Search for an explanation for neutralization rates of atomic ion-ion reactions.*

We have measured well over a hundred rate coefficients $k$ for cation-anion mutual neutralization reactions at thermal energies. For molecular ions, the $k$ at 300 K tend not to vary more than a factor of two or three, presumably because a great many neutral states cross the incoming Coulombic potential energy curve. Atomic-atomic systems, for which there are few favorable curve crossings between the neutral and Coulombic curves, show variation of at least a factor of 60 in the measured $k$ values at 300 K. For reactions involving the noble-gas cations, we assume that the final state is the lowest excited state of the neutral, plus the ground state of the neutralized anion, because otherwise the crossing distance $R$ is so small that the curve-crossing probability is nil. We plotted measured $k$ values (in cm$^{\mathrm{3}}$/s) vs the distance $R$ (in bohr) at which the neutral and Coulombic curves cross, the found that the data are fairly well fit by a power law for $k$, 10$^{\mathrm{-4}}R^{\mathrm{-2.8}}$. The question is, is there a physical explanation for the observed dependence on $R$? We will discuss the data and the expectations of Landau-Zener theory.