Fine Structure of the Helium Negative Ion

Abstract

Fine-structure transitions have been observed in the metastable $(1s2s2p)^{4}P_{J}$ state of the negative helium ion $^{4}\mathrm{He}^{\ensuremath{-}}$ using an rf resonance technique. Preliminary determinations have been made of the energy separations: $|{E}_{\frac{5}{2}}\ensuremath{-}{E}_{\frac{3}{2}}|=825.23\ifmmode\pm\else\textpm\fi{}0.82$ MHz and $|{E}_{\frac{5}{2}}\ensuremath{-}{E}_{\frac{1}{2}}|=8663\ifmmode\pm\else\textpm\fi{}56$ MHz. We have shown that the $J=\frac{3}{2}$ level lies between the $\frac{5}{2}$ and \textonehalf{} levels. These results are to be compared with the current theoretical estimates of ${E}_{\frac{5}{2}}\ensuremath{-}{E}_{\frac{3}{2}}=\ensuremath{-}2030$ MHz and ${E}_{\frac{5}{2}}\ensuremath{-}{E}_{\frac{1}{2}}=\ensuremath{-}9410$ MHz.