Possible Formation and Destruction of the OD+ Ions in the Interstellar Medium

Capture Theory Models: An overview of their development, experimental verification, and applications to ion-molecule reactions

Since Arrhenius first proposed an equation to account for the behaviour of thermally activated reactions in 1889, significant progress has been made in our understanding of chemical reactivity. A number of capture theory models have been developed over the past several decades to predict the rate coefficients for reactions between ions and molecules-ranging from the Langevin equation (for reactions between ions and non-polar molecules) to more recent fully quantum theories (for reactions at ultra-cold temperatures). A number of different capture theory methods are discussed, with the key assumptions underpinning each approach clearly set out. The strengths and limitations of these capture theory methods are examined through detailed comparisons between low-temperature experimental measurements and capture theory predictions. Guidance is provided on the selection of an appropriate capture theory method for a given class of ion-molecule reaction and set of experimental conditions-identifying when a capture-based model is likely to provide an accurate prediction. Finally, the impact of capture theories on fields such as astrochemical modelling is noted, with some potential future directions of capture-based approaches outlined.

The reaction of O+(4S) ions with H2, HD, and D2 at low temperatures: Experimental study of the isotope effect

The reactions of the O⁺ ions in the ⁴S electronic ground state with D₂ and HD were studied in a cryogenic 22-pole radio-frequency ion trap in the temperature range of 15 K-300 K. The obtained reaction rate coefficients for both reactions are, considering the experimental errors, nearly independent of temperature and close to the values of the corresponding Langevin collisional reaction rate coefficients. The obtained branching ratios for the production of OH⁺ and OD⁺ in the reaction of O⁺(⁴S) with HD do not change significantly with temperature and are consistent with the results obtained at higher collisional energies by other groups. Particular attention was given to ensure that the O⁺ ions in the trap are in the ground electronic state.