Electron collisions with diatomic anions

Electron collisions with negative ions (${\mathrm{B}}_2^{\mathrm{-}},$ ${\mathrm{C}}_2^{\mathrm{-}},$ ${\mathrm{O}}_2^{\mathrm{-}},$ ${\mathrm{BN}}^{\mathrm{-}},$ and OH) have been performed in a merged- beams experiment at the heavy-ion storage ring ASTRID (Aarhus Storage Ring, Denmark). Absolute cross sections for detachment ${(e}^{-}{+AB}^{-}\to {\mathrm{AB}}^0{+2e}^{-})$ and detachment plus dissociation ${(e}^{-}{+AB}^{-}\to A^0{+B}^0{+2e}^{-})$ are reported for energies below 40 eV. For the homonuclear ions, $X_2^{-}$ $(X=\mathrm{B},\mathrm{}\mathrm{C},\mathrm{}\mathrm{O}),$ cross sections are also presented for dissociative reactions leading to an $X^{-}$ particle in the final state. The detachment process is dominant for all investigated systems. The detachment cross section is characterized by an effective threshold larger than the electron binding energy. The shapes of the detachment cross sections are similar for the studied anions pointing to a common nonresonant mechanism for detachment. Also, the cross section for detachment plus dissociation and pure dissociation show regularities pointing to a general nonresonant mechanism. Structures are observed in the detachment cross section of ${\mathrm{C}}_2^{\mathrm{-}}$ and ${\mathrm{BN}}^{\mathrm{-}}$ and in the dissociation cross section of ${\mathrm{B}}_2^{\mathrm{-}}$ and ${\mathrm{C}}_2^{\mathrm{-}}.$ The structures are attributed to short-lived dianions (resonances) formed during the collision process. The results are supported by ab initio calculations, and it is inferred that electronic excitations are important in the resonant reactions.