Absorbing-Sphere Model for Calculating Ion—Ion Recombination Total Cross Sections

An ``absorbing-sphere'' model based on the Landau—Zener method is set up for calculating the upper limit thermal energy (300°K) reaction rate and the energy dependence of the total cross sections for the ion—ion recombination reaction, A++B−→ A*+B+Δ E, in which A or B may be atomic or molecular species. The crucial parameter needed for the calculation is the electron detachment energy for the outer electron on B−. For a given system and energy, we find that, as the electron detachment energy is decreased because of the existence of metastable electronic states or excited vibrational states, the cross sections increase. The calculated cross sections and rates are compared with afterglow results and with those obtained from ``merged-beam'' experimental data. We find good agreement with the existing results for atomic negative ions. There are differences for molecular negative ions, and the possible cause of these differences is discussed.