Statistical Theory of Four-Body Bimolecular Resonant Ion–Molecule Reactions

A four‐body phase space theory of ion–molecule reactions of the type A₂⁺ + A₂ ⇆ (A₄⁺) → A₃⁺ + A is presented. A modified definition of strong coupling is introduced by including the asymptotic resonance or chemical bonding potential in the reactant channel. It is shown that: (a) this leads to a larger cross section for the backward reaction than would be predicted using only the ion–induced dipole (polarization) potential, and (b) leads to cross sections for both the forward and backward reactions using hydrogen which are in better agreement with experiment than previously predicted. A computer program has also been developed following Nikitin's presentation of the four‐body phase‐space theory. Cross sections are presented as functions of the barycentric kinetic energy $E$ in the range $\text{0.01\hspace{0.17em}≤\hspace{0.17em}E\hspace{0.17em}≤\hspace{0.17em}10}\mathrm{eV}$ , and comparison is made between the statistical theories using the resonance potential and the polarization potential. Calculations with vibrationally excited reactants were also performed. Inclusion of the resonance potential shows a larger dependence of the cross section in the forward direction on the reactant vibrational energy than is predicted using the induced‐dipole potential only. These results also tend to give better agreement with existing experiments and yield predictions that may be tested by future experimentation.