Relativistic R-matrix calculations of photoionization cross sections for Na-like Cr¹³⁺ ions

The direct and resonant photoionization processes of Na-like Cr¹³⁺ ions from the ground state 1s²2s²2p⁶3s ²S_1/2 and two lowly excited 1s²2s²2p⁶3p ²P_{1/2, 3/2} states are explored by means of relativistic R-matrix calculations. Particular emphasis is placed on analysis of resonance structures and determination of direct and resonant ionization limits. For this purpose, we calculated the energy levels of Cr¹³⁺ and adjacent Cr¹⁴⁺ ions, together with the photoionization cross sections over a wide photon energy range on a fine grid. The obtained ionization limits are found to be in good accordance with the NIST values. For the ground state, 25 well-resolved resonance peaks are identified, with their respective total angular momentum and parity determined. By comparing the energy levels with the resonance positions extracted from the cross sections, these resonances are assigned to the double-excitation channels 1s²2s²2p⁵3lnl' (l = p,d). In contrast, for the two excited states, only the direct ionization limits can be reliably determined. Owing to high density and complexity of the resonance structures, individual resonances and their corresponding resonant ionization limits cannot be unambiguously identified. This work provides a comprehensive set of photoionization cross-section data for Na-like Cr¹³⁺ ions, filling a critical gap in direct and resonant photoionization studies and offering valuable support for diagnostic analysis and numerical modeling of chromium-containing astrophysical and laboratory plasmas. In addition, the photoionization dataset supporting the present findings is publicly retrievable from the Science Data Bank at https://www.scidb.cn/s/JFjQnm (https://doi.org/10.57760/sciencedb.j00113.00331).