Search

Liquid polymorphism is an intriguing phenomenon which has been found in a few single-component systems, the most famous being water. By supercooling liquid Te to more than 130 K below its melting point and performing simultaneous small-angle and wide-angle X-ray scattering measurements, we observe clear maxima in its thermodynamic response functions around 615 K, suggesting the possible existence of liquid polymorphism. A close look at the underlying structural evolution shows the development of intermediate-range order upon cooling, most strongly around the thermodynamic maxima, which we attribute to bond-orientational ordering. The striking similarities between our results and those of water, despite the lack of hydrogen-bonding and tetrahedrality in tellurium, indicate that water-like anomalies may be a general phenomenon among liquid systems with competing bond- and density-ordering., Comment: Main text: 10 pages, 5 figures; supplementary materials: 14 pages, 13 figures

Review of scientific instruments 92(3), 039901 (2021). doi:10.1063/5.0043951, This article was originally published online on 4 January 2021 with an error in the title and Ref. 6 should have been deleted from the reference section and the text. The title is correct above. All online and printed versions of the article were corrected on 7 January 2021. AIP Publishing apologizes for this error., Published by American Institute of Physics, [S.l.]

X-ray free-electron lasers can generate radiation pulses with extreme peak intensities at short wavelengths. This enables the investigation of laser–matter interactions in a regime of high fields, yet at a non-relativistic ponderomotive potential, where ordinary rules of light–matter interaction may no longer apply and nonlinear processes are starting to become observable. Despite small cross-sections, first nonlinear effects in the hard x-ray regime have recently been observed in solid targets, including x-ray-optical sum-frequency generation (XSFG), x-ray second harmonic generation (XSHG) and two-photon Compton scattering (2PCS). Nonlinear interactions of bound electrons in the x-ray range are fundamentally different from those dominating at optical frequencies. Whereas in the optical regime nonlinearities are predominantly caused by anharmonicities of the atomic potential in the chemical bonds, x-ray nonlinearities far above atomic resonances are expected to be due to nonlinear oscillations of quasi-free electrons, including inner-shell atomic electrons. While the quasi-free-electron model agrees reasonably well with the experimental data for XSFG and XSHG, 2PCS measurements have led to unexpected results: the energy of the nonlinearly scattered photons from non-relativistic electrons shows a substantial unexpected red shift in addition to the Compton shift that is well beyond that predicted by a nonlinear quantum electrodynamics model for free electrons. A potential explanation for the spectral broadening is based on a previously unexplored scattering process that involves the whole atom rather than just quasi-free electrons. A first simulation that includes the atomic binding potential was successful in describing a broadening of the spectrum of the nonlinearly scattered photons to longer wavelengths for soft x-rays. However, the same model does not show any broadening at hard x-ray wavelengths, which is in agreement with other simulation approaches. To this point no calculation has been able to reproduce the experimentally observed broadening. Here we present further experimental data of 2PCS for an extended parameter range using additional diagnostics. In particular, we present measurements of the electron momentum distribution during the interaction that strongly suggest that the spectral broadening is not caused by an increased plasma temperature. We extend our measurement of the magnitude of the red shift in beryllium to 1.9\enspace \mathrm{k}\mathrm{e}\mathrm{V}$?> > 1.9 k e V in addition to the Compton shift expected for free electrons and expand the measurement of the angular distribution to include forward scattering angles. We also present first measurements of 2PCS from diamond.

Applied physics letters 112(22), 221907 - (2018). doi:10.1063/1.5031907, In this work, we demonstrate simultaneous phase-contrast imaging (PCI) and X-ray diffraction from shock compressed matter at the Matter in Extreme Conditions endstation, at the Linac Coherent Light Source (LCLS). We utilize the chromaticity from compound refractive X-ray lenses to focus the 24.6 keV 3rd order undulator harmonic of the LCLS to a spot size of 5 μm on target to perform X-ray diffraction. Simultaneous PCI from the 8.2 keV fundamental X-ray beam is used to visualize and measure the transient properties of the shock wave over a 500 μm field of view. Furthermore, we demonstrate the ability to extend the reciprocal space measurements by 5 Å$^{−1}$, relative to the fundamental X-ray energy, by utilizing X-ray diffraction from the 3rd harmonic of the LCLS., Published by American Inst. of Physics, Melville, NY

International audience; We describe a setup for performing inelastic X-ray scattering and X-ray diffraction measurements at the Matter in Extreme Conditions (MEC) endstation of the Linac Coherent Light Source. This technique is capable of performing high-, meV-resolution measurements of dynamic ion features in both crystalline and non-crystalline materials. A four-bounce silicon (533) monochromator was used in conjunction with three silicon (533) diced crystal analyzers to provide an energy resolution of ∼50 meV over a range of ∼500 meV in single shot measurements. In addition to the instrument resolution function, we demonstrate the measurement of longitudinal acoustic phonon modes in polycrystalline diamond. Furthermore, this setup may be combined with the high intensity laser drivers available at MEC to create warm dense matter and subsequently measure ion acoustic modes.

International audience

Using simultaneous spectrally, angularly, and temporally resolved x-ray scattering, we measure the pronounced ion-ion correlation peak in a strongly coupled plasma. Laser-driven shock-compressed aluminum at ∼3× solid density is probed with high-energy photons at 17.9keV created by molybdenum He-α emission in a laser-driven plasma source. The measured elastic scattering feature shows a well-pronounced correlation peak at a wave vector of k=4Å-1. The magnitude of this correlation peak cannot be described by standard plasma theories employing a linear screened Coulomb potential. Advanced models, including a strong short-range repulsion due to the inner structure of the aluminum ions are however in good agreement with the scattering data. These studies have demonstrated a new highly accurate diagnostic technique to directly measure the state of compression and the ion-ion correlations. We have since applied this new method in single-shot wave-number resolved S(k) measurements to characterize the physical properties of dense plasmas. © 2014 AIP Publishing LLC.

Applied physics letters 109(3), 031108 (2016). doi:10.1063/1.4959256, We present results from time-resolved X-ray imaging and inelastic scattering on collective excitations. These data are then employed to infer the mass density evolution within laser-driven shock waves. In our experiments, thin carbon foils are first strongly compressed and then driven into a dense state by counter-propagating shock waves. The different measurements agree that the graphite sample is about twofold compressed when the shock waves collide, and a sharp increase in forward scattering indicates disassembly of the sample 1 ns thereafter. We can benchmark hydrodynamics simulations of colliding shock waves by the X-ray scattering methods employed., Published by American Inst. of Physics, Melville, NY

The extraordinary mechanical and optical properties of diamond are the basis of numerous technical applications and make diamond anvil cells a premier device to explore the high-pressure behavior of materials. However, at applied pressures above a few hundred GPa, optical probing through the anvils becomes difficult because of the pressure-induced changes of the transmission and the excitation of a strong optical emission. Such features have been interpreted as the onset of a closure of the optical gap in diamond, and can significantly impair spectroscopy of the material inside the cell. In contrast, a comparable widening has been predicted for purely hydrostatic compressions, forming a basis for the presumed pressure stiffening of diamond and resilience to the eventual phase change to BC8. We here present the first experimental evidence of this effect at geo-planetary pressures, exceeding the highest ever reported hydrostatic compression of diamond by more than 200 GPa and any other measurement of the band gap by more than 350 GPa. We here apply laser driven-ablation to create a dynamic, high pressure state in a thin, synthetic diamond foil together with frequency-resolved x-ray scattering as a probe. The frequency shift of the inelastically scattered x-rays encodes the optical properties and,more » thus, the behavior of the band gap in the sample. Using the ultra-bright x-ray beam from the Linac Coherent Light Source (LCLS), we observe an increasing direct band gap in diamond up to a pressure of 370 GPa. This finding points to the enormous strains in the anvils and the impurities in natural Type Ia diamonds as the source of the observed closure of the optical window. Our results demonstrate that diamond remains an insulating solid to pressures approaching its limit strength.« less

The Paul Scherrer Institute (PSI) in Switzerland held a three-day workshop from April 10–12, 2017, on the upcoming Athos soft X-ray branch at the SwissFEL beginning operation in 2020 and the upgrad...

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.

The Linac Coherent Light Source has added a self-seeding capability to the soft x-ray range using a grating monochromator system. We report the demonstration of soft x-ray self-seeding with a measured resolving power of 2000–5000, wavelength stability of 10$^{−4}$, and an increase in peak brightness by a factor of 2–5 across the photon energy range of 500–1000 eV. By avoiding the need for a monochromator at the experimental station, the self-seeded beam can deliver as much as 50-fold higher brightness to users.

A description of the Matter in Extreme Conditions instrument at the Linac Coherent Light Source is given. Recent scientific highlights illustrate phase-contrast imaging of shock waves, X-ray Thomson scattering and X-ray diffraction of shocked materials., The LCLS beam provides revolutionary capabilities for studying the transient behavior of matter in extreme conditions. The particular strength of the Matter in Extreme Conditions instrument is that it combines the unique LCLS beam with high-power optical laser beams, and a suite of dedicated diagnostics tailored for this field of science. In this paper an overview of the beamline, the capabilities of the instrumentation, and selected highlights of experiments and commissioning results are presented.

The use of bent asymmetric Laue crystals to sagittally focus high-energy synchrotron X-rays calls for an understanding of the mechanisms affecting X-ray diffraction by such crystals. The rocking-curve width, a measurable quantity directly related to the distortion of the lattice planes, is the necessary first step towards such an understanding. A model is formulated for assessing the rocking-curve widths of sagittally bent Laue crystals, considering the elastic anisotropy. A method for depth-resolved measurement of the rocking curves was also developed to verify the model. The model successfully explains the wide range of rocking-curve widths of a large number of reflections from silicon crystals with two different orientations.

We report observation of x-ray and optical sum frequency generation. An ultrafast optical pulse drives charge oscillations to the chemical bonds in diamond. A co-propagating x-ray pulse probes the accompanying atomic-scale chemical bond distortion.

We report the first experimental evidence for second harmonic generation at hard x-ray wavelengths. A second harmonic beam is produced in a diamond crystal using a 1.77 A pump from the SACLA free-electron laser.

The Matter in Extreme Conditions end station at the Linac Coherent Light Source holds great promise for novel pump-probe experiments to make new discoveries in high- energy density science. In recent experiments we have demonstrated the first spectrally- resolved measurements of plasmons using a seeded 8-keV x-ray laser beam. Forward x-ray Thomson scattering spectra from isochorically heated solid aluminum show a well-resolved plasmon feature that is down-shifted in energy by 19 eV from the incident 8 keV elastic scattering feature. In this spectral range, the simultaneously measured backscatter spectrum shows no spectral features indicating observation of collective plasmon oscillations on a scattering length comparable to the screening length. This technique is a prerequisite for Thomson scattering measurements in compressed matter where the plasmon shift is a sensitive function of the free electron density and where the plasmon intensity provides information on temperature.

We report the first observation of x-ray/optical sum frequency generation. An ultrafast optical pulse drives charge oscillations to the chemical bonds in diamond. A co-propagating x-ray pulse micro-probes the accompanying chemical bond distortion.

The SLAC National Accelerator Laboratory (SLAC), in collaboration with Argonne National Laboratory (ANL), Lawrence Livermore National Laboratory (LLNL), and the University of California at Los Angeles (UCLA), is constructing a Free-Electron Laser (FEL) research facility. The FEL has already met its performance goals in the wavelength range 1.5 nm - 0.15 nm. This facility, the Linac Coherent Light Source (LCLS), utilizes the SLAC 2-Mile Linear Accelerator (linac) and will produce sub-picosecond pulses of short wavelength X-rays with very high peak brightness and almost complete transverse coherence. The final one-third of the SLAC linac is used as the source of electrons for the LCLS. The high energy electrons are transported across the SLAC Research Yard, into a tunnel which houses a long undulator. In passing through the undulator, the electrons are bunched by the force of their own synchrotron radiation and produce an intense, monochromatic, spatially coherent beam of X-rays. By varying the electron energy, the FEL X-ray wavelength is tunable from 1.5 nm to 0.15 nm. The LCLS includes two experimental halls as well as X-ray optics and infrastructure necessary to create a facility that can be developed for research in a variety of disciplines such as atomic physics, materials science, plasma physics and biosciences. This Conceptual Design Report, the authors believe, confirms the feasibility of designing and constructing an X-ray instrument in order to exploit the unique scientific capability of LCLS by creating extreme conditions and study the behavior of plasma under those controlled conditions. This instrument will address the Office of Science, Fusion Energy Sciences, mission objective related to study of Plasma and Warm Dense Matter as described in the report titled LCLS, the First Experiments, prepared by the LCLS Scientific Advisory Committee (SAC) in September 2000. The technical objective of the LCLS Matter in Extreme Conditions (MEC) Instrument project is to design, build, and install at the LCLS an X-ray instrument that will complement the initial instrument suite included in the LCLS construction and the LUSI Major Item of Equipment (MIE) Instruments. As the science programs advance and new technological challenges appear, instrumentation must be developed and ready to conquer these new opportunities. The MEC concept has been developed in close consultation with the scientific community through a series of workshops team meetings and focused reviews. In particular, the MEC instrument has been identified as meeting one of the most urgent needs of the scientific community based on the advice of the LCLS Scientific Advisory Committee (SAC) in response to an open call for letters of intent (LOI) from the breadth of the scientific community. The primary purpose of the MEC instrument is to create High Energy Density (HED) matter and measure its physical properties. There are three primary elements of the MEC instrument: (A) Optical laser drivers that will create HED states by irradiation in several ways and provide diagnostics capability; (B) The LCLS x-ray free electron laser, which will provide the unique capability to create, probe and selectively pump HED states; and, (C) A suite of diagnostic devices required to observe the evolution of the HED state. These elements when combined in the MEC instrument meet the 'Mission Need' as defined in CD-0. For the purposes of the description we separate the types of experiments to be performed into three categories: (1) High pressure: Here we are interested in the generation of high pressure using the optical lasers to irradiate a surface that ablates and drives a pressure wave into a sample, similar to a piston. The pressures that can be reached exceed 1 Mbar and the properties of interest are for example, the reflectivity, conductivity, opacity as well as the changes driven by the pressure wave on, e.g., condensed matter structure. These phenomena will be studied by means of diffraction measurements, measurements of the pressure wave characteristics, in situ probing by x-ray scattering of various types all time resolved. The necessary diagnostics are discussed.

Many of the important challenges facing humanity, including developing alternative sources of energy and improving health, are being addressed by advances that demand the improved understanding and control of matter. While the visualization, exploration, and manipulation of macroscopic matter have long been technological goals, scientific developments in the twentieth century have focused attention on understanding matter on the atomic scale through the underlying framework of quantum mechanics. Of special interest is matter that consists of natural or artificial nanoscale building blocks defined either by atomic structural arrangements or by electron or spin formations created by collective correlation effects (Figure 1.1). The essence of the challenge to the scientific community has been expressed in five grand challenges for directing matter and energy recently formulated by the Basic Energy Sciences Advisory Committee [1]. These challenges focus on increasing our understanding of, and ultimately control of, matter at the level of atoms, electrons. and spins, as illustrated in Figure 1.1, and serve the entire range of science from advanced materials to life sciences. Meeting these challenges will require new tools that extend our reach into regions of higher spatial, temporal, and energy resolution. X-rays with energies above 10 keV offer capabilities extending beyond the nanoworld shown in Figure 1.1 due to their ability to penetrate into optically opaque or thick objects. This opens the door to combining atomic level information from scattering studies with 3D information on longer length scales from real space imaging with a resolution approaching 1 nm. The investigation of multiple length scales is important in hierarchical structures, providing knowledge about function of living organisms, the atomistic origin of materials failure, the optimization of industrial synthesis, or the working of devices. Since the fundamental interaction that holds matter together is of electromagnetic origin, it is intuitively clear that electromagnetic radiation is the critical tool in the study of material properties. On the level of atoms, electrons, and spins, x-rays have proved especially valuable. Future advanced x-ray sources and instrumentation will extend the power of x-ray methods to reach greater spatial resolution, increased sensitivity, and unexplored temporal domains. The purpose of this document is threefold: (1) summarize scientific opportunities that are beyond the reach of today's x-ray sources and instrumentation; (2) summarize the requirements for advanced x-ray sources and instrumentation needed to realize these scientific opportunities, as well as potential methods of achieving them; and (3) outline the R&D required to establish the technical feasibility of these advanced x-ray sources and instrumentation.

Many of the important challenges facing humanity, including developing alternative sources of energy and improving health, are being addressed by advances that demand the improved understanding and control of matter. While the visualization, exploration, and manipulation of macroscopic matter have long been technological goals, scientific developments in the twentieth century have focused attention on understanding matter on the atomic scale through the underlying framework of quantum mechanics. Of special interest is matter that consists of natural or artificial nanoscale building blocks defined either by atomic structural arrangements or by electron or spin formations created by collective correlation effects (Figure 1.1). The essence of the challenge to the scientific community has been expressed in five grand challenges for directing matter and energy recently formulated by the Basic Energy Sciences Advisory Committee. These challenges focus on increasing our understanding of, and ultimately control of, matter at the level of atoms, electrons, and spins, as illustrated in Figure 1.1. Meeting these challenges will require new tools that extend our reach into regions of higher spatial, temporal, and energy resolution. Since the fundamental interaction that holds matter together is of electromagnetic origin, it is intuitively clear that electromagnetic radiation is the critical tool in the study of material properties. On the level of atoms, electrons and spins, x rays have proved especially valuable.

Initial tests are described of a high-resolution small-angle X-ray scattering apparatus adapted for operation at high photon energies. The energy chosen was 50 keV, and was limited by the available source. All orders of scattering out to the seventh were observed from a sample of 0.9 μm diameter latex spheres in a 4 h experiment. Phase separation in a binary Al–Li alloy was studied by the observation of spherical Al3Li precipitates. This demonstrates that the use of perfect-crystal collimators and analyzers yields a resolution function which does not depend on photon energy. New sources of high-energy photons will make such an apparatus invaluable for the study of highly absorbing samples, or samples in difficult environments such as ovens or high-pressure cells.

The Stanford Linear Accelerator Center (SLAC), along with Argonne National Laboratory (ANL), Lawrence Livermore National Laboratory (LLNL), and the University of California at Los Angeles (UCLA), is constructing a Free-Electron Laser (FEL) facility, which will operate in the wavelength range 1.5 nm - 0.15 nm. This FEL, the Linac Coherent Light Source (LCLS), utilizes the SLAC linac and will produce sub-picosecond pulses of short wavelength X-rays with very high peak brightness and almost complete transverse coherence. The final one-third of the SLAC linac will be used as the source of electrons for the LCLS. The high energy electrons will be transported across the SLAC Research Yard, into a tunnel which will house a long undulator. In passing through the undulator, the electrons will be bunched by the force of their own synchrotron radiation and produce an intense, monochromatic, spatially coherent beam of X-rays. By varying the electron energy, the FEL X-ray wavelength will be tunable from 1.5 nm to 0.15 nm. The LCLS will include two experimental halls as well as X-ray optics and infrastructure necessary to create a facility that can be developed for research in a variety of disciplines such as atomic physics, materials science, plasma physics and biosciences. This Conceptual Design Report, the authors believe, confirms the feasibility of designing and constructing three X-ray instruments in order to exploit the unique scientific capability of this new LCLS facility. The technical objective of the LCLS Ultrafast Science Instruments (LUSI) project is to design, build, and install at the LCLS three hard X-ray instruments that will complement the initial instrument included in the LCLS construction. As the science programs advance and new technological challenges appear, instrumentation needs to be developed and ready to conquer these new opportunities. The LCLS instrument concepts have been developed in close consultation with the scientific community through a series of workshops team meetings and focused reviews. In particular, the LUSI project instruments have been identified as meeting the most urgent needs of the scientific community based on the advice of the LCLS Scientific Advisory Committee (SAC) in response to an open call for letters of intent (LOI) from the breadth of the scientific community.

Strong plasmon resonances characteristics of electron density fluctuations have recently been observed in dynamically compressed diamond for the first time at the Linac Coherent Light Source. These experiments observe the forward scattering spectra from 8 keV x-ray pulses at record peak brightness to probe laser-compressed diamond foils at the Matter in Extreme Conditions instrument. We demonstrate single-shot measurements of the x-ray scattering spectrum, which are sensitive to the temperatures and densities of the compressed samples. The inferred values from the inelastic scattering are compared to simulations, finding good agreement with the temperature and demonstrating the need to include solid state effects in the modeling of the plasmon oscillation.

Recent experiments performed at the Matter in Extreme Conditions end station of the Linac Coherent Light Source (LCLS) have demonstrated the first spectrally resolved measurements of plasmons from isochorically heated aluminum. The experiments have been performed using a seeded 8-keV x-ray laser beam as a pump and probe to both volumetrically heat and scatter x-rays from aluminum. Collective x-ray Thomson scattering spectra show a well-resolved plasmon feature that is down-shifted in energy by 19 eV. In addition, Mbar shock pressures from laser-compressed aluminum foils using velocity interferometer system for any reflector have been measured. The combination of experiments fully demonstrates the possibility to perform warm dense matter studies at the LCLS with unprecedented accuracy and precision.

The Sub‐Picosecond Photon Source (SPPS), in operation since May 2003, makes x‐ray pulses of 100 fs. Spontaneous radiation originates from a short pulsed electron beam that passes through an undulator at the Stanford Linear Accelerator Center. New techniques characterize the x‐rays, the electrons, and the timing jitter in real time. These results provide a framework for discussion of possible strong field experiments at SPPS and at future fourth‐generation photon sources like the recently funded Linac Coherent Light Source.

We present some measurements of nuclear fluorescence which were made while exploring the potential of a unique source of very hard x‐rays which was installed at ESRF until recently. It produced radiation of energies up to 1MeV. In this range there are many nuclear resonances, and so we made a small survey of some plausible candidates to see if they could indeed be excited using this source. Along with short‐lived resonances, we were also able to indirectly populate long‐lived isomeric states in several nuclei. Thus, simply exposing the samples to the white beam for a period of time caused them to be activated. The samples could them be removed from the beam and their emissions observed using an energy‐resolving detector. Isomeric states in isotopes of indium, cadmium and silver were excited and their subsequent decay observed.

Plasmon measurements hold great promise for providing highly accurate data on the physical properties of plasmas in the high-energy density physics regime. To this end we demonstrate in recent experiments at the Linac Coherent Light Source the first spectrally-resolved measurements of plasmons using a seeded 8-keV x-ray laser beam. Forward x-ray Thomson scattering spectra from isochorically heated solid aluminum show a well-resolved plasmon feature that is down-shifted in energy by 19 eV from the incident 8 keV elastic scattering feature. In this spectral range, the simultaneously measured backscatter spectrum shows no spectral features indicating observation of collective plasmon oscillations on a scattering length comparable to the screening length. This technique is a prerequisite for Thomson scattering measurements in compressed matter where the plasmon shift is a sensitive function of the free electron density and where the plasmon intensity provides information on temperature.

We report the optical design and performance of the phase II inelastic scattering beamline at the National Synchrotron Light Source. The new beamline consists of a four-crystal Si(220) monochromator followed by a bent cylinder mirror. The monochromator is tunable from 5 to 10 keV with about 0.2 eV energy resolution throughout the tuning range. The size of the focused beam is about 0.5 mm (H)×0.3 mm (V).

The successful lasing and operation of the LCLS hard X-ray FEL has brought tremendous interest to the user community spanning a wide range of scientific disciplines including physics, chemistry, structural biology, and material science. It created demand on beam time that is often left unfulfilled. Here we report experimental measurements of ultra-thin silicon single-crystal membranes for potentially beam-sharing the LCLS beam. The samples included the (111), (220), and (400) orientations with thicknesses ranging from 5 to 20 μm. Both high-resolution rocking curves and topographic data were first obtained using synchrotron X-rays, demonstrating near ideal diffraction qualities. Subsequent tests using the full LCLS beam revealed lattice distortions from beam-induced membrane vibrations, which were shown to be effectively reduced by ambient air and smaller membrane dimensions. High diffraction quality thin-diamonds in the (111) orientation are also being pursued as a parallel effort. Both approaches are paving a way for a practical beam-sharing implementation at LCLS in the near future.

We describe the design and operation of a transmissive single-shot spectrometer for the measurement of hard x-ray free electron laser (FEL) source spectrum at the Linac Coherent Light Source (LCLS). The spectrometer was built around a 10 μm thick near-perfect silicon single crystal that was cylindrically bent. Its energy range covered the full FEL bandwidth while its resolution was sufficient for resolving single spectral spikes characteristics of the FELs. Its application will not only greatly facilitate the understanding and optimization of the x-ray FEL sources, but can also serve as an invaluable inline diagnostic tool for experiments where the spectral content of the source plays an important role in data interpretation.

As the field of nuclear resonant scattering using synchrotron radiation matures, we see a shift from experiments designed to verify theoretical predictions towards those designed to use the phenomenon to elucidate the behavior of a sample. In parallel with this trend towards applications, we have seen further instrumental developments that extend the range of possible applications. This paper will review the recent work in this field with an emphasis on new sources, new isotopes, and new instrumentation.

EXAFS measurements have been made on the octahedral Jahn—Teller complexes, Cu(H 2 O) 6 2+ and Cr(H 2 O) 6 2+ , in aqueous solution. Two distinct metal—oxygen bond lengths are not clearly resolvable in the EXAFS Fourier transforms for each species; lengths and Deuye—Waller factors obtained suggest that the axial ligands are loosely bonded.

Accurate values of the absolute integrated reflections from a powder sample of Ni have been determined from measurements with synchrotron radiation as a function of scattering angle over a range of energies from 290 to 3 eV below the Ni K-absorption edge, 8332 eV. From these values, the dispersion factor f' has been determined by comparison of the experimental scattering factors with the theoretical values of Wang & Callaway [Phys. Rev. B. (1977), 15, 298- 306] based upon a LCAO calculation with the KSG (Kohn & Sham and Gaspar) approximation for exchange [Kohn & Sham (1965). Phys. Rev. 140, A1133-1138; Gaspar (1954). Acta Phys. Acad. Sci. Hung. 3, 263-286]. The values obtained for f' as a function of energy are within about 0.2 electron units of those calculated from measured and tabulated absorption coefficients by Bonse & Hartmann-Lotsch [Nucl. Instrum. Methods (1984), 222, 185-188] and within about 0.5 electron units of their direct measurements by interferometric techniques in the range 30 to 3 eV below threshold. The variation of f' with scattering angle is negligibly small (less than 0.2 electron units on average). The present experiment demonstrates that powder techniques provide a simple and convenient method for the measurement of f' for many materials that are not available in forms suitable for the application of other techniques such as interferometry.

Accurate measurements of the integrated intensities of several reflections from a standard powder sample of Ni have been made using monochromatic synchrotron X-ray radiation of wavelength 1.5413 A from a perfect double-crystal Si(111) monochromator. A perfect Ge(111) analyzer crystal was mounted on the detector arm of the diffractometer to serve as a narrow 'angular' receiving slit. The intensities were placed on an absolute scale by application of the appropriate powder diffraction expressions, which require the incident photon counts, the axial and equatorial openings of the receiving 'slit', and the polarization factor to be known. The procedure for evaluating these instrumental parameters is described in some detail. The quantitative agreement between these and previous absolute measurements on a standard Ni sample with Cu Kα radiation demonstrates that the synchrotron technique can be used for absolute measurements of high accuracy.

A neutron-scattering study of the structural phase transition in hydrogen-reduced crystal of SrTiO/sub 3/ shows a systematic enhancement of the central-peak intensity with increasing defect concentration. T/sub c/ is found to decrease linearly with increasing carrier concentration. The results provide direct experimental evidence for the involvement of a defect mechanism in central-peak formation.

The extended x ray absorption fine structure (EXAFS) phenomenon has become a powerful tool for structure determination in the immediate vicinity of an atom of identified Z. In this paper, the power of EXAFS for the study of kinetics and reaction mechanisms in solution are demonstrated. Of particular concern here are the metal-ligand bond distances and the associated Debye-Waller factors of M(H/sub 2/O)/sub 6//sup n+/ complexes in aqueous solution. The metal-oxygen interatomic distances for Cr/sup 3 +/, Cr/sup 2 +/, Mn/sup 2 +/, Fe/sup 2 +/, Fe/sup 3 +/ and Cu/sup 2 +/ obtained by EXAFS are tabulated and compared with reference values of those obtained from solution x-ray scattering experiments and single crystal diffraction work. Although it appears that for these ions in solution the EXAFS r values are slightly smaller than those from diffraction measurements, the differences are within experimental error. With the bond lengths and Debye-Waller information obtained directly from ions in solution the parameters relevant to the kinetics of the electron-exchange reaction can be estimated. For Fe(H/sub 2/)/sub 6//sup 2 +/ and Fe(H/sub 2/O)/sub 6//sup 3 +/, the difference between the Fe to 0 radii of the two ions, r/sup 2/ - r/sup 3/ is themore » most critical parameter. The relationship connecting the EXAFS parameters and the electron-exchange reaction is shown mathematically. The Debye-Waller factor derived from the EXAFS data also contains important bonding information.« less

Synchrotron radiation has a number of special characteristics that make electron storage rings and synchrotrons powerful tools for research with photons. Applications in both basic research and technology are possible and there are exciting prospects for the future with the design and construction of electron storage rings dedicated to synchrotron radiation production. Synchrotron radiation has a continuous spectrum ranging from the infrared to X-ray wavelengths. It is both intense and strongly polarized in the plane of the electron orbit. The light is emitted in pulses that are typically 1 × 10-9 sec long and have repetition frequencies of the order of 1 × 106 pulses per second. With the current designs, beam cross sections of 1 mm2 or less can be achieved and the photons are sharply collimated in a narrow cone about the tangent to the orbit of the emitting particle.

Although the theory describing the production of x rays from relativistic particles has been known for some forty years, x rays from electron synchrotrons and storage rings have not been routinely available until the last few years. These broadband, extremely intense, and collimated sources of x rays have made possible new and exciting studies heretofore untried. One such area is the scattering of x rays at angles near normal incidence. There have been studies carried out in the laboratory at near normal incidence, but these have been confined to the precise determination of d-spacings and strain in several prototypical systems.

Since the first observations of the variation of the absorption coefficient for X-rays above the energy thresholds in the thirties until the early seventies, measurements and analysis of these variations were merely intended for the understanding of the underlying physics. Recently, with the understanding of the information available about the local atomic structure in the neighborhood of the absorbing species and the availability of high intensity synchrotron radiation sources, EXAFS has become a powerful structural tool. In the discussions that follow the details of the measurement for very dilute species will be presented. It is shown that for the more dilute systems the measurement of the emission rather than the direct absorption is a more favorable technique.

The structure of stage 2 potassium intercalated graphite, KC24, is discussed in both the ordered and disordered phases. A one-dimensional model is used to illustrate the qualitative features of the KC24 diffraction patterns.

We present the first detailed measurements of the interatomic potential of photoexcited bismuth using ultrafast optical and x-ray scattering. Our results show that electronic softening is responsible for the strong chirp in phonon frequency.