The importance of kinetically excited ions in the synthesis of interstellar molecules

It is known that several ion–molecule reactions which are potentially important in the synthesis of interstellar molecules are only slightly endoergic or are inhibited by small activation energy barriers. Therefore such reactions will not proceed at significant rates for thermalized reactants at the very low temperatures of cold interstellar clouds. However, it is to be expected that when the ion–molecule centre-of-mass energy, KE_cm, exceeds that value appropriate to the thermal energy, then these reactions could proceed at appreciable rates. It has previously been shown that energetic ions – and hence elevated KE_cm – can be produced in MHD shocks. We draw attention to the fact that energetic ions can also be produced in exoergic ion–molecule reactions and present new laboratory data, obtained with our selected ion flow drift tube, which demonstrate that the rates of the H-atom abstraction reactions of $$\text{C}^+,\text{N}^+,\text{C}_2\text{H}^+_2,\text{C}_2\text{H}^+_3,\text{C}_3\text{H}^+$$ and $$\text{C}_3\text{H}^+_2$$ with H₂, which are very slow at low temperatures, do become appreciable as KE_cm is increased. Using some of these data and also invoking several other potentially important reactions, we investigate the role of energetic ions in the synthesis of NH₃ and H₂S in cold clouds. We conclude that additional laboratory data are required, especially those relating to the reactions of ions with N and S atoms, to supplement ion–chemical models of NH₃ and H₂S synthesis. However, our new laboratory data do support the preveious proposal that the C⁺–H₂ reaction is an efficient source of CH⁺ in shocked regions of the interstellar gas.