Surface EXAFS

The most straightforward method is to use a substrate of high area and to measure the absorption coefficient directly, as has been done for Br2 on graphite. 

This approach is possible when the ratio of the absorption coefficients of adsorbate and substrate is high, as in the case of Br2/C but for low-Z adsorbates the relative strength of background scattering from the bulk may make surface measurements impracticable.  

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SEXAFS Surface EXAFS Same as EXAFS Same as EXAFS... 

Rehr J J 1995 Multiple-scattering approach to surface EXAFS—theory versus experiment Surf. Rev. Lett 2 63-9... 

Blum L, Abruna FI D, White J, Gordon J G, Borges G L, Samant M G and Melroy 1986 Study of underpotentially deposited copper on gold by fluorescence detected surface EXAFS J. Chem. Phys. 85 6732-8... 

EXAFS is part of the field of X-ray absorption spectroscopy (XAS), in which a number of acronyms abound. An X-ray absorption spectrum contains EXAFS data as well as the X-ray absorption near-edge structure, XANES (alternatively called the near-edge X-ray absorption fine structure, NEXAFS). The combination of XANES (NEXAFS) and EXAFS is commonly referred to as X-ray absorption fine structure, or XAFS. In applications of EXAFS to surface science, the acronym SEXAFS, for surface-EXAFS, is used. The principles and analysis of EXAFS and SEXAFS are the same. See the article following this one for a discussion of SEXAFS and NEXAFS. 

Figure 2 Surface EXAFS spectra above the Pd L edge for a 1.5 monolayer evaporated film of Pd on Sid 11) and for bulk palladium ailicide, Pd2Si and metallic Pd.

XANES spectroscopy is also the basis of chemically sensitive X-ray imaging, as well as qualitative and quantitative microspectroscopy [306], ptXANES is attractive for chemical analysis, with its spatial resolution down to 10 ptm. Variations on the theme are surface EXAFS (SEXAFS), grazing incidence XAS and in situ time-resolved XAS investigations. Grazing angle XAFS can be used for the study of ultrathin multilayer systems. 

Polarization-dependent surface EXAFS measurements have provided some of the best-defined characterizations of adsorbate structures. 

There are a number of experimental factors to be considered in a surface EXAFS experiment. First of all, one needs access to a synchrotron source (for the reasons previously mentioned) with significant flux in the hard X-ray region. In the United States, three such facilities exist and these are ... 

The use of surface EXAFS in the study of passive films represents a natural application of the technique and, in fact, the studies by Kruger and co-workers70 73 on the passive film on iron represent the first reported. 

Most recently, we have been able to obtain the in situ surface EXAFS spectrum of a half-monolayer of underpotentially deposited copper on a bulk Pt(lll) single crystal pretreated with iodine. The spectrum shown in Fig. 23 is a bit noisy (due to limited number of scans) but at least five well-defined oscillations can be observed. Preliminary data analysis indicates that the copper adatoms sit on threefold hollow sites with copper neighbors at 2.80 0.03 A. This distance is very close to the Pt—Pt distance in the (111) direction and indicates the presence of a commensurate... 

In addition to surface EXAFS and X-ray standing waves, X-ray diffraction can be employed in the study of electrochemical interfaces. Although an extensive treatment of X-ray diffraction techniques is beyond the scope of this chapter, some brief statements are appropriate. 

Since the surface atoms are likely to be relatively strong backscatterers the extended fine structure will be dominated by the scattering from the substrate. This relatively weak modulation at higher energy is referred to as the surface EXAFS, or SEXAFS. An analysis of the SEXAFS can give further structural information, most valuably, on the adsorption site (for further details, see Ref. [2]). 

The surface EXAFS and near edge structure of electropolymerized films of [M(v-bpy)3]+2 (v-bpy is 4-vinyl-4 -methyl,-2,2 -bipyrdine and M = Ru,... 

Figure 1. Electrochemical cell for in situ surface EXAFS at chemically modified electrodes.

Structural information on the atomic arrangements at the early stage of formation of metal-metal, metal-semiconductor interfaces and semiconductor-semiconductor heterojunctions is needed along with the determination of the structure of the electron states in order to put on a complete experimental ground the discussion of the formation of solid-solid junctions. Amongst the structural tools that have been applied to the interface formation problem Surface-EXAFS is probably the best... 

Recently we have published integral equation predictions for a flexible model of water next to a planar interface. Experimental motivation for this work includes electrochemical experiments on ultra-pure (Oj-free) water, surface EXAFS studies of the oxygen-metal distance for water at an electrode, and the tunnel junction device measurements of Porter and Zinn." Vossen and Forstmann have published a related calculation using a different model of water and a different approximation for the bulk water bridge functions. Below we compare the input to the two calculations. First we review some results in bulk water and solutions of non-polar solutes. 

O.R. Melroy, M.G. Samant, G.L. Borges, J.G. Gordon, L. Blum, J.H. White, M.J. Albarelli, M. McMillan, H.D. Abruna, In-plane structure of underpotentially deposited copper on gold(lll) determined by surface EXAFS. Langmuir 4, 728-732, 1988. 

Kendelewicz T, Liu P, Labiosa WB, Brown GE (1995) Surface EXAFS and X-ray standing-wave study of the cleaved CaO(lOO) surface. PhysicaB209 441-442 Kitamura H (1998) Present status of SPring-8 insertion devices. J Synchrot Radiat 5 184-188 King HP, Alexander LE (1974) X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials 2nd- edition. Wiley Interscience, New York... 

For studies on single crystals, surface EXAFS offers an additional experimental handle and this refers to the polarization dependence of the... 

Electron yield—Auger, partial, or total—can be employed as a means of detection since again these techniques depend on the generation of core holes. Because of the very small mean free paths of electrons, electron yield detection is very well suited for surface EXAFS measurements. However, due to this very same reason, in-situ studies of electrochemical interfaces are precluded. Details of electron yield EXAFS have been discussed by a number of authors. ... 

The use of surface EXAFS in the study of passive films represents a natural application of the technique, and in fact, the studies by Kruger and co-workers on the passive film on iron represent the first reported. In their first studies, they employed vacuum-deposited iron films on glass slides and subsequently oxidized the films in either nitrite or chromate solution. They obtained the EXAFS spectra for the oxidized films, employing a photocathode ionization chamber (detecting the emitted electron current) and compared these with spectra for y-FeO(OH), y-Fe203, and Fc304. Although these studies were not in-situ, they did not require evacuation of... 

As with EXAFS and XSW experiments, the use of synchrotron radiation greatly facilitates surface diffraction experiments. Since diffraction experiments benefit greatly from an enhancement in the x-ray flux density (photons/cm sec), a toroidal focusing mirror is often employed in order to focus the incoming beam (which is typically 6x2 mm) to a tight spot. The other optical elements present are similar to those employed in surface EXAFS and XSW experiments (e.g.. Fig. 28). 

The requirements for an electrochemical cell to be employed in surface diffraction experiments are quite similar to those for surface EXAFS and XSW measurements. Thus, very similar cell designs have been employed. In order to minimize background scattering by the electrolyte, its thickness must be kept as thin as possible. This is because at the low angles of incidence typically employed, the effective path length can be quite long. Figure 40 shows a schematic of the cell employed by Samant et al. ... 

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