Probing Non-Equilibrium Electron Dynamics in Warm Dense Copper with Two-Color X-ray Pump–Probe Thomson Scattering

Recent developments in X-ray Thomson scattering (XRTS) using X-ray Free Electron Lasers (XFELs) now enable femtosecond-resolved studies of dense plasmas driven far from equilibrium. We report an X-ray pump–X-ray probe experiment on solid-density copper conducted at the SACLA XFEL at the RIKEN Harima Institute in Japan. Using SACLA's two-color mode, a 9.4 keV pump pulse was focused to a 1 μm spot with on-target intensities exceeding 10¹⁷ W/cm², while a 7 keV probe with variable femtosecond delays captured the subsequent evolution of the system. Energy deposition just above the copper K-edge created a well-defined population of hot electrons, and the time-resolved XRTS measurements tracked their thermalization dynamics from 20 - 300 femtoseconds after initial X-ray heating. By combining stochastic spectroscopy with high-resolution spectrometers, the experiment resolved both collective and non-collective scattering features, providing direct measurements of electron temperature and ionization state during the earliest stages of relaxation. The ability to isolate electron thermalization before significant lattice motion or hydrodynamic expansion provides new insight into the microscopic processes governing non-equilibrium, highly correlated, states of matter. These results demonstrate a powerful platform for studying ultrafast energy redistribution in high-density matter and provide new benchmarks for modeling non-equilibrium electron dynamics in mid-Z materials.