The onset of condensation: association reactions

Association reactions are important in a variety of astronomical regions for increasing molecular complexity and as a prelude to condensation. Theories are presented for the two mechanisms - three-body and radiative - by which association reactions occur in the gas phase. The theory for three-body association is in satisfactory agreement with a variety of laboratory studies. This agreement lends confidence to the idea that both theories can be utilized for association reactions of astrophysical importance. (Auth.)