A Feshbach resonance associated with the ${1s3s4s}^{4}S$ state of He${}^{-}$ has been observed in the He${(1s2s}^{3}S)$ + $e^{-}\left(\epsilon s\right)$ 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 $E_{\mathrm{r}}=2.959\mathrm{}255\left(7\right)\mathrm{}$ eV and a width $\Gamma =0.19\mathrm{}\left(3\right)\mathrm{}$ meV, in agreement with a recent calculation.

• Received 23 January 1997

DOI:https://doi.org/10.1103/PhysRevA.56.2788