De-excitation of $K$-shell hollow atoms with $12 \le Z \le 20$: transition rates and branching ratios

Investigating $K$-shell hollow atom spectra enhances our understanding of femtosecond phenomena in atomic physics, chemistry, and biology. Synchrotron measurements of two-electron one-photon (TEOP) transitions in low-$Z$ atoms have revealed discrepancies between experimental results and theoretical predictions of TEOP relative intensities. These discrepancies appear to originate from an incomplete description of an atom's response to the strong perturbation caused by $K$-shell double photoionization (DPI). The multiconfiguration Dirac-Hartree-Fock relativistic configuration interaction method has been applied for studying the TEOP spectra of Mg, Al, Si, S, Ar, and Ca atoms. The results show that branching ratios can be accurately reproduced by accounting for the effects of core and valence electron correlations, as well as the outer-shell ionization and excitation processes following $K$-shell DPI.