Evolution of ac conductivity in nonequilibrium warm dense gold

Ultrafast Heating-Induced Suppression of d-Band Dominance in the Electronic Excitation Spectrum of Cuprum

The combination of isochoric heating of solids by free-electron lasers (FELs) and in situ diagnostics by X-ray Thomson scattering (XRTS) allows for measurements of material properties at warm dense matter (WDM) conditions relevant for astrophysics, inertial confinement fusion, and materials science. In the case of metals, the FEL beam pumps energy directly into electrons with the lattice structure of ions being nearly unaffected. This leads to a unique transient state that gives rise to a set of interesting physical effects, which can serve as a reliable testing platform for WDM theories. In this work, we present extensive linear-response time-dependent density functional theory (TDDFT) results for the electronic dynamic structure factor of isochorically heated copper with a face-centered cubic lattice. At ambient conditions, the plasmon is heavily damped due to the presence of d-band excitations, and its position is independent of the wavenumber. In contrast, the plasmon feature starts to dominate the excitation spectrum and has a Bohm-Gross-type plasmon dispersion for temperatures T ≥ 4 eV, where the quasi-free electrons in the interstitial region are in the WDM regime. In addition, we analyze the thermal changes in the d-band excitations and outline the possibility to use future XRTS measurements of isochorically heated copper as a controlled testbed for WDM theories.

Ab initio-simulated optical response of hot electrons in gold and ruthenium

Optical femtosecond pump-probe experiments allow to measure the dynamics of ultrafast heating of metals with high accuracy. However, the theoretical analysis of such experiments is often complicated because of the indirect connection of the measured signal and the desired temperature transients. Establishing such a connection requires an accurate model of the optical constants of a metal, depending on both the electron temperature Te and the lattice temperature Tl. In this paper, we present first-principles simulations of the two-temperature scenario with Te ≫ Tl, showing the optical response of hot electrons to laser irradiation in gold and ruthenium. Comparing our simulations with the Kubo-Greenwood approach, we discuss the influence of electron-phonon and electron-electron scattering on the intraband contribution to optical constants. Applying the simulated optical constants to the analysis of ultrafast heating of ruthenium thin films we highlight the importance of the latter scattering channel to understand the measured heating dynamics.

How to read optical properties of matter via the Kubo-Greenwood approach

Substances with a complex electronic structure exhibit non-Drude optical properties that are challenging to interpret experimentally and theoretically. In our recent paper [Phys. Rev. E 105, 035307 (2022)2470-004510.1103/PhysRevE.105.035307], we offered a computational method based on the continuous Kubo-Greenwood formula, which expresses dynamic conductivity as an integral over the electron spectrum. In this Letter, we propose a methodology to analyze the complex conductivity using liquid Zr as an example to explain its nontrivial behavior. To achieve this, we apply the continuous Kubo-Greenwood formula and extend it to include the imaginary part of the complex conductivity into the analysis. Our method is suitable for a wide range of substances, providing an opportunity to explain optical properties from ab initio calculations of any difficulty.

Dynamic and transient processes in warm dense matter

In this paper, we discuss some of the key challenges in the study of time-dependent processes and non-equilibrium behaviour in warm dense matter. We outline some of the basic physics concepts that have underpinned the definition of warm dense matter as a subject area in its own right and then cover, in a selective, non-comprehensive manner, some of the current challenges, pointing along the way to topics covered by the papers presented in this volume. This article is part of the theme issue 'Dynamic and transient processes in warm dense matter'.

Dynamics of femtosecond heated warm dense copper with time-resolved L3-edge XANES

Combining experimental set up and ab initio molecular dynamics simulations, we were able to follow the time evolution of the X-ray absorption near edge spectrum (XANES) of a dense copper plasma. This provides a deep insight into femtosecond laser interaction with a metallic copper target. This paper presents a review of the experimental developments we made to reduce the X-ray probe duration, from approximately 10 ps to fs duration with table-top laser systems. Moreover, we present microscopic scale simulations, performed with Density Functional Theory, as well as macroscopic simulations considering the Two-Temperature Model. These tools allow us to get a complete picture of the evolution of the target at a microscopic level, from the heating process to the melting and expansion stages, with a clear view of the physics involved during these processes. This article is part of the theme issue 'Dynamic and transient processes in warm dense matter'.

Ultrafast Energy Transfer Process in Confined Gold Nanospheres Revealed by Femtosecond X-ray Imaging and Diffraction

Femtosecond laser pulses drive nonequilibrium phase transitions via reaction paths hidden in thermal equilibrium. This stimulates interest to understand photoinduced ultrafast melting processes, which remains incomplete due to challenges in resolving accompanied kinetics at the relevant space-time resolution. Here, by newly establishing a multiplexing femtosecond X-ray probe, we have successfully revealed ultrafast energy transfer processes in confined Au nanospheres. Real-time images of electron density distributions with the corresponding lattice structures elucidate that the energy transfer begins with subpicosecond melting at the specimen boundary earlier than the lattice thermalization, and proceeds by forming voids. Two temperature molecular dynamics simulations uncovered the presence of both heterogeneous melting with the melting front propagation from surface and grain boundaries and homogeneous melting with random melting seeds and nanoscale voids. Supported by experimental and theoretical results, we provide a comprehensive atomic-scale picture that accounts for the ultrafast laser-induced melting and evaporation kinetics.

Nonequilibrium band occupation and optical response of gold after ultrafast XUV excitation

Free electron lasers offer unique properties to study matter in states far from equilibrium as they combine short pulses with a large range of photon energies. In particular, the possibility to excite core states drives new relaxation pathways that, in turn, also change the properties of the optically and chemically active electrons. Here, we present a theoretical model for the dynamics of the nonequilibrium occupation of the different energy bands in solid gold driven by exciting deep core states. The resulting optical response is in excellent agreement with recent measurements and, combined with our model, provides a quantitative benchmark for the description of electron-phonon coupling in strongly driven gold. Focusing on sub-picosecond time scales, we find essential differences between the dynamics induced by XUV and visible light.

Femtosecond Resolution of the Nonballistic Electron Energy Transport in Warm Dense Copper

The ultrafast electron energy transport is investigated in laser-heated warm dense copper in a high flux regime (2.5±0.7×10^{13}  W/cm^{2} absorbed). The dynamics of the electron temperature is retrieved from femtosecond time-resolved x-ray absorption near-edge spectroscopy near the Cu L3 edge. A characteristic time of ∼1  ps is observed for the increase in the average temperature in a 100 nm thick sample. Data are well reproduced by two-temperature hydrodynamic simulations, which support energy transport dominated by thermal conduction rather than ballistic electrons.

Electron Kinetics Induced by Ultrafast Photoexcitation of Warm Dense Matter in a 30-nm-Thick Foil

We report on the study of electron kinetics induced by intense femtosecond (fs) laser excitation of electrons in the 5d band of Au. Changes in the electron system are observed from the temporal evolution of ac conductivity and conduction electron density. The results reveal an increase of electron thermalization time with excitation energy density, contrary to the Fermi-liquid behavior of the decrease of thermalization time associated with the heating of conduction electrons. This is attributed to the severe mitigation of photoexcitation by Auger decay. The study also uncovers the shortening of 5d hole lifetime with the increase of photoexcitation rates. These unique findings provide valuable insights for understanding electron kinetics under extreme nonequilibrium conditions.

Ultrafast multi-cycle terahertz measurements of the electrical conductivity in strongly excited solids

Key insights in materials at extreme temperatures and pressures can be gained by accurate measurements that determine the electrical conductivity. Free-electron laser pulses can ionize and excite matter out of equilibrium on femtosecond time scales, modifying the electronic and ionic structures and enhancing electronic scattering properties. The transient evolution of the conductivity manifests the energy coupling from high temperature electrons to low temperature ions. Here we combine accelerator-based, high-brightness multi-cycle terahertz radiation with a single-shot electro-optic sampling technique to probe the evolution of DC electrical conductivity using terahertz transmission measurements on sub-picosecond time scales with a multi-undulator free electron laser. Our results allow the direct determination of the electron-electron and electron-ion scattering frequencies that are the major contributors of the electrical resistivity.

The electrical conductivity is critical to understand warm dense matter, but the accurate measurement is extremely challenging. Here the authors use multi-cycle THz pulses to measure the conductivity of gold foils strongly heated by free-electron laser, determining the individual contributions of electron-electron and electron-ion scattering.

Finite-temperature density-functional-theory investigation on the nonequilibrium transient warm-dense-matter state created by laser excitation

We present a finite-temperature density-functional-theory investigation of the nonequilibrium transient electronic structure of warm dense Li, Al, Cu, and Au created by laser excitation. Photons excite electrons either from the inner shell orbitals or from the valence bands according to the photon energy, and give rise to isochoric heating of the sample. Localized states related to the 3d orbital are observed for Cu when the hole lies in the inner shell 3s orbital. The electrical conductivity for these materials at nonequilibrium states is calculated using the Kubo-Greenwood formula. The change of the electrical conductivity, compared to the equilibrium state, is different for the case of holes in inner shell orbitals or the valence band. This is attributed to the competition of two factors: the shift of the orbital energies due to reduced screening of core electrons, and the increase of chemical potential due to the excitation of electrons. The finite-temperature effect of both the electrons and the ions on the electrical conductivity is discussed in detail. This work is helpful to better understand the physics of laser excitation experiments of warm dense matter.

Ultrafast Thermal Melting in Nonequilibrium Warm Dense Copper

The ultrafast dynamics of the loss of crystalline periodicity is investigated in femtosecond laser heated warm dense copper, by the original use of x-ray absorption near-edge specific structures just above the L3 edge. The characteristic time is observed near 1 ps, for specific energy density ranging from 1 to 5  MJ/kg, using ps-resolution x-ray absorption spectroscopy. The overall experimental data are well reproduced with two-temperature hydrodynamic simulations, supporting a thermal phase transition.

Heat-release equation of state and thermal conductivity of warm dense carbon by proton differential heating

Warm dense carbon is generated at 0.3-2.0 g/cc and 1-7 eV by proton heating. The release equation of state (EOS) after heating and thermal conductivity of warm dense carbon are studied experimentally in this regime using a Au/C dual-layer target to initiate a temperature gradient and two picosecond time-resolved diagnostics to probe the surface expansion and heat flow. Comparison between the data and simulations using various EOSs and thermal conductivity models is quantified with a statistical χ^{2} analysis. Out of seven EOS tables and five thermal conductivity models, only L9061 with the Lee-More model provides a probability above 50% to match all data.

First-Principles Determination of Electron-Ion Couplings in the Warm Dense Matter Regime

We present first-principles calculations of the rate of energy exchanges between electrons and ions in nonequilibrium warm dense plasmas, liquid metals, and hot solids, a fundamental property for which various models offer diverging predictions. To this end, a Kubo relation for the electron-ion coupling parameter is introduced, which includes self-consistently the quantum, thermal, nonlinear, and strong coupling effects that coexist in materials at the confluence of solids and plasmas. Most importantly, like other Kubo relations widely used for calculating electronic conductivities, the expression can be evaluated using quantum molecular dynamics simulations. Results are presented and compared to experimental and theoretical predictions for representative materials of various electronic complexity, including aluminum, copper, iron, and nickel.

Toward quasi-DC conductivity of warm dense matter measured by single-shot terahertz spectroscopy

We present an experimental setup capable of measuring the near DC conductivity of laser generated warm dense matter using single-shot terahertz time-domain spectroscopy. The setup uses a reflective echelon and balanced detection to record THz waveforms with a minimum detectable signal of 0.2% in a single laser pulse. We describe details of the experimental setup and the data analysis procedure and present single-shot terahertz transmission data on aluminum that has been laser heated to an electron temperature of 0.5 eV.

Nonequilibrium optical properties of transition metals upon ultrafast electron heating

Femtosecond laser excitation of metals triggers swift modifications of the electronic distribution within the band structure. This has direct consequences on optical transitions transiently modifying the optical properties of materials. Influencing in real time the action of the pulse, these changes lead to substantial variations of the amount and the distribution in the energy deposited during the laser irradiation. The effect of the laser pulse can be described considering electrons heated to a range of electronic temperatures. In order to evaluate the dielectric response of ultrafast heated electrons, we performed ab initio molecular dynamic simulations coupled to the Kubo-Greenwood formalism and determined electronic temperature dependent optical properties. A series of representative transition metals was investigated: Cu, Ni, Cr, W, Ti, and Fe. The evolution of the optical properties is optically-pumped based on electronic redistribution within the density of electronic states. The proposed interpretation rely on modifications of the energy range of occupied states undergoing optical electronic transitions. It is found that the degree of filling and the shape of the d-block drive the dynamics of optical processes. Nonequilibrium optical indices, reflectivities and skin depths are reported for electron thermal excitation relevant to near-threshold laser ablation regimes. The effect of the electron temperature on optical properties allows to reconstruct and model ultrafast excitation dynamics in time-resolved diagnostics with relevance in laser micro- and nano-processing.

Interatomic Potential in the Nonequilibrium Warm Dense Matter Regime

We present a new measurement of lattice disassembly times in femtosecond-laser-heated polycrystalline Au nanofoils. The results are compared with molecular dynamics simulations incorporating a highly optimized, embedded-atom-method interatomic potential. For absorbed energy densities of 0.9-4.3  MJ/kg, the agreement between the experiment and simulation reveals a single-crystal-like behavior of homogeneous melting and corroborates the applicability of the interatomic potential in the nonequilibrium warm dense matter regime. For energy densities below 0.9  MJ/kg, the measurement is consistent with nanocrystal behavior where melting is initiated at the grain boundaries.

Thermal conductivity measurements of proton-heated warm dense aluminum

Thermal conductivity is one of the most crucial physical properties of matter when it comes to understanding heat transport, hydrodynamic evolution, and energy balance in systems ranging from astrophysical objects to fusion plasmas. In the warm dense matter regime, experimental data are very scarce so that many theoretical models remain untested. Here we present the first thermal conductivity measurements of aluminum at 0.5–2.7 g/cc and 2–10 eV, using a recently developed platform of differential heating. A temperature gradient is induced in a Au/Al dual-layer target by proton heating, and subsequent heat flow from the hotter Au to the Al rear surface is detected by two simultaneous time-resolved diagnostics. A systematic data set allows for constraining both thermal conductivity and equation-of-state models. Simulations using Purgatorio model or Sesame S27314 for Al thermal conductivity and LEOS for Au/Al release equation-of-state show good agreement with data after 15 ps. Discrepancy still exists at early time 0–15 ps, likely due to non-equilibrium conditions.

Crossover from Classical to Fermi Liquid Behavior in Dense Plasmas

We explore the crossover from classical plasma to quantum Fermi liquid behavior of electrons in dense plasmas. To this end, we analyze the evolution with density and temperature of the momentum lifetime of a test electron introduced in a dense electron gas. This allows us (1) to determine the boundaries of the crossover region in the temperature-density plane and to shed light on the evolution of scattering properties across it, (2) to quantify the role of the fermionic nature of electrons on electronic collisions across the crossover region, and (3) to explain how the concept of the Coulomb logarithm emerges at a high enough temperature but disappears at a low enough temperature.

Photocatalytic activities and photoinduced fusion of gold-modified titania nanoparticle

Gold nanoparticles measuring 3–30 nm deposited on semiconductors result in an effective photocatalyst against several pollutants. Its photocatalytic activities are significant under both UV and solar irradiation. In a photocatalytic system, the oxidation of pollutants takes place on the gold surface as the electron donor, while the electron is consumed by the reduction of oxygen as the electron acceptor on the semiconductor’s surface. This promotes not only increased photocatalytic activities but also the green transformation of pollutant compounds to harmless compounds. The photosensitivity of semiconductors can be modified by tuning the size, shape, and contact of gold nanoparticles. This review highlights the function of gold nanoparticles in overcoming the limitation of transition metal oxide materials in photocatalytic applications.

Equation of state, phonons, and lattice stability of ultrafast warm dense matter

Using the two-temperature model for ultrafast matter (UFM), we compare the equation of state, pair-distribution functions g(r), and phonons using the neutral pseudoatom (NPA) model with results from density functional theory (DFT) codes and molecular dynamics (MD) simulations for Al, Li, and Na. The NPA approach uses state-dependent first-principles pseudopotentials from an "all-electron" DFT calculation with finite-T exchange-correlation functional (XCF). It provides pair potentials, structure factors, the "bound" and "free" states, as well as a mean ionization Z[over ¯] unambiguously. These are not easily accessible via DFT+MD calculations which become prohibitive for T/T_{F} exceeding ∼0.6, where T_{F} is the Fermi temperature. Hence, both DFT+MD and NPA methods can be compared up to ∼8eV, while higher T can be addressed via the NPA. The high-T_{e} phonon calculations raise the question of UFM lattice stability and surface ablation in thin UFM samples. The ablation forces in a UFM slab are used to define an "ablation time" competing with phonon formation times in thin UFM samples. Excellent agreement for all properties is found between NPA and standard DFT codes, even for Li where a strongly nonlocal pseudopotential is used in DFT codes. The need to use pseudopotentials appropriate to the ionization state Z[over ¯] is emphasized. The effect of finite-T XCF is illustrated via its effect on the pressure and the electron-density distribution at a nucleus.

Pair potentials for warm dense matter and their application to x-ray Thomson scattering in aluminum and beryllium

Ultrafast laser experiments yield increasingly reliable data on warm dense matter, but their interpretation requires theoretical models. We employ an efficient density functional neutral-pseudoatom hypernetted-chain (NPA-HNC) model with accuracy comparable to ab initio simulations and which provides first-principles pseudopotentials and pair potentials for warm-dense matter. It avoids the use of (i) ad hoc core-repulsion models and (ii) "Yukawa screening" and (iii) need not assume ion-electron thermal equilibrium. Computations of the x-ray Thomson scattering (XRTS) spectra of aluminum and beryllium are compared with recent experiments and with density-functional-theory molecular-dynamics (DFT-MD) simulations. The NPA-HNC structure factors, compressibilities, phonons, and conductivities agree closely with DFT-MD results, while Yukawa screening gives misleading results. The analysis of the XRTS data for two of the experiments, using two-temperature quasi-equilibrium models, is supported by calculations of their temperature relaxation times.

A single-shot spatial chirp method for measuring initial AC conductivity evolution of femtosecond laser pulse excited warm dense matter

To study the rapid evolution of AC conductivity from ultrafast laser excited warm dense matter (WDM), a spatial chirp single-shot method is developed utilizing a crossing angle pump-probe configuration. The pump beam is shaped individually in two spatial dimensions so that it can provide both sufficient laser intensity to excite the material to warm dense matter state and a uniform time window of up to 1 ps with sub-100 fs FWHM temporal resolution. Temporal evolution of AC conductivity in laser excited warm dense gold was also measured.

Atomic bound state and scattering properties of effective momentum-dependent potentials

Effective classical dynamics provide a potentially powerful avenue for modeling large-scale dynamical quantum systems. We have examined the accuracy of a Hamiltonian-based approach that employs effective momentum-dependent potentials (MDPs) within a molecular-dynamics framework through studies of atomic ground states, excited states, ionization energies, and scattering properties of continuum states. Working exclusively with the Kirschbaum-Wilets (KW) formulation with empirical MDPs [C. L. Kirschbaum and L. Wilets, Phys. Rev. A 21, 834 (1980)0556-279110.1103/PhysRevA.21.834], optimization leads to very accurate ground-state energies for several elements (e.g., N, F, Ne, Al, S, Ar, and Ca) relative to Hartree-Fock values. The KW MDP parameters obtained are found to be correlated, thereby revealing some degree of transferability in the empirically determined parameters. We have studied excited-state orbits of electron-ion pair to analyze the consequences of the MDP on the classical Coulomb catastrophe. From the optimized ground-state energies, we find that the experimental first- and second-ionization energies are fairly well predicted. Finally, electron-ion scattering was examined by comparing the predicted momentum transfer cross section to a semiclassical phase-shift calculation; optimizing the MDP parameters for the scattering process yielded rather poor results, suggesting a limitation of the use of the KW MDPs for plasmas.

dc conductivity of two-temperature warm dense gold

Using recently obtained ac conductivity data we have derived dc conductivity together with free electron density and electron momentum relaxation time in two-temperature warm dense gold with energy density up to 4.1 MJ/kg (0.8×10^{11}J/m^{3}). The derivation is based on a Drude interpretation of the dielectric function that takes into account contributions of intraband and interband transitions as well as atomic polarizability. The results provide valuable benchmarks for assessing the extended Ziman formula for electrical resistivity and an accompanying average atom model.

Linear dependence of surface expansion speed on initial plasma temperature in warm dense matter

Recent progress in laser-driven quasi-monoenergetic ion beams enabled the production of uniformly heated warm dense matter. Matter heated rapidly with this technique is under extreme temperatures and pressures, and promptly expands outward. While the expansion speed of an ideal plasma is known to have a square-root dependence on temperature, computer simulations presented here show a linear dependence of expansion speed on initial plasma temperature in the warm dense matter regime. The expansion of uniformly heated 1–100 eV solid density gold foils was modeled with the RAGE radiation-hydrodynamics code, and the average surface expansion speed was found to increase linearly with temperature. The origin of this linear dependence is explained by comparing predictions from the SESAME equation-of-state tables with those from the ideal gas equation-of-state. These simulations offer useful insight into the expansion of warm dense matter and motivate the application of optical shadowgraphy for temperature measurement.

Measurement of Electron-Ion Relaxation in Warm Dense Copper

Experimental investigation of electron-ion coupling and electron heat capacity of copper in warm and dense states are presented. From time-resolved x-ray absorption spectroscopy, the temporal evolution of electron temperature is obtained for non-equilibrium warm dense copper heated by an intense femtosecond laser pulse. Electron heat capacity and electron-ion coupling are inferred from the initial electron temperature and its decrease over 10 ps. Data are compared with various theoretical models.

Uniform heating of materials into the warm dense matter regime with laser-driven quasimonoenergetic ion beams

In a recent experiment at the Trident laser facility, a laser-driven beam of quasimonoenergetic aluminum ions was used to heat solid gold and diamond foils isochorically to 5.5 and 1.7 eV, respectively. Here theoretical calculations are presented that suggest the gold and diamond were heated uniformly by these laser-driven ion beams. According to calculations and SESAME equation-of-state tables, laser-driven aluminum ion beams achievable at Trident, with a finite energy spread of ΔE/E∼20%, are expected to heat the targets more uniformly than a beam of 140-MeV aluminum ions with zero energy spread. The robustness of the expected heating uniformity relative to the changes in the incident ion energy spectra is evaluated, and expected plasma temperatures of various target materials achievable with the current experimental platform are presented.

Free-electron X-ray laser measurements of collisional-damped plasmons in isochorically heated warm dense matter

We present the first highly resolved measurements of the plasmon spectrum in an ultrafast heated solid. Multi-keV x-ray photons from the Linac Coherent Light Source have been focused to one micrometer diameter focal spots producing solid density aluminum plasmas with a known electron density of n_{e}=1.8×10^{23}  cm^{-3}. Detailed balance is observed through the intensity ratio of up- and down-shifted plasmons in x-ray forward scattering spectra measuring the electron temperature. The plasmon damping is treated by electron-ion collision models beyond the Born approximation to determine the electrical conductivity of warm dense aluminum.

Ab initio calculation of the ion feature in x-ray Thomson scattering

The spectrum of x-ray Thomson scattering is proportional to the dynamic structure factor. An important contribution is the ion feature which describes elastic scattering of x rays off electrons. We apply an ab initio method for the calculation of the form factor of bound electrons, the slope of the screening cloud of free electrons, and the ion-ion structure factor in warm dense beryllium. With the presented method we can calculate the ion feature from first principles. These results will facilitate a better understanding of x-ray scattering in warm dense matter and an accurate measurement of ion temperatures which would allow determining nonequilibrium conditions, e.g., along shock propagation.

Resistivity saturation in warm dense matter

Electrical resistivity is shown to saturate in solid-density aluminum in the warm dense matter regime. Calculations are done using the average-atom model SCAALP and the finite-temperature Ziman-Evans formula for electrical resistivity. The mean free path is estimated using the Drude law. This mean free path is shown to present a minimum of the order of the interatomic spacing.

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-ray burst of 92-eV photon energy. A second pulse probes the sample via x-ray scattering at jitter-free variable time delay. We show that the initial molecular structure dissociates within (0.9 ± 0.2) ps, allowing us to infer the energy transfer rate between electrons and ions. We evaluate Saha and Thomas-Fermi ionization models in radiation hydrodynamics simulations, predicting plasma parameters that are subsequently used to calculate the static structure factor. A conductivity model for partially ionized plasma is validated by two-temperature density-functional theory coupled to molecular dynamic simulations and agrees with the experimental data. Our results provide important insights and the needed experimental data on transport properties of dense plasmas.

Resolving ultrafast heating of dense cryogenic hydrogen

We report on the dynamics of ultrafast heating in cryogenic hydrogen initiated by a ≲300  fs, 92 eV free electron laser x-ray burst. The rise of the x-ray scattering amplitude from a second x-ray pulse probes the transition from dense cryogenic molecular hydrogen to a nearly uncorrelated plasmalike structure, indicating an electron-ion equilibration time of ∼0.9  ps. The rise time agrees with radiation hydrodynamics simulations based on a conductivity model for partially ionized plasma that is validated by two-temperature density-functional theory.

Ultrafast short-range disordering of femtosecond-laser-heated warm dense aluminum

We have probed, with time-resolved x-ray absorption near-edge spectroscopy (XANES), a femtosecond-laser-heated aluminum foil with fluences up to 1  J/cm2. The spectra reveal a loss of the short-range order in a few picoseconds. This time scale is compared with the electron-ion equilibration time, calculated with a two-temperature model. Hydrodynamic simulations shed light on complex features that affect the foil dynamics, including progressive density change from solid to liquid (∼10  ps). In this density range, quantum molecular dynamics simulations indicate that XANES is a relevant probe of the ionic temperature.

Laser wakefield generated X-ray probe for femtosecond time-resolved measurements of ionization states of warm dense aluminum

We have developed a laser wakefield generated X-ray probe to directly measure the temporal evolution of the ionization states in warm dense aluminum by means of absorption spectroscopy. As a promising alternative to the free electron excited X-ray sources, Betatron X-ray radiation, with femtosecond pulse duration, provides a new technique to diagnose femtosecond to picosecond transitions in the atomic structure. The X-ray probe system consists of an adjustable Kirkpatrick-Baez (KB) microscope for focusing the Betatron emission to a small probe spot on the sample being measured, and a flat Potassium Acid Phthalate Bragg crystal spectrometer to measure the transmitted X-ray spectrum in the region of the aluminum K-edge absorption lines. An X-ray focal spot size of around 50 μm was achieved after reflection from the platinum-coated 10-cm-long KB microscope mirrors. Shot to shot positioning stability of the Betatron radiation was measured resulting in an rms shot to shot variation in spatial pointing on the sample of 16 μm. The entire probe setup had a spectral resolution of ~1.5 eV, a detection bandwidth of ~24 eV, and an overall photon throughput efficiency of the order of 10(-5). Approximately 10 photons were detected by the X-ray CCD per laser shot within the spectrally resolved detection band. Thus, it is expected that hundreds of shots will be required per absorption spectrum to clearly observe the K-shell absorption features expected from the ionization states of the warm dense aluminum.