Warm dense matter (WDM), a state bridging plasma and condensed phases, is crucial in astrophysics, planetary science, and inertial confinement fusion. This study utilizes an intense, ultrafast X-ray free-electron laser (XFEL) pulse to generate and characterize warm dense copper via L-edge X-ray absorption spectroscopy. Below 10¹⁵ W cm⁻², a pre-edge absorption peak emerges due to 3d band depletion, shifting to lower energy with increasing intensity. At higher intensities, ionization and collisions cause a transition from reverse saturable absorption (RSA) to saturable absorption (SA), promising for X-ray pulse shaping. Theoretical calculations combining kinetic Boltzmann equations and finite-temperature density-functional theory support these findings, providing benchmarks for non-equilibrium WDM models.

Laurent Mercadier, Andrei Benediktovitch, Špela Krušič, Joshua J. Kas, Justine Schlappa, Marcus Agåker, Robert Carley, Giuseppe Fazio, Natalia Gerasimova, Young Yong Kim, Loïc Le Guyader, Giuseppe Mercurio, Sergii Parchenko, John J. Rehr, Jan-Erik Rubensson, Svitozar Serkez, Michal Stransky, Martin Teichmann, Zhong Yin, Matjaž Žitnik, Andreas Scherz, Beata Ziaja, Nina Rohringer