Relativistic structure and transition probabilities of O-like ions along the Ζ =18-22 isoelectronic sequence

Using the Multiconfiguration Dirac-Hartree-Fock (MCDHF) method and the corresponding GRASP2018 computational package, energies of the ground state 1s 2 2s 2 2p 4 (2p 4 ), inner-shell 1s excited state 1s2s 2 2p 5 (1s2p 5 ), and the 2s excited state 1s 2 2s2p 5 (2s2p 5 ), 1s 2 2p 6 of Ar 10+ , K 11+ , Ca 12+ , Sc 13+ , and Ti 14+ were investigated. Systematic E1 and M2 transition data of 1s-2p (1s2p⁵-2p⁴ and 1s2p⁵-1s²2p⁶) and 2s-2p transitions (2s2p⁵-2p⁴ and 1s²2p⁶-2s2p⁵) are studied. To the best of our knowledge, experimental data for the inner-shell 1s excited states 1s2p⁵ of Ar 10+ -Ti 14+ are currently limited to the wavelengths and spectra of Ar 10+ . Furthermore, no other reports regarding the M2 transition wavelengths and probabilities involved in this study have been found. Electron correlation effects are gradually incorporated through the active space approach. To improve computational accuracy, higher-order relativistic effects [Quantum Electrodynamics (QED) and Breit interactions], were included in the calculations. The calculated energy levels are in good agreement with the National Institute of Standards and Technology (NIST) experimental values, with deviations within 0.01 eV. For the radiative transition probabilities of the 1s2p⁵-2p⁴ transitions in Ar¹⁰⁺, the maximum relative deviation from the relativistic Hartree-Fock (HFR) results is 6.8%, and the differences between the Babushkin and Coulomb gauges are mostly below 2%, with a maximum of 5% (except for the two weak transitions affected by cancellation effects).