The reaction of O+(S-4) and N-2(X (1)Sigma(+)(g)) revisited: Recoil velocity analysis of the NO+ product

The guided-ion beam method has been used to measure time-of-flight (TOF) sp ectra of the NO+ produced in the reaction of O+(S-4) and N-2(X (1)Sigma(g)( +)). Particular care was taken to minimize the participation of the O+(D-2, P-2) metastables in the reactions, the success of which is validated by mea suring the absolute cross section for the reaction as a function of collisi on energy in the near-thermal to 1.5 eV range. In this range, the cross sec tion exhibits a region of low reactivity at near-thermal energies, which ca n be obscured by reactions of metastables and other effects, and a region o f sharply increasing reactivity above a threshold of collision energy appro ximate to 0.3 eV. The velocity-transformed TOF spectra, at collision energi es of 0.51 eV, 0.80 eV, 3.2 eV, and 6.3 eV, indicate two reaction mechanism s: in the first, product ions are scattered backward with inefficient trans lational to internal energy conversion; in the second, product ions arise f rom intermediate collision complexes with efficient translational energy co nversion. The first mechanism is associated with the passage through the sp in-forbidden doublet ground state N2O+ intermediate, while the second invol ves passage through an endothermic quartet intermediate. (C) 1999 American Institute of Physics. [S0021-9606(99)02233-3].