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Nature Communications 12(1), 1638 (2021). doi:10.1038/s41467-021-21756-6, 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., Published by Nature Publishing Group UK, [London]

Spatially encoded measurements of transient optical transmissivity became a standard tool for temporal diagnostics of free-electron-laser (FEL) pulses, as well as for the arrival time measurements in X-ray pump and optical probe experiments. The modern experimental techniques can measure changes in optical coefficients with a temporal resolution better than 10 fs. This, in an ideal case, would imply a similar resolution for the temporal pulse properties and the arrival time jitter between the FEL and optical laser pulses. However, carrier transport within the material and out of its surface, as well as carrier recombination may, in addition, significantly decrease the number of carriers. This would strongly affect the transient optical properties, making the diagnostic measurement inaccurate. Below we analyze in detail the effects of those processes on the optical properties of XUV and soft X-ray irradiated Si$${_3}$$ 3 N$$_4$$ 4 , on sub-picosecond timescales. Si$${_3}$$ 3 N$$_4$$ 4 is a wide-gap insulating material widely used for FEL pulse diagnostics. Theoretical predictions are compared with the published results of two experiments at FERMI and LCLS facilities, and with our own recent measurement. The comparison indicates that three body Auger recombination strongly affects the optical response of Si$${_3}$$ 3 N$$_4$$ 4 after its collisional ionization stops. By deconvolving the contribution of Auger recombination, in future applications one could regain a high temporal resolution for the reconstruction of the FEL pulse properties measured with a Si$${_3}$$ 3 N$$_4$$ 4 -based diagnostics tool.

Optics express 28(18), 25664-25681 (2020). doi:10.1364/OE.396755, Proper diagnostics of intense free-electron laser (FEL) X-ray pulses is indisputably important for experimental data analysis as well as for the protection of beamline optical elements. New challenges for beam diagnostic methods are introduced by modern FEL facilities capable of delivering powerful pulses at megahertz (MHz) repetition rates. In this paper, we report the first characterization of a defocused MHz 13.5-nm beam generated by the free-electron laser in Hamburg (FLASH) using the method of multi-pulse desorption imprints in poly(methyl methacrylate)(PMMA). The beam fluence profile is reconstructed in a novel and highly accurate way that takes into account the nonlinear response of material removal to total dose delivered by multiple pulses. The algorithm is applied to experimental data of single-shot ablation imprints and multi-shot desorption imprints at both low (10 Hz) and high (1 MHz) repetition rates. Reconstructed response functions show a great agreement with the theoretical desorption response function model., Published by Soc., Washington, DC

Journal of synchrotron radiation 28(3), 688 - 706 (2021). doi:10.1107/S1600577521002551, The high-precision X-ray diffraction setup for work with diamond anvil cells (DACs) in interaction chamber 2 (IC2) of the High Energy Density instrument of the European X-ray Free-Electron Laser is described. This includes beamline optics, sample positioning and detector systems located in the multipurpose vacuum chamber. Concepts for pump–probe X-ray diffraction experiments in the DAC are described and their implementation demonstrated during the First User Community Assisted Commissioning experiment. X-ray heating and diffraction of Bi under pressure, obtained using 20 fs X-ray pulses at 17.8 keV and 2.2 MHz repetition, is illustrated through splitting of diffraction peaks, and interpreted employing finite element modeling of the sample chamber in the DAC., Published by Wiley-Blackwell, [S.l.]

The European physical journal / D 75, 224 (2021). doi:10.1140/epjd/s10053-021-00235-z, In this paper, we report on the Boltzmann kinetic equation approach adapted for simulations of warm dense matter created by irradiation of bulk gold with intense ultrashort X-ray pulses. X-rays can excite inner-shell electrons, which triggers creation of deep-lying core holes. Their relaxation, especially in heavier elements such as gold (atomic number Z=79) takes complicated pathways, involving collisional processes, and leading through a large number of active configurations. This number can be so high that solving a set of evolution equations for each configuration becomes computationally inefficient, and another modeling approach should be used instead. Here, we use the earlier introduced ���predominant excitation and relaxation path��� approach. It still uses true atomic configurations but limits their number by restricting material relaxation to a selected set of predominant pathways for material excitation and relaxation. With that, we obtain time-resolved predictions for excitation and relaxation in X-ray irradiated bulk of gold, including the respective change of gold optical properties. We compare the predictions with the available data from high-energy-density experiments. Their good agreement indicates ability of the Boltzmann kinetic equation approach to describe warm dense matter created from high-Z materials after their irradiation with X rays, which can be validated in future experiments., Published by Springer, Heidelberg

Physical review / A covering atomic, molecular, and optical physics and quantum information 101(6), 063412 (2020). doi:10.1103/PhysRevA.101.063412, Focused short-wavelength free-electron laser (FEL) pulses interacting with gas phase samples can induce by inner-shell ionization a short-lived population inversion, followed by coherent collective emission of directed, short, and strong radiation bursts. We extend our studies into the warm-dense matter (WDM) regime by investigating the nanoplasmas produced in an ensemble of nanometer-sized clusters by FEL irradiation. Here, additional pathways can also lead to strong, laserlike emission: Electron-ion collisions can yield a long-lived population inversion, and subsequent amplified spontaneous emission. We observe amplified spontaneous emission (ASE) in the extreme ultraviolet in xenon clusters excited by soft x-ray FEL pulses, we diagnose the generated nanoplasmas by fluorescence spectroscopy, and we study under various cluster and FEL parameters the directed ASE from the Xe$^{2+}$ 65 nm line. We show its exponential increase as a function of FEL irradiation power, and an accompanying collisional broadening of the emission spectra. These findings are corroborated by extensive numerical simulations based on theory, combining detailed hydrodynamic and kinetic simulations with time-dependent calculations of radiation transport, amplification, and collective emission in the WDM nanoplasma. Our theoretical findings underline that population inversion is due to electron-ion collisions and that the observed decoherence processes can be empirically characterized by a phenomenological decoherence time in the range of 100–200 fs., Published by Inst., Woodbury, NY

The free-free opacity in plasmas is fundamental to our understanding of energy transport in stellar interiors and for inertial confinement fusion research. However, theoretical predictions in the challenging dense plasma regime are conflicting and there is a dearth of accurate experimental data to allow for direct model validation. Here we present time-resolved transmission measurements in solid-density Al heated by an XUV free-electron laser. We use a novel functional optimization approach to extract the temperature-dependent absorption coefficient directly from an oversampled pool of single-shot measurements, and find a pronounced enhancement of the opacity as the plasma is heated to temperatures of order of the Fermi energy. Plasma heating and opacity enhancement are observed on ultrafast timescales, within the duration of the femtosecond XUV pulse. We attribute further rises in the opacity on ps timescales to melt and the formation of warm dense matter.

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 n$_e$ = 5 × 10$^{27}$ 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 × 10$^4$ 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.

Silicon, being one of the most abundant elements in nature, attracts wide-ranging scientific and technological interest. Specifically, in its elemental form, crystals of remarkable purity can be produced. One may assume that this would lead to silicon being well understood, and indeed, this is the case for many ambient properties, as well as for higher-pressure behaviour under quasi-static loading. However, despite many decades of study, a detailed understanding of the response of silicon to rapid compression—such as that experienced under shock impact—remains elusive. Here, we combine a novel free-electron laser-based X-ray diffraction geometry with laser-driven compression to elucidate the importance of shear generated during shock compression on the occurrence of phase transitions. We observe lowering of the hydrostatic phase boundary in elemental silicon, an ideal model system for investigating high-strength materials, analogous to planetary constituents. Moreover, we unambiguously determine the onset of melting above 14 GPa, previously ascribed to a solid–solid phase transition, undetectable in the now conventional shocked diffraction geometry; transitions to the liquid state are expected to be ubiquitous in all systems at sufficiently high pressures and temperatures.

The process of damage accumulation in thin ruthenium films exposed to multiple femtosecond extreme ultraviolet (XUV) free-electron laser (FEL) pulses below the critical angle of reflectance at the FEL facility in Hamburg (FLASH) was experimentally analyzed. The multi-shot damage threshold is found to be lower than the single-shot damage threshold. Detailed analysis of the damage morphology and its dependence on irradiation conditions justifies the assumption that cavitation induced by the FEL pulse is the prime mechanism responsible for multi-shot damage in optical coatings.

Optics express 26(15), 19665 - 19685 (2018). doi:10.1364/OE.26.019665, Ruthenium is a perspective material to be used for XUV mirrors at free-electron laser facilities. Yet, it is still poorly studied in the context of ultrafast laser-matter interaction. In this work, we present single-shot damage studies of thin Ru films irradiated by femtosecond XUV free-electron laser pulses at FLASH. Ex-situ analysis of the damaged spots, performed by different types of microscopy, shows that the weakest detected damage is surface roughening. For higher fluences we observe ablation of Ru. Combined simulations using Monte-Carlo code XCASCADE(3D) and the two-temperature model reveal that the damage mechanism is photomechanical spallation, similar to the case of irradiating the target with optical lasers. The analogy with the optical damage studies enables us to explain the observed damage morphologies., Published by OSA, Washington, DC

Applied physics letters 113(11), 114102 (2018). doi:10.1063/1.5046070, Recent studies confirmed that the materials used in the extreme UV and soft X-ray regime for precise characterization of intense free-electron laser pulses (e.g., Si$_3$N$_4$) do not work efficiently in the hard X-ray regime, which is due to the fact that the impact of a hard X-ray photon is followed by a series of electron cascading processes. Following theoretical indication, we show that this limitation can be circumvented and the cascading time can be significantly reduced if the X-ray photon energy is double the ionization energy. We investigate an alternative material for pulse diagnostics, SnO$_2$, using the Linac Coherent Light Source at photon energies of 5 keV and 9 keV. We prove the validity of the concept and show that it has a large potential for practical applications. By applying the proposed criteria, the temporal accuracy of the non-invasive pulse diagnostic tools can be improved in current and emerging hard X-ray facilities., Published by American Inst. of Physics, Melville, NY

Nature Communications 9(1), 2860 (2018). doi:10.1038/s41467-018-05365-4, The physics and chemistry of liquid solutions play a central role in science, and our understanding of life on Earth. Unfortunately, key tools for interrogating aqueous systems, such as infrared and soft X-ray spectroscopy, cannot readily be applied because of strong absorption in water. Here we use gas-dynamic forces to generate free-flowing, sub-micron, liquid sheets which are two orders of magnitude thinner than anything previously reported. Optical, infrared, and X-ray spectroscopies are used to characterize the sheets, which are found to be tunable in thickness from over 1 μm down to less than 20 nm, which corresponds to fewer than 100 water molecules thick. At this thickness, aqueous sheets can readily transmit photons across the spectrum, leading to potentially transformative applications in infrared, X-ray, electron spectroscopies and beyond. The ultrathin sheets are stable for days in vacuum, and we demonstrate their use at free-electron laser and synchrotron light sources., Published by Nature Publishing Group UK, [London]

Physical review letters 109, 245003 (2012). doi:10.1103/PhysRevLett.109.245003, Published by APS, College Park, Md.

We report on soft X-ray scattering experiments on cryogenic hydrogen and simple metal samples. As a source of intense, ultrashort soft X-ray pulses we have used free-electron laser radiation at 92 eV photon energy from FLASH at DESY, Hamburg. X-ray pulses with energies up to 150 μJ and durations 15-50 fs provide interaction with the sample leading simultaneously to plasma formation and scattering. Experiments exploiting both of these interactions have been carried out, using the same experimental setup. Firstly, recording of soft X-ray inelastic scattering from near-solid density hydrogen plasmas at few electron volt temperatures confirms the feasibility of this diagnostics technique. Secondly, the soft X-ray excitation of few electron volt solid-density plasmas in bulk metal samples could be studied by recording soft X-ray line and continuum emission integrated over emission times from fs to ns. © 2009 Elsevier B.V.

We report on the x-ray absorption of Warm Dense Matter experiment at the FLASH Free Electron Laser (FEL) facility at DESY. The FEL beam is used to produce Warm Dense Matter with soft x-ray absorption as the probe of electronic structure. A multilayer-coated parabolic mirror focuses the FEL radiation, to spot sizes as small as 0.3μm in a ∼15fs pulse of containing >10 12 photons at 13.5 nm wavelength, onto a thin sample. Silicon photodiodes measure the transmitted and reflected beams, while spectroscopy provides detailed measurement of the temperature of the sample. The goal is to measure over a range of intensities approaching 10 18 W/cm 2. Experimental results will be presented along with theoretical calculations. A brief report on future FEL efforts will be given. © 2009 SPIE.

International audience; An irreversible response of inorganic scintillators to intense soft x-ray laser radiation was investigated at the FLASH (Free-electron LASer in Hamburg) facility. Three ionic crystals, namely, Ce:YAG (cerium-doped yttrium aluminum garnet), PbWO4 (lead tungstate), and ZnO (zinc oxide), were exposed to single 4.6 nm ultra-short laser pulses of variable pulse energy (up to 12 μJ) under normal incidence conditions with tight focus. Damaged areas produced with various levels of pulse fluences, were analyzed on the surface of irradiated samples using differential interference contrast (DIC) and atomic force microscopy (AFM). The effective beam area of 22.2 ± 2.2 μm2 was determined by means of the ablation imprints method with the use of poly(methyl methacrylate) - PMMA. Applied to the three inorganic materials, this procedure gave almost the same values of an effective area. The single-shot damage threshold fluence was determined for each of these inorganic materials. The Ce:YAG sample seems to be the most radiation resistant under the given irradiation conditions, its damage threshold was determined to be as high as 660.8 ± 71.2 mJ/cm2. Contrary to that, the PbWO4 sample exhibited the lowest radiation resistance with a threshold fluence of 62.6 ± 11.9 mJ/cm2. The threshold for ZnO was found to be 167.8 ± 30.8 mJ/cm2. Both interaction and material characteristics responsible for the damage threshold difference are discussed in the article.

International audience; Taking advantage of the new opportunities provided by x-ray free electron laser (FEL) sources when coupled to a long laser pulse as available at the Linear Coherent Light Source (LCLS), we have performed x-ray absorption near-edge spectroscopy (XANES) of laser shock compressed iron up to 420 GPa (±50) and 10 800 K (±1390). Visible diagnostics coupled with hydrodynamic simulations were used to infer the thermodynamical conditions along the Hugoniot and the release adiabat. A modification of the pre-edge feature at 7.12 keV in the XANES spectra is observed above pressures of 260 GPa along the Hugoniot. Comparing with ab initio calculations and with previous laser-heated diamond cell data, we propose that such changes in the XANES pre-edge could be a signature of molten iron. This interpretation then suggests that iron is molten at pressures and temperatures higher than 260 GPa (±29) and 5680 K (±700) along the principal Fe Hugoniot.

International audience; The time evolution of the electron structure is investigated in a molybdenum foil heated up to the warm dense matter regime by a femtosecond laser pulse, through time-resolved x-ray absorption near-edge spectroscopy. Spectra are measured with independent characterizations of temperature and density. They are successfully compared with ab initio quantum molecular dynamic calculations. We demonstrate that the observed white line in the L-3 edge reveals the time evolution of the electron density of state from the solid to the hot (a few eV) and expanding liquid. The data indicate a highly nonequilibrated state, 5 ps after heating.

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 â‰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. © 2014 American Physical Society.

International audience; The rapidly growing ultrafast science with X-ray lasers unveils atomic scale processes with unprecedented time resolution bringing the so called "molecular movie'' within reach. X-ray absorption spectroscopy is one of the most powerful x-ray techniques providing both local atomic order and electronic structure when coupled with ad-hoc theory. Collecting absorption spectra within few x-ray pulses is possible only in a dispersive setup. We demonstrate ultrafast time-resolved measurements of the LIII-edge x-ray absorption near-edge spectra of irreversibly laser excited Molybdenum using an average of only few x-ray pulses with a signal to noise ratio limited only by the saturation level of the detector. The simplicity of the experimental set-up makes this technique versatile and applicable for a wide range of pump-probe experiments, particularly in the case of non-reversible processes.

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. © 2014 American Physical Society.

A theoretical study of the ultrafast electron kinetics in solid SiO2, irradiated with a femtosecond X-ray laser pulse, is presented. A Monte Carlo code for event-by-event simulations of individual particles is applied to model the electron kinetics within the irradiated SiO2 bulk. The simulation includes photoionization, elastic and inelastic scatterings of electrons, Auger decays of core holes, and electron-hole recombination via exciton self-trapping mechanism. Transient electron density is followed, at different photon energies and pulse durations. The change of the optical properties (reflectance, transmittance) of the material is estimated with the help of the Drude model. The analysis of the results allows us to conclude that within the X-ray excited dielectric, the holes in the valence band give the predominant contribution to the transient changes of optical properties within the material. The increase rate of the free electron density is limited by the duration of secondary electron cascading. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; Recently, few-femtosecond pulses have become available at hard X-ray free-electron lasers. Coupled with the available sub-10 fs optical pulses, investigations into few-femtosecond dynamics are not far off. However, achieving sufficient synchronization between optical lasers and X-ray pulses continues to be challenging. We report a 'measure-and-sort' approach, which achieves sub-10 fs root-mean-squared (r.m.s.) error measurement at hard X-ray FELs, far beyond the 100-200 fs r.m.s. jitter limitations. This timing diagnostic, now routinely available at the Linac Coherent Light Source (LCLS), is based on ultrafast free-carrier generation in optically transparent materials. Correlation between two independent measurements enables unambiguous demonstration of ~6 fs r.m.s. error in reporting the optical/X-ray delay, with single shot error suggesting the possibility of reaching few-femtosecond resolution.

We studied experimentally and theoretically the structural transition of diamond under an irradiation with an intense femtosecond extreme ultraviolet laser (XUV) pulse of 24–275 eV photon energy provided by free-electron lasers. Experimental results obtained show that the irradiated diamond undergoes a solid-to-solid phase transition to graphite, and not to an amorphous state. Our theoretical findings suggest that the nature of this transition is nonthermal, stimulated by a change of the interatomic potential triggered by the excitation of valence electrons. Ultrashort laser pulse duration enables to identify the subsequent steps of this process: electron excitation, band gap collapse, and the following atomic motion. A good agreement between the experimentally measured and theoretically calculated damage thresholds for the XUV range supports our conclusions.

In this article, we describe the experimental station and procedures for investigating the interaction of short-wavelength free-electron lasers (FELs) pulses with solids. With the advent of these sources, a unique combination of radiation properties (including wavelength range from tens of nanometers down to sub-Angstroms, femtosecond pulse duration, and high pulse energy reaching milli-Joules level) creates new research possibilities for the systematic studies of radiation-induced structural changes in solids. However, the properties of the intense FEL radiation generate, apart from the new experimental opportunities, extreme demands on the experimental set-up (mostly in terms of radiation hardness of detectors and their saturation levels). Thus, radiation-induced phase transitions in solids, beyond the fundamental scientific interest, are of importance for the design of FEL beamlines and instruments which interact with the direct beam. In this report, we focus on the instrumentation and experimental techniques used in the recent studies performed at the FLASH facility in Hamburg.

Nature Communications 4, 1731 (2013). doi:10.1038/ncomms2754, The resolution of ultrafast studies performed at extreme ultraviolet and X-ray free-electron lasers is still limited by shot-to-shot variations of the temporal pulse characteristics. Here we show a versatile single-shot temporal diagnostic tool that allows the determination of the extreme ultraviolet pulse duration and the relative arrival time with respect to an external pump-probe laser pulse. This method is based on time-resolved optical probing of the transient reflectivity change due to linear absorption of the extreme ultraviolet pulse within a solid material. In this work, we present measurements performed at the FLASH free-electron laser. We determine the pulse duration at two distinct wavelengths, yielding (184±14) fs at 41.5 nm and (21±19) fs at 5.5 nm. Furthermore, we demonstrate the feasibility to operate the tool as an online diagnostic by using a 20-nm-thin Si3N4 membrane as target. Our results are supported by detailed numerical and analytical investigations., Published by Nature Publishing Group, London

We have used the Linac Coherent Light Source to generate solid-density aluminum plasmas at temperatures of up to 180 eV. By varying the photon energy of the x rays that both create and probe the plasma, and observing the $K\mathrm{\text{\ensuremath{-}}}\ensuremath{\alpha}$ fluorescence, we can directly measure the position of the $K$ edge of the highly charged ions within the system. The results are found to disagree with the predictions of the extensively used Stewart-Pyatt model, but are consistent with the earlier model of Ecker and Kr\"oll, which predicts significantly greater depression of the ionization potential.

The interaction of free electron laser pulses with grating structure is investigated using 4.6 +/- 0.1 nm radiation at the FLASH facility in Hamburg. For fluences above 63.7 +/- 8.7 mJ/cm(2), the interaction triggers a damage process starting at the edge of the grating structure as evidenced by optical and atomic force microscopy. Simulations based on solution of the Helmholtz equation demonstrate an enhancement of the electric field intensity distribution at the edge of the grating structure. A procedure is finally deduced to evaluate damage threshold. (C) 2012 Optical Society of America

We introduce a time-domain approach to explore rotational dynamics caused by intramolecular coupling or the interaction with dissipative media. It pushes the time resolution toward the ultimate limit determined by the rotational period. Femtosecond pulses create a coherent superposition of two rotational states of carbon monoxide. The wave-packet motion is observed by subsequent Coulomb explosion, which results in a time-dependent asymmetry of spatial fragmentation patterns. The asymmetry oscillation prevails for at least 1 ns, covering more than 300 periods with no decoherence. Long time scans will allow weak perturbations of the order of $\ensuremath{\Delta}E/E={10}^{\ensuremath{-}4}$ to be discerned. Our conclusions are confirmed by a fully quantum-mechanical model.

Pulse intensities greater than 10(17) Watt/cm(2) were reached at the FLASH soft X-ray laser in Hamburg, Germany, using an off-axis parabolic mirror to focus 15 fs pulses of 5-70 mu J energy at 13.5 nm wavelength to a micron-sized spot. We describe the interaction of such pulses with niobium and vanadium targets and their deuterides. The beam produced craters in the solid targets, and we measured the kinetic energy of ions ejected from these craters. Ions with several keV kinetic energy were observed from craters approaching 5 mu m in depth when the sample was at best focus. We also observed the onset of saturation in both ion acceleration and ablation with pulse intensities exceeding 10(16) W/cm(2), when the highest detected ion energies and the crater depths tend to saturate with increasing intensity. A general difficulty in working with micron and sub-micron focusing optics is finding the exact focus of the beam inside a vacuum chamber. Here we propose a direct method to measure the focal position to a resolution better than the Rayleigh length. The method is based on the correlation between the energies of ejected ions and the physical dimensions of the craters. We find that the focus position can be quickly determined from the ion time-of-flight (TOF) data as the target is scanned through the expected focal region. The method does not require external access to the sample or venting the vacuum chamber. Profile fitting employed to analyze the TOF data can extend resolution beyond the actual scanning step size. (C) 2011 Elsevier B.V. All rights reserved.

We present results of damage studies performed at the Free-Electron LASer in Hamburg (FLASH) on amorphous carbon (a-C). The experiment was performed in the total external reflection geometry representing the working configuration of X-ray mirrors. The 177 eV photon laser beam was focused on a 40 nm thin layer of a-C coated on a silicon substrate. Single-shot damages were produced at two different grazing angles (4.3 degrees and 7.7 degrees) below the critical angle (0(c)=8 degrees) calculated for this photon energy. The corresponding damage fluence thresholds were determined to be 43.7 and 74 mJ/cm(2). No evidence of delamination or mechanical damages (cracks) under grazing angles on the 40 nm thin sample was observed. The results are compared with the results of the previous experiment done on thicker layers (900 nm) of a-C with the same experimental conditions [1]. (C) 2010 Elsevier B.V. All rights reserved.

As liver fibrosis is the result of persistent necroinflammation in the liver, pro-inflammatory cytokines secreted in response to cell injury have a central role in the pathogenesis of liver fibrosis. We aimed to investigate the association of cytokine gene polymorphism and liver fibrosis among Chinese patients with chronic hepatitis B.Polymorphisms at interleukin-10 (IL-10-627, -1117), interleukin-1-beta (IL-1beta-511, -31, -3964), interleukin-1 receptor antagonist (IL-1RN), and tumor necrosis factor-alpha (TNF-alpha-308, -238) among Chinese chronic hepatitis B patients were determined. Severe liver fibrosis was defined as Ishak fibrosis score = 4 (of 6).Fifty-nine of 273 (22%) patients had severe fibrosis. The distribution of genotypes for IL-10-627 was CC (11%), CA (41%), and AA (48%). The CC genotype at IL-10-627 was protective against severe fibrosis (odds ratio (OR) 0.11; 95% CI 0.014-0.82; P = 0.032). After adjusted for baseline variables, the adjusted OR of CC genotypes at IL-10-627 for severe fibrosis was 0.063 (95% CI 0.06-0.64; P = 0.063). Other gene polymorphisms at IL-1beta, IL-1RN, TNF-alpha, and IL-10 had no significant association with severe fibrosis. Weak linkage disequilibrium was observed between IL-10-627 and IL-10-1117 with linkage disequilibrium coefficient of 0.12 (P < 0.001). The distribution of haplotypes of IL-10-1117 and IL-10-627 was A-A (69%), A-C (26%), and G-C (5%). High and intermediate IL-10 production (A-C and G-C) haplotypes were protective against severe fibrosis (OR 0.62; 95% CI 0.39-0.99; P = 0.046).High production genotype and haplotypes of IL-10 were associated with less severe liver fibrosis in chronic hepatitis B in Chinese.

We investigate ultrafast (fs) electron dynamics in a liquid hydrogen sample, isochorically and volumetrically heated to a moderately coupled plasma state. Thomson scattering measurements using 91.8 eV photons from the free-electron laser in Hamburg (FLASH at DESY) show that the hydrogen plasma has been driven to a nonthermal state with an electron temperature of 13 eV and an ion temperature below 0.1 eV, while the free-electron density is 2.8x10{20} cm{-3}. For dense plasmas, our experimental data strongly support a nonequilibrium kinetics model that uses impact ionization cross sections based on classical free-electron collisions.

We present a new technique for the characterization of non-Gaussian laser beams which cannot be described by an analytical formula. As a generalization of the beam spot area we apply and refine the definition of so called effective area (A(eff)) [1] in order to avoid using the full-width at half maximum (FWHM) parameter which is inappropriate for non-Gaussian beams. Furthermore, we demonstrate a practical utilization of our technique for a femtosecond soft X-ray free-electron laser. The ablative imprints in poly(methyl methacrylate) - PMMA and amorphous carbon (a-C) are used to characterize the spatial beam profile and to determine the effective area. Two procedures of the effective area determination are presented in this work. An F-scan method, newly developed in this paper, appears to be a good candidate for the spatial beam diagnostics applicable to lasers of various kinds. (C) 2010 Optical Society of America

The introduction of brilliant free-electron lasers enables new pump-probe experiments to characterize warm dense matter states. For instance, a short-pulse optical laser irradiates a liquid hydrogen jet that is subsequently probed with brilliant soft x-ray radiation. The strongly inhomogeneous plasma prepared by the optical laser is characterized with particle-in-cell simulations. The interaction of the soft x-ray probe radiation for different time delays between pump and probe with the inhomogeneous plasma is also taken into account via radiative hydrodynamic simulations. We calculate the respective scattering spectrum based on the Born-Mermin approximation for the dynamic structure factor considering the full density and temperature-dependent Thomson scattering cross section throughout the target. We can identify plasmon modes that are generated in different target regions and monitor their temporal evolution. Therefore, such pump-probe experiments are promising tools not only to measure the important plasma parameters density and temperature but also to gain valuable information about their time-dependent profile through the target. The method described here can be applied to various pump-probe scenarios by combining optical lasers and soft x ray, as well as x-ray sources.

A compact and light weight soft X-ray spectrograph covering 5−35 nm and employing a toroidal mirror and a variable line space reflection grating has been newly developed. Particular emphasis has been placed on achieving a large collection solid angle (1.9 × 10−3 sr) and a high efficiency of the components in order to enable Thomson Scattering plasma diagnostics which has a small total cross section (6.65 × 10−25 cm2). The instrument achieves a signal-to-noise ratio of 5 with a 13.5 nm source which isotropically emits 2.5 × 105 photons. A resolution λ/Δλ = 330 was measured at 21 nm and the dispersion was calibrated. The instrument is housed inside a DN 100 CF ultra high vacuum manipulator (43 × 46 × 47 cm3) which allows positioning relative to the source within ±5 mm and ±50 mm in X,Y and Z direction, respectively. It can be used with or without entrance pinhole and is equipped with a motorized grating, a filter wheel with five filters, and a shutter. Altogether, these features make the spectrograph a versatile instrument which can be employed in a variety of physics applications such as line and bremsstrahlung spectroscopy or Thomson scattering.