J. White

Study of underpotentially deposited copper on gold by fluorescence detected surface EXAFS

Using grazing incidence geometry and fluorescence detection, surface EXAFS of a monolayer of underpotentially deposited copper on epitaxially deposited gold (111) on mica was observed. Both Cu–Au and Cu–O scattering are observed. The results are interpreted in terms of models in which the Cu–O distance is 2.08±0.03 A and the Cu–Au distance is 2.5±0.06 A. The copper and three gold atoms at the metallic surface form an elongated tetrahedron, with the oxygen on top of the copper. Two plausible models for the oxygen arrangement are proposed, one involving only one oxygen, the other with a sulfate ion adsorbed. This second model yields a slightly better fit of the data.

Liquid gallium cooling of silicon crystals in high intensity photon beams (invited)

The high‐brilliance, insertion‐device‐based photon beams of the next generation of synchrotron sources (Argonne’s APS and Grenoble’s ESRF) will deliver large thermal loads (1–10 kW) to the first optical elements. Considering the problems that present synchrotron users are experiencing with beams from recently installed insertion devices, new and improved methods of cooling these first optical elements, particularly when they are diffraction crystals, are clearly needed. A series of finite element calculations were performed to test the efficiency of new cooling geometries and various cooling fluids. The best results were obtained with liquid Ga metal flowing in channels just below the surface of the crystal. Ga was selected because of its good thermal conductivity and thermal capacity, low melting point, high boiling point, low kinetic viscosity, and very low vapor pressure. Its very low vapor pressure, even at elevated temperatures, makes it especially attractive in UHV conditions. A series of experiment...

Influence of competing adsorbates on the underpotential deposition of copper on Pt(111)

Abstract We present a survey of the voltammetry for the underpotential deposition (UPD) of copper from aqueous sulfuric acid solution on a flame annealed Pt(111) electrode pretreated with or in the presence of aqueous solutions of a variety of adsorbales: because of its sensitivity to the presence of competing adsorbales, variations in the Cu UPD voltammetric features are used to infer substrate / adsorbate interactions. The adsorbales studied included both ionic and molecular species and were: sulfate, cyanide, thiocyanate and cis-1,2-dicyanoethylene-1,2-dithiolate anions; dimethylformamide (DMF), acetonitrile, dimethylsulfoxide (DMSO), pyridine (from 0.1 M H2SO4 and PDW (pyrolytically distilled water)); formic acid, acetic acid, hydroquinone (HQ); pyrocatechol, 2,2 - bipyridyl (0.1 M H2SO4 and PDW), 4,7-dihydroxy-1,10-phenanthroline, and 1,3-propanedithiol. Anionic adsorbates caused a diminution in the underpotential shift (relative to that in sulfuric acid alone), dependent on the strength of adsorption of the anion. Neutral adsorbales gave a variety of responses in terms of underpotential shifts and generally produced changes in the voltammetric peak shapes. Sulfur containing ligands gave rise to the most dramatic effects, generally causing a large reduction in the underpotential shift and in some cases (e.g. propanedithiol) complete inhibition of the UPD process. DMSO, acetonitrile, and DMF did not appreciably affect the potential of UPD but caused substantial kinetic effects in terms of variations in peak heights and widths. For adsorbates that could undergo protonation (e.g. pyridine, bipyridine and dihydroxy phenanthroline) the observed behavior was strongly dependent on pH, reflecting the fact that protonation of the nitrogen can lead to profound alteration in the mode of binding. Deposition in the presence of formic or acetic acid suggests the presence of discrete coexisting domains.

An in-situ surface EXAFS study of copper underpotential deposition on Pt (111): Part I. The observation of strong in-plane scattering at submonolayer coverage

Abstract The underpotential deposition of copper from a dilute (50 μM) aqueous solution of Cu2+ on an iodine pretreated Pt (111) electrode has been studied using fluorescence detected surface EXAFS (extended X-ray absorption fine structure) with the plane of polarization of the X-ray beam parallel to the electrode surface. It was found that at coverages of copper of about 0.3 monolayer, the absorption fine structure at the Cu K edge was very pronounced, yielding a Cu-Cu bond length of about 0.288 ± 0.004 nm. The intensity of the EXAFS oscillations and near edge structure suggested a high number of copper backscatterers in the first coordination shell. The data are interpreted in terms of a model where clustering or nucleation occurs on certain regions of the electrode surface.

Performance of a hard x-ray undulator at CHESS (invited)

A 3.3‐cm period Nd‐Fe‐B hybrid undulator has been designed and successfully operated in the Cornell Electron Storage Ring (CESR). This 2‐m‐long, 123‐pole insertion device is a prototype of one of the undulators planned for the Advanced Photon Source. In dedicated operation, the undulator produced the expected brightness at 5.437 GeV with the fundamental x‐ray energy ranging from 4.3 to 7.9 keV corresponding to a change in gap from 1.5 to 2.8 cm.

Microchannel water cooling of silicon x‐ray monochromator crystals

The use in silicon x‐ray monochromator crystals of water cooling channels with dimensions optimized for efficient heat transfer from silicon to water has been investigated. Such channels are typically about 40 μm wide and 400 μm deep. Procedures have been found for reliably producing microchannel‐cooled crystals with very small amounts of residual strain. These crystals have been tested at a high‐power wiggler beam line at the Cornell High Energy Synchrotron Source, using an x‐ray beam having total power in excess of 250 W and normal‐incidence power density greater than 5 W/mm2. Under these conditions, the surface‐temperature rise of a typical microchannel‐cooled crystal was less than 5 °C, and degradation of the (111) rocking curve at 12 keV was very slight. The cooling efficiency is consistent with analytic calculations.

Structural studies of electrochemical interfaces with x rays

We have employed x‐ray absorption spectroscopy, including surface EXAFS (extended x‐ray absorption fine structure) and x‐ray standing waves (XSW) in the in‐situ structural study of electrochemical interfaces. The SEXAFS technique was used to study the structure of half a monolayer of copper underpotentially deposited on a Pt(111) electrode. From analysis of the data a Cu–Cu distance of 2.85 A and a coordination number of six were determined, strongly suggesting that the deposit is largely composed of clustered sites. The electrosorption isotherm of iodide on a Pt(111) electrode surface was measured in situ using x‐ray absorption spectroscopy, and it exhibited two plateaus which we ascribe to changes in packing density and surface structure. Using x‐ray standing waves, we have studied the potential dependence of the adsorption of iodide onto a Pt/C LSM and have interpreted the results in terms of distributional changes in the vicinity of the electrode surface.

Adaptive silicon monochromators for high power insertion devices: Tests at CHESS, ESRF, and HASYLAB (abstract)

X-ray wigglers which produce tens of kilowatts of photon power within the white beam will soon become available at third-generation sources of synchrotron radiation. Insertion devices that produce several kilowatts already exist and we have used those at CHESS, ESRF and HASYLAB to test adaptive 111 silicon water-jet-cooled monochromators at up to 2 kW total incident beam power. This development from earlier work at the Brookhaven National Synchrotron Light Source (NSLS) uses the pressure in the water coolant to provide active compensation of the strain field in the thermal footprint, nulling its effect to within residual variations in Bragg angle of only a few arc s. The design is robust, vacuum compatible and uses no moving mechanical parts.