A novel X-ray free electron laser (XFEL) diffraction setup for use with diamond anvil cells (DACs) at the Linac Coherent Light Source (LCLS) is described. The new diamond window at the Matter at Extreme Conditions (MEC) instrument allows hard X-ray experiments on DACs to be performed in air. The platform is described along with alignment and calibration procedures, and details of the X-ray beam and diagnostics. Example data are presented, including a reversible XFEL-induced phase transition in CsPbI3. The DAC setup was commissioned at MEC, but is applicable to most LCLS instruments where the unique pulse structures available at LCLS offer access to new ultrafast experimental techniques at high pressure.

Color centers in diamond are promising single-photon sources for quantum technologies and biomedical applications. We studied the formation dynamics of negatively charged silicon-vacancy (SiV) centers during diamond nucleation using time-resolved X-ray pump-probe experiments at the European X-ray Free-Electron Laser Facility. A silicon-containing adamantane precursor was flash-heated within diamond anvil cells using ultrashort femtosecond X-ray pulses at a 2.2-MHz repetition rate. We captured the structural evolution of diamond with X-ray diffraction patterns separated by 443 ns. Correlating these dynamics with postexperiment SiV photoluminescence reveals, for the first time, a link between ultrafast diamond nucleation and color-center inclusion. SiV formation is contingent on diamond formation and occurs only above pressure-dependent energy-delivery-rate thresholds: 34.3 J·s-1 at 17.8 GPa and 178.4 J·s-1 at 14.4 GPa. Our findings define synthesis windows for generating SiV and reveal a kinetic regime in which diamond nucleates without optically active SiV, informing rate- and pressure-aware strategies for producing various color centers.

The exceptional brilliance of X-ray free electron lasers (XFELs) combined with quasi-static compression in diamond anvil cells (DACs) has enabled novel time-resolved methods for probing extreme states of matter. Coupling serial X-ray techniques with dynamic drivers for rapidly changing pressure and temperature, these studies offer insight into phase equilibria, chemical reaction kinetics, and geophysical processes. Here, we present the current state-of-the-art XFEL techniques used for DAC research, including serial X-ray heating, pulsed laser heating, and rapid compression. Essential questions for future investigation are also highlighted, regarding the interactions of intense X-rays with samples at high pressure and associated non-equilibrium states.

Understanding how laser pulses compress solids into high-energy-density states requires diagnostics that simultaneously resolve macroscopic geometry and nanometer-scale structure. Here we present a combined X-ray imaging (XRM) and small-angle X-ray scattering (SAXS) approach that bridges this diagnostic gap. Using the Matter in Extreme Conditions end station at LCLS, we irradiated 25 μm copper wires with 45 fs, 0.9 J, 800 nm pulses at 3.5×1019 W/cm2 while probing with 8.2 keV XFEL pulses. XRM visualizes the evolution of ablation, compression, and inward-propagating fronts with ∼200 nm resolution, while SAXS quantifies their nanometer-scale sharpness via the time-resolved evolution of scattering streaks. The joint analysis reveals that an initially smooth compression steepens into a nanometer-sharp shock front after tsh≈(18±3) ps, consistent with an analytical steepening model and hydrodynamic simulations. The front reaches a velocity of csh≈25 km/s and a lateral width of several tens of microns, demonstrating direct observation of shock formation and decay at solid density for the first time with few-nanometer precision. This integrated XRM–SAXS method establishes a quantitative, multi-scale diagnostic of laser-driven shocks in dense plasmas relevant to inertial confinement fusion, warm dense matter, and planetary physics.

Gold ist ein unreaktives Metall und seine chemische Wechselwirkung mit Wasserstoff wird erst seit Kurzem untersucht. Unter Verwendung verschiedener Kohlenwasserstoffe als Wasserstoffquelle beobachten wir hier die Bildung von festem Goldhydrid in der Diamantstempelzelle, die mit einem Freie‐Elektronen‐Laser beheizt wird. Bei Drücken oberhalb von 40 GPa und bei Temperaturen in der Nähe des Schmelzpunkts bildet sich eine hexagonale Phase, die einem Hydrid mit der Stöchiometrie entspricht, wobei x mit dem Druck (40 bis 80 GPa) von 0 auf nahezu 1 ansteigt. Es handelt sich um eine Hochtemperaturphase, die sich beim Abkühlen auf 295 K in flächenzentriertes kubisches Gold umwandelt. Begleitende DFT‐MD‐Simulationen stimmen hervorragend mit den Ergebnissen der Experimente überein und zeigen, dass die Struktur aus einem hexagonal dicht gepackten Goldgitter mit ungeordneten Wasserstoffatomen in den Zwischenräumen besteht. Der Wasserstoff ist superionisch, d.h. er weist eine hohe Diffusionsfähigkeit durch das kristalline Goldgitter auf. Unsere Ergebnisse weisen die erste binäre Verbindung aus Gold und Wasserstoff im festen Zustand nach.

Gold is an unreactive metal and its chemical interactions with hydrogen have only recently been explored. Here, we report the formation of gold hydride above 40 GPa and 2200 K in X‐ray free electron laser heated diamond anvil cells using various hydrocarbons as hydrogen sources. Above 40 GPa, a hexagonal phase emerges close to the gold melting point, corresponding to a hydride with stoichiometry , with increasing from 0 to near 1 with pressure from 40 to 80 GPa. This is a high‐temperature phase which reverts to face centered cubic gold on cooling to 295 K. Accompanying DFT‐MD simulations are in excellent agreement with experiment and reveal the structure to consist of an hexagonal close packed gold lattice with atomic hydrogen disordered in the interstices. The hydrogen is superionic and exhibits high diffusivity through the crystalline gold lattice. Our results present the first solid‐state binary compound of gold and hydrogen.

Various icy moons, such as Europa and Ganymede, have thin oxygen atmospheres and exhibit spectral features attributed to oxygen held in their surface ices. The oxygen forms from the radiolysis of water. The interiors of these bodies are subject to high pressures and it is not known how deep into icy moons oxygen-bearing ices can penetrate, or the structures formed by the oxygen-water system at high pressure. Here, we show that oxygen hydrates are stable to 2.6 GPa, allowing them to penetrate deep into icy moons, both above and below proposed sub-surface liquid-water oceans. Similarities between oxygen and hydrogen hydrates indicate potentially enhanced recombination rates, transforming them back into water and offering a resolution to the discrepancy between predicted and measured radiolysis rates. In addition to the low-pressure CS-II clathrate, our results find three high-pressure phases in the oxygen-water system: an ST clathrate, a C0 hydrate, and a filled ice isomorphous with methane hydrate III. This shows a vast storage potential for molecular oxygen in icy moons and indicates that Europa could still be absorbing oxygen into its crustal ice.

We report time resolved observations of the crystallization from liquid hydrogen, supercooled to temperatures below the melting point, using 11.2 keV X-ray diffraction from the Linac Coherent Light Source (LCLS). Changes to the metastable solid and liquid structure factors have been dynamically measured. This allows for a direct determination of the lowest energy crystal polymorphs, the stacking probabilities, as well as the liquid and solid densities and temperatures. Such measurements provide experimental evidence of an Arrhenius-like growth kinetics along the stacking direction during supercooling.

Phonon scattering in metals is one of the most fundamental processes in materials science. However, understanding such processes has remained challenging and requires detailed information on interactions between phonons and electrons. We use an ultrafast electron diffuse scattering technique to resolve the nonequilibrium phonon dynamics in femtosecond–laser-excited tungsten in both time and momentum. We determine transient populations of phonon modes which show strong momentum dependence initiated by electron-phonon coupling. For phonons near Brillouin zone border, we observe a transient rise in their population on a timescale of approximately 1 picosecond driven by the strong electron-phonon coupling, followed by a slow decay on a timescale of approximately 8 picosecond governed by the weaker phonon-phonon relaxation process. We find that the exceptional harmonicity of tungsten is needed for isolating the two processes, resulting in long-lived nonequilibrium phonons in a pure metal. Our finding highlights that electron-phonon scattering can be the determinant factor in the phonon thermal transport of metals.

Silicon-vacancy (SiV) centers in diamond are emerging as promising quantum emitters in applications such as quantum communication and quantum information processing. Here, we demonstrate a sub-μs pulsed annealing treatment that dramatically increases the photoluminescence of SiV centers in diamond. Using a silane-functionalized adamantane precursor and a laser-heated diamond anvil cell, the temperature and energy conditions required to form SiV centers in diamond were mapped out via an optical thermometry system with an accuracy of ±50 K and a 1 μs temporal resolution. Annealing scheme studies reveal that pulsed annealing can obviously minimize the migration of SiV centers out of the diamond lattice, and a 2.5-fold increase in the number of emitting centers was achieved using a series of 200-ns pulses at a 50 kHz repetition rate via acousto-optic modulation. Our study provides a novel pulsed annealing treatment approach to improve the efficiency of the creation of SiV centers in diamond. Silicon-vacancy (SiV) centers in diamond are emerging as promising quantum emitters for various applications. Here, authors devised a novel pulsed annealing scheme that improves the conversion efficiency of SiV centers in diamond.

The pressure and temperature conditions at which precipitation of diamond occurs from hydrocarbon mixtures is important for modelling the interior dynamics of icy planets. However, there is substantial disagreement from laboratory experiments, with those using dynamic compression techniques finding much more extreme conditions are required than in static compression. Here we report the time-resolved observation of diamond formation from statically compressed polystyrene, (C 8 H 8 ) n , heated using the 4.5 MHz X-ray pulse trains at the European X-ray Free Electron Laser facility. Diamond formation is observed above 2,500 K from 19 GPa to 27 GPa, conditions representative of Uranus’s and Neptune’s shallow interiors, on 30 μs to 40 μs timescales. This is much slower than may be observed during the ∼10 ns duration of typical dynamic compression experiments, revealing reaction kinetics to be the reason for the discrepancy. Reduced pressure and temperature conditions for diamond formation has implications for icy planetary interiors, where diamond subduction leads to heating and could drive convection in the conductive ice layer that has a role in their magnetic fields.

AbstractH2O transforms to two forms of superionic (SI) ice at high pressures and temperatures, which contain highly mobile protons within a solid oxygen sublattice. Yet the stability field of both phases remains debated. Here, we present the results of an ultrafast X-ray heating study utilizing MHz pulse trains produced by the European X-ray Free Electron Laser to create high temperature states of H2O, which were probed using X-ray diffraction during dynamic cooling. We confirm an isostructural transition during heating in the 26-69 GPa range, consistent with the formation of SI-bcc. In contrast to prior work, SI-fcc was observed exclusively above ~50 GPa, despite evidence of melting at lower pressures. The absence of SI-fcc in lower pressure runs is attributed to short heating timescales and the pressure-temperature path induced by the pump-probe heating scheme in which H2O was heated above its melting temperature before the observation of quenched crystalline states, based on the earlier theoretical prediction that SI-bcc nucleates more readily from the fluid than SI-fcc. Our results may have implications for the stability of SI phases in ice-rich planets, for example during dynamic freezing, where the preferential crystallization of SI-bcc may result in distinct physical properties across mantle ice layers.

Understanding the dynamics of electron-phonon and phonon-phonon interactions is important to unravel the complex behavior of materials subject to ultrafast laser excitation. We report the results of studying these interactions in femtosecond laser-excited tungsten (W) using the technique of ultrafast electron diffuse scattering (UEDS). By tracking changes of diffuse scattering signal over time, we resolve the dynamics of phonon populations across the Brillouin zone in W. Our results shed light on both electron-phonon and phonon-phonon coupling dynamics in W [Mo et al. Science Advances 10, eadk9051 (2024)]. This paper outlines the fundamental principle behind the UEDS technique, provides a brief overview of the experimental setup, and presents selected results of time-resolved diffuse scattering patterns.

High-intensity, short-pulse lasers are crucial for generating energetic electrons that produce high-energy-density (HED) states in matter, offering potential applications in igniting dense fusion fuels for fast ignition laser fusion. High-density targets heated by these electrons exhibit spatially non-uniform and highly transient conditions, which have been challenging to characterize due to limitations in diagnostics that provide simultaneous high spatial and temporal resolution. Here, we employ an X-ray Free Electron Laser (XFEL) to achieve spatiotemporally resolved measurements at sub-micron and femtosecond scales on a solid-density copper foil heated by laser-driven fast electrons. Our X-ray transmission imaging reveals the formation of a solid-density hot plasma localized to the laser spot size, surrounded by Fermi degenerate, warm dense matter within a picosecond, and the energy relaxation occurring within the hot plasma over tens of picoseconds. These results validate 2D particle-in-cell simulations incorporating atomic processes and provide insights into the energy transfer mechanisms beyond current simulation capabilities. This work significantly advances our understanding of rapid fast electron heating and energy relaxation in solid-density matter, serving as a key stepping stone towards efficient high-density plasma heating and furthering the fields of HED science and inertial fusion energy research using intense, short-pulse lasers.

The pressure–volume equations of state of palladium and rhodium statically compressed in neon are presented. Vinet fits give parameters for palladium: V0=58.678(73) Å3, B0=189.3(30) GPa, B0′=5.473(63), and rhodium: V0=55.062(63) Å3, B0=241.3(65) GPa, B0′=5.34(24). Both metals are observed to react with hydrocarbons under pressure to form hydrides. Existing equations of state are discussed with regard to potential inadvertent hydrogen contamination as a source for discrepancies and anomalous fitted parameters.

Dehydrogenation of alkanes is of increasing importance in fulfilling global demand for olefins and offers a potential source of carbon‐neutral hydrogen as a co‐product. Currently commercial dehydrogenation processes occur at high‐temperatures (500–900 °C) which is energy intensive and results in side reactions and rapid coking of the catalysts. In addition, the hydrogen produced is often burned to maintain temperature and to inhibit the back reaction. Here, pressure is utilized as a parameter to enable novel chemical catalytic processes, and ambient‐temperature dehydrogenation of alkanes by palladium is observed at 50–100 MPa, with both hydrogen gas and olefins recovered on decompression. This reaction follows a fundamentally different path to current commercial high‐temperature low‐pressure dehydrogenation processes with the palladium catalyst reversibly forming a hydride intermediate.

High-power, short-pulse laser-driven fast electrons can rapidly heat and ionize a high-density target before it hydrodynamically expands. The transport of such electrons within a solid target has been studied using two-dimensional (2D) imaging of electron-induced Kα radiation. However, it is currently limited to no or picosecond scale temporal resolutions. Here, we demonstrate femtosecond time-resolved 2D imaging of fast electron transport in a solid copper foil using the SACLA x-ray free electron laser (XFEL). An unfocused collimated x-ray beam produced transmission images with sub-micron and ∼10 fs resolutions. The XFEL beam, tuned to its photon energy slightly above the Cu K-edge, enabled 2D imaging of transmission changes induced by electron isochoric heating. Time-resolved measurements obtained by varying the time delay between the x-ray probe and the optical laser show that the signature of the electron-heated region expands at ∼25% of the speed of light in a picosecond duration. Time-integrated Cu Kα images support the electron energy and propagation distance observed with the transmission imaging. The x-ray near-edge transmission imaging with a tunable XFEL beam could be broadly applicable for imaging isochorically heated targets by laser-driven relativistic electrons, energetic protons, or an intense x-ray beam.