Beyond-EUV spectrum of highly-charged gadolinium

Abstract

We report ab initio relativistic calculations on the complex Gd$^{18+}$ and Gd$^{26+}$ highly-charged ions, which are relevant to beyond-extreme-ultraviolet (BEUV) light sources based on laser-produced gadolinium plasmas. Using the particle-hole configuration-interaction with many-body perturbation theory (CI+MBPT) method, we systematically saturate the $n=4$ configuration space and determine the influence of multiply-excited states on the emissivity spectrum. For Gd$^{26+}$ we find behavior analogous to that of its isoelectronic counterpart Sn$^{12+}$, with strong contributions from multiply-excited states and a significant appearance of even quadruply-excited states at certain effective temperatures. In contrast, the industry-relevant 6.7\,nm emission of Pd-like Gd$^{18+}$ is dominated by the singly excited $4d^{10}$ $^{1}S_{0}$ - $4d^{9}4f$ $^{1}P_{1}$ transition, while multiply-excited states primarily contribute to out-of-band emission.