Energy and fine structure of 1s2np states (n = 2, 3, 4 and 5) for the lithium isoelectronic sequence

The non-relativistic energies of 1s2np (n = 2, 3, 4 and 5) states for the lithium isoelectronic sequence from Li I to Ne VIII are calculated using the full-core plus correlation method. Relativistic and mass polarization effects on the energy are evaluated using first order perturbation theory. The fine structures are determined from the expectation values of the spin-orbit and spin-other-orbit interaction opertors in the Pauli-Breit approximation. The higher order relativistic effects are estimated using the hydrogenic solution to the Dirac equation with an effective nuclear charge. The QED correction is also included. Our results are compared with the experimental and theoretical data in the literature. The fine structure results agree well with experiment. For 1s2np energies with n≥3, it appears that our results are quite accurate for all Z investigated. However, for the 1s22p systems, the discrepancy with experiment grows monotonically from 0.5 cm-1 for Li I to 29 cm-1 for Ne VIII. This is very different from all the other 1s2n systems we have investigated using the same method. What separates 1s22p apart from the others is the unusually large orbit-orbit interaction and mass polarization effects. For Z>6, the expectation values of these perturbation operators are opposite in sign to those of the 1s2 core. This energy increases quickly with Z. (orig.)