Formation of Diatomic Molecules and Radicals in the Gas Phase

In low to medium density clouds (10 m 103 particles cm- 3) radiative association is one important mechanism for producing the diatomic radicals CH and CH+ as has been shown by Solomon and Klemperer (1972). These primary reactions are followed by exchange reactions which produce CN, CO, 

C2 but not H2, NO, N2 and 02. Thus Solomon and Klemperer were able to quantitatively account for Herbig s (1968) optical observations made in the direction of ij-Ophiuchi. They made the assumptions that i) the UV radiation field is intense enough that the major constituents of the gas are atoms or ions  

Two types of gas phase reactions are considered for the formation of diatomic molecules i) radiative association, ii) chemical exchange reactions. The rate constants6 are either taken from laboratory experiments or estimated by comparison with similar reactions and if possible they are corrected for low temperature conditions. Since rate constants are often not well known and subject to major revision as new experimental data become available they are a source of considerable uncertainty. For radiative association the reaction rate is equal to the product of the rate of collisions and the probability that a photon is emitted during the collision process (Bates, 1951). The general reaction scheme is of the form 

For a reaction A + B -+C + D, the rate constant k is defined by the differential equation 

for example, the formation of H2 molecules is computed, all possible binary reactions between different molecules, atoms, ions, photons, etc. have to be considered. This yields according to the above-mentioned definitions, a set of steady state equations which involve the rate constants and the densities of the various reactants. The latter quantities are then computed under the assumption of known rate constants. 

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