Translational energy spectrum of O₂⁻ions formed by 4 keV O₂⁺ions impacting on argon

The translational energy spectrum of O 2 - arising from single and double collision double charge transfer between 4 keV O 2 + ions and Ar atoms has been recorded. The incident O 2 + ions are found to be either in the ground x 2 Pi g state or in the metastable a 4 Pi u state. The single-collision double charge transfer is observed only from metastable O 2 + ions, ground state O 2 + leading to autodetaching levels of O 2 - . This result gives evidence on the validity of Franck-Condon principle for molecular transitions occurring in double charge transfer processes.