Threshold Detachment of Negative Ions by Electron Impact

The description of threshold fragmentation under long range repulsive forces is presented. The dominant energy dependence near threshold is isolated by decomposing the cross section into a product of a background part and a barrier penetration probability resulting from the repulsive Coulomb interaction. This tunneling probability contains the dominant energy variation and it can be calculated analytically based on the same principles as Wannier's description [Phys. Rev. 90, 817 (1953)] for threshold ionization under attractive forces. Good agreement is found with the available experimental cross sections on detachment by electron impact from ${\mathrm{D}}^{-}$, ${\mathrm{O}}^{-}$, and ${\mathrm{B}}^{-}$.