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I K Robinson

Publications and source records attributed to I K Robinson.

11 recordsLinked to original sources
PubMed2006

Visualizing soft tissue in the mammalian cochlea with coherent hard X-rays.

This paper concerns an important aspect of current developments in medical and biological imaging: the possibility for imaging soft tissue at relatively high resolution in the micrometer range or better, without tedious and/or entirely destructive sample preparation. Structures with low absorption contrast have been visualized using in-line phase contrast imaging. The experiments have been performed at the Advanced Photon Source, a third generation source of synchrotron radiation. The source provides highly coherent X-ray radiation with high photon flux (>10(14) photons/s) at high photon energies (5-70 keV). Thick gerbil cochlear slices have been imaged and were compared with those obtained by light microscopy. Furthermore, intact gerbil cochleae have been imaged to identify the soft tissue structures involved in the hearing process. The present experimental approach was essential for visualizing the inner ear structures involved in the hearing process in an intact cochlea.

Animals↗
PubMed2005

Shearing interferometer for quantifying the coherence of hard x-ray beams.

We report a quantitative measurement of the full transverse coherence function of the 14.4 keV x-ray radiation produced by an undulator at the Swiss Light Source. An x-ray grating interferometer consisting of a beam splitter phase grating and an analyzer amplitude grating has been used to measure the degree of coherence as a function of the beam separation out to 30 microm. Importantly, the technique provides a model-free and spatially resolved measurement of the complex coherence function and is not restricted to high resolution detectors and small fields of view. The spatial characterization of the wave front has important applications in discovering localized defects in beam line optics.

Journal Article↗
PubMed2005

Adsorbate-geometry specific subsurface relaxation in the CO/Pt(111) system.

A dramatic multilayer substrate relaxation is observed for the (square root 19 x square root 19)-13CO adlayer phase on a Pt(111) electrode by surface X-ray scattering. Within the (square root 19 x square root 19) unit cell, a vertical expansion of 0.28 A was determined for the Pt atoms under near-top-site CO molecules, whereas only 0.04 A was found under near-bridge-site CO molecules. The lateral displacements involve small rotations toward more symmetric bonding. Both the expansions and rotations extend into the bulk with a decay length of 1.8 Pt layers. This nonuniform layer expansion, hitherto unseen, appears to be a manifestation of the differential stress induced by CO adsorption at different sites.

Adsorption↗
PubMed2003

Imaging of quantum array structures with coherent and partially coherent diffraction.

Recent achievements in experimental and computational methods open the possibility of measuring and inverting the diffraction pattern from a finite object of submicrometer size. In this paper the possibilities of such experiments for two-dimensional arrays of quantum dots are discussed. The diffraction pattern corresponding to coherent and partial coherent illumination of a sample was generated. Test calculations based on the iterative algorithms were applied to reconstruct the shape of the individual islands in such a quantum structure directly from its diffraction pattern. It is demonstrated that, in the case of coherent illumination, the correct shape and orientation of an individual island can be obtained. In the case of partially coherent illumination, the correct shape of the particle can be obtained only when the coherence of the incoming beam is reduced to match the size of the island.

Algorithms↗
PubMed2003

Three-dimensional imaging of microstructure in Au nanocrystals.

X-ray diffraction using a coherent beam involves the mutual interference among all the extremities of small crystals. The continuous diffraction pattern so produced can be phased because it can be oversampled. We have thus obtained three-dimensional images of the interiors of Au nanocrystals that show 50 nm wide bands of contrast with [111] orientation that probably arise from internal twinning by dynamic recrystallization during their formation at high temperature.

Journal Article↗
PubMed2002

Surface relaxation in protein crystals.

Surface x-ray diffraction measurements were performed on (111) growth faces of crystals of the cellular iron-storage protein, horse spleen ferritin. Crystal truncation rods (CTR) were measured. A fit of the measured profile of the CTR revealed a surface roughness of 48+/-4.5 A and a top layer spacing contraction of 3.9+/-1.5%. In addition to the peak from the CTR, the rocking curves of the crystals displayed unexpected extra peaks. Multiple scattering is demonstrated to account for them. Future applications of the method could allow the exploration of hydration effects on the growth of protein crystals.

Journal Article↗
PubMed2002

Structure of quantum wires in Au/Si(557).

The structure of the Au/Si(557) surface is determined from three-dimensional x-ray diffraction measurements, which directly mandate a single Au atom per unit cell. We use a "heavy atom" method in which the Au atom images the rest of the structure. Au is found to substitute for a row of first-layer Si atoms in the middle of the terrace, which then reconstructs by step rebonding and adatoms. The structure is consistent with the 1D metallic behavior seen by photoemission.

Journal Article↗
PubMed2001

Reconstruction of the shapes of gold nanocrystals using coherent x-ray diffraction.

Inverse problems arise frequently in physics: The magnitude of the Fourier transform of some function is measurable, but not its phase. The "phase problem" in crystallography arises because the number of discrete measurements (Bragg peak intensities) is only half the number of unknowns (electron density points in space). Sayre first proposed that oversampling of diffraction data should allow a solution, and this has recently been demonstrated. Here we report the successful phasing of an oversampled hard x-ray diffraction pattern measured from a single nanocrystal of gold.

Journal Article↗
PubMed2001

Adsorption configuration and local ordering of silicotungstate anions on Ag(100) electrode surfaces.

X-ray reflectivity, cyclic voltammetry, and scanning tunneling microscopy (STM) are used to examine the structure of alpha-SiW12O4(4-) or silicotungstic acid (STA) adsorbed on Ag(100) in acid solution. The voltammetry shows that STA passivates the Ag surface relative to electron transfer to a solution redox species. STM images reveal the formation of a series of lattice structures, one of which can be associated with a commensurate ( radical13x radical13)R33.69 degrees structural model. X-ray reflectivity measurements show uniquely that STA orients with its four-fold axis perpendicular to the Ag(100) surface and that the center of the STA molecule is 4.90 A above the top layer of the Ag substrate. Analysis of bond lengths leads to a footprint of STA on Ag(100), in which the four terminal O atoms are located near the hollow sites and have a Ag-O bond length of 2.06 A. This bond length is consistent with a strong covalent interaction between STA and the Ag surface.

Journal Article↗
PubMed1997

Asymmetric Fraunhofer diffraction from roller-blade slits.

X-ray diffraction experiments under coherent conditions have been made possible by the development of new sources of synchrotron radiation, but make tough demands on the experimental apparatus. Here we describe the design and initial testing of a precision aperture that uses polished molybdenum rods as the slit blades. The device has an inherent asymmetry which is accurately accounted for by a simple modification to the Fraunhofer diffraction function.

Journal Article↗