Collision-Energy Dependence of the Ion-Molecule Charge Exchange Reaction Ar+ + NO.

High-lying quantum states of molecule are apt to be populated by translational-to-internal energy transfer in the collisions with atom, which usually becomes more significant with the increase of collision energy. However, in the charge exchange reaction Ar+ + NO → Ar + NO+, the products NO+ prefer a dominant population at the lowest triplet state a3Σ+, in particular, in the higher energy collisions; the higher states b3Π and w3Δ of NO+ are accessed only at the lower collision energies. Such a striking collision-energy dependence is attributed to two distinctly different processes: the former is controlled with an energetically resonant charge-transfer mechanism; while the latter experiences an intermediate complex (Ar-NO)+ which permits a more efficient translational-to-internal energy transfer.