Ion and Electron Momentum Distributions from Single and Double Ionization of Helium Induced by Compton Scattering.

We present the momentum distributions of the nucleus and of the electrons from double ionization of the helium atom by Compton scattering of photons with hν=40 keV. We find that the doubly charged ion momentum distribution is very close to the Compton profile of the nucleus in the ground state of the helium atom, and the momentum distribution of the singly charged ion to give a precise image of the electron Compton profile. To reproduce these results, nonrelativistic calculations require the use of highly correlated initial- and final-state wave functions.

Hydrogen-bond network distortion of water in the soft confinement of Nafion membrane.

A Compton spectroscopy investigation is carried out in hydrated Nafion membranes, enabling identification of distortions in the hydrogen-bond distribution of the polymer hydrating water by means of the subtle changes reflected by the Compton profiles. Indeed, deformations of the Compton profiles are observed when varying hydration, and two different bonding kinds are associated with the water molecules: at low hydration, water surrounds the sulfonic groups, while on increasing hydration, water molecules occupy the interstitial cavities formed upon swelling of the membrane. The analysis is proposed in terms of averaged OH bond length variation. A sizable contraction of the OH distance is observed at low hydration (∼0.09 Å), while at higher hydration levels, the contraction is smaller (∼0.02 Å) and the OH bond length is closer to bulk water. An evaluation of the electron kinetic energy indicates that the spatial changes associated with the water distribution correspond to a consistent binding energy increase. Distinct temperature dependences of each water population are observed, which can be straightly related to water desorption into ice on cooling below the freezing point.

Complex Geometric Structure of a Simple Solid-Liquid Interface: GaN(0001)-Ga.

The equilibrium atomic interface structure between Ga and GaN(0001) is shown to contain substrate surface vacancies followed by substrate-induced layering and preferential lateral ordering in the liquid. The uncovered presence of point defects, in the form of vacancies at both sides of the solid-liquid interface, is an important structural feature which governs the local physical properties. Our x-ray diffraction study reveals that the layering is very stable and persists up to a temperature of 1123 K and a nitrogen pressure of 32 bar. The Ga layer spacing agrees remarkably well with the Friedel oscillation period for this system.

Photon-Momentum-Induced Molecular Dynamics in Photoionization of N_{2} at hν=40 keV.

We investigate K-shell ionization of N_{2} at 40 keV photon energy. Using a cold target recoil ion momentum spectroscopy reaction microscope, we determine the vector momenta of the photoelectron, the Auger electron, and both N^{+} fragments. These fully differential data show that the dissociation process of the N_{2}^{2+} ion is significantly modified not only by the recoil momentum of the photoelectron but also by the photon momentum and the momentum of the emitted Auger electron. We find that the recoil energy introduced by the photon and the photoelectron momentum is partitioned with a ratio of approximately 30∶70 between the Auger electron and fragment ion kinetic energies, respectively. We also observe that the photon momentum induces an additional rotation of the molecular ion.

The atomic scale structure of CXV carbon: wide-angle x-ray scattering and modeling studies.

The disordered structure of commercially available CXV activated carbon produced from finely powdered wood-based carbon has been studied using the wide-angle x-ray scattering technique, molecular dynamics and density functional theory simulations. The x-ray scattering data has been converted to the real space representation in the form of the pair correlation function via the Fourier transform. Geometry optimizations using classical molecular dynamics based on the reactive empirical bond order potential and density functional theory at the B3LYP/6-31g* level have been performed to generate nanoscale models of CXV carbon consistent with the experimental data. The final model of the structure comprises four chain-like and buckled graphitic layers containing a small percentage of four-fold coordinated atoms (sp(3) defects) in each layer. The presence of non-hexagonal rings in the atomic arrangement has been also considered.

Simultaneous small- and wide-angle scattering at high X-ray energies.

Combined small- and wide-angle X-ray scattering (SAXS/WAXS) is a powerful technique for the study of materials at length scales ranging from atomic/molecular sizes (a few angstroms) to the mesoscopic regime ( approximately 1 nm to approximately 1 microm). A set-up to apply this technique at high X-ray energies (E > 50 keV) has been developed. Hard X-rays permit the execution of at least three classes of investigations that are significantly more difficult to perform at standard X-ray energies (8-20 keV): (i) in situ strain analysis revealing anisotropic strain behaviour both at the atomic (WAXS) as well as at the mesoscopic (SAXS) length scales, (ii) acquisition of WAXS patterns to very large q (>20 A(-1)) thus allowing atomic pair distribution function analysis (SAXS/PDF) of micro- and nano-structured materials, and (iii) utilization of complex sample environments involving thick X-ray windows and/or samples that can be penetrated only by high-energy X-rays. Using the reported set-up a time resolution of approximately two seconds was demonstrated. It is planned to further improve this time resolution in the near future.

Modelling the atomic structure of Al92U8 metallic glass.

The local atomic structure of the glassy Al(92)U(8) alloy was modelled by the reverse Monte Carlo (RMC) method, fitting x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) signals. The final structural model was analysed by means of partial pair correlation functions, coordination number distributions and Voronoi tessellation. In our study we found that the most probable atomic separations between Al-Al and U-Al pairs in the glassy Al(92)U(8) alloy are 2.7 Å and 3.1 Å with coordination numbers 11.7 and 17.1, respectively. The Voronoi analysis did not support evidence of the existence of well-defined building blocks directly embedded in the amorphous matrix. The dense-random-packing model seems to be adequate for describing the connection between solvent and solute atoms.

Radiation-induced premelting of ice at silica interfaces.

The existence of surface and interfacial melting of ice below 0 degrees C has been confirmed by many different experimental techniques. Here we present a high-energy x-ray reflectivity study of the interfacial melting of ice as a function of both temperature and x-ray irradiation dose. We found a clear increase of the thickness of the quasiliquid layer with the irradiation dose. By a systematic x-ray study, we have been able to unambiguously disentangle thermal and radiation-induced premelting phenomena. We also confirm the previously announced very high water density (1.25 g/cm(3)) within the emerging quasiliquid layer.

The structure of liquid carbon dioxide and carbon disulfide.

We present neutron and x-ray scattering data (a 2N+X experiment) of liquid CO(2) and CS(2) at a density of about 10 molecules/nm(3). Because the scattering length contrast of the carbon isotope is very small and, in fact, smaller than anticipated from standard scattering length tables, a direct partial structure factor determination via matrix inversion gives unconvincing results. Instead we search for the best representation of the three independent scattering data sets by a simulation of rigid molecules interacting via a 12-6-1 potential, furthermore restricting the pressure p, the density rho, and the temperature T to the experimental values. We show that a 12-6-1 potential is completely adequate to describe the structure of CO(2); for CS(2) we find that the best 12-6-1 potential still slightly overestimates the height of the sulfur-sulfur pair-distribution function g(SS). Orientational correlations reflect the similarities much more than the differences of the two molecular systems. The distinct differences in the atom-atom pair distribution functions of CO(2) and CS(2) do not mean that their structures are radically different and the comparison with the crystalline structures is somewhat deceptive. A linear transformation, wherein all the parameters describing the interaction and the geometry of CS(2) are changed to those of CO(2), allows us to point out the physical parameters which may be responsible for the differences or similarities in thermodynamic behavior (pressure) and structures (orientations) between the two liquids.

In situ high-energy X-ray diffraction study of a bioactive calcium silicate foam immersed in simulated body fluid.

The method of in situ time-resolved high-energy X-ray diffraction, using the intrinsically highly collimated X-ray beam generated by the European Synchrotron Radiation Facility, is demonstrated. A specially designed cell, which allows the addition of liquid components, has been used to study the reaction mechanisms of a foamed bioactive calcia-silica sol-gel glass immersed in simulated body fluid. Analysis of the X-ray diffraction data from this experiment provides atomic distances, via the pair correlation functions, at different stages of the dissolution of the glass and of the associated calcium phosphate, and ultimately hydroxyapatite, i.e. bone mineral, formation. Hence, changes in the atomic scale structure can be analysed as a function of reaction time, giving an insight into the evolution of the structure of both the glass matrix and the hydroxyapatite surface growth.

Giant metal compression at liquid-solid (Pb-Si, In-Si) Schottky junctions.

Using a high-energy x-ray transmission-reflection scheme we have studied the density profile of solid-liquid Schottky contacts close to the interface. We found a massive disturbance of the electronic system on the liquid metal side at different interfaces with pronounced density anomalies on a new length scale. The liquid metal at the interface forms a strongly compressed layer followed by a density depleted layer. The experimental evidence points to a charge transfer phenomenon in the metallic system. Control experiments performed at a metal-insulator interface confirm this picture.

X-ray optics for liquid surface/interface spectrometers.

A new X-ray optics which enables precise structural investigations of liquid surfaces/interfaces is introduced. The new device is based on the use of high-energy microbeams and gives access to large momentum transfer values perpendicular to the liquid surface/interface. The performance of a prototype of this new optics, which has been constructed and implemented at the high-energy diffraction beamline ID15A at the European Synchrotron Radiation Source, is demonstrated.

Competition between order and phase separation in Au-Ni.

We have measured and theoretically analyzed the diffuse scattering in the binary alloy system Au-Ni, which has been proposed as a testing ground for theories of alloy phase stability. We found strong evidence that in the alloys Au3Ni and Au3Ni2, fluctuations of both ordering- and clustering-type are competing with each other. Our results resolve a long-standing controversy on the balance of relaxation and mixing energies in this alloy system and explain recent findings of ordering in thin Au-Ni films.

Performance of a dispersion-compensating scanning X-ray spectrometer for Compton profile measurements.

A new X-ray spectrometer has been constructed for Compton profile measurements at beamline ID15B of the ESRF. The spectrometer is based on a novel idea, dispersion compensation, which was proposed earlier. A cylindrically bent Laue monochromator focuses approximately 90 keV synchrotron radiation at about 0.7 m before the sample, and produces a well defined energy or wavelength gradient on the sample. A cylindrically bent Laue analyser almost perfectly compensates this wavelength gradient. Using an Al sample, it has been confirmed that the new spectrometer improves the counting rate by a factor of two compared with the previously constructed 30 keV and 60 keV spectrometers, with a comparable momentum resolution. Because of reduced absorption owing to use of high-energy X-rays, the enhancement of the counting rate is spectacular for heavy-element materials.

Interfacial melting of ice in contact with SiO(2).

The physical behavior of condensed matter can be drastically altered in the presence of interfaces. Using a high-energy x-ray transmission-reflection scheme, we have studied ice-SiO2 model interfaces. We observed the formation of a quasiliquid layer below the bulk melting temperature and determined its thickness and density as a function of temperature. The quasiliquid layer has stronger correlations than water and a large density close to rho(HDA)=1.17 g/cm(3) of high-density amorphous ice suggesting a structural relationship with the postulated high-density liquid phase of water.

Observation of five-fold local symmetry in liquid lead.

The local point symmetry of the short-range order in simple monatomic liquids remains a fundamental open question in condensed-matter science. For more than 40 years it has been conjectured that liquids with centrosymmetric interactions may be composed of icosahedral building blocks. But these proposed mobile, randomly orientated structures have remained experimentally inaccessible owing to the unavoidable averaging involved in scattering experiments, which can therefore determine only the isotropic radial distribution function. Here we overcome this limitation by capturing liquid fragments at a solid-liquid interface, and observing the scattering of totally internally reflected (evanescent) X-rays, which are sensitive only to the liquid structure at the interface. Using this method, we observe five-fold local symmetry in liquid lead adjacent to a silicon wall, and obtain an experimental portrait of the icosahedral fragments that are predicted to occur in all close-packed monatomic liquids. By shedding new light on local bond order in disordered structures such as liquids and glasses, these results should lead to a better microscopic understanding of melting, freezing and supercooling.

Focusing Optics for High-Energy X-ray Diffraction.

Novel focusing optical devices have been developed for synchrotron radiation in the energy range 40-100 keV. Firstly, a narrow-band-pass focusing energy-tuneable fixed-exit monochromator was constructed by combining meridionally bent Laue and Bragg crystals. Dispersion compensation was applied to retain the high momentum resolution despite the beam divergence caused by the focusing. Next, microfocusing was achieved by a bent multilayer arranged behind the crystal monochromator and alternatively by a bent Laue crystal. A 1.2 micro m-high line focus was obtained at 90 keV. The properties of the different set-ups are described and potential applications are discussed. First experiments were performed, investigating with high spatial resolution the residual strain gradients in layered polycrystalline materials. The results underline that focused high-energy synchrotron radiation can provide unique information on the mesoscopic scale to the materials scientist, complementary to existing techniques based on conventional X-ray sources, neutron scattering or electron microscopy.

High-energy magnetic Compton scattering experiments at ESRF.

Investigations of spin densities in ferromagnetic materials using magnetic Compton scattering are reported. At the high-energy beamline ID15 at the ESRF, experiments have been carried out utilizing the high flux at very high photon energies. Energies from 60 up to 1000 keV have been used for investigations of experimental resolution, cross section, spin moments and momentum distribution. Optimized conditions are found for photon energies from 200 to 250 keV with a momentum resolution < 0.4 a.u. and a doubled magnetic effect compared with earlier measurements. In the determination of absolute spin moments multiple scattering has to be taken into account.