2018MNRAS.476.2931H

R-matrix calculations combined with the adiabatic-nuclei-rotation and Coulomb-Born approximations are used to compute electron-impact rotational rate coefficients for two open-shell diatomic cations of astrophysical interest: the hydoxyl and sulphanyl ions, OH⁺ and SH⁺. Hyperfine resolved rate coefficients are deduced using the infinite-order-sudden approximation. The propensity rule ΔF = Δj = ΔN = ±1 is observed, as is expected for cations with a large dipole moment. A model for OH⁺ excitation in the Orion Bar photon-dominated region is presented which nicely reproduces Herschel observations for an electron fraction x_e = 10^–4 and an OH⁺ column density of 3 x 10¹³ cm^–2. Electron-impact electronic excitation cross-sections and rate coefficients for the ions are also presented.