Uniform treatment of direct and indirect mechanisms in low-energy dissociative recombination of CF⁺ and CH⁺

The dissociative recombination (DR) of molecular ions is an important process in several molecular plasmas, such as for plasma-based technologies and tracking the evolution of interstellar clouds. Calculating DR cross sections, needed to determine DR rate coefficients to understand the evolutions of such molecular plasmas, often becomes difficult when the direct and indirect DR mechanisms are simultaneously important. This typically occurs in ions with low-energy electronic resonances, e.g., open-shell molecular ions. The present theoretical method treats the direct and indirect mechanisms uniformly in the case of a diatomic ion, with or without low-energy electronic resonances, while resolving electronic, vibrational, and rotational degrees of freedom, and can likely be scaled to study the DR of triatomic ions. Our method is based on R-matrix scattering calculations, performed at several internuclear distances, rovibrational frame transformation, and multichannel quantum-defect theory. The K-matrices obtained from such scattering calculations do not exhibit electronic Rydberg resonances due to the excited states of the ion, but the physics is contained implicitly in our scattering matrices so that we can more accurately describe the scattering of the electron at low collision energies. The method is easy to implement; it does not require explicit calculation of, e.g., Rydberg-Rydberg couplings or bound dissociative states of the neutral molecule. We apply this approach to the CH+ and CF+ ions and compare our results to experimental measurements from storage-ring experiments.