Collisions between Electrons and the Cl$_2^-$ Ion

Collisions between electrons and molecular negative ions result in the following fragmentation processes: detachment, dissociation and detachment plus dissociation. In some cases attachment also occurs, resulting in resonance structure associated with the transient formation of a doubly charged negative ion. We report on a measurement of the cross sections for detachment, dissociation and detachment plus dissociation for the e$^-$ - Cl$_2^-$ collision system over a collision energy range 0-200 eV. The merged electron-ion beam experiment was performed at the heavy ion magnetic storage ring, CRYRING, situated at the MSL in Stockholm [1]. Over the energy range studied, the dominant breakup mechanism is dissociation. A prominent structure was observed just above threshold in the Cl$^-$ + Cl + e$^-$ dissociation channel. It is proposed that the structure is a resonance associated with the production and rapid decay of an excited state of the doubly charged Cl$_2$$^{2-}$ ion.\newline [1] G.F.Collins et al, Phys. Rev. A 72, 042708 (2005).