Total electron-detachment cross sections for collisions of negative halogen ions and rare gases for energies around threshold

The threshold behavior of the absolute total electron-detachment cross section for collisions of ${\mathrm{Cl}}^{-}$ and ${\mathrm{Br}}^{-}$ with rare gases has been determined in an ion-beam—gas-target apparatus. The relative collision energies investigated range from below the electron affinity of the halogen up to about 100 eV. For all of the reactants studied, the threshold for detachment is found to occur at about twice the electron affinity of the negative ion. The general features of measurements are in agreement with the ideas of a simple collision model. Upper limits to detachment-rate constants which are based upon the measurements are presented.