Article

Ionization energies for several of the lower lying S- and P-states of helium are deduced from a combination of theory and a variety of high-precision measurements of transition energies. High-precision variational methods are reviewed and used to subtract the nonrelativistic energy and lowest order α 2 au relativistic corrections from the ionization energies. The remaining quantum electrodynamic (QED) shift of order α 3 au and higher is calculated in an extended Kabir–Salpeter formalism and compared with experiment. The comparison verifies the usefulness of the Kabir–Salpeter formalism for terms at least up to order α 4 au, and it verifies an asymptotic 1/n 3 scaling law for the two-electron corrections to the Bethe logarithm. The asymptotic scaling law is used to obtain improved semi-empirical estimates for the ionization energies of the higher lying 1sns 1 S and 3 S states up to n = 10. A revised comprehensive listing is given for the ionization energies of all states of helium up to n = 10 and angular momentum L = 7, together with quantum defect extrapolations for the S-states. PACS Nos.: 31.15.Pf, 31.30.Jv, and 32.10.Hq