Equilibration dynamics and conductivity of warm dense hydrogen

We investigate subpicosecond dynamics of warm dense hydrogen at the XUV free-electron laser facility(FLASH) at DESY (Hamburg). Ultrafast impulsive electron heating is initiated by a < 300-fs short x-rayburst of 92-eV photon energy. A second pulse probes the sample via x-ray scattering at jitter-free variabletime delay. We show that the initial molecular structure dissociates within (0.9 ± 0.2) ps, allowing us to inferthe energy transfer rate between electrons and ions. We evaluate Saha and Thomas-Fermi ionization modelsin radiation hydrodynamics simulations, predicting plasma parameters that are subsequently used to calculatethe static structure factor. A conductivity model for partially ionized plasma is validated by two-temperaturedensity-functional theory coupled to molecular dynamic simulations and agrees with the experimental data. Ourresults provide important insights and the needed experimental data on transport properties of dense plasmas