E. Dartyge

X-ray absorption in dispersive mode: A new spectrometer and a data acquisition system for fast kinetics

Abstract The photodiode array provided a position sensitive detector for powerful X-ray sources. Then the development of the energy dispersive scheme for the X-ray absorption experiments became realistic. The data acquisition system and elements of optics are discussed. Experiments in transmission mode and in total reflection mode illustrate the dramatic gain in data acquisition time as well as advantages dealing with the focussing optics.

Energy-Dispersive Phase Plate for Magnetic Circular Dichroism Experiments in the X-ray Range

A 220 diamond phase plate was combined with an energy-dispersive absorption spectrometer to convert linearly polarized X-rays into circularly polarized photons and to detect circular magnetic X-ray dichroism (CMXD) from ferromagnetic samples. In these experiments, carried out at LURE, the energy-dispersive spectrometer was equipped with a bent Si (311) polychromator and vertically collimating slits accepting essentially a linearly polarized incident beam. The quarter-wave plate was operated in the Bragg geometry but well outside the range of quasi total reflection, with the consequence that the forward-diffracted beam was circularly polarized with a polarization rate approaching 80% over the whole energy bandpass of the polychromator. CMXD spectra of GdFe2 and GdCo2 intermetallic compounds were recorded at ca 7.2 keV near the Gd LIII absorption edge: they are essentially identical to the spectra commonly recorded with elliptically polarized X-ray photons collected out of the orbit plane of the storage ring. It is suggested that the energy-dispersive phase plate will be very useful to detect CMXD spectra with energy-dispersive spectrometers exploiting the well collimated linearly polarized emission of standard undulators installed on the storage rings of the third generation.

Electron yield X-ray absorption spectroscopy at atmospheric pressure

Abstract The electron yield at atmospheric pressure for X-ray absorption spectroscopy is a new technique able to study the very top surface of thick samples in real environments. A combined detector has been designed at LURE which yields EXAFS spectra in transmission, electron and fluorescence modes. A variable probed thickness is obtained, depending on the polarization. With a positively biased collector under He atmosphere, the analyzed thickness is of the order of 300 A while it decreases down to 20 A with a negative polarization.

XANES study of underpotential deposited copper on carbon-supported platinum

Abstract Underpotential deposited (UPD) copper on carbon-supported platinum was investigated in-situ in 0.5 M H2SO4 at 0.05 V (SCE) by XANES (X-ray Absorption Near Edge Structure). By taking XANES spectra at the Cu K-edge and the Pt LIII-edge it was possible to determine the valence state of the copper and observe modifications in the electronic structure of the platinum on adsorption of copper. The XANES for UPD copper shows that the adsorbed copper had an oxidation state close to Cu+. XANES features indicate strongly a tetrahedral coordination for the adsorbed Cu species. A reduction in intensity of the white line in the platinum XANES is consistent with a partial filling of empty Pt d-band vacancies on adsorption of copper. Thus, UPD species can modify the electronic structure of a platinum catalyst. The adsorbed Cu+ species are apparently associated with HSO−4 ions.

Aberration-free and harmonic-free optics for time-resolved X-ray absorption spectroscopy using synchrotron radiation

Abstract To obtain aberration-free optics, we tailored a new crystal shape which achieves an ellipsoidal profile. The theoretical shape has been designed using elastic equations and a ray-tracing technique. Cutting has been carried out and evaluation of the on-site performance has been achieved for a set of crystals. The size of the image turns out to be limited only by the demagnification factor and is restricted to about 300 μm. In order to get rid of the harmonic contamination and to get a reliable signal out of an absorbing sample we use a flat fused quartz mirror at grazing incidence as a low-pass filter. The performance of this filter has been studied experimentally by scanning the angle of the mirror relative to the beam and by a ray-tracing simulation. An adjustment of the grazing incident angle to reflect the fundamental without any harmonic contamination turns out to be an efficient and easy task in our current setup.

X-ray absorption spectroscopy in dispersive mode and by total reflection

Abstract The association of the total reflection scheme with the dispersive mode allows one to record in short time the EXAFS and XANES spectra of the first 30 A of a surface. In situ time resolved observations of the surface modifications under chemical or electrochemical treatments are thus accessible. This promising technique is illustrated through two examples: (i) the thermal oxidation of a metallic copper surface and (ii) the electrochemical inclusion of Cu + ions within a 5000 A organic conducting polymer (poly-3-methylthiophene) electrochemically deposited on a platinum surface.

Perfect crystal and mosaic crystal quarter‐wave plates for circular magnetic x‐ray dichroism experiments

Circular magnetic x‐ray dichroism experiments require circularly polarized x rays usually provided by bending magnet synchrotron radiation below or above the orbit plane. It is shown here that a very promising alternative consists of using linearly polarized x rays converted to circularly polarized ones by quarter‐wave plates made of nonabsorbing crystals like good diamond crystals and also mosaic beryllium crystals. Convincing experimental results obtained on the energy‐dispersive absorption spectrometer at LURE (Orsay‐France) are presented.

Electrochemical inclusion of metallic clusters in organic conducting polymers: An in situ dispersive X-ray absorption study

Conducting polymers containing metallic clusters are of considerable interest either from a fundamental point of view or by their potential applications. We report here, the first direct kinetic study of the different electrochemical steps occurring during the inclusion of metallic clusters in an organic matrix by using X-ray absorption spectroscopy from a synchrotron radiation source. The system described concerns the growth of copper clusters in poly-3-methyl-thiophene, an organic conducting polymer stable in air and in aqueous medium. The spectra in the vicinity of the Cu K edge (8991 eV) have been recorded continuously from the starting (CuCl2 solution) to the final state (copper cluters in the organic material) during the cathodic polarization of the polymer immersed in the solution. The main points seen through this work can be underlined as follow: (1) The CuCl2 solution is characterized by a strong white line shifted by + 8.5 eV from the edge of the Cu foil, consistent with solvated Cu2+. (2) When the polymer is cathodically polarized, several modifications in the Cu edge spectrum are observed depending on the electrolysis time, (a) A few seconds after the current is applied, Cu + ions are formed and stabilized by the polymeric matrix. (b) After several minutes, the system is evolving to the appearance of Cu0 species on the polymer. (c) For longer polarization time (15–20 min), a new shift toward lower energy is observed and seems to be related with the formation of very small copper clusters. These inclusion processes are completly reversible by changing the potential value. The size and location of the copper aggregates depend on the electrical parameters and on the CuCl2 concentration in the aqueous solution: at high concentration (1–3 M/l), the inclusion along the polymeric fibers is very homogeneous and the cluster sizes lie in the range 1–10 nm; at low concentration (10−2 M/l), copper particles of a few microm are mainly deposited near the metallic electrode.

X-ray absorption spectroscopy of iron-(II) and -(III) basket-handle porphyrins

X-Ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopies have been used to characterize iron basket-handle porphyrins in various co-ordinations, stereochemistries and oxidation states. Attention is paid to the square-planar iron(II) complexes, a dioxygen adduct of FeII and a hydroxyiron(III) complex. The results are consistent with a FeII(O2) rather than a FeIII(O2–) formulation for the dioxygen adduct. A Fe–O distance of 1.94 A was found in the hydroxo complex. The kinetics of oxidation of a dioxygen form to the hydroxo complex has been studied using a dispersive-mode X-ray absorption spectrometer (half-life ≈ 6 min).

Two recent developments in XMCD.

This paper reports on two new technical developments concerning sample environments for X-ray magnetic circular dichroism (XMCD). The first measurements under high pressures of up to 30 GPa are described. The difficulties of combining the techniques of high pressure and XMCD are commented on. The second development involves the use of a fast-switching magnetic field. A new superconducting device is used to perform XMCD measurements on paramagnetic compounds in magnetic fields of up to 6 T. The small amplitude of the XMCD signal imposes, for a given signal-to-noise ratio, a noise less than a few 10(-5). The signal-to-noise ratio is improved by the use of a series of acquisitions, switching the magnetic field between each acquisition. A very fast switching mechanism has been built based on mechanical rotation of a superconducting coil, with the sample kept in place inside the coil. The XMCD signals at the L(II,III)-edges of paramagnetic rare-earth compounds have been measured at 4.5 K in fields of up to 6 T with a switching time of 11 s.