Observation of the Molecular Helium Negative Ion ${\mathrm{He}}_2^{-}$

Long-lived metastable ${\mathrm{He}}_2^{-}$ negative ions have been observed for the first time. They were produced from ${\mathrm{He}}_2^{+}$ by two-step electron capture in a Cs vapor target. The apparent autodetachment decay rates have been measured at several delay times after formation, and indicate the presence of more than one state, with the lifetime of the major component exceeding 100 μs. The most likely configurations are ${{}_{}{}^4{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}$ and/or ${}_{}{}^4{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$ states bound below ${\mathrm{He}}_2\left({{}_{}{}^3{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}\right)$.