Predominant contributions of doubly and triply excited atomic states to the radiative opacity of Gd plasmas

A large-scale configuration-interaction calculation is performed for Gd18+ and nearby ionization stages using a multi-configuration Dirac–Hartree–Fock method. Multiply excited configurations up to quadrupole excitations from the 4d orbital are included, resulting in nearly 2.3 × 105 fine-structure levels for each particular ion stage. Electron correlation effects of 4d − 4f are demonstrated by taking Gd18+ as an example, leading to an evident redistribution of oscillator strengths. For the first time, absorptions from doubly and triply excited configurations are presented, which significantly enhance absorption near 6.5 nm and account for almost 90% of the total absorption. The spectrally resolved radiative opacities of a Gd plasma at 60 eV and 1021 cm−3 are demonstrated, which presented complicated absorption structures around 6.5 nm. This study highlights the important role of multiply excited configurations, offering insights for optimizing next-generation EUV light sources.