Extended fine-structure techniques

The Cu adsorbate structure was studied using STM and EXAFS (extended x-ray absorption fine structure) techniques, but it is not yet well understood. UPD-OPD transition is in the range —82 to —71 mV. Bulk fee Cu spacing is reached after deposition of about lOCu monolayers. Holzle et al. (72) have shown that UPD Cu deposition on Au(l 11) is a combined adsorption-nucleation and growth process. 

The Cu adsorbate structure was studied using STM and EXAFS (extended X-ray absorption fine structure) techniques, but it is not yet well understood. 

Extended X-ray Absorption Fine Structure) technique, concerning nanocrystalline materials, is reviewed. The potentialities of the technique to characterise hulk materials, thin films, and structural evolution during the preparation of nanoparticles using in situ set-ups are presented. 

A non exhaustive description of the history of X-ray Absorption Spectroscopy (XAS) can be found in Ref. 1. The modem EXAFS (Extended X-ray Absorption Fine Structure) technique began in the early seventies of the last century. It corresponds to the concomitance of both theoretical and experimental developments. Between 1969 and 1975, Stem, Sayers and Lytle succeeded in interpreting theoretically the X-ray Absorption Structures observed above an absorption edge [2], while during the same period, the advent of synchrotron radiation (SR) sources reduced drastically the acquisition time of a spectrum if compared to data obtained with conventional X-ray tubes. XAS provides essential information about the local atomic geometry and the electronic and chemical state of a specific atom, for almost any element of the periodic table (Z>5). This prime tool for... 

EXELFS Extended X-ray Energy Loss Fine Structure A fine-structure technique similar to EXAFS, except that 60-300 KeV electrons rather than photons excite core-holes. Like EXAFS, this techniques is not explicitly surface sensitive. 

Lytle, F. W., Sayers, D. E., and Stern, E. A. Extended X-ray-absorption fine-structure technique. II. Experimental practice and selected results. Phys. Rev., Bll, 4825-4835 (1975). 

X-ray scattering and extended X-ray absorption fine-structure techniques. Data indicate that the indium ion is coordinated by six water molecules, and the In—O bond distance in the first hydration sphere is 2.131(7) A. Changes in concentration or the anion had no influence on this... 

Spectroscopy also provides structural information about supported metals. The EXAFS (extended X-ray absorption fine structure) technique helps to define average crystallite structures, and infrared spectroscopy provides structural characterization of chemisorbed species. The challenges of determining structures of supported metals are great because of the nonuniformities of the metal crystallites in almost all catalysts. 

It is not always easy to characterize the electronic and crystallographic structures of very small aggregates. Their size (a few nanometers) is due to the fact that as many atoms as possible must be active and therefore must be at the surface. Moreover, the analysis has to be done in situ, under the true reaction conditions, in order to build a physical model for the role of the catalyst. Then, many experimental techniques have been used, including most recently electron microscopy and X-ray absorption. We focus our attention here on the EXAFS (Extended X-ray Absorption Fine Structure) technique and its possibilities for the study of supported metal catalysts. Most of the examples come from a collaboration between LURE and some public CNRS laboratories (Strasbourg, Meudon) and a private one (IFF — Rueil Malmaison). We begin with some generalities about the technique and the type of catalysts studied, then move to several examples of application. 

Lytle FW, Sayers DE, Stem EA (1975) Extended X-ray-absorption fine-structure technique. II. 

The X rays diffraction techniques such as the X rays Extended Fine Structure and Electron Radial Distribution which permit the elucidation of the structure of possible cluster compounds within the zeolite pores and/or channels might be used. Unfortunatly these techniques are only applicable to high symmetry zeolites. 

EXAFS Extended X-ray absorption fine structure spectroscopy. A spectroscopic technique which can determine interatomic distances very precisely. 

This chapter contains articles on six techniques that provide structural information on surfaces, interfeces, and thin films. They use X rays (X-ray diffraction, XRD, and Extended X-ray Absorption Fine-Structure, EXAFS), electrons (Low-Energy Electron Diffraction, LEED, and Reflection High-Energy Electron Diffraction, RHEED), or X rays in and electrons out (Surfece Extended X-ray Absorption Fine Structure, SEXAFS, and X-ray Photoelectron Diffraction, XPD). In their usual form, XRD and EXAFS are bulk methods, since X rays probe many microns deep, whereas the other techniques are surfece sensitive. There are, however, ways to make XRD and EXAFS much more surfece sensitive. For EXAFS this converts the technique into SEXAFS, which can have submonolayer sensitivity. 

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