Importance of Relativistic Effects for Intermediate-Z Elements: Photoionization Process of Excited Na

Using a modified R-matrix code, the fine-structure-resolved partial photoionization cross sections of excited Na (Z=11) are calculated within the Breit--Pauli approximation. Our calculated energy levels of Na⁺ and Na are in good agreement with the experimental values within 1% and the branching ratios of the J-resolved partial cross sections are consistent with the recent measurements within the experimental uncertainties. The agreements are impossible to be obtained without adequately taking into account the relativistic effects and the electron correlations together. Therefore, even for the intermediate-Z elements (e.g. Na with Z=11), the relativistic effects (mainly the spin-orbit interactions) should not be neglected.