The reaction of O+(4S) and N2(X 1Σg+) revisited: Recoil velocity analysis of the NO+ product

The guided-ion beam method has been used to measure time-of-flight (TOF) spectra of the NO+ produced in the reaction of O+(4S) and N2(X 1Σg+). Particular care was taken to minimize the participation of the O+(2D,2P) metastables in the reactions, the success of which is validated by measuring the absolute cross section for the reaction as a function of collision energy in the near-thermal to 1.5 eV range. In this range, the cross section exhibits a region of low reactivity at near-thermal energies, which can be obscured by reactions of metastables and other effects, and a region of sharply increasing reactivity above a threshold of collision energy ≈0.3 eV. The velocity-transformed TOF spectra, at collision energies of 0.51 eV, 0.80 eV, 3.2 eV, and 6.3 eV, indicate two reaction mechanisms: in the first, product ions are scattered backward with inefficient translational to internal energy conversion; in the second, product ions arise from intermediate collision complexes with efficient translational energy conversion. The first mechanism is associated with the passage through the spin-forbidden doublet ground state N2O+ intermediate, while the second involves passage through an endothermic quartet intermediate.