Extended Fine Structure Research Articles

Experiments with an Imaging Filter in a CTEM

The installation of an imaging energy filter extends the imaging and analytical capabilities of a conventional transmission electron microscope (CTEM) considerably. We report here on experiments which demonstrate the versatility of such a device. The arrangement of our system is shown in Fig. 1. The experiments performed pertain to:The improvement of contrast in thick specimens by exclusion of inelastically scattered electrons. The spatial resolution of the electron microscope can be preserved by the correction of second-order aberrations with additional electromagnetic hexapoles.The determination of the concentration of light elements with EELS at a spatial resolution better than 10 nm. The chemical structure of ceramic material (SI3N4and SiC with additions) could thus be elucidated. A study on the extended fine structure in the energy loss spectra of the oxygen and magnesium K edges in MgO (Fig. 2) did confirm the theoretical prediction for that material.

X-ray K-absorption spectra of some aminoacid and carbonato complexes of copper(II)

X-ray (K-) absorption edge spectrometric (XAES) investigations of some copper(II) chelates involving amino acids and a carbonato complex have been carried out employing a curved crystal spectrograph. Experimental edgewidths have been used to bear on the number of nearest neighbours around the central metal atom in the complexes in the solid state. Comparisons in respect of the nature of the metal-ligand bond have been made between inferences drawn on the basis of XAES, i.r. and optical spectral data. Average radii of the coordination sphere in the amino acid chelates have been reported using extended fine structure energy separations (Levy's method). In the carbonato complex, the possibility of determining different metal-donor distances within the coordination sphere has been suggested for the first time.

Summary Abstract: Oxidation studies by extended appearance potential fine structure (EAPFS)

Summary Abstract: Oxidation studies by extended appearance potential fine structure (EAPFS)

Extended fine structure in electron energy loss spectra of MgO crystallites

EXAFS-like features in energy loss spectra (known as EXELFS) were observed by using an imaging energy filter built into an electron microscope. Single crystal micro-platelets of MgO were used as test samples. Extended fine structures were obtained reproducibly in the energy range up to 150 eV beyond the O-K, Mg-K and Mg-L edges. Comparison with single scattering calculations for the K edges showed a fair agreement, indicating the feasibility of this type of analysis.

Extended fine structure in electron energy loss spectra of MgO crystallites

EXAFS-like features in energy loss spectra (known as EXELFS) were observed by using an imaging energy filter built into an electron microscope. Single crystal micro-platelets of MgO were used as test samples. Extended fine structures were obtained reproducibly in the energy range up to 150 eV beyond the OK, MgK and MgL edges. Comparison with single scattering calculations for the K edges showed a fair agreement, indicating the feasibility of this type of analysis.

X-ray absorption spectroscopy of xanthine oxidase. The molybdenum centres of the functional and the desulpho forms.

X-ray absorption spectra have been recorded for the molybdenum K-edge region of xanthine oxidase. Both the absorption edge and the extended fine structure (e.x.a.f.s.) regions were investigated. Spectra were obtained for samples of the desulpho enzyme as well as for mixtures of this with the active enzyme. The spectrum of the pure active form was then obtained by difference. The desulpho enzyme shows a pronounced step in the absorption edge, of a type previously associated terminal oxygen ligands. In the active enzyme this step has decreased markedly. Satisfactory simulations of the e.x.a.f.s. spectrum of the desulpho enzyme could be obtained by assuming the molybdenum to be bonded to two terminal oxygen atoms (Mo = O about .175 nm), two sulphur atoms (presumably from cysteine residues, Mo-S about .0250 nm) and one sulphur atom (presumably from a methionine residue, Mo-S about 0.290 nm). E.x.a.f.s. of the active enzyme differed appreciably from this. In keeping with earlier proposals [Gutteridge, Tanner & Bray (1978) Biochem. J. 175, 887-897], the spectrum of the active enzyme could be simulated if a sulphur atom at about 0.225 nm (i.e. presumably a terminal sulphur atom) replaced one of the terminal oxygen atoms of the desulpho from, with small changes in the other bond distances. Validity of the interpretative procedures, which involved phase shift and amplitude calculations ab initio, was demonstrated by using low molecular weight compounds of known structure.

Photon-energy dependence of the core-level peak intensity in photoelectron spectra: A study of the extended fine structure

Note: Ecole polytech fed lausanne,phys appl lab,ch-1007 lausanne,switzerland. Margaritondo, g, univ wisconsin,dept phys,madison,wi 53706.ISI Document Delivery No.: KK449 Reference LSE-ARTICLE-1980-006doi:10.1103/PhysRevB.22.2777View record in Web of Science Record created on 2006-10-03, modified on 2016-08-08

Anisotropy of the white line in the gallium K-absorption spectrum: A part of EXAFS?

Anisotropy effects of the so-called white line in K-absorption spectra of a single crystal of gallium can be explained by EXAFS theories. From this point of view, the first absorption maximum in the gallium K spectrum forms part of the extended fine structure.

Surface spacings from the secondary electron yield

Small inflections in plots of the total yield of secondary electrons versus primary electron energy were reported more than fifty years ago and attributed variously to thresholds for the excitation of core states, and to diffraction of the incident electrons. Although the existence of these inflections was discounted by later researchers, recent studies of the derivative of the yield leave no doubt as to their reality or their origin. The spectrum of core excitation thresholds provides information on the composition of the surface region. Near-threshold structure is related to the electronic state of the atoms. Extended fine structure above the threshold can be inverted to obtain interatomic spacings even from relatively disordered surfaces. The principal limitation in these measurements results from diffraction of the incident electron beam, which produces variations in electron reflectivity. The extreme simplicity of the equipment required makes this technique available to essentially every surface science laboratory.

Effect of the central atom potential on the extended fine structure above appearance potential thresholds

The formalism previously given for describing the extended fine structure above appearance-potential-spectroscopy (APS) thresholds is extended by incorporating the effects of the excited "central" atom potential in an exact manner. The excitation-matrix elements are expressed in terms of the exact wave functions of the central atom potential. This introduces a "phase renormalization" into the excitation-matrix elements and eliminates a previously noted "phase difference" between single- and multiple-scattering calculations employing a plane-wave basis set. A series of approximations is then made which leads to an expression for the APS extended fine structure in terms of sinusoidal functions and hence provides a rationale for a Fourier-transform analysis. Simple model calculations assuming a constant "bare" excitation-matrix element, a spherically symmetric electronic density of states, and only $S$-wave scattering from the atomic cores are performed for a cluster of atoms having the atomic geometry of bulk vanadium. These calculations display the major predictions of the formalism and indicate that for a given system there may be some optimal energy range for data analysis. The problem of electron characteristic losses is considered, and it is pointed out that in the small-momentum-transfer limit the simple dipole selection rules appropriate to a photon-excitation process again apply. This may obviate many of the problems introduced by multiple angular momentum final states in the APS process.

Structural studies on small areas using stem

Three techniques for investigating the microstructure of amorphous materials are described. These include incoherent dark field microscopy, analysis of extended fine structure on the elemental core excitations, and analysis of the elastic diffraction pattern. All of these can be implemented with a Scanning Transmission Electron Microscope, allowing comparison of complementary results obtained under similar conditions.

Recent progress in surface analysis

The most basic requirements in surface analysis are to identity the chemical constituents and to determine their structural arrangement. Progress in chemical analysis has been rapid since the introduction of Auger electron spcctroscopy, but the extreme complexity of low energy electron diffraction analysis has limited surface crystallography to a few relatively simple structures. It may, however, be possible to extract structural information, even from relatively disordered surfaces, using extended fine structure techniques. Fine structure variations have been observed to extend hundreds of electron volts above appearance potential thresholds. This structure is analogous to extended X-ray absorption fine structure (EXAFS), and can be interpreted in terms of interatomic distances in the surface region. The recent development of extended appearance potential fine structure (EAPFS) analysis is reviewed. The technique is of potential benefit in the resolution of LEED analysis problems on single crystal surfaces, as well as in the study of less ideal surfaces. The present limitations of the technique are stressed and analytical and experimental approaches to minimizing these limitations are discussed.

Abstract: Use of Fourier transforms for analyziang the extended fine structure above appearance potential thresholds

Abstract: Use of Fourier transforms for analyziang the extended fine structure above appearance potential thresholds

The use of Fourier transforms for analyzing the extended fine structure above appearance potential thresholds

The problem of calculating the geometrically-induced, extended fine structure above electron-excited apprearance potential spectroscopy (APS) thresholds is described and it is shown that differentiating the result with respect to the incident beam energy leads to an expression similar to that for extended X-ray absorption fine structure (EXAFS). The dominant contribution to the APS extended fine structure occurs when one of the final state electrons is “pinned” at threshold. The momentum of the other final state electron is thus a unique Fourier transform variable for data analysis. On this basis, it was assumed that the APS extended fine structure could be represented as a sum of sinusoidally varying terms as in EXAFS. Model calculations assuming a constant “bare” excitation matrix element, a spherically symmetric electronic density of states, and S-wave scattering from the atomic cores were performed to test this assumption. The results show that multiple scattering effects primarily introduce an overall phase shift into the calculated curves which makes them look quite different than curves calculated in the single scattering limit. However, this does not affect the Fourier transform procedure and the model calculations indicate that it may be feasible to simply extract at least the nearest-neighbor spacing from the APS extended fine structure.

White line structure in the X-ray LIIIabsorption edge of holmium

The white line at the Ho LIII absorption edge has been recorded in Ho metal, Ho2O3 and HoCl3. The white line structure in Ho2O3 has been analysed by regarding it as due to the transition into bound states of the LIII excited ion. The extended fine structure has been used to obtain information on the bond lengths in the compounds.

Extended fine structure above vanadium L-shell appearance potential thresholds

The probability of electron scattering from atomic core states as a function of electron energy, can be extracted from secondary electron yields by differentiation. Fine structure variations above the thresholds for excitation of the vanadium 2p and 2s states are found to extend for several hundred electron volts. The fine structure is altered significantly by reaction of the surface with CO, but is insensitive to long-range order. The structure exhibits periodicities in k, and presumably results from interference of an outgoing spherical wave of a scattered electron with backscattered components from neighboring atoms. The structure therefore appears to be analogous to extended X-ray absorption fine structure (EXAFS). Attempts to extract interatomic spacings by Fourier inversion, however, have been frustrated by multiple scattering effects and other complications. The extreme experimental simplicity provides ample motivation for dealing with these theoretical complexities.

Extended Fine Structure of Copper K-Absorption Discontinuity in the Metal and in its Alloy with Antimony

The extended fine structure associated with the K-absorption discontinuity of copper in pure metal and in the alloy Cu 82.5 Sb 17.5 is studied using a 40 cm Cauchois type bent crystal X-ray spectrograph. The extended fine structure has been explained on the basis of the theory proposed by Lytle, Sayers and Stern. The theory has also been used to determine the bond length for the alloy. Levy's method has also been used to determine bond lengths for the systems under study. The results obtained from both the theories have been compared.

X‐Ray K‐Absorption Structure of Zirconium Metal and Its Compounds

AbstractThe K‐absorption spectra of zirconium are studied in the following substances: Zr, ZrO2, ZrC, ZrBr4, Zr(SO4)2 · 4 H2O, Zr(NO3)4 · 5 H2O, ZrOCl2 · 8 H2O, and K2Zr(C2O4)3. The fine structure within the main edge is discussed on the basis of band structure and is in general accordance with the overlap of d‐ and sp‐bands in transition metals. The observed extended fine structure is analysed in each case to give information on the bond length.

Short-Range Order in Metallic Glasses

From a measurement and analysis of the extended fine structure on the Ge $K$-shell absorption cross section, we determine that the metalloid (Ge) nearest-neighbor environments in arc-quenched glassy ${\mathrm{Pd}}_{78}$${\mathrm{Ge}}_{22}$ and in sputtered amorphous ${\mathrm{Pd}}_{80}$${\mathrm{Ge}}_{20}$ are essentially identical. Conclusive evidence is presented for the occurrence of chemical ordering. The glass structure is shown to be substantially modified by the presence of the metalloids, bringing into question the applicability of models based upon a structure determined by the metal atoms alone.

Abstract: Extended fine structure above the vanadium 2s appearance potential edge

Abstract: Extended fine structure above the vanadium 2<i>s</i> appearance potential edge