The huge ${\mathrm{He}}^{\mathrm{-}}$ ${}_{}{}^4{}_{}{}^{}$${\mathit{P}}^{\mathit{e}}$ shape resonance was reexamined in photodetachment using a titanium:sapphire laser tuned over the energy range 9740–10 300 ${\mathrm{cm}}^{\mathrm{-}1}$. This range covered well below the He (2 ${}_{}{}^3{}_{}{}^{}$P) threshold and included the entire resonance region. The uncertainties in the earlier data [Peterson et al., Phys. Rev. Lett. 55, 692 (1985)] were greatly reduced. The new (old) experimental value for the electron affinity of He (2 ${}_{}{}^3{}_{}{}^{}$S) is 77.67±0.12 meV (77.6±1.0 meV), in fair agreement with the most accurate theoretical value, 77.51±0.04 meV, of Bunge and Bunge [Phys. Rev. A 30, 2179 (1984)]. This resonance lies 10.80±0.07 meV above the He 2 ${}_{}{}^3{}_{}{}^{}$P threshold, and has a decay width of Γ=7.16±0.07 meV. The ratio of the cross section at resonance to the continuum at the threshold is ${\mathrm{\sigma }}_{\mathit{R}}$/${\mathrm{\sigma }}_0$=89±5. This study provides by far the most detailed experimental view of an electronic shape resonance.

• Received 19 May 1994

DOI:https://doi.org/10.1103/PhysRevA.50.2257