Relativistic and Magnetic Spin Interactions in Helium-Like Atoms

Relativistic and magnetic spin corrections to the ionization energy of He and ${\mathrm{O}}^{6+}$ are given. The magnetic spin interaction given by Sessler and Foley has been derived by a careful evaluation of the Breit operator. The relativistic corrections obtained by reduction of the Dirac equation differ from some previous expressions. With all known corrections taken into account, there remains a discrepancy with experiment of about 30 ${\mathrm{cm}}^{-1\mathrm{}}$ in the case of He. Within the rather large experimental and theoretical uncertainties, there appears to be no residual discrepancy in the case of ${\mathrm{O}}^{6+}$.