[Show abstract][Hide abstract] ABSTRACT: Physical, biological, and chemical transformations are initiated by changes in the electronic configuration of the species involved. These electronic changes occur on the timescales of attoseconds (1E−18 s) to femtoseconds (1E−15 s) and drive all subsequent electronic reorganization as the system moves to a new equilibrium or quasi-equilibrium state. The ability to detect the dynamics of these electronic changes is crucial for understanding the potential energy surfaces upon which chemical and biological reactions take place. Here, we report on the determination of the electronic structure of matter using a single self-seeded femtosecond x-ray pulse from the Linac Coherent Light Source hard x-ray free electron laser. By measuring the high energy resolution off-resonant spectrum (HEROS), we were able to obtain information about the electronic density of states with a single femtosecond x-ray pulse. We show that the unoccupied electronic states of the scattering atom may be determined on a shot-to-shot basis and that the measured spectral shape is independent of the large intensity fluctuations of the incoming x-ray beam. Moreover, we demonstrate the chemical sensitivity and single-shot capability and limitations of HEROS, which enables the technique to track the electronic structural dynamics in matter on femtosecond time scales, making it an ideal probe technique for time-resolved X-ray experiments.
[Show abstract][Hide abstract] ABSTRACT: Next-generation X-ray sources, based on the X-ray Free Electron Laser (XFEL) concept, will provide highly coherent, ultrashort pulses of soft and hard X-rays with peak intensity many orders of magnitude higher than that of a synchrotron. These pulses will allow studies of femtosecond dynamics at nanometer resolution and with chemical selectivity. They will produce diffraction images of organic and inorganic nanostructures without deleterious effects of radiation damage.
CHIMIA International Journal for Chemistry 02/2014; 68(1-2):73-8. · 1.09 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The new X‐ray free‐electron laser source (SwissFEL) that is currently being developed at PSI will provide a broad‐bandpass mode with an energy bandwidth of about 4%. By using the full energy range, a new option for structural studies of crystalline materials may become possible. The proof of concept of broad‐bandpass diffraction presented here is based on Laue single‐crystal microdiffraction and the experimental setup on BL12.3.2 at the Advanced Light Source in Berkeley. Diffraction patterns for 100 randomly oriented stationary crystallites of the MFI‐type zeolite ZSM‐5 were simulated assuming several bandwidths, and the statistical and structural results are discussed. With a 4% energy bandwidth, the number of reflection intensities measured in a single shot is significantly higher than with monochromatic radiation. Furthermore, the problem of partial reflection measurement, which is inherent to the monochromatic mode with stationary crystals, can be overcome.
Journal of Applied Crystallography 06/2013; 46(3). · 3.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Knowledge of the structure of biological macromolecules, especially in their native environment, is crucial because of the close structure-function relationship. X-ray small-angle scattering is used to determine the shape of particles in solution, but the achievable resolution is limited owing to averaging over particle orientations. In 1977, Kam proposed to obtain additional structural information from the cross-correlation of the scattering intensities. Here we develop the method in two dimensions, and give a procedure by which the single-particle diffraction pattern is extracted in a model-independent way from the correlations. We demonstrate its application to a large set of synchrotron X-ray scattering images on ensembles of identical, randomly oriented particles of 350 or 200 nm in size. The obtained 15 nm resolution in the reconstructed shape is independent of the number of scatterers. The results are discussed in view of proposed 'snapshot' scattering by molecules in the liquid phase at X-ray free-electron lasers.
[Show abstract][Hide abstract] ABSTRACT: In preparation for picosecond pump-probe experiments at the SwissFEL X-ray laser facility, the feasibility of collectively initiating surface chemical reactions using energetic pulses of terahertz radiation is being tested.
CHIMIA International Journal for Chemistry 05/2011; 65(5):323-5. · 1.09 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present an investigation of the near-surface tetragonal phase transition
in SrTiO3, using the complementary techniques of beta-detected nuclear magnetic
resonance and grazing-incidence X-ray diffraction. The results show a clear
depth dependence of the phase transition on scales of a few microns. The
measurements support a model in which there are tetragonal domains forming in
the sample at temperatures much higher than the bulk phase transition
temperature. Moreover, we find that these domains tend to form at higher
temperatures preferentially near the free surface of the crystal. The details
of the tetragonal domain formation and their depth/lateral dependencies are
[Show abstract][Hide abstract] ABSTRACT: The issue of beam-induced damage on diffractive hard X-ray optics is addressed. For this purpose a systematic study on the radiation damage induced by a high-power X-ray beam is carried out in both ambient and inert atmospheres. Diffraction gratings fabricated by three different techniques are considered: electroplated Au gratings both with and without the polymer mold, and Ir-coated Si gratings. The beam-induced damage is monitored by X-ray diffraction and evaluated using scanning electron microscopy.
Journal of Synchrotron Radiation 11/2010; 17(6):786-90. · 2.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Next-generation X-ray sources, based on the X-ray Free Electron Laser (XFEL) concept, will
provide highly coherent, ultrashort pulses of soft and hard X-rays with peak intensity many
orders of magnitude above that of a synchrotron. These pulses will allow studies of
femtosecond dynamics at nanometer resolution and with chemical selectivity. They will
produce coherent-diffraction images of organic and inorganic nanostructures without the
deleterious effects of radiation damage.
[Show abstract][Hide abstract] ABSTRACT: The Paul Scherrer Institute is planning the construction of an X-ray free electron laser (SwissFEL), which will produce 20-fs pulses of coherent X-rays in the wavelength range 0.1–7nm, with peak brightness approximately 1010 times that of a third-generation synchrotron beamline. The brightness, the coherence and the short pulses will provide opportunities for performing novel science in the fields of chemistry, biochemistry, solid-state physics and materials science. It is planned that the SwissFEL will be operational by the end of 2016. The present article focuses on promising SwissFEL applications in the field of time-resolved X-ray scattering from chemical systems.
[Show abstract][Hide abstract] ABSTRACT: Time-dependent X-ray Absorption Spectroscopy (XAS) measurements of chemical reaction dynamics have a time resolution which is limited by: (a) the speed and efficiency of the reaction initiation; (b) the duration of the X-ray pulses used for the measurement; and (c) the brightness of the X-ray source. X-Ray Free Electron Lasers (XFEL), which will deliver 20-100 fs pulses of X-rays, with a peak brightness which is 10(10) times that of a synchrotron, will alleviate limitations (b) and (c). Furthermore, by including a synchronized source of UV, visible, IR or THz pump radiation, the XFEL will contribute to the solution of limitation (a). The present article describes the XFEL operating principle and the generic design of an XFEL facility, emphasizing the features of particular interest to the XAS investigator.
Physical Chemistry Chemical Physics 06/2010; 12(21):5647-52. · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Paul Scherrer Institute is planning the construction of a hard-x-ray free-electron laser, the SwissFEL, by 2016, which will produce intense, ultrashort pulses of transversely coherent radiation in the wavelength range 0.1–7 nm, with future extensions to cover the range 0.08–30 nm. Special design considerations include (a) a compact construction, compatible with the status of a national facility, (b) a uniform 100 Hz repetition rate, well suited to sample manipulations and detector readout, (c) flexible wavelength tuning by the electron beam energy and undulator gaps, (d) soft x-rays at approximately 1 nm wavelength, with circular polarization and Fourier-transform-limited pulses, (e) hard x-rays of pulse duration 5–20 fs and (f) an independent source of high-energy, half-cycle terahertz pump pulses. The science case for the Swiss FEL project, which emphasizes the dynamics of condensed matter systems and the damage-free imaging of nanostructures, includes novel considerations that make optimal use of these features.
New Journal of Physics 03/2010; 12(3):035012. · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The structure of epitaxially grown hexagonal boron nitride (h-BN) on the surface of a Ru(0001) single crystal was investigated using surface X-ray diffraction, which showed the system to form a commensurate 14-on-13 superstructure. This result disagrees with previous reports on superstructures of the same system and arguments based on simple thermal expansion coefficient calculations. We argue that the larger observed superstructure forms because of the strong bonding of h-BN to Ru. In comparison to h-BN/Rh(111) it can accommodate more induced lateral in-plane strain- or lock-in energy over larger regions (referred to as the holes) within the superstructure, which itself can consequently become larger.
[Show abstract][Hide abstract] ABSTRACT: We present a high-resolution surface X-ray diffraction study of hexagonal boron nitride (h-BN) on the surface of Rh(111). The previously observed commensurate 13-on-12 superstructure for this system is stable in the temperature range between room temperature and 830°C. Surface X-ray diffraction measurements up to 830°C on the superstructure show no sign of a shift towards a different superstructure, demonstrating the high thermal stability and strong bonding between film and substrate. At lower temperatures, an anomalous thermal expansion behaviour of the topmost surface region of rhodium is observed, where the rhodium in-plane lattice constant remains invariant. This can be explained by the (h-BN) single-layer being compressively strained, whereby the strong bonding to the substrate causes the latter to be tensile strained.
[Show abstract][Hide abstract] ABSTRACT: We present a structural analysis of the graphene/Ru(0001) system obtained by surface x-ray diffraction. The data were fit using Fourier-series expanded displacement fields from an ideal bulk structure, plus the application of symmetry constraints. The shape of the observed superstructure rods proves a reconstruction of the substrate, induced by strong bonding of graphene to ruthenium. Both the graphene layer and the underlying substrate are corrugated, with peak-to-peak heights of (0.82 +/- 0.15) A and (0.19 +/- 0.02) A for the graphene and topmost Ru-atomic layer, respectively. The Ru-corrugation decays slowly over several monolayers into the bulk. The system also exhibits chirality, whereby in-plane rotations of up to 2.0 degrees in those regions of the superstructure where the graphene is weakly bound are driven by elastic energy minimization.
New Journal of Physics 08/2009; · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigated with synchrotron x-ray diffraction and reflectometry the formation of structural domains in the near-surface region of single crystalline SrTiO3 (001) substrates with Y0.6Pr0.4Ba2Cu3O7/La2/3Ca1/3MnO3 superlattices grown on top. We find that the antiferrodistortive cubic to tetragonal transition, which occurs at TSTO=104K in the bulk and at a considerably higher temperature of at least 120 K in the surface region of SrTiO3 , has only a weak influence on the domain formation. The strongest changes occur instead in the vicinity of the tetragonal to orthorhombic transition in SrTiO3 around 65 K where pronounced surface facets develop that reach deep (at least several micrometers) into the SrTiO3 substrate. These micrometer-sized facets are anisotropic and tilted with respect to one another by up to 0.5° along the shorter direction. Finally, we find that a third structural transition below 30 K gives rise to significant changes in the spread of the c -axis parameters. Overall, our data provide evidence for a strong mutual interaction between the structural properties of the SrTiO3 surface and the multilayer grown on top.
[Show abstract][Hide abstract] ABSTRACT: The structure of a single layer of graphene on Ru(0001) has been studied using surface x-ray diffraction. A surprising superstructure containing 1250 carbon atoms has been determined, whereby 25 x 25 graphene unit cells lie on 23 x 23 unit cells of Ru. Each supercell contains 2 x 2 crystallographically inequivalent subcells caused by corrugation. Strong intensity oscillations in the superstructure rods demonstrate that the Ru substrate is also significantly corrugated down to several monolayers and that the bonding between graphene and Ru is strong and cannot be caused by van der Waals bonds. Charge transfer from the Ru substrate to the graphene expands and weakens the C-C bonds, which helps accommodate the in-plane tensile stress. The elucidation of this superstructure provides important information in the potential application of graphene as a template for nanocluster arrays.
[Show abstract][Hide abstract] ABSTRACT: The MYTHEN detector is a one-dimensional microstrip detector with single photon counting readout optimized for time resolved powder diffraction experiments at the Swiss Light Source (SLS). The system has been successfully tested for many different synchrotron radiation applications including phase contrast and tomographic imaging, small angle scattering, diffraction and time resolved pump and probe experiments for X-ray energies down to 5keV and counting rate up to 3MHz. The frontend electronics is designed in order to be coupled to 50μm pitch microstrip sensors but some interest in enhancing the spatial resolution is arising for imaging and powder diffraction experiments.A test structure with strip pitches in the range 10–50μm has been tested and the gain and noise on the readout electronics have been measured for the different strip pitches, observing no large difference down to 25μm. Moreover, the effect of the charge sharing between neighboring strips on the spatial resolution has been quantified by measuring the Point Spread Function (PSF) of the system for the different pitches.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 06/2008; 591(1):163-166. · 1.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Paul Scherrer Institut is planning the construction of an X-ray Free Electron Laser. The PSI-XFEL will produce 60 fs pulses of coherent X-rays in the wavelength range 0.1 – 10 nm, with a peak brightness approximately 10 10 times that of a third-generation synchrotron. The brightness, coherence and short pulses provide opportunities for performing novel science in the fields of chemistry, biochemistry, solid state physics and materials science. The PSI-XFEL could be operational in the year 2016. What is an XFEL, and how is the PSI-XFEL special? The active medium in an X-ray free electron laser consists of a 100 femtosecond pulse of 10 9 relativistic electrons moving in the sinusoidal field of an undulator: a periodic linear array of alternately-poled permanent magnets. As in a synchrotron light source, the transverse acceleration from the Lorentz force causes the electrons to emit X-radiation, but in an XFEL, the undulator is sufficiently long that the growing radiation field influences the trajectory of the electrons. At the "resonance condition", where the radiation overtakes an electron by exactly one wavelength per undulator period, certain of the electrons gain energy and others lose energy, thus splitting the pulse into 10 5 "microbunches". As it moves along the undulator, the microbunched electron pulse then radiates as if it were a single charge of 10 9 e, producing an intense, coherent pulse of "superradiant" X-rays. Besides the PSI-XFEL there are presently three other projected XFELs worldwide: in Stanford, USA (2009), Hyogo, Japan (2011) and Hamburg, Germany (2014). The maximum electron energy and hence the overall XFEL length (800 m) are significantly lower at PSI than for the other projects. This is made possible by PSI innovations in the high-brightness electron source technology, including nanometer-scale field-emitting tips and initial acceleration in a pulsed field of 1 MV across a 4 mm gap, followed by a novel two-frequency RF-cavity. The individual X-ray pulses will be very similar to those of the larger projects (see Table 1). While the Swiss, US and Japanese XFELs will emit 60-120 pulses per second, the Hamburg machine, due to the use of superconducting accelerator technology, will produce 10 trains of 3000 pulses per second, with a minimum pulse spacing of 200 ns.
[Show abstract][Hide abstract] ABSTRACT: We present new results on the application of the x-ray phase retrieval method, Coherent Bragg Rod Analysis (COBRA), to heteroepitaxial interfaces in LaAlO3 thin films grown on SrTiO3 substrates, a system known to form an interfacial quasi-2D electron gas. We observe a dilated, mixed-valence interface which modifies the electronic band structure, lowering the minimum of the conduction band below the Fermi level and thereby rendering the dilated interface conducting. In particular the COBRA measurements reveal the formation of an interfacial La,SrTiO3 layer with an accumulation of trivalent Ti at the interface which is responsible for the lattice dilation and minimizes the electrostatic energy at the TiO2-terminated SrTiO3 substrate surface. The work presented here establishes a structural basis for the formation of the conducting interface.