Concerning a long-lived $\left({}_{}{}^2{}_{}{}^{}P_{}^e{}_{}{}^{}\right){\mathrm{He}}^{-}$ state: Lifetime and energy-level measurements of ${\mathrm{He}}^{-}$

The question concerning the existence of a long-lived doublet $\left({}_{}{}^2{}_{}{}^{}P_{}^e{}_{}{}^{}\right){\mathrm{He}}^{-}$ state has been addressed by measuring the lifetime and energies of autodetaching electrons from 20—120-keV ${\mathrm{He}}^{-}$-ion beams formed by charge exchange in Ca vapor—a vapor conducive with $\left({}_{}{}^2{}_{}{}^{}P_{}^e{}_{}{}^{}\right){\mathrm{He}}^{-}$ formation. The measured lifetime and electron energy levels agree closely with accepted values for components of the $\left({}_{}{}^4{}_{}{}^{}P_J^o{}_{}{}^{}\right){\mathrm{He}}^{-}$ states. No other structures were detected in the electron energy spectrum encompassing the anticipated energy level of the autodetached electron from the $\left({}_{}{}^2{}_{}{}^{}P_{}^e{}_{}{}^{}\right){\mathrm{He}}^{-}$ state.