The detachment of electrons from negative ions

Detachment cross-sections for collisions of O - , Cl - , F - ions with rare-gas atoms are measured between 50 and 4000 eV incident-ion energy. The negative ions are produced (i) by thermal dissociation of molecules at a filament, followed by radiative attachment into the ground state; (ii) by electron bombardment of molecules, giving ions previously believed to be in an excited state of long lifetime. In every case the detachment cross-sections for the ions produced in the two ways are the same, so that we must conclude either (i) that the excited state has a short lifetime which is unlikely but not impossible, or (ii) that no excited state exists, and that the appearance potentials for ions believed to be in this state must be explained by another process, such as the formation of O neutrals followed by radiative attachment at the source filament.