Electron-Detachment in Slow Collisions between Atoms and Negative Ions

The collision of a negative ion $A^{-}$ and neutral atom $B$ at a very low kinetic energy (a few eV or less) may lead to the release of an electron through the reaction $A^{-}+B\to \mathrm{}\left(\mathrm{AB}\right)+e$, known as "associative detachment." At higher energies, $A^{-}+B\to A+B+e$ also becomes possible. A theory of these processes is formulated by assuming that the electronic state is stable at large separations $R$ of $A^{-}$ and $B$, and changes adiabatically as $R$ decreases; at very small $R$, of the order of ${10}^{-8\mathrm{}}$ cm, the electronic state turns into an unstable compound state able to emit an electron. Expressions are derived for the total cross section for electron detachment, and for the cross sections for detachment leaving the nuclei in a single discrete final state. At thermal energies, the total cross section can become very large because of Langevin spiralling, arising from the long-range polarization between $A^{-}$ and $B$. For example, in the reaction ${\mathrm{H}}^{-}+\mathrm{H}\to {\mathrm{H}}_2+e$, the cross section is estimated to be of the order of ${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$ at a relative kinetic energy of $k\times 400°$K.