Extended Data Fig. 1: Simulated Au spectrum.

Simulated Au spectrum corresponding to solid flat gold target irradiated at an intensity of 4×10²¹ Wcm⁻². Best agreement with the experimental spectrum is obtained when lines from F-like to N-like Au are shifted down in energy by ~0.4%. Similar shifts were necessary in ref. ⁷ when comparing the calculated Au line energies with the measurement. The spectrum was computed using transient calculations performed with the atomic physics/hydrodynamic code RADEX using initial plasma parameters computed by the PIC simulations. The RADEX code uses detailed atomic data from the Hebrew University Lawrence Livermore Atomic Code (HULLAC) (ref. ²⁵). It was run using 200 to 1500 levels per ion from Cu-like (Au⁵⁰⁺) to B-like (Au⁷⁴⁺) to accurately simulate the large number of transitions from the L shell configuration. The simulated spectrum shows emission of all the observed lines for charge states from Ne-like to N-like Au.