We present a detailed theoretical study of the rotational excitation of CH⁺ due to reactive and non-reactive collisions involving C⁺(²P), H₂, CH⁺, H and free electrons. Specifically, the formation of CH⁺ proceeds through the reaction between C⁺(²P) and H|$_2(\nu _{\rm H_2} = 1, 2)$|⁠, while the collisional (de)excitation and destruction of CH⁺ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10–3000 K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH⁺ with H atoms at kinetic temperatures below 50 K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our non-LTE calculations confirm that the formation pumping due to vibrationally excited H₂ has a substantial effect on the excitation of CH⁺ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH⁺ towards the Orion Bar and the planetary nebula NGC 7027. Our results further suggest that the population of |$\nu _{\rm H_2} = 2$| might be significant in the photon-dominated region of NGC 7027.

A new series of computations has been performed to obtain cross-sections and rate coefficients for state-to-state rotational transitions in the H$$_{2}^{+}$$ ion, induced by collisions with very low-energy electrons. Following our recent work on the HD⁺ ion (Motapon et al. 2014), and using the same molecular structure data sets, excitations $$N_{i}^{+} \rightarrow$$ $$N_{i}^{+}+2$$ for $$N_{i}^{+}=0$$ to 10, and de-excitations $$N_{i}^{+}$$ → $$N_{i}^{+}-2$$, for $$N_{i}^{+}=2$$ to 10, in the energy range 0.01 meV–0.3 eV, have been explored. The calculated cross-sections have been convolved in order to obtain Maxwell rate coefficients for electronic temperatures up to a few hundred of Kelvin. Moreover, Maxwell rate coefficients for dissociative recombination have been calculated for the same initial rotational levels.

Cross sections and thermal rate coefficients are computed for electron-impact dissociative recombination and vibrational excitation/de-excitation of the N${}_2^{+}$ molecular ion in its lowest six vibrational levels, for collision energies/temperatures up to 2.3 eV/5000 K.

The collision-energy resolved rate coefficient for dissociative recombination of HD+ ions in the vibrational ground state is measured using the photocathode electron target at the heavy-ion storage ring TSR. Rydberg resonances associated with rovibrational excitation of the HD+ core are scanned as a function of the electron collision energy with an instrumental broadening below 1 meV in the low-energy limit. The measurement is compared to calculations using multichannel quantum defect theory, accounting for rotational structure and interactions and considering the six lowest rotational energy levels as initial ionic states. Using thermal-equilibrium-level populations at 300 K to approximate the experimental conditions, close correspondence between calculated and measured structures is found up to the first vibrational excitation threshold of the cations near 0.24~eV. Detailed assignments, including naturally broadened and overlapping Rydberg resonances, are performed for all structures up to 0.024 eV. Resonances from purely rotational excitation of the ion core are found to have similar strengths as those involving vibrational excitation. A dominant low-energy resonance is assigned to contributions from excited rotational states only. The results indicate strong modifications in the energy dependence of the dissociative recombination rate coefficient through the rotational excitation of the parent ions, and underline the need for studies with rotationally cold species to obtain results reflecting low-temperature ionized media.

A quantitative theoretical study of the dissociative recombination of SH⁺ with electrons has been carried out. Multireference, configuration interaction calculations were used to determine accurate potential energy curves for SH⁺ and SH. The block diagonalization method was used to disentangle strongly interacting SH valence and Rydberg states and to construct a diabatic Hamiltonian whose diagonal matrix elements provide the diabatic potential energy curves. The off-diagonal elements are related to the electronic valence-Rydberg couplings. Cross sections and rate coefficients for the dissociative recombination reaction were calculated with a stepwise version of the multichannel quantum defect theory, using the molecular data provided by the block diagonalization method. The calculated rates are compared with the most recent measurements performed on the ion Test Storage Ring (TSR) in Heidelberg, Germany.