Electron-detachment cross sections for O− + N2 near the free-collision-model velocity threshold

We present measurements of the total projectile-electron-loss cross sections in O^- + N₂ collisions in the energy range from 2.5 to 8.5 keV. Two different techniques, beam attenuation and the growth rate, are employed. The cross sections measured using the growth-rate method show a threshold behavior. We analyze the cross-section velocity dependence in the framework of a collision between a quasifree electron, loosely bound to the projectile, and the molecular target. Within the free-collision model, we deduce and test a simple analytical expression for the expected velocity threshold taking into account the angular distribution of electron velocities within the anion.