Calculation of the energy levels and hyperfine structure for Xe~II, Rn~II, and Og~II ions
Energy levels, Landé $g$-factors, and hyperfine-structure constants are calculated for the singly ionized noble-gas atoms Xe II, Rn II, and Og II. The calculations are performed using the configuration-interaction method with perturbative treatment of high-lying configurations. Core polarization effects in the hyperfine interaction are included within the time-dependent Hartree-Fock method. Calculations for Xe II are used to test the accuracy of the approach by comparison with available experimental data. The agreement is at the level of about one percent for the energies and typically about ten percent for the hyperfine constants, with better accuracy for states with large hyperfine constants. Predictions are then presented for Rn II and Og II, for which experimental spectroscopic data are limited or absent. Special attention is paid to Breit and quantum-electrodynamic corrections to the hyperfine structure in heavy many-electron ions. We show that these corrections may be strongly enhanced by configuration mixing when interacting states with very different hyperfine matrix elements are separated by small energy intervals. This effect is demonstrated explicitly for odd-parity $J=1/2$ states of Og II. The calculated hyperfine-structure constants for Rn II and Og II provide electronic factors needed for extracting nuclear magnetic dipole and electric quadrupole moments from future spectroscopic measurements. These results may be useful for experimental studies of radon and oganesson ions and for testing nuclear models in the superheavy region.