Term values of 1s2s2p4P° for lithiumlike ions

The best-known nonrelativistic energy of the term 1s2s2p ${}_{}{}^4{}_{}{}^{}$P° of neutral lithium is improved to an accuracy of less than 1 ${\mathrm{cm}}^{\mathrm{-}1}$ by using the Rayleigh-Ritz variational method with Slater-type basis functions. The relativistic, mass-polarization, and Lamb-shift effects are calculated perturbatively as first-order corrections to the nonrelativistic energy. Combined with best-known data from experiment, we attain the ionization potential 56 460.0(2) ${\mathrm{cm}}^{\mathrm{-}1}$ for the term 1s2s2p ${}_{}{}^4{}_{}{}^{}$P° of neutral lithium, compared to the experimental value of 56 473(5) ${\mathrm{cm}}^{\mathrm{-}1}$ by Mannervik and Cederquist [Phys. Scr. 27, 175 (1983)]. The discrepancy between theory and experiment is discussed. The relative term value of 1s2s2p ${}_{}{}^4{}_{}{}^{}$P° for neutral lithium is calculated to be 463 062.4(3.3) ${\mathrm{cm}}^{\mathrm{-}1}$ above the lithium ground state, compared with 463 039(122) ${\mathrm{cm}}^{\mathrm{-}1}$ by Rassi et al. [J. Phys. B 10, 3535 (1977)] from the ejected-electron spectrum. We have also extended the calculation to Li-like ions up to Z=9, where no experimental values are available.