The consistencies of calculated term values (ionization energies) for various 1snl terms having n=5–8, l=1–5 are tested by microwave-spectroscopic and other available data. The most accurate n P, n D, and n F term values obtained from published variational calculations are generally found to be consistent, within the estimated uncertainties, with more accurate n G and n H energies from core-polarization theory [R. J. Drachman, Phys. Rev. A 26, 1228 (1982)]. The n ${\mathrm{}}^3$D and n ${\mathrm{}}^1$D term values from variational calculations [A. Kono and S. Hattori, Phys. Rev. A 34, 1727 (1986)] are confirmed within uncertainties as small as ${10}^{\mathrm{-}4}$ ${\mathrm{cm}}^{\mathrm{-}1}$ (n=8), although the ${}_{}{}^1{}_{}{}^{}\mathrm{D}$ values are systematically too large by amounts within the uncertainties. The ionization energy $E_I$ has accordingly been reevaluated by using only the calculated n ${\mathrm{}}^3$D term values (n=4,5) and available experimental measurements: The resulting $E_I$ value is 198 310.772 27(40) ${\mathrm{cm}}^{\mathrm{-}1}$ with respect to the 2 ${}_{}{}^3{}_{}{}^{}S$ level at 159 856.077 60 ${\mathrm{cm}}^{\mathrm{-}1}$.

The available data and calculated term energies allow determination of the entire 1snl excited energy-level system with much improved accuracy for the higher levels. Calculated energies are used instead of experimental results in several cases involving apparently underestimated experimental uncertainties. The levels are given explicitly through n=8. Experimental term values based on the new $E_I$ value are combined with calculated term values not including QED contributions to obtain ‘‘experimental’’ Lamb shifts for a number of n S and n P terms. The experimental Lamb shifts of the 2 ${}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$–2 ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ and 2 ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$–2 ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ separations are also reevaluated. Results of comparisons of these experimental shifts with various calculated Lamb shifts vary from agreement within 0.3% experimental uncertainties (for the 2 ${}_{}{}^3{}_{}{}^{}S$ shift and shift of the 2 ${}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$–2 ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ separation) to relatively large discrepancies for the higher n S terms (∼50% for 5 ${}_{}{}^3{}_{}{}^{}S$). Two-electron QED contributions calculated for 2 ${}_{}{}^3{}_{}{}^{}S$, 2 ${}_{}{}^1{}_{}{}^{}S$, and 2 ${}_{}{}^1{}_{}{}^{}P$ [G. W. F. Drake and A. J. Makowski, J. Phys. B 18, L103 (1985)] are confirmed within uncertainties of about 35% by the data in each case, the level of confirmation for 2 ${}_{}{}^3{}_{}{}^{}S$ and 2 ${}_{}{}^1{}_{}{}^{}P$ being subject to additional uncertainties from as-yet uncalculated contributions.

• Received 12 September 1986

DOI:https://doi.org/10.1103/PhysRevA.36.3575