Electron detachment from O$_{2}^{-}$ ions in oxygen and air in a strong electric field

Electron detachment from O$_{2}^{-}$ ions have been theoretically studied in oxygen and O$_{2}$-N$_{2}$ mixtures when the ions are heated in a strong external electric field. Properties of the ions were studied by a Monte Carlo simulation technique. Collisional cross sections for ion-molecule scattering was calculated on the basis of the statistical approach for the vibrational transfer and relaxation in collisions between O$_{2}^{-}$ ions and O$_{2}$ molecules. To validate the statistical approach used, we calculated ion mobility and diffusion coefficients under conditions under which experiments are available and obtained good agreement with measurements in pure oxygen. The detachment rate was determined under the assumption that electron detachment proceeds via the formation of vibrationally excited temporary O$_{2}^{-}$ ions. The obtained detachment rate constants turned out to agree well with available measurements in oxygen. This method was extended to calculate detachment rates in air and other O$_{2}$:N$_{2}$ mixtures. It was shown that, for a given value of the reduced electric field, the detachment rate coefficient increases with decreasing mole fraction of oxygen in mixtures. In particular, the detachment rate in air is much higher than that in oxygen. The reason is that the effect of resonant charge transfer in collisions between O$_{2}^{-}$ and O$_{2}$ is less profound in the mixtures with lower fraction of oxygen; as a result, the average ion energy is higher.