Threshold Behavior near an Electronic Shape Resonance: Analysis of theHe(P3)Threshold inHe−Photodetachment and Determination of theHe(2S3)Electron Affinity

${\mathrm{He}}^{-}$ photodetachment cross sections near the $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}P\right)$ threshold have been measured and analyzed. The data were found to deviate rapidly from the Wigner threshold law because of the strong influence of the nearby ${}_{}{}^4{}_{}{}^{}P_{}^e{}_{}{}^{}$ shape resonance. A modified formula has been derived, which accounts for this effect. It successfully fits the data not only in the threshold region but over the resonance itself. The threshold data yield an electron affinity for $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S\right)$ of 77.5±0.8 meV, in agreement with the accurate calculations of Bunge and Bunge.