Absolute ionization energy of the 21Slevel of helium

Abstract

We have measured the absolute wave numbers of a series of transitions from the metastable 2${\mathrm{}}^1$S level of helium to the n ${}_{}{}^1{}_{}{}^{}$P (n=7–74) excited states. From these data we determine the binding energy of the 2 ${}_{}{}^1{}_{}{}^{}$S level to 2.2 parts in ${10}^{10}$ by using a Ritz series formula. This high-precision determination of the 2 ${}_{}{}^1{}_{}{}^{}$S binding energy does not depend on theoretical calculation of the binding energy of any helium level. The result, 32 033.228 855(7) ${\mathrm{cm}}^{\mathrm{-}1}$, confirms our earlier finding of a relatively large discrepancy with the predicted two-electron Lamb shift for the 2 ${}_{}{}^1{}_{}{}^{}$S level.