Theory of dissociative recombination of molecular ions with competing direct and indirect mechanisms applied to CH+

This study presents a theoretical approach to model the dissociative recombination (DR) of molecular ions in which direct and indirect mechanisms are competing with each other. This is often the case for the ions having low-energy electronic resonances, such as open-shell molecular ions. The approach combines the UK R-matrix method for fixed-nuclei electron-ion scattering, the vibronic frame transformation with outgoing-wave dissociative functions obtained using a complex absorbing potential, and molecular quantum-defect theory. The present first-principles approach is applied to the CH+ ion. The contribution of the Rydberg series converging to the two lowest excited electronic states of the ion, a(3) Pi and A(1)Pi, plays a significant role in the DR cross section. The obtained DR cross section is in good agreement with experimental measurements at energies between 0.3 and 3 eV, but is much higher for energies below 0.3 eV. The disagreement is probably due to the rotational structure neglected in the present theory. The nature of prominent resonances in the computed results is analyzed by considering DR probabilities for different partial waves of the incident electron. It was found that the d-type partial waves (d sigma and d pi) contribute considerably to the DR of CH+ in its ground vibronic state.