Ray Absorption Spectrum

The (pseudo ) tetragonal structure of Nd4Se7 does not change on application of pressures up to 100 kbar, in contrast to the behavior of NdSe g, Tikhomirova et al. [1]. The specific paramagnetic susceptibility was measured from 77 to 600 K. The Curie-Weiss law is followed with 0p = -14K, Heff = 3.60 (Xg, corresponding to the value expected for Nd . The atomic susceptibility is 5210x10 cm /mol Nd at 290 K, Pechennikov et al. [2]. 

The composition of the diselenide is given as NdSe 93. A range of homogeneity is indicated by the change of the lattice constants during thermal decomposition, Slovyanskikh et al. [1]. Computer calculations predict anionic vacancies corresponding to the formula NdSe 88 0.12. Kutolin et al. [2]. 

NdSei 93 is obtained by heating starting material of composition NdSe 2.5 to 670 K [1]. Preparation of the diselenide from Nd203 and Se in the presence of H2 has been successfully attempted by Lashkarev, Savitskii [3]. Single crystals were grown by a chemical transport method with I2 as the transport agent [1], Eliseev, Kuznetsov [4], Kalitin et al. [5]. The crystals were truncated tetragonal pyramids [5]. 

The Nd diselenide is isostructural with LaTe2 according to single crystal investigations. It contains two types of Se atoms. The atomic positions are as follows  

The lattice constants of NdSe 93 decrease with increasing temperature to a = 8.24, c = 8.38 A at 850 K due to thermal decomposition [1]. 

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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 Molybdenum K-edge X-ray absorption spectrum, ln(i /i ) versus X-ray energy (eV), for molybdenum metal foil (25- jjn thick), obtained by transmission at 77 K with synchrotron radiation. The energy-dependent constructive and destructive interference of outgoing and backscattered photoelectrons at molybdenum produces the EXAFS peaks and valleys, respectively. The preedge and edge structures marked here are known together as X-ray absorption near edge structure, XANES and EXAFS are provided in a new compilation of literature entitled X-rsy Absorption Fine Structure (S.S. Hasain, ed.) Ellis Norwood, New York, 1991.

However, mathematics is essential to explain how structural data are derived from EXAFS. The EXAFS function, x(k), is extracted from the X-ray absorption spectrum in Fig. 4.10 by removing the approximately parabolic background and the step, i.e. the spectrum of the free atom. As in any scattering experiment, it is customary to express the signal as a function of the wavenumber, k, rather than of energy. The relation between k and the kinetic energy of the photoelectron is ... 

Figure 1. X-ray absorption spectrum of a silica supported ruthenium-copper catalyst at 100 K In the vicinity of the K absorption edge of ruthenium. Reproduced with permission from Ref. 8. Copyright 1980, American Institute of Physics.

EXAFS analysis is a powerful spectroscopic method for structural analysis which has been extensively applied to the problem of structure determination in nanoparticles, and especially bimetallic nanoparticles [170-172]. The X-ray absorption spectrum of an element contains absorption edges corresponding to the excitation of electrons from various electronic states at energies characteristic of that element, i.e., K edges arise from the excitation of electrons from Is states, and LI, II, III edges from excitations from 2s, 2p 1/2, and 2p3/2 states. When the X-ray energy is increased above an edge, oscillations (fine... 

Figure 6. Figure depicting the various regions in an X-ray absorption spectrum. 

Figure 19. In situ X-ray absorption spectrum for a copper upd monolayer on a gold (111) electrode with the polarization of the X-ray beam being perpendicular (A) or parallel (B) to the electrode surface.

Figure 2.73(a) shows the X-ray absorption spectrum of copper. The K edge is the minimum energy required to ionise an electron from the Is orbital ... 

Figure 2.73 (a) The X-ray absorption spectrum of copper showing the K- and L-absorption edges, (b) The K.-edge in more detail. From A.R. West, Solid State Chemistry and its Applications, John Wiley and Sons, Chichester (1984). Reprinted by permission of John Wiley and Sons, Ltd. 

The electronic spin-state crossover in [Fe(HB(pz)3)2] has also been observed in the fine structure of its fC-edge x-ray absorption spectrum [38]. The changes in the x-ray absorption spectra of [Fe(HB(pz)3)2] are especially apparent between 293 and 450 K at ca. 25 eV, as is shown in Fig. 5. The 293 K x-ray absorption spectral profile observed in Fig. 5 for [Fe(HB(pz)3)2] has been reproduced [39] by a multiple photoelectron scattering calculation, a calculation that indicated that up to 33 atoms at distances of up to 4.19 A are involved in the scattering. As expected, the extended x-ray absorption fine structure reveals [38] no change in the average low-spin iron(II)-nitro-gen bond distance of 1.97 A in [Fe(HB(pz)3)2] upon cooling from 295 to 77 K. 

Mo EXAFS. In Figure 2a we have shown an X-ray absorption spectrum near the Mo K-edge of the unsupported catalyst with Co/Mo = 0.125. The spectrum has been obtained in situ and at room temperature. After background subtraction, multiplication by k and normalization,... 

Figure 2. a) X-ray absorption spectrum near the Mo K-edge of the Co/Mo = 0.125 unsupported Co-Mo catalyst recorded in situ at room temperature b) normalized Mo EXAFS spectrum c) absolute magnitude of the Fourier transform d) fit of the first shell e) fit of the second shell. The solid line in d) and e) is the filtered EXAFS, and the dashed line is the least squares fit. 

Figure k. X-ray absorption spectrum near the Co K-edge of the Co/Mo = 0.125 catalyst after exposure to air at room temperature. 

X-ray absorption near edge structure (XANES) The X-ray absorption spectrum, as for EXAFS, may also show detailed structure below the absorption edge. This arises from excitation of core electrons to high level vacant orbitals, and can be used to estimate the oxidation state of the metal ion. 

It is understood that the x-ray absorption spectrum should be treated as divided into near-edge and extended fine structures. The x-ray absorption near-edge structure... 

Figure 2. Typical X-ray absorption spectrum. Inset is schematic illustration of out-going and backscattered photoelectron wave for energies E (left) and E (nght).

These results suggest that oxidation state is not solely responsible for catalyst deactivation but that other factors such as V location and mobility may play an important role. Basic alkaline earth oxide passivators such as MgO, admixed to the catalyst, interact strongly with vanadium during the regeneration period. Although the oxidation state of vanadium is essentially unaffected, MgO structurally modifies V as evidenced by a unique X-ray absorption spectrum. 

Fig. 1. X-ray absorption spectrum (XAS) of Cu—Zn metallothionein at the Cu and Zn K-edges. The structure near the edge, referred to as XANES is dominated by multiple scattering events while the extended structure, referred to as EXAFS, at photoelectron energies greater than 30-50 eV is primarily due to single scattering events...

Fig, 11. Photoacoustic X-ray absorption spectrum and X-ray absorption spectrum for copper foil (5 pm thick). Photoacoustic signal is normalized by ion chamber current. Chopping frequency 10 Hz. Ring current 145-142 mA... 

The EXAFS function is obtained from the X-ray absorption spectrum by subtracting the absorption due to the free atom. A Fourier transform of the EXAFS data gives a radial distribution function which shows the distribution of the neighbouring atoms as a function of internuclear distance from the absorbing atom. Shells of neighbours, known as coordination shells, surround the absorbing atom. Finally, the radial distribution function is fitted to a series of trial structural models until a structure which best fits the... 

Fig. 4. a) a) Static /.m-edge x- ray absorption spectrum of ground state [Rul (bpy)3]2+ (trace R), and excited state absorption spectrum (trace P) generated from the transient data curve T in (b) via eq. 2. b) Transient x-ray absorption spectrum (T) of photoexcited aqueous [Ru"(bpy)3]2+ measured 50 ps after laser excitation. 

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