Theoretical studies on M-shell relocalization of K-hole Mg ions produced by x-ray free-electron laser

In hot dense plasmas, orbital delocalization and relocalization considerably influence the ionization balance, equation of state and radiative properties of matter. A self-consistent plasma screening potential is applied in atomic structure calculations for solid-density Mg plasmas, generated by an x-ray free electron laser (XFEL). As the charge state increases, the M-shell orbitals of K-hole Mg ions gradually relocalize, beginning with Mg^{6+} at solid density. The level population distributions are determined by solving a time-dependent rate equation using fine-structure level accounting approximations. The predicted K_{α} emission spectra align well with XFEL experiments only when orbital relocalization is carefully accounted for, revealing that the 3d orbital of Mg^{7+} is delocalized, consistent with density functional theory calculations.