Computer Calculations of Ion—Molecule Reactions

Abstract

A computer program has been developed which follows Light's phase-space theory of three-body ion—molecule reactions. The program predicts cross sections for 10 possible reaction branches as functions of the relative kinetic energy of the colliding particles in the range from thermal energies up to 20 eV. It also yields partial cross sections for the populations of the vibrational levels of the diatomic molecules in the product channel. Included in the results are reactions for the following ion—molecule pairs: [O++N2], [N++O2], [O+N2+], [He++N2], and [18O++O2]. The effects of varying the level of excitation, both vibrationally and electronically, of the reactants are presented. Increasing the excitation level always results in greater cross sections for reactions which are otherwise endothermic and slightly smaller cross sections for reactions which are otherwise exothermic. All endothermic cross sections reach maximum values as functions of the barycentric kinetic energy of the reactants. All exothermic cross sections decrease with increasing barycentric kinetic energy at rates faster than predicted by the Gioumousis and Stevenson theory. This is due to the greater competition between the products for the available phase space which occurs at elevated energies. Good agreement is found between the theory and experiment in the high-energy region above 5 eV and in some cases throughout the whole energy range. Disagreement is most pronounced for low energies. It is felt that this indicates the presence of a small activation energy or, in the case where the products differ from the reactants by a charge transfer A++BC→A+BC+, the neglect of including resonance forces in the calculation.