Ion momentum imaging study of the ion-molecule reaction Ar + + O 2 → Ar + O 2 .

Charge exchange reactions between Ar ⁺ ( ² P) and O ₂ (X ³ Σ) are investigated in the collision energy range of 3.40-9.24 eV within the center-of-mass coordinate, by using the ion momentum imaging technique. The internal energy of the product O ₂ ⁺ is enhanced gradually with the increase of collision energy, and the forward-scattered O ₂ ⁺ ions are distributed in the broader range of scattering angle at higher collision energies. At the low collision energy of 3.40 eV, the resonant charge transfer, similar to a photon ionization process, leads to the Franck-Condon-like vibrational state population of O ₂ ⁺ at the a ⁴ Π _u state. At the higher collision energies, besides a ⁴ Π _u and the high-lying states that are visible in the photoionization process, the O ₂ ⁺ products could be populated at some electronically bound states in the non-Franck-Condon region. The present observations indicate again the strong collision-energy dependences of the charge exchange reactions, but distinctly different from our previous findings for Ar ⁺ + NO → Ar + NO ⁺ .