Modeling radiative association kinetics

New generic modeling results are shown for the radiative association of ions with neutrals. The approach used was based on the standard-hydrocarbon model of typical properties of ions and neutrals. A canonical phase space theory method was applied to calculate the kinetics with more confidence than previous RRKM-based results, and a hopefully more realistic set of vibrational properties of the association complex was used. Modeling is reported for ion-neutral systems having between 30 and 150 degrees of freedom, binding energies between about 0.4 and 5 eV, and temperatures from 100 to 500 K. The range of validity of the low-efficiency approximation is explored, and it is found that it is sometimes valid at efficiencies as high as 1%, but is often only valid for efficiencies of 10(-4) or even lower. The atomic-ion effect is confirmed, whereby an association reaction involving an atomic-ion reactant is about two orders of magnitude faster than an otherwise similar reaction involving molecular reactants. An accurate prescription is given for predicting the shift in binding energy corresponding to the atomic-ion effect, specifying that a given atomic-ion association is equivalent to a corresponding molecular-ion association whose binding energy is 1.4 times larger. Graphical modeling results are given which should be useful for predicting radiative association rates, for predicting size, temperature and binding energy dependences, and for estimating binding energy values from observed association kinetics. (C) 1997 Elsevier Science B.V.