Local atomic structure, analysis

ReflEXAES can be used for near-surface structural analysis of a wide variety of samples for which no other technique is appropriate. As with EXAES, ReflEXAES is particularly suited for studying the local atomic structure around particular atomic species in non-crystalline environments. It is, however, also widely used for the analysis of nanocrystalline materials and for studying the initial stages of crystallization at surfaces or interfaces. ReflEXAES was first proposed by Barchewitz [4.135], and after several papers in the early nineteen-eighties [4.136, 4.168-4.170] it became an established (although rather exotic) characterization technique. Most synchrotron radiation sources now have beam-lines dedicated to ReflEXAES experiments. 

For more sophisticated spectroelectrochemical analysis, electrochemical cycling is coupled with X-ray absorption spectroscopy (XAS) methods, which probe electronic and local atomic structures with element specificity.266-234 Because XAS techniques are compatible with in situ investigations and do not require long-range structural order for analysis, they are... 

In addition, using EXAFS analysis, this group showed importantly that the local atomic structure of MoS2 was preserved in the sulfided catalyst... 

Figure 4 A schematic representation of the experimentai approach for time-resoived XAS measurements. XAS provides local structural and electronic information about the nearest coordination environment surrounding the catalytic metal ion within the active site of a metalloprotein in solution. Spectral analysis of the various spectral regions yields complementary electronic and structural information, which allows the determination of the oxidation state of the X-ray absorbing metal atom and precise determination of distances between the absorbing metal atom and the protein atoms that surround it. Time-dependent XAS provides insight into the lifetimes and local atomic structures of metal-protein complexes during enzymatic reactions on millisecond to minute time scales, (a) The drawing describes a conventional stopped-flow machine that is used to rapidly mix the reaction components (e.g., enzyme and substrate) and derive kinetic traces as shown in (b). (b) The enzymatic reaction is studied by pre-steady-state kinetic analysis to dissect out the time frame of individual kinetic phases, (c) The stopped-flow apparatus is equipped with a freeze-quench device. Sample aliquots are collected after mixing and rapidly froze into X-ray sample holders by the freeze-quench device, (d) Frozen samples are subjected to X-ray data collection and analysis.

Analysis of the Local Atomic Structure of Surface Layers from SEFS Experimental Data... 

At the present time, of all EXAFS-like methods of analysis of local atomic structure, the SEES method is the least used. The reason is that the theory of the SEES process is not sufficiently developed. However the standard EXAES procedure of the Fourier transformation has been applied also to SEES spectra. The Fourier transforms of MW SEES spectra of a number of pure 3d metals have been compared with the corresponding Fourier transforms of EELFS and EX-AFS spectra. Besides the EXAFS-like nature of SEES oscillations shown by this comparison, parameters of the local atomic structure of studied surfaces (the interatomic distances and the mean squared atomic deviations from the equilibrium positions [12, 13, 15-17, 21, 23, 24]) have been obtained from an analysis of Fourier transforms of SEES spectra. The results obtained have, at best, a semi-quantitative character, since the Fourier transforms of SEES spectra differ qualitatively from both the bulk crystallographic atomic pair correlation functions and the relevant Fourier transforms of EXAFS and EELFS spectra. 

Compared to the diffraction methods, the main advantages of the spectral methods of structural analysis, in particular SEFS spectroscopy, are due to their physical nature, namely, their sensitivity to the local atomic structure only. That is, no matter what the structural state of the matter (crystalline or amorphous), the oscillating signal in the spectrum is determined by the nearest atomic environment of the ionized atom of the particular chemical element. Owing to this, the spectral methods of structural analysis make it possible to use the results obtained from a sample with known atomic structure as standards in the study of the structure of unknown objects by determining all parameters of the extended fine structure... 

But in spite of all the advantages of the SEFS method as compared to both diffraction and spectroscopic methods of structure analysis, this technique has not yet been applied to analyze the local atomic structure of surfaces and thin films. This is explained by the diversity and complexity of the processes forming the secondary electron spectrum and the corresponding fine structures, and the resulting difficulties in their theoretical description and in the mathematical formalization of the problem of determining local atomic structure parameters from the experimental data. 

ANALYSIS OF THE SURFACE LOCAL ATOMIC STRUCTURE BY THE SEFS METHOD... 

Like other EXAFS-like spectral features, the SEFS spectrum can be used for the experimental analysis of the local atomic structure. The implementation of extended spectral fine structure as a method of structural analysis came into use in EXAFS spectroscopy nearly 40 years ago. Nowadays EXAFS spectroscopy is a well-developed and intensively used technique for structural analysis. By contrast, the EELFS and SEFS methods are less developed and hence less employed, though the structural information the SEFS contains is often unique, especially for thin surface layers of condensed matter. 

In addition, using EXAFS analysis, this group showed that the local atomic structure of M0S2 was preserved in the sulfided catalyst (26), suggesting that the Co atoms are located in the same plane as the Mo atoms, but not at perfect Mo edge sites. 

Analysis of Local Atomic Structure by the Pulsed Neutron Atomic Pair-Density Function (PDF) Method. 117... 

Crystal structure analysis of 43 revealed that the radical is perfectly trigonal planar, implying the sp -hybridization of the central Si atom and, hence, the localization of the unpaired electron on its 3pz-orbital. The remarkable planarity of the radical 43 was explained by the great steric bulk of the voluminous t-Bu2MeSi groups, which prefered to move away from each other as far as possible to avoid the steric repulsion between them. On the contrary, the significant electron donation of the positive silyl substituents... 

Local surface structure and coordination numbers of neighbouring atoms can be extracted from the analysis of extended X-ray absorption fine structures (EXAFS). The essential feature of the method22 is the excitation of a core-hole by monoenergetic photons modulation of the absorption cross-section with energy above the excitation threshold provides information on the distances between neighbouring atoms. A more surface-sensitive version (SEXAFS) monitors the photoemitted or Auger electrons, where the electron escape depth is small ( 1 nm) and discriminates in favour of surface atoms over those within the bulk solid. Model compounds, where bond distances and atomic environments are known, are required as standards. 

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