The formation of interstellar molecules: the rôle of radiative association

Abstract

A brief overview is presented of the ionic reactions that occur in interstellar gas clouds which lead to the production of the complex molecules detected in those regions. It is shown that the interstellar clouds are weakly ionized plasmas and that the chemistry is largely plasma chemistry involving sequences of ion/molecule reactions producing polyatomic ions which they undergo dissociative recombination with electron resulting in the observed neutral molecules. Whilst many normal bimolecular reactions are involved, another class of reactions — radiative association reactions — is very important in the overall chemistry. These are bimolecular reactions of ions with molecules in which the excitation energy of the associated ion is released in the form of radiation thus stabilising the ion against decomposition back to the reactants. These reactions uniquely occur in the low density, low temperature interstellar clouds. They are very involved in the synthesis of polyatomic molecules in these regions of space and so they are given special attention in this Review. That ion/molecule radiative association was likely to be important in interstellar clouds was deduced from laboratory (SIFT) studies of the analogous process of termolecular association, but now a significant body of data is becoming available from low pressure ion trap experiments on the kinetics of radiative association reactions. Some of these data are referred to and their importance to interstellar chemistry is considered. Attention is drawn to some other potentially important radiative association reactions, based on the SIFT studies of termolecular association reactions, which could profitably be studied at low pressures.