Investigation of Nonequilibrium Electronic Dynamics of Warm Dense Copper with Femtosecond X-Ray Absorption Spectroscopy.

Ultrafast optical excitation of matter leads to highly excited states that are far from equilibrium. In this study, femtosecond x-ray absorption spectroscopy was used to visualize the ultrafast dynamics in photoexcited warm dense Cu. The rich dynamical features related to $d$ vacancies are observed on femtosecond timescales. Despite the success in explaining x-ray absorption data in the picosecond regime, the new femtosecond data are poorly understood through the traditional two-temperature model based on the fast thermalization concept and the static electronic structure for high-temperature metals. An improved understanding can be achieved by including the recombination dynamics of nonthermal electrons and changes in the screening of the excited $d$ block. The population balance between the $4sp$ and $3d$ bands is mainly determined by the recombination rate of nonthermal electrons, and the underpopulated $3d$ block is initially strongly downshifted and recovered in several hundreds of femtoseconds.