Crossed‐Molecular beam studies of the state‐to‐state reaction dynamics of charge transfer at low and intermediate energy

The microscopic, state‐to‐state reaction dynamics of charge transfer reactions are reviewed for the two system N   2 + (N 2 , N 2 )N   2 +and Ar + (N 2 , Ar)N   2 + . The crossed molecular beam method demonstrates that the symmetric, molecular case proceeds by two mechanisms at low collision energy (< 0.7 eV). One of these involves an orbiting complex in which the total available energy is redistributed, essentially statistically. The second mechanism is a direct mechanism which involves nearly rectilinear trajectories and corresponds to exactly resonant charge transfer at low energy (0.7 eV) and charge transfer accompanied by vibrational excitation at higher energy (> 10 eV). The orbiting mechanism is dominant at low energies and disappears entirely with increasing collision energy. The unsymmetric, nearly resonant charge transfer reaction is direct at all energies. At very low collision energy (0.6 eV) and moderate collision energy (> 1.5 eV) the dominant reaction channel is the generation of N 2 + (X 2 g, v = 1) with little rotational excitation. At 0.6 eV substantial angular scattering (including back‐scattering) is observed; above 1.5 eV the trajectories are essentially rectilinear. At intermediate energies (0.8 < E < 1.3 eV) a very different mechanism is observed. All the energetically accessible vibrational states of N   2 +are formed, each with a preferred scattering angle at a given energy. Related experiments and theoretical models which rationalize some, but not all, of these results are described. The most intriguing puzzle is the “energy window” in which quantum‐state specific, angular‐specific reactive scattering is observed.