Momentum-Resolved X-Ray Thomson Scattering Benchmark of Electronic-Response Models in Warm Dense Aluminium

The robust diagnosis of conditions generated in warm dense matter experiments remains a persistent challenge. Here, we describe the measurement of shock-compressed aluminium at 50 GPa with angle-resolved femtosecond x-ray Thomson scattering (XRTS) over a wide range of scattering wave vectors at the European X-Ray Free-Electron Laser. The measured plasmon dispersion and line shape show that the de facto standard approach for analyzing XRTS spectra, using uniform-electron-gas models, systematically overestimates the resonance energy by up to 8 eV. We present an approach using ab initio methods that agrees within the experimental uncertainty and demonstrates how accounting for shock-induced disorder in shock-compressed systems is critical for their understanding, providing evidence that ab initio treatments are required for reliable XRTS inference in warm dense aluminium.