G. Tourillon

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.

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.

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.

Two time‐dependent, focus‐dependent experiments using the energy‐dispersive spectrometer at LURE

Rather than a brief review of what has been achieved at LURE [A. Fontaine, E. Dartyge, J. P. Itie, A. Polian, H. Tolentino, and G. Tourillon, Topics in Current Chemistry 151, (Springer, Berlin, 1989), Vol. 151] since 1985 with the fully dedicated, energy dispersive x‐ray absorption fine structure beam line, we will present two classes of experiments plus a discussion on the present ultimate source of noise of our mechanical movement‐free spectrometer. Moreover, the readers interested in this field, can find two other studies with the new born ports at Hamburg [M. Hagelstein, S. Cunis, R. Frahm, W. Niemann, and P. Rabe, Phys. B 158, 324 (1989)] and Daresbury [G. Baker, C. Richard, A. Catlow, J. Couves, A. J. Dent, G. Derbyshire, G. N. Greaves, and J. M. Thomas, XAFS VI York 1990]. Related to this field are the quick extended x‐ray absorption fine structure spectrometers developed initially at Hamburg under R. Frahm [R. Frahm, Nucl. Instrum. Methods A 270, 578 (1988)] and at LURE under P. Lagarde and C. Pri...

Directly illuminated photodiode array as a position sensitive detector for synchrotron X-ray radiation

Abstract A system of a cooled directly illuminated photodiode array of 1024 pixels 25 μm apart is described. The spatial resolution has been measured and is better than 50 μm (2 pixels). The dynamic range is shown to be larger than four orders of magnitude. The downstream setup allows us to record 60 spectra before transfer to a mass storage. The radiation damage observed after irradiation is crudely estimated to be due to 10 15 −10 16 10-keV photons. Attempts to cure it using UV illumination or thermal annealing have been persued.

Time-resolved X-ray absorption spectroscopy.

Time‐resolved experiments based on X‐ray absorption spectroscopies involve generally mass transportation which means relatively slow processes. Selected experiments in electrochemistry are presented and different options to improve the time‐resolution are reviewed.

Electronic and structural modifications of organic molecules under very high pressure: A dispersive x ray absorption study

Abstract 3-ethyl selenophene has been invest igated by x-ray absorption spectroscopy (XAS) up to 32 GPa at the Se K edge. A solidification of the material is observed at the beginning of the pressure cycle, while the aromatic ring is broken at pressure above 16 GPa. The compound recovered after the pressure cycle reacts with the environment and is insoluble and infusible. On the basis of the radial distribution function surrounding the Se atoms, it is thought to have a R-Se-Se-R structure, where R could be C, O or H atoms. Le 3-ethyl selenophene a ete etudie sous pression jusqu a 32 CPa par spectroscopie dabsorption X en mode dispersif au seuil K du selenium au debut de la compression, une solidification du compose est observee, alors que le cyle aromatique est detruit pour des pressions superieures a 16 GPa. Le produit recuphre apres le cycle de pression reagit avec Ienvironment et est insoluble et infusible. Sur la base de la fonction de distribution radial de Iatome de selenium, une st ruc t ure du...