactions of N+4 with H2, HD, and D2: An evaluation of pseudo-Arrhenius analyses of ion–molecule reaction systems.

Reactions of N+4 ions with H2, HD, and D2 are studied from thermal to 5 eV kinetic energy under single-collision conditions in a guided-ion beam mass spectrometer. Reactant ions are formed in a flow tube source to ensure thermalization. Despite being exothermic by 1.5 eV, formation of N2H+ (N2D+) is observed to proceed with an activation barrier that we measure to be 0.09±0.03 eV at 0 K, independent of the hydrogen isotopomer used. Possible reaction mechanisms are discussed, and the present results are compared to previous flow and drift tube measurements. Effects of collisional reheating on the derivation of thermochemistry in drift-tube experiments are discussed. We also discuss the difficulty of deriving reliable thermochemistry from pseudo-Arrhenius plots when the mathematical form of the cross section excitation function is unknown. [ABSTRACT FROM AUTHOR]