Electronic Structure of the Helium Molecular Anion ${\mathrm{He}}_2^{-}$

The recently discovered ${\mathrm{He}}_2^{-}$ ion is examined by means of configuration-interaction wave functions constructed from an extended Slater-type-orbitals basis. We find that the ${}_{}{}^4{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$ state of ${\mathrm{He}}_2^{-}$ is bound relative to the $a{}_{}{}^2{}_{}{}^{}\Sigma _{\mathrm{u}}^{+}{}_{}{}^{}$ state of ${\mathrm{He}}_2$. The calculated electron affinity is 0.233 eV compared to 0.077 eV for the ${\mathrm{He}}^{-}\left[{}_{}{}^4{}_{}{}^{}P_{}^{\circ }{}_{}{}^{}\right]$ ion. This anion decays through the long-lived $\frac{5}{2}\mathrm{g}$ component with $\tau \sim 350$ μsec and the much shorter-lived $\frac{3}{2}\mathrm{g}$, ½g components with $\tau \sim 10$ μsec.