Nonequilibrium electron-phonon and electron-ion couplings in warm dense copper

Electron–phonon and electron–ion coupling dynamics in warm dense copper are investigated using femtosecond time-resolved X-ray absorption near-edge structure spectroscopy at the Cu L3 edge. Measurements performed with 0.5 eV spectral and 100 fs temporal resolutions reveal pronounced transient changes in pre-edge absorption, indicating strong nonequilibrium behavior in the electronic system. By comparing experimental spectra with simulations based on a modified two-temperature model coupled with density functional theory-calculated density of states, the temporal evolution of coupling strength is quantified across solid and liquid phases. Nonequilibrium electron distributions are found to significantly enhance electron–phonon coupling in the solid phase due to broadened electron energy distributions immediately following laser excitation. Upon melting, the coupling becomes weaker and less sensitive to electron distributions, reflecting a transition to a more disordered atomic structure and increasingly thermalized electrons. These experimental observations are accurately reproduced by theoretical modeling that explicitly incorporates electron relaxation dynamics. The critical role of nonequilibrium electron kinetics in understanding ultrafast energy transfer processes in laser-excited materials is thus highlighted, providing benchmark data essential for future modeling of material behavior under extreme conditions.