XAS Measurement

XAS requires a very good measurement of absorption coefficient p E). To measure the energy dependence of p E) in the X-ray region, first of all, a continuous and tunable X-ray source is needed. The X-ray source typically used is a synchrotron, which provides a full range of X-ray wavelengths, and a monochromator made from silicon that uses Bragg diffraction to select a particular energy. 

The absorption coefficient p E) is most commonly measured either in transmission or fluorescence mode according to the following for-mulas.Transmission mode  

In XAS measurement, multiple spectra are generally collected to improve signal-to-noise ratios, from a minimum of two spectra for concentrated samples to many hours, for example, for dilute biological samples. As beam intensity increases, the signal-to-noise ratio improves and less measurement time is required. Since other elements in the sample can cause scattering of X-rays, filters are sometimes used to remove this scatter. 

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X-Ray Absorption Spectroscopy (XAS). The XAS measurements were similar to those described elsewhere.Grazing incidence (GI)-XAS measurements were performed at beamline 11-2 at Stanford Synchrotron Radiation Laboratory (SSRL). A double Si(220) crystal spectrometer was used to select the energy of the synchrotron X-rays, and the beam size was set to 400 pm x 2 mm. The bandwidth of the spectrometer was about 1 eV. Routine procedures were used to optimize the positions of the samples so that the angle of incidence was about 0.17°, with the X-ray... 

Effect of SAN. We also investigated the contribution of SAN to the growth of SiNW. In these experiments, extra Co left on the substrate after the synthesis of SAN was removed from the SAN samples by washing it in Piranha solution. The Co in those washed samples was in the form of almost pure CoSi2, as verified by XAS measurements (vide infra). These cleaned samples were then used to grow SiNW at 1100°C. It was found that moderate yields of SiNW were produced. 

The XANES region of the Pt Lm and Ln absorption edges can be used to determine the fractional d-electron occupancy of the Pt atoms in the catalyst sample by a so-called white line analysis. Figure 2 shows the XAS spectrum collected at both Pt Lm and Lii absorption edges of Na2Pt(OH)e. The sharp features at the absorption edges are called white lines after the white line observed in early photographic film based XAS measurements. Mansour and coworkers have shown that comparison of the white line intensities of a sample with those of a reference metal foil provides a measure of the fractional d-electron vacancy, f, of the absorber atoms in the sample. is defined as follows ... 

As described above, XAS measurements can provide a wealth of information regarding the local structure and electronic state of the dispersed metal particles that form the active sites in low temperature fuel cell catalysts. The catalysts most widely studied using XAS have been Pt nanoparticles supported on high surface area carbon powders,2 -27,29,so,32,33,38-52 represented as Pt/C. The XAS literature related to Pt/C has been reviewed previ-ously. In this section of the review presented here, the Pt/C system will be used to illustrate the use of XAS in characterizing fuel cell catalysts. 

Ru provides sites for water activation as well as having an electronic effect on the Pt atoms, such that CO is less strongly adsorbed. In situ XAS measurements have been used to determine the structure of PtRu catalysts, to assess the magnitude of any electronic effect that alloy formation may have on the Pt component of the catalyst, and to provide evidence in support of the bifunctional mechanism. 

The following examples represent only a minor fraction of the published literature involving XAS measurements of biological samples. They have been chosen in order to draw attention to the specific kinds of questions which can, or cannot, be answered using XAS. 

On the other hand, there are plans to constmct new synchrotron sources optimized to provide high brighmess (photons/unit area) X-ray sources. These will permit so-called micro-probe XAS measurements, involving very small san le volumes. The corresponding reduction in the amount of protein required will permit studies which are presently impractical. 

Nitridation was also shown to be specifically associated with V with respect to A1 in AlVONs. Using a combination of DRIFTS, XPS and TGA, evidence has been reported for the formation of an intermediate metal-dinitrogen or azide species in the nitridation of AIVON. " XAS measurements have demonstrated that mixed tetrahedra of the form XOxNy (where X = P, Al, or Ga) are formed in the AlGaPON system. ... 

Figure 2.5. L-edge XAS measurements of Ar adsorbed on Ag(110). The projected Ar As states becomes the 4s using the Z + 1 approximation for the core hole state.

The Deutsche Forschungsgemeinschaft (Fr 1372/1-1 and Fr 1372/2-1) and the Fonds der Chemischen Industrie is gratefully acknowledged. We thank HASYLAB DESY for allocating beamtime and Dr. M. Tischer (HASYLAB) for help during the XAS measurements. Finally, we would like to thank Dr. M. Thommes (Quantachrome, Germany) for valuable discussions and assistance in the analysis of the physisorption data. 

In Figure 15.13, the slopes P=A/(X2-XA), measured in end-linked PDMS networks with bimodal distributions of precursor chain lengths and various types of crosslinking agents,... 

Two special electrochemical cells are used for XRD and XAS measurements. In one case a polymer membrane is pressed on the specimen surface after its electrochemical treatment to reduce the water layer on top, but still permitting potential control during the measurements. In an other case the beam penetrates an electrolyte layer in front of the electrode, which corresponds to the specimen s dimensions, but which is thick enough to reduce the danger of ohmic drops and crevices. Beam lines often provide the exact orientation of the samples with the cell by a goniometer. For XAS measurements a special low cost refraction stage has been constructed which permits the orientation of the sample within 0.01 degrees and which has been used for the study of several systems [108]. 

This example illustrates the type of information that can be learned about the transformation of one species to another by XAS measurements of catalysts. There are many other examples that could be cited. Often, it is only the XANES that is measured, but sometimes the quantitative EXAFS data are presented. In the large majority of cases the EXAFS information has been valuable in allowing understanding of the structure of the catalyst, and often this information could not be obtained by any other method. 

FIGURE 25 All-silica environmental cell for XAS measurements of fluorescence yield at temperatures up to 1273 K with sample in an atmosphere of corrosive gas (Moggridge et al, 1995). Reprinted from (Moggridge et al., 1995), Copyright 1995, with permission from Elsevier. 

Surface analytical methods — X-ray absorption spectroscopy, XAS — Figure. Electrochemical cell for in situ XAS measurements in reflection, set up with a gracing incident X-ray beam, beam shaping slit, ionization chambers for the intensity measurement of incoming (ii) and reflected beam (I2) and beam stop for the direct nonreflected beam [vii]... 

Figure 4. The characteristic time xa, measured from several chain microstructures of polybutadiene, is represented for A = 0.6 vinyl contents are XI,2 = 0.22 ( ), 0.40 (A), 0.58 (A) and 0.82 ).

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.

Unlike other copper oxide superconductors which have higher cja ratio, the cj a ratio in the non-defective infinite-layer compound is close to 1. However, because of a lack of apical oxygen, the electronic structure is expected to be more anisotropic than all other high oxides, despite the similar c and a lattice constants of the unit cell. This supposition was recently confirmed by X-ray absorption spectroscopy (XAS) measurements [8.27], in which an anisotropic upper Hubbard band was reported. In this section, we will discuss the question of whether the electronic transitions near the Fermi surface behave anisotropically with respect to the a6-plane and the c-axis. 

The first excitation in Figure 8.4 along the ah-plane is associated with the CUO2. In all of the undoped cuprates, there is a sharp excitation below the broad 3 eV excitation. In the present case, due to the energy resolution (0.5 eV) of the spectrometer, the 2 spectrum for the ah-plane represents a mixture of the sharp excitation and the broad excitation. The anisotropic dielectric function indicates that the Hubbard band lies mostly in the ah-plane. This is consistent with the very anisotropic O K-pre-edge data reported from XAS measurements, which indicate that only 1 1% of the upper Hubbard band is out of the ah-plane for the infinite-layer compound [8.27]. The reason for this significant anisotropy in the Hubbard band for the infinite-layer compound is clearly associated with the lack of apical oxygen in the structure. 

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