Measurement of the $1s2p2p^{\prime }$ $4P^e$ Resonance in ${\mathrm{He}}^{-}$ Photodetachment

A large resonance in the photodetachment spectrum of metastable ${\mathrm{He}}^{-}$, which was recently predicted theoretically, has been observed experimentally. Located at 1.2344 eV with a width of 7.0 meV, the detachment cross section reaches a maximum of ∼70 ${\mathrm{\AA }}^2$. Its size results from the large oscillator strength associated with the first allowed transition in ${\mathrm{He}}^{-}$, $\mathrm{}\left(1s2s2p\right)\mathrm{}{}_{}{}^4{}_{}{}^{}P_{}^{\circ }{}_{}{}^{}\to \left(1s2p2\mathrm{}{p}^{\prime }\right){}_{}{}^4{}_{}{}^{}P_{}^e{}_{}{}^{}$.