Ionization potential of the lithiumlike 1s22sstates from lithium to neon

The ionization potentials of the 1${\mathit{s}}^2$2s states are calculated for Z=3 to 10 with a method of full core plus correlation using multiconfiguration interaction wave functions. Relativistic and mass polarization effects are included using first-order perturbation theory. The QED contributions to the ionization potential are calculated using effective nucleus charges. The results of this work generally agree with those of experiments. For example, our predicted ionization potentials for Li i and Be i i are 43 487.15(9) and 146 883.03(18) ${\mathrm{cm}}^{\mathrm{-}1}$ in comparison with the experimental results of 43 487.15 and 146 882.86 ${\mathrm{cm}}^{\mathrm{-}1}$, respectively. The predicted nonrelativistic energies in this work agree with those of King [Phys. Rev. A 40, 1735 (1989); 38, 6017 (1988)]. Our results show that the higher-order relativistic perturbation tends to reduce the ionization potential. It becomes appreciable for Z≥7, increases from less than 1 ${\mathrm{cm}}^{\mathrm{-}1}$ for carbon to about 14 ${\mathrm{cm}}^{\mathrm{-}1}$ for Ne. Although the agreement between our predictions and experiments is good for most systems, there is a clear discrepancy of more than 1 ${\mathrm{cm}}^{\mathrm{-}1}$ in the case of boron. The reason for this discrepancy is not understood at this time.