Theoretical Study of Relativistic Retardation Effects: the Abnormal Fine Structure of OII

Using multi-configuration Dirac--Fock and relativistic configuration interaction methods with high-order corrections, we report our precise calculation results of the fine-structure energy levels of the ground-state configuration of OII (1s²2s²2p³). Our calculated fine-structure splittings of ²D _3/2,5/2 and ²P _1/2,3/2 are abnormal. We elucidate that the transverse (Breit) interaction, i.e. relativistic retardation effect, plays an important role for the abnormal fine-structure splittings. Our calculation results are in good agreement with experimental measurements.