Fig. 1

Numerical simulation of the ultrafast nonequilibrium transition of copper from solid to WDM. The energy of a femtosecond optical laser pulse is suddenly deposited in the electrons of the system (femtosecond scale), then progressively transferred to the lattice (picosecond scale). a Cold solid lattice before heating: the electron temperature T_e equals the ion one T_i. b Just after heating, a strongly out-of-equilibrium situation is transiently produced where electrons are hot while the lattice is still cold and solid-like. c A few picoseconds after, the lattice disappears as electrons and ions progress up to their thermal equilibration. d–f Calculated absorption spectra in the XANES region corresponding respectively to a–c. The cold XANES signal shown in d is reported in dashed line in e, f. See the Methods section for simulation details