Fine-Structure Splittings of Nitrogen Isoelectronic Sequence: Competitions among Spin--Orbit Interactions, Breit Interactions and Electron Correlations

Using the multi-configuration Dirac--Fock self-consistent field method and the relativistic configuration interaction method with quantum-electrodynamics
corrections performed by the GRASP code, we calculate the fine-structure energy levels of the ground-state configuration (1s²²s²²p³) of the nitrogen isoelectronic sequence, according to the L-S coupling scheme with atomic number Z up to 22. Based on the calculated results, we elucidate the mechanism of the orderings of fine-structure energy levels of ²D_3/2,5/2 and
²P_1/2,3/2 respectively, i.e. for ²D_3/2,5/2 orderings, the competition between the spin--orbit interactions and the Breit interactions; for ²P_1/2,3/2 orderings, the electron correlations, especially the electron correlations owing to the
2p⁵ configuration interactions.