The Formation of Diatomic Molecules in Interstellar Clouds

The rates of a number of homogeneous gas-phase chemical reactions likely to occur in clouds composed primarily of atomic hydrogen, with 10 < flH < 1000, are estimated. It is shown that the primary mechanism for producing diatomic molecules from atoms is the radiative association of C + H and C + + H yielding CR and CR +. Dielectronic recombination of CR + is important in producing CR, destroying CR + and increasing the ratio CR/CR + relative to C/C +. Other important processes, including exothermic chemical exchange and charge-exchange reactions, lead to the production of C2, CN, and CO, but not NO, N2, or 02. The steady-state abundance of electrons, atoms, ions, and diatomic molecules composed of H, C, N, and 0 is determined as a function of the H density, the kinetic temperature, and the ultraviolet optical depth, which governs the photodissociation and ionization rates. A comparison of the theory with optical observations of the interstellar medium, particularly the well-studied features in the Oph spectrum, shows good qualitative and quantitative agreement. A molecular composition dominated by carbon monoxide and with a variety of diatomic species can be established in the interstellar medium by pure gasphase reactions.