Abstract
The electrical conductivity of water under extreme temperatures and densities plays a central role in modeling planetary magnetic fields. Experimental data are vital to test theories of high-energy-density water and assess the possible development and presence of extraterrestrial life. These states are also important in biology and chemistry studies when specimens in water are confined and excited using ultrafast optical or free-electron lasers (FELs). Here we utilize femtosecond optical lasers to measure the transient reflection and transmission of ultrathin water sheet samples uniformly heated by a 13.6 nm FEL approaching a highly conducting state 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 changes in the index of refraction, charge carrier densities, and AC electrical conductivity at optical frequencies. At excitation energy densities exceeding 10 MJ/kg, the index of refraction falls to n = 0.7, and the thermally excited free-carrier density reaches ne = 5 × 1027 m−3, which is over an order of magnitude higher than that of the electron carriers produced by direct photoionization. Significant specular reflection is observed owing to critical electron density shielding of electromagnetic waves. The measured optical conductivity reaches 2 × 104 S/m, a value that is one to two orders of magnitude lower than those of simple metals in a liquid state. At electron temperatures below 15 000 K, the experimental results agree well with the theoretical calculations using density-functional theory/molecular-dynamics simulations. With increasing temperature, the electron density increases and the system approaches a Fermi distribution. In this regime, the conductivities agree better with predictions from the Ziman theory of liquid metals.
Highlights
Water has been extensively studied because of its ubiquity and importance for a variety of fundamental processes and technologies.In its liquid phase, water exhibits many thermodynamic anomalies that originate from its well-connected hydrogen bond network.1–3Water plays a prominent role as a solvent in chemistry and scitation.org/journal/mre biology,4,5 but is used as a reactant in energy research and technology.6 Further, water is a key ingredient for many cosmic and planetary processes.7 For instance, water and water–ice mixtures are highly abundant in ice giant planets such as Uranus or Neptune.8–10The high pressures and temperatures in the cores of these planets can create diamond from methane,11 and the polymorphs of water under such conditions might become conductors capable of generating magnetic fields
Water under extreme conditions22 is important for a vast number of x-ray free-electron-laser (XFEL) experiments in physics,23 chemistry,24 and biology,25 where the interaction of short and intense XFEL pulses will lead to warm dense conditions
By monitoring the free-electron lasers (FELs)-induced fluorescence intensity on a yttrium aluminum garnet (YAG) screen with and without the water sample, we found that the XUV transmission ratios through 200 and 300 nm thick water were (82.2 ± 2.6) and (77.4 ± 2.7)%, respectively, which are in close agreement with the values of 83.1 and 75.9% predicted by the wellknown XUV transmission model
Summary
Water has been extensively studied because of its ubiquity and importance for a variety of fundamental processes and technologies. Water under extreme conditions is important for a vast number of x-ray free-electron-laser (XFEL) experiments in physics, chemistry, and biology, where the interaction of short and intense XFEL pulses will lead to warm dense conditions. While there has been some experimental effort to understand the structural properties of water using x-ray diffraction techniques, measurements of its electrical conductivity under well-characterized WDM conditions are still lacking. To determine the electrical conductivity of WDM generally requires measurements of the optical reflection and transmission of an excited sample.. The AC electrical conductivity at the frequency of an optical probe pulse is deduced These measurements require the sample surface to be optically flat. The AC electrical conductivity was determined from simultaneous measurement of the optical reflection and transmission of ultrafast probe pulses at two different wavelengths, namely, 750 and. The experimental results agree well with theoretical calculations using densityfunctional theory coupled with molecular dynamics (DFT-MD). At temperatures exceeding 15 000 K, the measurements agree better with the predictions from Ziman theory, which is a manifestation of the liquid-metal-like electrical properties
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