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This paper describes how to efficiently solve time-dependent X-ray dynamic diffraction problems in distorted crystals with a fast Fourier transform based beam propagation method. Examples are given of using the technique to simulate the propagation of X-ray beams in deformed crystals in space and time domains relevant to the cavity-based X-ray free-electron lasers and X-ray free-electron laser self-seeding systems.

We present an x-ray regenerative amplifier free-electron laser design capable of producing fully coherent hard x-ray pulses across a broad tuning range at a high steady state repetition rate. The scheme leverages a strong undulator taper and an apertured diamond output-coupling cavity crystal to produce both high peak and average spectral brightness radiation that is 2 to 3 orders of magnitude greater than conventional single-pass self-amplified spontaneous emission free-electron laser amplifiers. Refractive guiding in the postsaturation regime is found to play a key role in passively controlling the stored cavity power. The scheme is explored both analytically and numerically in the context of the Linac Coherent Light Source II High Energy upgrade.

Physical review applied 14(3), 034057 (2020). doi:10.1103/PhysRevApplied.14.034057, Solids irradiated by energetic photons can be eroded in two modes, depending on the radiation intensity. High average, low-peak power sources, e.g., synchrotron radiation and high-order harmonics, induce desorption of the material at a low etch rate. In contrast, high-peak-power radiation from extreme ultraviolet and x-ray lasers usually causes a massive removal of the material even by a single shot. In this contribution, an effective material erosion is reported in PMMA exposed to multiple accumulated pulses generated by the free-electron x-ray-laser Linac Coherent Light Source (LCLS, tuned at a photon energy of 830 eV in this study, operated in Menlo Park at Stanford, CA, USA) at a fluence below the single-pulse ablation threshold. The effect is caused by polymer-chain scissions initiated by single photons carrying enough energy to break the C$—$C bounds. High efficiency of the erosion is supposed to occur due to a correlation of the single-photon effects. The subthreshold damage exhibits a nonlinear dose dependence resulting from a competition between chain scissions and cross-linking processes. The cross-linking is proven by Raman spectroscopy of the irradiated polymer. Two theoretical models of the x-ray free-electron-laser-induced erosion are suggested, which provide an excellent agreement with the experimental results., Published by American Physical Society, College Park, Md. [u.a.]

This presentation will focus on solving certain x-ray dynamic diffraction problems using a beam propagation method (BPM). I will show examples of using the BPM method in: 1) simulating performance of non-perfect multilayer mirrors and focusing optics, such as multilayer Laue lenses (MML), 2) predicting damage thresholds of diffraction elements such as diffraction gratings, 3) modeling scattering of x-rays form deformed crystal. A special emphasis will be placed on including imperfection and thermal deformation in simulations of performance of x-ray optical elements. Some of simulation results will be related to experimental data. I will also compare the BPM method with other approaches to theory of x-ray dynamic diffraction.

We discuss how by using the wake fields generating in a corrugating plate we may increase the resolution of a transverse diagnostic cavity while producing an extremely short X-ray pulse at LCLS.

A growing number of X-ray sources based on the free-electron laser (XFEL) principle are presently under construction or have recently started operation. The intense, ultrashort pulses of these sources will enable new insights in many different fields of science. A key problem is to provide x-ray optical elements capable of collecting the largest possible fraction of the radiation and to focus into the smallest possible focus. As a key step towards this goal, we demonstrate here the first nanofocusing of hard XFEL pulses. We developed diamond based Fresnel zone plates capable of withstanding the full beam of the world's most powerful x-ray laser. Using an imprint technique, we measured the focal spot size, which was limited to 320 nm FWHM by the spectral band width of the source. A peak power density in the focal spot of 4×10(17)W/cm(2) was obtained at 70 fs pulse length.

Measurements of the soft x-ray, self-seeding spectrum at the Linac Coherent Light Source (LCLS) free-electron laser generally display a pedestal-like distribution around the central seeded wavelength. This pedestal limits the spectral purity and can negatively affect some user applications not employing a post-undulator monochromator. In this paper, we investigate the detailed experimental characteristics of both the amplified seed and its accompanying pedestal using data from a number of separate LCLS shifts over the 2015-2018 time period. We find that the amplified seed shows excellent wavelength stability and an exponential growth rate whose dependence upon energy detuning is consistent with theory. The pedestal's spectral distribution and integrated strength vary strongly shot by shot, independent of electron beam energy jitter. Its shot-averaged strength relative to that of the seed grows at least linearly with $z$ and can approach values of 15% or more. The pedestal is comprised of two separate components: (1) normal self-amplified spontaneous emission (SASE) whose total strength is nominally insensitive to energy detuning and laser heater (LH) strength; (2) sideband-like emission whose strength positively correlates with that of the amplified seed and negatively with energy detuning and LH strength. We believe this latter, non-SASE component arises from comparatively long wavelength (i.e., $\ensuremath{\lambda}\ensuremath{\sim}0.3--3\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$) amplitude and phase modulations of the main seeded radiation line. Its shot to shot variability and LH sensitivity suggests an origin connected to growth of the longitudinal microbunching instability on the electron beam.

In this work we present the application of a 2D single grating wavefront sensor to align and characterize the 100 nm focus at the Coherent X-ray Imaging (CXI) endstation at the Linac Coherent Light Source (LCLS). The results agree well with a model of the system, indicating that the mirrors perform as designed when alignment is optimized. In addition, a comparison with the imprint technique confirms the validity of the results, which showed that wavefront-based alignment resulted in negligible astigmatism. Analysis of the retrieved focus profile indicates that intensities

For the LCLS-II X-ray instruments, we have developed laser power meters as compact X-ray power monitors. A calibration of the responsivity of the power meters was carried out against a silicon photodiode with synchrotron radiation and a gas monitor detector with FEL X-rays. A manipulator with two power meters was installed in various locations at the LCLS. In the LCLS front end, the power meters were compared with the gas detectors, which are calibrated by the electron energy loss method. The agreement between the power meters and the gas detectors was better than 20% at 1500 eV with the pulse energy measured by the gas detectors higher than that from the power meters. In the AMO instrument, the power meters evaluated the improvement in beamline transmission caused by the oxygen plasma cleaning of the Kirkpatrick-Baez mirrors. Measurements were also conducted one and two years later to observe the effect of further contamination of the optical surfaces. Finally at the SXR instrument, the power meters determined the pulse energy at the sample for a beamtime, where the X-ray intensity was an important parameter.

The quantum mechanical motion of electrons in molecules and solids occurs on the sub-femtosecond timescale. Consequently, the study of ultrafast electronic phenomena requires thegeneration of laser pulses shorter than 1 fs and of sufficient intensity to interact with their targetwith high probability. Probing these dynamics with atomic-site specificity requires the extensionof sub-femtosecond pulses to the soft X-ray spectral region. Here we report the generation of iso-lated soft X-ray attosecond pulses with an X-ray free-electron laser. Our source has a pulse energythat is a million times larger than any other source of isolated attosecond pulses in the soft X-rayspectral region, with a peak power exceeding 100 GW. This unique combination of high intensity,high photon energy and short pulse duration enables the investigation of electron dynamics withX-ray non-linear spectroscopy and single-particle imaging, unlocking a path towards a new era ofattosecond science.

Physical review / A covering atomic, molecular, and optical physics and quantum information 100(5), 053420 (2019). doi:10.1103/PhysRevA.100.053420, Highly brilliant, coherent, femtosecond x-ray pulses delivered by free-electron lasers (FELs) constitute oneof the pillars of modern ultrafast science. Next generation FEL facilities provide up to megahertz repetitionrates and pulse durations down to the attosecond regime utilizing self-amplification of spontaneous emission.However, the stochastic nature of this generation mechanism demands single-shot pulse characterization toperform meaningful experiments. Here we demonstrate a fast yet robust online analysis technique capableof megahertz-rate mapping of the temporal intensity structure and arrival time of x-ray FEL pulses withfew-femtosecond resolution. We performed angular streaking measurements of both neon photo- and Augerelectrons and show their applicability for a direct time-domain feedback system during ongoing experiments.The fidelity of the real-time pulse characterization algorithm is corroborated by resolving isolated x-ray pulsesand double pulse trains with few-femtosecond substructure, thus paving the way for x-ray-pump–x-ray-probeFEL science at repetition rates compatible with the demands of LCLS-II and European XFEL., Published by Inst., Woodbury, NY

The journal of chemical physics 149(23), 234707 (2018). doi:10.1063/1.5044579, We report on atom-specific activation of CO oxidation on Ru(0001) via resonant X-ray excitation. We show that resonant 1s core-level excitation of atomically adsorbed oxygen in the co-adsorbed phase of CO and oxygen directly drives CO oxidation. We separate this direct resonant channel from indirectly driven oxidation via X-ray induced substrate heating. Based on density functional theory calculations, we identify the valence-excited state created by the Auger decay as the driving electronic state for direct CO oxidation. We utilized the fresh-slice multi-pulse mode at the Linac Coherent Light Source that provided time-overlapped and 30 fs delayed pairs of soft X-ray pulses and discuss the prospects of femtosecond X-ray pump X-ray spectroscopy probe, as well as X-ray two-pulse correlation measurements for fundamental investigations of chemical reactions via selective X-ray excitation., Published by American Institute of Physics, Melville, NY

A sustained filamentation or density depression phenomenon in an argon gas attenuator servicing a high-repetition femtosecond X-ray free-electron laser has been studied using a finite-difference method applied to the thermal diffusion equation for an ideal gas. A steady-state solution was obtained by assuming continuous-wave input of an equivalent time-averaged beam power and that the pressure of the entire gas volume has reached equilibrium. Both radial and axial temperature/density gradients were found and describable as filamentation or density depression previously reported for a femtosecond optical laser of similar attributes. The effect exhibits complex dependence on the input power, the desired attenuation, and the geometries of the beam and the attenuator. Time-dependent simulations were carried out to further elucidate the evolution of the temperature/density gradients in between pulses, from which the actual attenuation received by any given pulse can be properly calculated.

The silicon sample irradiated with femtosecond soft X-ray pulses at the Linac Coherent Light Source has been studied with several synchrotron X-ray diffraction topographic methods at HASYLAB. The irradiations were performed for two different wavelengths combined with various impact energy controlled by means of the gas attenuator. The topographic investigation revealed characteristic images of the created craters included the inner region reflecting the X-rays at lower angle, coming most probably from part of the silicon melted during the irradiation. The melted region was surrounded by strained outer region, similar to those observed in the case of rod-like inclusion but less regular in view of some irregularity of the beam used for generation of the craters. It was observed that the higher impact energy higher dose of the irradiating pulses resulted in increasing diameter of the melted area of the crater and the range of the strained region around it. Some features of the monochromatic and white beam back reflection section images of the craters were reproduced in numerically simulated images approximating the strain field in the crater by a droplet containing uniformly distributed point inclusions.

s Using a beam propagation method the focusing properties of a one-dimensional wedged multilayer Laue lens with imperfections have been investigated theoretically. The calculations were done for a lens focusing 20 keV X-rays to

A description of the Atomic, Molecular and Optical Sciences (AMO) instrument at the Linac Coherent Light Source is presented. Recent scientific highlights illustrate the imaging, time-resolved spectroscopy and high-power density capabilities of the AMO instrument., The Atomic, Molecular and Optical Science (AMO) instrument at the Linac Coherent Light Source (LCLS) provides a tight soft X-ray focus into one of three experimental endstations. The flexible instrument design is optimized for studying a wide variety of phenomena requiring peak intensity. There is a suite of spectrometers and two photon area detectors available. An optional mirror-based split-and-delay unit can be used for X-ray pump–probe experiments. Recent scientific highlights illustrate the imaging, time-resolved spectroscopy and high-power density capabilities of the AMO instrument.

We demonstrate a novel multistage amplification scheme for self-amplified spontaneous-emission free electron lasers for the production of few femtosecond pulses with very high power in the soft x-ray regime. The scheme uses the fresh-slice technique to produce an x-ray pulse on the bunch tail, subsequently amplified in downstream undulator sections by fresh electrons. With three-stages amplification, x-ray pulses with an energy of hundreds of microjoules are produced in few femtoseconds. For single-spike spectra x-ray pulses the pulse power is increased more than an order of magnitude compared to other techniques in the same wavelength range.

Nature photonics 12(3), 150 - 153 (2018). doi:10.1038/s41566-018-0110-y, Ultrafast X-ray imaging on individual fragile specimens such as aerosols1, metastable particles2, superfluid quantum systems3 and live biospecimens4 provides high-resolution information that is inaccessible with conventional imaging techniques. Coherent X-ray diffractive imaging, however, suffers from intrinsic loss of phase, and therefore structure recovery is often complicated and not always uniquely defined4,5. Here, we introduce the method of in-flight holography, where we use nanoclusters as reference X-ray scatterers to encode relative phase information into diffraction patterns of a virus. The resulting hologram contains an unambiguous three-dimensional map of a virus and two nanoclusters with the highest lateral resolution so far achieved via single shot X-ray holography. Our approach unlocks the benefits of holography for ultrafast X-ray imaging of nanoscale, non-periodic systems and paves the way to direct observation of complex electron dynamics down to the attosecond timescale., Published by Nature Publ. Group, London [u.a.]

Optica 5(8), 967 - 975 (2018). doi:10.1364/OPTICA.5.000967, Systematic understanding and real-time feedback capability for x-ray free electron laser (FEL) accelerator and optical components are critical for scientific experiments and instrument performance. Single-shot wavefront sensing enables characterization of the intensity and local electric field distribution at the sample plane, something that is important for understanding scientific experiments such as nonlinear studies. It can also provide feedback for alignment and tuning of the FEL beam and instrumentation optics, leading to optimal instrument performance and greater operational efficiency. A robust, sensitive, and accurate single-shot wavefront sensor for x-ray FEL beams using single grating Talbot interferometry has been developed. Experiments performed at the Linac Coherent Light Source (LCLS) demonstrate 3𝜎 sensitivity and accuracy, both better than 𝜆/100, and retrieval of hard x-ray (𝜆=0.13 nm, 𝐸=9.5 keV) wavefronts in 3D. Exhibiting high performance from both unfocused and focused beams, the same setup can be used to systematically study the wavefront from the FEL output, beam transport optics, and endstation focusing optics. This technique can be extended for use with softer and harder x rays with modified grating configurations., Published by OSA, Washington, DC

Physical review / B 96(21), 214101 (2017). doi:10.1103/PhysRevB.96.214101, All carbon materials, e.g., amorphous carbon (a-C) coatings and $C_{60}$ fullerene thin films, play an important role in short-wavelength free-electron laser (FEL) research motivated by FEL optics development and prospective nanotechnology applications. Responses of a-C and $C_{60}$ layers to the extreme ultraviolet (SPring-8 Compact SASESource in Japan) and soft x-ray (free-electron laser in Hamburg) free-electron laser radiation are investigated by Raman spectroscopy, differential interference contrast, and atomic force microscopy. A remarkable difference in the behavior of covalent (a-C) and molecular ($C_{60}$) carbonaceous solids is demonstrated under these irradiation conditions. Low thresholds for ablation of a fullerene crystal (estimated to be around 0.15 eV/atom for $C_{60}$ vs 0.9 eV/atom for a-C in terms of the absorbed dose) are caused by a low cohesive energy of fullerene crystals. An efficient mechanism of the removal of intact $C_{60}$ molecules from the irradiated crystal due to Coulomb repulsion of fullerene-cage cation radicals formed by the ionizing radiation is revealed by a detailed modeling., Published by Inst., Woodbury, NY

A simple method for generating single-spike hard x-ray pulses in free-electron lasers (FELs) has been developed at the Linac Coherent Light Source (LCLS). This is realized by nonlinear bunch compression using 20-pC bunch charge, demonstrated in the hard x-ray regime at 5.6 and 9 keV, respectively. Measurements show about half of the FEL shots containing a single-spike spectrum. At 5.6-keV photon energy, the single-spike shots have a mean pulse energy of about 10 μJ with 70% intensity fluctuation and the pulse full width at half maximum is evaluated to be at 200-as level.

Development of X-ray Free Electron Lasers (XFEL) opens new era in X-ray science. The full exploitation of unique properties of the XFEL radiation require challenging solutions that preserves radiation properties from a coherent, diffraction limited source under unprecedented instantaneous and average power load. We will present properties of simulated XFEL radiation such as coherence, source shape, divergence and longitudinal location inside the undulator. Recently, a construction of the LCLS II project has started as a major upgrade to the LCLS facility that will increase the average power of the XFEL up to 1 kW level. We will show how the X-ray simulations are used for minimizing thermal distortions on focusing of the LCLS II X-ray beams by 1 meter long Kirkpatrick-Baez mirrors. We will discuss and compare accuracy of simulations using different simulation methods and packages applied to focusing optics. The design of instruments should mitigate the damage to the optics caused by the tremendous instantaneous XFEL power. We will present X-ray simulation of the damage to the diffraction grating coatings and compare it with experimental results obtained at LCLS. The self-seeded mode of the LCLS operation increases temporal coherence and reduces greatly the bandwidth of the X-ray radiation. The results of time dependent X-ray simulations of the LCLS radiation passing through the seeding monochromator will be presented. We will compare two different approaches: Fourier Optics and an approach based on a dispersive system described by 6x6 pulse ray matrixes.

Performance tests of parabolic beryllium refractive lenses, considered as X-ray focusing elements in the future X-ray free-electron laser oscillator (XFELO), are reported. Single and double refractive lenses were subject to X-ray tests, which included: surface profile, transmissivity measurements, imaging capabilities and wavefront distortion with grating interferometry. Optical metrology revealed that surface profiles were close to the design specification in terms of the figure and roughness. The transmissivity of the lenses is >94% at 8 keV and >98% at 14.4 and 18 keV. These values are close to the theoretical values of ideal lenses. Images of the bending-magnet source obtained with the lenses were close to the expected ones and did not show any significant distortion. Grating interferometry revealed that the possible wavefront distortions produced by surface and bulk lens imperfections were on the level of ∼λ/60 for 8 keV photons. Thus the Be lenses can be succesfully used as focusing and beam collimating elements in the XFELO.

Interaction of short-wavelength free-electron laser (FEL) beams with matter is undoubtedly a subject to extensive investigation in last decade. During the interaction various exotic states of matter, such as warm dense matter, may exist for a split second. Prior to irreversible damage or ablative removal of the target material, complicated electronic processes at the atomic level occur. As energetic photons impact the target, electrons from inner atomic shells are almost instantly photo-ionized, which may, in some special cases, cause bond weakening, even breaking of the covalent bonds, subsequently result to so-called non-thermal melting. The subject of our research is ablative damage to lead tungstate (PbWO4) induced by focused short-wavelength FEL pulses at different photon energies. Post-mortem analysis of complex damage patterns using the Raman spectroscopy, atomic-force (AFM) and Nomarski (DIC) microscopy confirms an existence of non-thermal melting induced by high-energy photons in the ionic monocrystalline target. Results obtained at Linac Coherent Light Source (LCLS), Free-electron in Hamburg (FLASH), and SPring-8 Compact SASE Source (SCSS) are presented in this Paper.

We have experimentally measured and characterized a Self-Amplified Spontaneous Emission (SASE) Free Electron Laser (FEL) in the spectral domain. Spectra were captured for hard X-rays using a pair of transmissive bent-crystal spectrometers on a single-shot basis. The probability distributions of the spectral intensity as a function of an increasing bandwidth were studied in different SASE regimes. The number of spectral modes was found to grow linearly in the exponential growth regime, but the growth became super-linear at saturation and in deeper saturation, consistent with the current theoretical understanding and simulations. The spectral intensity fluctuations were found to decrease when a wider portion of the beam (transverse to the direction of the dispersion) was integrated, indicating a partial spatial coherence, the degree of which was then estimated.

X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn ∼ 6–15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn4CaO5) in Photosystem II (Mn

Atomic clusters can serve as ideal model systems for exploring ultrafast (∼100 fs) laser-driven ionization dynamics of dense matter on the nanometer scale. Resonant absorption of optical laser pulses enables heating to temperatures on the order of 1 keV at near solid density conditions. To date, direct probing of transient states of such nano-plasmas was limited to coherent x-ray imaging. Here we present the first measurement of spectrally resolved incoherent x-ray scattering from clusters, enabling measurements of transient temperature, densities, and ionization. Single shot x-ray Thomson scattering signals were recorded at 120 Hz using a crystal spectrometer in combination with a single-photon counting and energy-dispersive pnCCD. A precise pump laser collimation scheme enabled recording near background-free scattering spectra from Ar clusters with an unprecedented dynamic range of more than 3 orders of magnitude. Such measurements are important for understanding collective effects in laser-matter interactions on femtosecond time scales, opening new routes for the development of schemes for their ultrafast control.

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

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.

We have studied a solid-to-plasma transition by irradiating Al foils with the FLASH free electron laser at intensities up to 10(16) W/cm(2). Intense XUV self-emission shows spectral features that are consistent with emission from regions of high density, which go beyond single inner-shell photoionization of solids. Characteristic features of intrashell transitions allowed us to identify Auger heating of the electrons in the conduction band occurring immediately after the absorption of the XUV laser energy as the dominant mechanism. A simple model of a multicharge state inverse Auger effect is proposed to explain the target emission when the conduction band at solid density becomes more atomiclike as energy is transferred from the electrons to the ions. This allows one to determine, independent of plasma simulations, the electron temperature and density just after the decay of crystalline order and to characterize the early time evolution.

Lasing in a hard-X-ray free-electron laser is typically seeded from noise due to the self-amplification of spontaneous emission, which limits temporal coherence and spectral characteristics. Researchers now demonstrate self-seeding using X-rays from the first half of the magnetic undulator to seed the second half via a diamond-based monochromator at angstrom wavelengths.

The LCLS hard X-ray Free Electron Laser at SLAC reported first lasing in April 2009. Since then two successful user runs have been completed at the two soft X-ray stations. The first hard X-ray station has started commissioning in July 2010. Beam diagnostics play an essential role for tuning the machine and delivering the requested beam properties to the users. An overview of the LCLS photon diagnostics will be presented including some selected commissioning results. Plans for future improvements and upgrades will be briefly discussed.

X-ray lasers represent a powerful tool to explore matter under extreme conditions. A rigorous characterization of their output parameters is, therefore, of substantial importance for the purposes of the experiments being conducted at these sources. A profound knowledge of the spatial, temporal, spectral, statistical, coherence, and wavefront beam properties may protect us from an unwanted misinterpretation of the experimental data. We present an experimental technique of the spatial (transverse and longitudinal) characterization of the beam profile. Investigating ablative imprints in various materials, we evaluate the spatial properties of the incident beam, namely, the beam waist radius and position, the Rayleigh range, M2 parameter, and divergence. In this paper, we recall briefly our recent work at the transverse beam profile reconstruction. A newly developed method of the longitudinal beam profile characterization is the main subject of this work.

The propagation of X-rays through a compound refractive lens (CRL) with imperfect CRL elements is investigated. The trajectories of random rays within the geometrical optics regime are calculated in one plane using Monte Carlo methods. Three different lenses were simulated: Be, Al and Ni lenses designed for photon energies of 20 keV, 60 keV and 175 keV, respectively. The results show that while transverse displacements of single elements in a CRL do not influence imaging resolution, rotational errors can be important. Systematic calculations of aberrations owing to the deviation of the element's surface from a perfect parabolic shape are also presented.

The FLASH XUV-free electron laser has been used to irradiate solid samples at intensities of the order 1016 W cm-2 at a wavelength of 13.5 nm. The subsequent time integrated XUV emission was observed with a grating spectrometer. The electron temperature inferred from plasma line ratios was in the range 5-8 eV with electron density in the range 1021-1022 cm-3. These results are consistent with the saturation of absorption through bleaching of the L-edge by intense photo-absorption reported in an earlier publication. © 2009 Elsevier B.V. All rights reserved.

The article presents preliminary investigation results on the near-surface damage produced by single pulses of XUV free-electron laser in the amorphous α-SiO 2 , the monocrystalline silicon and the epitaxial films of gold. The irradiation was delivered with single pulses of only 25 fs at a wavelength of 32.5 nm and of energy up to 10 μJ. Structural modifications induced by irradiation were characterized by X-ray microdiffraction, as well as by the AFM and optical microscopy. Ablation craters of well-defined edges with smooth interiors and outer embankments surrounding the crater edges were found in the materials. Polycrystalline phases were revealed in Si and Au film samples, in the embankments and in central parts of some craters. In α-SiO 2 , a diffraction pattern typical of an amorphous material was observed without any traces of irradiation-initiated crystallization. A step-like, complete removal of gold film was evidenced inside craters, with only small gold residues in central part of craters exposed to higher fluences.

An ablation by a femtosecond laser pulses of polymethylmethacrylate was investigated in the near-infrared region. The morphological changes due to increasing laser fluency were examined. The size and shape as well as morphological forms occurring in the resulted craters were analyzed as a function of irradiation fluency. Various forms of surface modifications in ablated crater i.e. cracks and smaller features like honey-comb-like structures, and ripples with an average period of 800 nm were observed. The threshold fluency for observable changes of the sample surface, the ablation threshold and the threshold fluency for creation of the structure in the centre of the cavity were determined for 60 fs laser pulse. Obtained data were compared with available literature data.

Stimulated electronic x-ray Raman scattering is the building block for several proposed x-ray pump probe techniques, that would allow the study of electron dynamics at unprecedented timescales. We present high spectral resolution data on stimulated electronic x-ray Raman scattering in a gas sample of neon using a self-amplified spontaneous emission x-ray free-electron laser. Despite the limited spectral coherence and broad bandwidth of these sources, high-resolution spectra can be obtained by statistical methods, opening the path to coherent stimulated x-ray Raman spectroscopy. An extension of these ideas to molecules and the results of a recent experiment in CO are discussed.

Nature Communications 7, 11652 (2016). doi:10.1038/ncomms11652, New capabilities at X-ray free-electron laser facilities allow the generation of two-colour femtosecond X-ray pulses, opening the possibility of performing ultrafast studies of X-ray-induced phenomena. Particularly, the experimental realization of hetero-site-specific X-ray-pump/X-ray-probe spectroscopy is of special interest, in which an X-ray pump pulse is absorbed at one site within a molecule and an X-ray probe pulse follows the X-ray-induced dynamics at another site within the same molecule. Here we show experimental evidence of a hetero-site pump-probe signal. By using two-colour 10-fs X-ray pulses, we are able to observe the femtosecond time dependence for the formation of F ions during the fragmentation of XeF$_2$ molecules following X-ray absorption at the Xe site., Published by Nature Publishing Group, London

Scientific data 3, 160060 (2016). doi:10.1038/sdata.2016.60, Free-electron lasers (FEL) hold the potential to revolutionize structural biology by producing X-ray pules short enough to outrun radiation damage, thus allowing imaging of biological samples without the limitation from radiation damage. Thus, a major part of the scientific case for the first FELs was three-dimensional (3D) reconstruction of non-crystalline biological objects. In a recent publication we demonstrated the first 3D reconstruction of a biological object from an X-ray FEL using this technique. The sample was the giant Mimivirus, which is one of the largest known viruses with a diameter of 450 nm. Here we present the dataset used for this successful reconstruction. Data-analysis methods for single-particle imaging at FELs are undergoing heavy development but data collection relies on very limited time available through a highly competitive proposal process. This dataset provides experimental data to the entire community and could boost algorithm development and provide a benchmark dataset for new algorithms., Published by Nature Publ. Group, London

X-ray free electron lasers (XFELs) enable unprecedented new ways to study the electronic structure and dynamics of transition metal systems. L-edge absorption spectroscopy is a powerful technique for such studies and the feasibility of this method at XFELs for solutions and solids has been demonstrated. However, the required x-ray bandwidth is an order of magnitude narrower than that of self-amplified spontaneous emission (SASE), and additional monochromatization is needed. Here we compare L-edge x-ray absorption spectroscopy (XAS) of a prototypical transition metal system based on monochromatizing the SASE radiation of the linac coherent light source (LCLS) with a new technique based on self-seeding of LCLS. We demonstrate how L-edge XAS can be performed using the self-seeding scheme without the need of an additional beam line monochromator. We show how the spectral shape and pulse energy depend on the undulator setup and how this affects the x-ray spectroscopy measurements. (C) 2016 Optical Society of America