Femtosecond visualization of non-equilibrium warm dense copper using x-ray absorption spectroscopy

In this contribution, we present the femtosecond XANES measurements of highly non-equilibrium copper (E_d = 0.127~1.27 MJ/kg), which is heated with frequency-doubled Ti:Sapphire laser pulse and evolves to warm dense matter. The experiment is performed at the European XFEL facility. A transmission zone plate is used to split and focus two identical XFEL beams (±1st order) and allow a single-shot-based measurement with high S/N ratios. X-ray absorption is measured in a broad range of photon energy around Cu L3-edge (E-E_F = -4.5~10.5 eV) with 100 fs resolution. The evolution of pre-edge absorption visualizes the thermalization process between non-equilibrium electron and hole populations in the femtosecond regime. The detailed dynamics can be described with a modified multi-temperature model considering the dynamical band shifts and bond-hardening effect. The evolution of NEXAFS visualizes the lattice vibration and melting process in picosecond time scales.