A brief review of interstellar ion chemistry

Abstract

An overview is presented of the basic gas-phase ion chemistry which occurs in interstellar clouds and which results in the production of many of the molecular species observed in these regions. This chemistry is considered to involve many parallel and sequential ion-neutral reactions, which produce many simple and complex positive-ions species. These species can then undergo dissociative recombination with electrons to generate neutral molecules. To substantiate these ideas, quantitative ion-chemical modelling is necessary which requires a great deal of kinetic data on the aforementioned reaction processes. These kinetic data, some obtained at low temperatures, have been provided by the exploitation of several laboratory techniques. Examples of key reactions and reaction processes and of the magnitudes of their rate coefficients are presented, and the ways in which the reactions combine to describe the overall chemistry of interstellar clouds is discussed. This it is shown how far the qualitative and quantitative aspects of interstellar ion chemistry are understood. Finally, attention is drawn to some outstanding problems in this exciting area of research.