High conductivity warm dense water excited by free-electron lasers

In this study, we measured the transient optical reflection and transmission of ultra-thin water sheet uniformly heated by 13.6 nm free-electron laser (FEL) approaching highly conducting states at electron temperatures exceeding 20,000 K. The experiment probes the trajectory of water through the high-energy density phase space and provides insights into the changes of the refractive index, charge carrier densities, and the electrical conductivity at optical frequencies. Significant specular reflection is observed due to the critical electron density shielding, indicating that the carrier electrons density is much higher than the FEL ionized electrons. At electron temperatures below 15,000 K, the experimental results agree well with the density-functional-theory molecular-dynamics simulations. With increasing temperature the electron system approaches a Fermi distribution. The conductivities agree better with predictions from the Ziman theory of liquid metals.