Relativistic Modeling of K- and L‑Shell Decay in Sulfur Ions across Oxidation States.

Core-hole decay processes in sulfur ions from S to S were systematically investigated by using relativistic quantum electrodynamics. Fine-structure wave functions were generated via multiconfiguration Dirac-Fock methods to model decay pathways, including Auger and radiative channels. Auger processes were treated using configuration interaction, with Auger transition rates calculated through distorted-wave and isolated-resonance approximations. Radiative decay rates were determined for multipoles using relaxed-orbital oscillator strengths. The simulations reveal that Auger electron kinetic energies decrease monotonically with increasing oxidation state, while Auger transition intensities vary sensitively with ionization state. These variations are driven by changes in electronic configuration, which modulate Coulomb and Breit interactions, thereby altering energy gaps and intensity ratios across transitions. While radiative decay remains weak in all sulfur ionization states due to the low atomic number, K-shell decay exhibits a significantly higher radiative contribution than L-shell decay. Additionally, higher ionization leads to a slight reduction in the Auger-to-fluorescence decay ratio. Notably, spin-orbit coupling in 2p core-shells exerts a pronounced influence on Auger transition probabilities and X-ray fluorescence yields, though its impact diminishes with increasing ionization.