Photodetachment study of the $1s3s4s ^4S$ resonance in $He^-$

A Feshbach resonance associated with the 1s3s4s ^{4}S state of He^{-} has been observed in the $He (1s2s ^{3}S) + e^- (\epsilon s)$ partial photodetachment cross section. The residual He(1s2s ^{3}S) atoms were resonantly ionized and the resulting He^+ ions were detected in the presence of a small background. A collinear laser-ion beam apparatus was used to attain both high resolution and sensitivity. We measured a resonance energy $E_r = 2.959 255(7)$ eV and a width $\Gamma = 0.19(3)$ meV, in excellent agreement with a recent calculation