Kinetics of ion-ion mutual neutralization

We have measured rate coefficients for 87 mutual neutralization reactions between thermal energy anions and cations, a number of them as a function of temperature. In addition, in two cases we have observed a transfer ionization channel in which there is enough energy for the anion reactant to be doubly ionized, yielding a cation product rather than neutralization. We will summarize these results and note correlations, namely: (1) binary neutralization rate coefficients are primarily a function of the chemical nature of the system for atom-atom ionic pairs (with a wide range of rate coefficients), but quickly become dominated by physical aspects (i.e., relative velocity) as the number of atoms in the system increases. (2) Rate coefficients for atom-atom ionic pairs are well fit at 300 K by k $=$ 3 x 10$^{-4}$ R$_{x}^{-3.15}$, where R$_{x}$ is the curve crossing radius given by R$_{x} \quad =$ 27.2/$\Delta $E, with $\Delta $E being the electron transfer energy released in the reaction. ($\Delta $E in eV, R$_{x}$ in Bohr, and k in cm$^{3}$/s.) (3) Rate coefficients for systems of more than 4 or 5 atoms are well described by k $=$ 2.7 x 10$^{-7}$ (T/300)$^{-0.9} \quad \mu ^{-0.5}$. (T in K, and the reduced mass $\mu $ in amu.) (4) Triatomic systems have rate coefficients smaller than given by the expression in (3).