Doubly Charged Negative Ions of $B_2$ and $C_2$

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 ${\mathrm{B}}_2^{-}$ and ${\mathrm{C}}_2^{-}$ 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 ${\mathrm{B}}_2^{-}$ ( $\sim 5\mathrm{eV}$) is close to a calculated dianion ground-state energy and to the energy of some low-lying excited states of ${\mathrm{B}}_2^{2-}$. The ${\mathrm{C}}_2^{-}$ resonance energy ( $\sim 10\mathrm{eV}$) is close to predicted excited states of ${\mathrm{C}}_2^{2-}$ and significantly above the calculated dianion ground-state energy. From the width of the resonances a lifetime of about ${10}^{-16\mathrm{}}\mathrm{s}$ is obtained for these fragile gas-phase dianions.