We present a method to determine the bulk temperature of a single crystal diamond sample at an X-Ray free electron laser using inelastic X-ray scattering. The experiment was performed at the high energy density instrument at the European XFEL GmbH, Germany. The technique, based on inelastic X-ray scattering and the principle of detailed balance, was demonstrated to give accurate temperature measurements, within 8% for both room temperature diamond and heated diamond to 500 K. Here, the temperature was increased in a controlled way using a resistive heater to test theoretical predictions of the scaling of the signal with temperature. The method was tested by validating the energy of the phonon modes with previous measurements made at room temperature using inelastic X-ray scattering and neutron scattering techniques. This technique could be used to determine the bulk temperature in transient systems with a temporal resolution of 50 fs and for which accurate measurements of thermodynamic properties are vital to build accurate equation of state and transport models.From the thermal shield of a spacecraft during atmospheric re-entry 1 to the interior of Jovian planets 2 , matter is often found at pressures and temperatures that are at the limits of where conventional condensed matter and plasma physics formalisms are valid 3 . At such extreme conditions, the kinetic energy of the electrons is comparable to the potential energy of interaction between electrons and the nuclei. For these systems, direct and accurate measurements of thermodynamic and transport properties are vital. Extreme states of matter can be produced by dynamic laser-driven compression [4][5][6] and laser heating techniques 7,8 . These methods can excite a material into a transient state of simultaneously high density and temperature, and thereby enable access to previously unexplored regions of phase space. State-of-the-art experiments investigate the properties of materials driven into these conditions by coupling high-energy lasers with suitable probing techniques 9,10 . In particular, X-ray scattering has proven to be a powerful tool for determining the structure and density, and the development of

Extreme conditions inside ice giants such as Uranus and Neptune can result in peculiar chemistry and structural transitions, e.g., the precipitation of diamonds or superionic water, as so far experimentally observed only for pure C─H and H 2 O systems, respectively. Here, we investigate a stoichiometric mixture of C and H 2 O by shock-compressing polyethylene terephthalate (PET) plastics and performing in situ x-ray probing. We observe diamond formation at pressures between 72 ± 7 and 125 ± 13 GPa at temperatures ranging from ~3500 to ~6000 K. Combining x-ray diffraction and small-angle x-ray scattering, we access the kinetics of this exotic reaction. The observed demixing of C and H 2 O suggests that diamond precipitation inside the ice giants is enhanced by oxygen, which can lead to isolated water and thus the formation of superionic structures relevant to the planets’ magnetic fields. Moreover, our measurements indicate a way of producing nanodiamonds by simple laser-driven shock compression of cheap PET plastics.

The development of bright free-electron lasers (FEL) has revolutionized our ability to create and study matter in the high-energy-density (HED) regime. Current diagnostic techniques have been successful in yielding information on fundamental thermodynamic plasma properties, but provide only limited or indirect information on the detailed quantum structure of these systems, and on how it is affected by ionization dynamics. Here we show how the valence electronic structure of solid-density nickel, heated to temperatures of around 10 of eVon femtosecond timescales, can be probed by single-shot resonant inelastic x-ray scattering (RIXS) at the Linac Coherent Light Source FEL. The RIXS spectrum provides a wealth of information on the HED system that goes well beyond what can be extracted from x-ray absorption or emission spectroscopy alone, and is particularly well suited to time-resolved studies of electronicstructure dynamics.

ARTICLES YOU MAY BE INTERESTED INDemonstration of an x-ray Raman spectroscopy setup to study warm dense carbon at the high energy density instrument of European XFEL

We present a method to determine the bulk temperature of a single crystal diamond sample at an X-Ray free electron laser using inelastic X-ray scattering. The experiment was performed at the high energy density instrument at the European XFEL GmbH, Germany. The technique, based on inelastic X-ray scattering and the principle of detailed balance, was demonstrated to give accurate temperature measurements, within 8% for both room temperature diamond and heated diamond to 500 K. Here, the temperature was increased in a controlled way using a resistive heater to test theoretical predictions of the scaling of the signal with temperature. The method was tested by validating the energy of the phonon modes with previous measurements made at room temperature using inelastic X-ray scattering and neutron scattering techniques. This technique could be used to determine the bulk temperature in transient systems with a temporal resolution of 50 fs and for which accurate measurements of thermodynamic properties are vital to build accurate equation of state and transport models. From the thermal shield of a spacecraft during atmospheric re-entry 1 to the interior of Jovian planets 2 , matter is often found at pressures and temperatures that are at the limits of where conventional condensed matter and plasma physics formalisms are valid 3. At such extreme conditions, the kinetic energy of the electrons is comparable to the potential energy of interaction between electrons and the nuclei. For these systems, direct and accurate measurements of thermodynamic and transport properties are vital. Extreme states of matter can be produced by dynamic laser-driven compression 4-6 and laser heating techniques 7,8. These methods can excite a material into a transient state of simultaneously high density and temperature, and thereby enable access to previously unexplored regions of phase space. State-of-the-art experiments investigate the properties of materials driven into these conditions by coupling high-energy lasers with suitable probing techniques 9,10. In particular, X-ray scattering has proven to be a powerful tool for determining the structure and density, and the development of