Towards a spectroscopy of doubly charged ions

In the light of a recent demonstration that a two-electron symmetry rule governs final-state selection in the double photoionization of argon, we re-examine current interpretations of molecular doubly charged ion spectra. New double-ionization data from photoionization and charge-exchange experiments on 02 and C2H2 augment the number of centrosymmetric molecules on which the application of the selection rule can be tested. Results do not contradict a rule that photoionization of closed-shell molecules populates chiefly triplet gerade or singlet ungerade states of doubly charged ions.