Doubly Charged Negative Ions of B₂ and C₂

The formation of short-lived doubly charged negative diatomic molecular ions has been studied at a heavy-ion storage ring by bombarding singly charged negative ions of B₂ ^- and C₂ ^- by free electrons of well-defined energy. Structures are observed in the cross sections for detachment and fragmentation which are argued to be due to short-lived dianion resonances. The resonance energy of B₂ ^- ( ∼5 eV) is close to a calculated dianion ground-state energy and to the energy of some low-lying excited states of B₂ ^2-. The C₂ ^- resonance energy ( ∼10 eV) is close to predicted excited states of C₂ ^2- and significantly above the calculated dianion ground-state energy. From the width of the resonances a lifetime of about 10^-16 s is obtained for these fragile gas-phase dianions.