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, where even the quadruply excited states become significant 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. The calculated spectra show good agreement with experiment and demonstrate that the importance of multiply excited states depends strongly on both charge state and plasma conditions.