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 α2 au relativistic corrections from the ionization energies. The remaining quantum electrodynamic (QED) shift of order α3 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 α4 au, and it verifies an asymptotic 1/n3 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 1S and 3S 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