Showing papers on "Extended X-ray absorption fine structure published in 1979"

Extended x-ray absorption fine structure Debye-Waller factors. I. Monatomic crystals

Abstract: Debye-Waller factors in the extended x-ray absorption fine structure (EXAFS) are related to the mean-square fluctuations in interatomic distances, ${\ensuremath{\sigma}}_{R}^{2}$. Monte Carlo calculations of ${\ensuremath{\sigma}}_{R}^{2}$ based on lattice-dynamical models are presented for crystalline Cu, Fe, and Pt. The results are compared with correlated Einstein and Debye models, with experimental data and with the mean-square vibrational amplitudes ${{u}_{\stackrel{^}{q}}}^{2}$ which enter the Debye-Waller factor in x-ray diffraction.

Extended X-Ray—Absorption Fine Structure of Small Cu and Ni Clusters: Binding-Energy and Bond-Length Changes with Cluster Size

Abstract: Extended x-ray-absorption fine-structure measurements have been made on metal clusters of Cu and Ni which were formed by vapor deposition on amorphous carbon substrates. Small clusters of both elements show a substantial contraction of the nearest-neighbor metal-metal distance and an increase in binding energy for the onset of the $K$ absorption edge. The results are explained by the increasing surface-to-volume ratio as the cluster size decreases resulting in a more free-atom---like configuration of the metal atoms.

Physics of superionic conductors

Abstract: 1. Introduction.- References.- 2. Structure and Its Influence on Superionic Conduction: EXAFS Studies.- 2.1 Technique of EXAFS.- 2.1.1 Theory.- 2.1.2 Experiment.- 2.1.3 Data Reduction and Analysis.- 2.1.4 Contrast with Diffraction Studies.- 2.2 Structural Considerations for Superionic Conduction.- 2.2.1 General Considerations.- 2.2.2 Pair Potentials.- 2.2.3 Anharmonic Model.- 2.2.4 Excluded Volume Model and Cation-Anion Correlations.- 2.3 EXAFS Investigations of bcc Superionic Conductors: AgI.- 2.3.1 Early Structural Studies.- 2.3.2 EXAFS Study.- 2.3.3 Other Recent Structural Studies.- 2.3.4 Structural Model for Superionic Conduction in bcc Conductors.- 2.4 EXAFS Investigations of fcc Superionic Conductors: Cuprous Halides.- 2.4.1 CuI Structural Studies.- 2.4.2 EXAFS and Structural Models for CuI.- 2.4.3 CuBr.- 2.4.4 CuCl.- 2.4.5 Discussion.- 2.5 Summary.- References.- 3. Neutron Scattering Studies of Superionic Conductors.- 3.1 Neutron Scattering.- 3.1.1 Scattering function.- 3.1.2 Elastic Scattering.- 3.1.3 Inelastic Scattering.- 3.2 Structural Studies.- 3.2.1 AgI.- 3.2.2 Fluorites.- 3.2.3 ?-Alumina.- 3.3 Inelastic Studies.- 3.3.1 AgI.- 3.3.2 RbAg4I5.- 3.3.3 Fluorites.- 3.3.4 ?-Alumina.- 3.4 Conclusions.- References.- 4. Statics and Dynamics of Lattice Gas Models.- 4.1 General Theory of the Lattice Gas Model for Superionic Conductors.- 4.1.1 Definition of the Lattice Gas Model.- 4.1.2 Liouvillian Approach to Lattice Gas Dynamics.- 4.1.3 Master-Equation Approximation.- 4.1.4 High-Frequency Limit.- 4.1.5 Extension to All Frequencies.- 4.2 Extended Dynamical Theory.- 4.2.1 ?trap and Its Relation to a Soliton Model.- 4.2.2 Low-Frequency Conductivity.- 4.3 Applications to Silver Iodide and Hollandite.- 4.3.1 Silver Iodide: Structural Properties, Lattice Gas Representation.- 4.3.2 The Disorder Entropy of AgI.- 4.3.3 Dynami c Properties of ?-AgI.- 4.3.4 Collective Excitations in One-Dimensional Systems: Hollandite.- 4.4 Conclusions.- Appendix A.- Appendix B.- Appendix C.- References.- 5. Light Scattering in Superionic Conductors.- 5.1 Raman Scatteri ng.- 5.1.1 Silver Iodide.- 5.1.2 M+Ag4I5 (M+ = Rb+, K+, NH+4).- 5.1.3 Copper Halides.- 5.1.4 ?-Aluminas.- 5.1.5 Anion Conducting Fluorites.- 5.2 Low-Frequency Raman and Brillouin Scattering.- 5.2.1 Theoretical Considerations.- 5.2.2 Silver Halides.- 5.2.3 Other Superionic Conductors.- 5.3 Infrared Absorption and Frequency Dependent Conductivity.- 5.4 Conclusion.- References.- 6. Magnetic Resonance in Superionic Conductors.- 6.1 Theory of NMR Relaxation of and by Rapidly Diffusing Ions.- 6.1.1 General Correlation Functions and Interactions.- 6.1.2 Calculation of T1 and T2 from Correlation Functions.- 6.1.3 T1/T2 Ratio.- 6.1.4 Simple Random-Walk Values.- 6.1.5 Diffusion in Lower Dimensions.- 6.1.6 Effects of Correlated Hopping.- 6.2 Comparison with Experiment.- 6.2.1 Thermal Activation.- 6.2.2 Frequency Dependence.- 6.2.3 Prefactors.- 6.2.4 Coupling to Paramagnetic Impurities.- 6.3 Electron Paramagnetic Resonance.- 6.4 Structure Determination.- 6.5 Summary and Conclusions.- References.- 7. Phase Transitions in Ionic Conductors.- 7.1 Modern Theory of Phase Transitions.- 7.1.1 Landau Criteria.- 7.1.2 Renormalization Group.- 7.2 Models for Critical Behavior in Superionic Conductors.- 7.2.1 Quasi-Chemical Models.- 7.2.2 Lattice Gas Models.- 7.2.3 The Order Parameter for RbAg4I5.- 7.3 Critical Behavior of Physical Properties.- 7.3.1 Specific Heat.- 7.3.2 Ionic Conductivity.- 7.3.3 Acoustic Properties.- 7.3.4 Other Properties.- 7.4 Conclusions.- References.- 8. Continuous Stochastic Models.- 8.1 Models for Superionic Conductors.- 8.1.1 The Hamiltonian.- 8.1.2 Comparison of the Models from Microscopic Considerations.- 8.1.3 Correlation Functions.- 8.2 Continuous Models.- 8.2.1 Langevin Equation.- 8.2.2 Fokker-Planck Equation and Liouvillian.- 8.2.3 Continued-Fraction Expansion.- 8.2.4 Static Mobility, Diffusion Constant, dc Conductivity.- 8.2.5 Dynamic Mobility, ac Conductivity.- 8.2.6 Approximate Solutions and Similar Models.- 8.2.7 Dynamic Structure Factor for Jump Diffusion.- 8.2.8 Dynamic Structure Factor for Large Friction.- 8.2.9 Dynamic Structure Factor for General Friction.- 8.2.10 Light Scattering: Continuous and Continuum Models.- 8.2.11 Microscopic Foundation.- 8.3 Computer Simulations.- 8.4 Correlations Among the Mobile Ions.- References.- Additional References with Titles.

X-ray filter assembly for fluorescence measurements of x-ray absorption fine structure.

Abstract: Fluorescence detection, in principle, permits the detection of the extended x-ray absorption fine structure (EXAFS) of more dilute atoms than can be obtained in absorption. To take advantage of this it is necessary, in practice, to eliminate the background that normally accompanies the fluorescence signal. We describe an x-ray filter assembly that accomplishes this purpose. The unique characteristic of the assembly is a slit system that minimizes the fluorescence background from the filter. The theory of the slit assembly is presented and is found to agree with measurements made on the Fe EXAFS of a dilute sample. The filter assembly has a better effective counting rate in this case than that of a crystal monochromator design.

Absorption coefficient of silicon for solar cell calculations

Abstract: The optical absorption coefficient is an important parameter in calculating the performance characteristics of solar cells. For silicon solar cells it is desirable to know the absorption coefficient over the range of 1.1–4.0 eV and over a wide range of temperature, particularly when evaluating the concentration type systems. An analytical (empirical) expression has been developed for this purpose. We have interpreted the available experimental data in terms of three bands of silicon. With our fit, the experimental data can be explained to within an accuracy of 20% and its validity extends from 1.1 to 4.0 eV and over the temperature range of 20–500°K.

Optical absorptions of light and heavy water by laser optoacoustic spectroscopy

Abstract: A pulsed dye-laser optoacoustic spectroscopy technique has been used to measure the absorption spectra of light and heavy water at 21.5 degrees C in the visible region. Basic principles of pulsed optoacoustic spectroscopy technique and the procedure for absolute calibration are discussed with reference to its application in water. Experimental details of the application of optoacoustic spectroscopy to water are given. Our absorption coefficients, of accuracies about +/- 10%, are believed to be the most reliable so far. Light water has a broad absorption minimum near 475 nm where the absorption coefficient is 1.8 x 10(-4) cm(-1). Heavy water exhibits a totally different absorption spectrum and has a broad absorption minimum near 600 nm where the absorption coefficient is 1.9 x 10(-4) cm(-1). Previous measurements of the optical spectra of water were done mostly by long-path transmission measurements, and they display disagreement by factors as large as 10 near the green absorption minimum of light water. We give a critical comparison of our optoacoustic absorption spectra with other existing data.

Optical properties of Zn3P2

Abstract: The optical properties of bulk and thin‐film Zn3P2 have been measured at room temperature over the range 0.5–5.0 eV, with emphasis on the region of the interband absorption edge. The bulk absorption edge is found to be exponential in energy for values of absorption coefficient less than about 1500 cm−1. The thin‐film absorption edge, when freed of spurious absorptance due to scattering, is also found to be exponential over this range although shallower in slope. Analysis of the thin‐film data at higher values of absorption coefficient is inconclusive with respect to the nature of the edge. Nevertheless, indirect evidence suggests that the optical gap is probably direct and lies in the neighborhood of 1.6 eV. Measurements of refractive index in the near‐infrared yield n=3.3±0.1 in the long‐wavelength limit. The ultraviolet reflectivity spectrum is redetermined, and found to differ substantially from earlier reports. Results are discussed in terms of the Dow‐Redfield model of exponential absorption edges, t...

Extended x-ray absorption fine structure (EXAFS) of dispersed metal catalysts

Abstract: Extended x‐ray absorption fine structure (EXAFS) studies were made in a series of highly dispersed metal catalysts to obtain structural information on the metal clusters present. Clusters of osmium, iridium, and platinum dispersed on silica or alumina were investigated. The metal clusters in the catalysts constituted 1.0 wt.% of the total mass. Chemisorption measurements indicated that the metal dispersions approached unity, where dispersion is defined as the ratio of surface metal atoms to total metal atoms in the clusters. When EXAFS data on the metal clusters are compared with data on the corresponding bulk metals, the lower average coordination number of the metal atoms in a cluster is evident. The decrease in coordination number is accompanied by an increase in the root mean square deviation of interatomic distance about the equilibrium value.

Direct Measurement of Hot-Electron Relaxation by Picosecond Spectroscopy

Abstract: The time dependence of the optical absorption and the gain of a hot electron-hole plasma in GaAs is measured by means of tunable picosecond light pulses. The electrons and holes (\ensuremath{\sim}${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) are photoexcited via two-photon absorption. Our measurements provide a detailed picture of the temperature evolution, showing that in 250 ps the plasma cools from over 100 K down to \ensuremath{\sim} 40 K and does not reach the lattice temperature. The energy relaxation process is shown to be polar LO-phonon emission of hot electrons and holes.

Infrared optical absorption of Hg1−xCdxTe

Abstract: The absorption near the fundamental absorption edge of Hg1−xCdxTe was measured over the composition range 0.205⩽x⩽0.220 at temperatures from 80 to 300 K. The dispersion of the index of refraction of Hg1−xCdxTe was obtained from the interference pattern. It was found that the absorption tail obeys a modified Urbach’s rule and is expressed by α=α0 exp[σ (E−E0)/(T+T0)] for 20⩽α (cm−1) ⩽1000. The fitting parameters α0, σ, T0, and E0 vary regularly with x. The expression is used to obtain the absorption coefficient and the temperature coefficient of the gap as a function of x and T. Evidence is presented to show that these parameters may be extrapolated to calculate the absorption beyond the measured composition range.

Two-Photon Absorption in Zinc-Blende Semiconductors

Abstract: Two-photon absorption can become the dominant loss mechanism for semiconductors subjected to sufficiently intense laser light in the frequency range Svp&E, &28p, where +p is the laser frequency and E, is the energy gap. The proper understanding of this effect can yield information not accessible to one-photon transitions, and is important for the operation of a number of devices such as \"inducible absorbers, \" twophoton-pumped and spin-flip Raman lasers, and infrared detectors. There has been a long-standing discrepancy, in some cases of more than an order of magnitude, between reports of the measured frequency dependence of the two-photon absorption coefficient and the theoretically predicted values, in particular for two-photon energies much greater than E, (see, for example, Basov et al. ,' Lee and Fan, ' and Doviak et al. ') We show in the present work that a simple three-band model calculation provides a single comprehensive description of both the magnitude and the frequency dependence of the two-photon absorption coefficient for a wide range of III-V and II-VI zinc-blende semiconductors. For a given photon energy the magnitude of the coefficient d(An) Kal dt 28&p (2)

Extended x-ray absorption fine structure determination of thermal disorder in Cu: Comparison of theory and experiment

Abstract: The extended x-ray absorption fine structure (EXAFS) of Cu at temperatures from 10 to 683K has been used to evaluate the EXAFS disorder term and compare it with theory. The change in disorder for different temperatures was extracted from the slopes of $\mathrm{ln}(\frac{{\ensuremath{\chi}}_{1}}{{\ensuremath{\chi}}_{2}})$ plots and by using a least-squares curve-fitting program. The values of the disorder parameter vs temperature are compared to a pure Debye model without correlation, a Debye model with correlation, and a model incorporating the measured phonon spectrum of Cu. The pure Debye model predicts too large an effect, whereas there is good agreement between the experimental data and either of the other two theories. This indicates that models which account for the correlated motion of the absorbing and scattering atom can give $\ensuremath{\sigma}$ values which are in good agreement with those determined experimentally.

Synchrotron Radiation: Techniques and Applications

Abstract: 1. Introduction - Properties of Synchrotron Radiation.- 1.1 Historical Development.- 1.2 Quantitative Properties.- 1.2.1 Equations for Ideal Orbits.- 1.2.2 Considerations for Real Orbits.- a) Coherence.- b) Periodic Wigglers.- c) Synchrotron Accelerators.- d) Beam Cross Section and Divergency.- 1.2.3 Time Structure.- 1.3 Comparison with other Sources.- 1.3.1 Infrared and Visible Range.- 1.3.2 Vacuum Ultraviolet Range.- 1.3.3 X-rays.- 1.4 Acknowledgments.- References.- 2. The Synchrotron Radiation Source.- 2.1 Fundamental Concepts.- 2.1.1 Orbit Dynamics.- a) Betatron Oscillations.- b) Betatron Oscillations of Off-energy Particles.- c) Phase Focusing and Synchrotron Oscillations.- 2.1.2 Radiation Damping.- 2.1.3 Beam Lifetime.- 2.1.4 Beam Cross Section.- 2.2 Design Considerations.- 2.2.1 Magnetic Field and Energy.- 2.2.2 Lattice.- 2.2.3 Injector.- 2.2.4 Accelerating System.- 2.2.5 Energy Shifter Wigglers.- 2.2.6 Multipole Wigglers (Undulator).- 2.3. Design Examples.- 2.3.1 Aladdin.- a) Lattice.- b) Vacuum System.- c) Accelerating System.- d) Injector.- e) Computer Control.- 2.3.2 The National Synchrotron Light Source (NSLS).- References.- 3. Instrumentation for Spectroscopy and other Applications.- 3.1 Layout and Operation of Laboratories.- 3.1.1 VUV Laboratory at a Small Storage Ring.- 3.1.2 VUV and X-Ray Laboratory at a Large Storage Ring.- 3.1.3 Beam Line Optics.- a) General Considerations.- b) The Phase Space Method.- c) Magic Mirrors.- 3.2 Optical Components.- 3.2.1 Mirrors and Reflective Coatings.- a) General Remarks.- b) Reflectivity in the Vacuum Ultraviolett.- c) Coating Materials and Multilayer Coatings.- d)Sattering and Stray Light.- e) Mirror Substrate Materials.- f) Imaging in VUV.- 3.2.2 Dispersive Elements.- a) Reflection Grating Dispersors.- b) Spherical Concave Gratings.- c) Aspherical Concave Gratings.- d) Efficiency and Blaze.- e) Holographic Gratings.- f) Zone Plates and Transmission Gratings.- g) Crystals for Monochromators.- 3.2.3 Filters and Polarizers.- a) Filters and higher Order Problems.- b) Polarizers.- 3.3 VUV Monochromators.- 3.3.1 General Considerations.- 3.3.2 Normal Incidence Monochromators.- 3.3.3 Grazing Incidence Monochromators.- a) Plane Grating Monochromators.- b) Rowland Mountings.- c) Non-Rowland Monochromators.- 3.3.4 New Concepts.- 3.4 X-Ray Monochromators.- 3.4.1 Plane Crystal Instruments.- 3.4.2 Higher Order Rejection.- 3.4.3 Bent Crystal Monochromators.- 3.5 Photon Detectors.- 3.5.1 Detectors for the Vacuum Ultraviolet.- 3.5.2 X-Ray Detectors.- 3.6 Typical Experimental Arrangements.- 3.6.1 Experiments in the Vacuum Ultraviolet.- a) Absorption Reflection, Ellipsometry.- b) Luminescence, Fluorescence.- c) Photoionisation, Fotofragmentation.- d) Photoemission.- e) Radiometry.- f) Microscopy.- 3.6.2 Experiments in the X-Ray Range.- a) Single Crystal Diffraction.- b) Small Angle Diffraction.- c) Small Angle Scattering.- d) Mossbauer Scattering.- e) Energy Dispersive Diffraction.- f) Interferometry.- g) Absorption (EXAFS).- h) Topography.- i) Standing wave excited Fluorescence.- j) Fluorescence Excitation.- k) Compton Scattering.- 1) Resonant Raman Scattering.- m) Photoelectron Spectroscopy (XPS).- 3.7 Acknowledgements.- References.- 4. Theoretical Aspects of Inner-Level Spectroscopy.- 4. 1. Basic Concepts and Relations in Radiative Processes.- 4.1.1 Polarizability and Dielectric Function.- a) Self-consistent field Method.- b) Direct Method for Longitudinal Part.- 4.1.2 Absorption Coefficient and Oscillator Strength.- 4.1.3 Dispersion Relations and Sum Rules.- 4.2 Distribution of Oscillator Strength.- 4.2.1 Absorption Spectra in Atoms.- 4.2.2 A Unified Picture for Spectra in Atoms, Molecules and Solids.- a) Cancellation of Oscillator Strength, Giant and Subgiant Bands.- b) Pseudo Potential and Energy Band Effect.- c) Effect of Coulomb Attraction.- 4.2.3 Extended X-Ray Absorption Fine Structure (EXAFS).- 4.3 Electron-Hole Interactions.- 4.3.1 General Treatment of Excitons.- 4.3.2 Wannier and Frenkel Excitons.- a) Wannier Exciton.- b) Frenkel Exciton.- 4.3.3 Optical Absorption Spectra.- a) First Class Transition.- b) Second Class Transition.- 4.3.4 Effects of Spin and Orbital Degeneracies.- 4.4 Configuration Interactions.- 4.4.1 Aulger Process.- 4.4.2 Fano Effect.- 4.5 Simultaneous Excitations and Relaxations.- 4.5.1 Localized Excitation and Relaxation in Deformable Lattice.- 4.5.2 Host Excitation in Deformable Medium.- a) Slow Modulation Limit.- b) Rapid Modulation Limit.- 4.5.3 Sideband Structures.- 4.5.4 Relaxation in Exciton-Phonon Systems.- 4.6 Many Body Effects in Metals.- 4.6.1 Friedel Sum Rule and Anderson Orthogonality Theorem.- 4.6.2 Infrared Divergence.- 4.6.3 Fermi Edge Singularity.- 4.7 Final State Interactions Associated with Incomplete Shells.- 4.7.1 Multipiet Splitting.- 4.7.2 Local Versus Band Pictures.- 4.7.3 Correlation Effects in Narrow d-Band.- 4.8 Inelastic X-Ray Scattering.- 4.8.1 Compton and Raman Scattering.- 4.8.2 Resonant Raman Scattering.- 4.9. Topics of Recent and Future Interest.- References.- 5. Atomic Spectroscopy.- 5.1. Atomic Photoabsorption Spectroscopy in the Extreme Ultraviolet.- 5.2 The Basic Experiments in Photoabsorption Spectroscopy.- 5.2.1 Photoabsorption Spectroscopy.- 5.2.2 Photoelectron Spectroscopy.- 5.2.3 Mass Spectrometry.- 5.2.4 Fluorescence.- 5.3 Limitations of Photon Absorption Experiments.- 5.4 The General Theoretical Framework.- 5.5 Experimental Results.- 5.5.1 Photoabsorption Spectroscopy.- a) Discrete Resonances.- b) Gross Features.- 5.5.2 Photoelectron Spectroscopy.- a) Partial Photoionisation Cross Sections.- b) Angular Distributions of Photoelectrons.- 5.5.3 Mass Spectrometry.- 5.6 Future Work.- References.- 6. Molecular Spectroscopy.- 6.1 Concepts.- 6.2 Absorption Spectroscopy.- 6.2.1 Valence Spectra of Simple Di- and Tri-Atomic Molecules.- 6.2.2 Valence and Rydberg Excitations in N2.- 6.2.3 Rydberg Series in the Valence Absorption Spectrum of H2O and D2O.- 6.2.4 Core-Spectra of Simple Di-Atomic and Tri-Atomic Molecules.- a) N2.- b) NO.- 6.2.5 d-Spectra of Se2, Te2 and I2.- a) Se2.- b) Te2.- c) I2.- 6.2.6 Alkali Halides.- a) Li ls-absorption in LiF.- b) Cs-halides.- 6.2.7 Xenon Fluorides.- 6.2.8 Inner-well Resonances.- 6.2.9 EXAFS.- 6.2.10 Valence Shell Spectra of Organic Compounds.- a) Saturated Hydrocarbons: Alkanes, Neopentane.- b) Molecules with bonding o- and -?-orbitals.- 6.2.11 Core Spectra of Organic Compounds.- 6.3 Photoelectron Spectroscopy.- 6.3.1 Intensities of Photoelectron Spectra and Partial Photoionization Cross Sections.- 6.3.2 Photoionization Resonance Spectroscopy and Coincidence Measurements.- 6.4 Fluorescence.- 6.4.1 Fluorescence- and Excitation-Spectra.- 6.4.2 Time resolved Fluorescence Spectroscopy.- 6.5 Mass-Spectrometry.- 6.6. Acknowledgments.- 6.7. Appendix.- References.- 7. Solid-State Spectroscopy.- 7.1 Quantitative Description of Optical Properties.- 7.1.1 Macroscopic Optical Properties.- 7.1.2 Microscopic Description.- 7.1.3 Modulation Spectroscopy.- 7.1.4 Summary.- 7.2 Metals and Alloys.- 7.2.1 Vacuum Ultraviolet.- a) Simple Metals.- b) Noble Metals.- c) Transition Metals.- d) Rare Earths.- 7.2.2 Soft X-Ray.- a) Simple Metals.- b) Transition Metals.- c) Rare Earths.- 7.2.3 Summary.- 7.3 Semi conductors.- 7.3.1 Vacuum Ultraviolet.- a) II-VI Compounds.- b) Pb-Chalcogenides.- c) Other Semiconductors.- 7.3.2 Soft X-ray.- 7.3.3 Summary.- 7.4 Insulators.- 7.4.1 Rare Gas Solids.- 7.4.2 Alkali Hal ides.- 7.4.3 Other Metal Hal ides.- 7.4.4 Other Inorganic Insulators.- 7.4.5 Organic Insulators.- 7.4.6 Summary.- References.- Additional References with Titles.

The molybdenum site of xanthine oxidase. Structural evidence from x-ray absorption spectroscopy

Abstract: Xanthine oxidase was isolated from unpasteurized buttermilk by a standard procedure. Extended x-ray absorption fine structure (EXAFS) data indicate that reduced xanthine oxidase contains molybdenum in the four valence oxidation state. Curve fitting of the EXAFS data for oxidized xanthine oxidase suggests that there are two different Mo-ligand distances, 1.5 oxygen atoms at 1.71 A and 2 sulfur atoms at 2.54 A. Including a third sulfur atom at a longer distance significantly improved the fit. Several possible formulations are proposed. 2 figures, 1 table.

Al surface relaxation using surface extended x-ray--absorption fine structure

Abstract: The surface extended x-ray-absorption fine structure (EXAFS) of a single crystal has been measured for the first time. By comparison with parameters obtained from bulk aluminum EXAFS, a decrease of the interatomic distance (..delta..r = 0.15 +- 0.05 A) at the Al(111) surface has been found. No relaxation is found of the Al-Al separation on the (100) face.

Structure of the iron-containing core in ferritin by the extended x-ray absorption fine structure technique

Abstract: Extended X-ray absorption fine structure (EXAFS) measurements are used to determine the structure of the iron-containing core of ferritin. By comparing the EXAFS from ferritin with that from an Fe-glycine model compound, it is found that at room temperature the irons are surrounded by 6.4 +- 0.6 oxygens at 1.95 +- 0.02 A which are likely in a distorted octahedral arrangement. Each iron also has 7 +- 1 iron neighbors at an average distance of 3.29 +- 0.05 A. Considerable structural disorder was found which increased when the ferritin solution was frozen, indicating a possible phase transition occurring at lower temperatures. Combining these results with the known stoichiometry and density it is shown that the structure for the iron core is a layered arrangement with the iron in the interstices between two nearly close-packed layers of oxygens with approximate sixfold rotational symmetry, and that these compact O--Fe--O layers are only weakly bound to adjacent layers. The known phosphorus component is accounted for by terminating the layer into a strip whose width naturally explains the size of the core. The ferritin core consists, in this picture, of a strip folded back and forth upon itself in the form of a pleat.more » Measurements are also presented for two forms of the polymer of Spiro and Saltman, and it is found that only one form is possibly similar to ferritin.« less

Determination of the temperature dependence of the free carrier and interband absorption in silicon at 1.06μm

Abstract: Simultaneous determinations of the free-carrier absorption cross-section, the interband absorption coefficient, and the surface reflectivity, at the Nd:YAG laser wavelength 1.06 mu m and in the temperature interval 195-372K are reported, using a method developed previously. The influence of the free-carrier absorption is described by an analytical model which fits the experimental data very well. The values of the interband absorption coefficient measured with short high-intensity laser pulses agree with the literature values measured at low intensity. The free-carrier absorption cross-section sigma was found to be proportional to the absolute temperature, sigma = sigma n+ sigma p=1.7*10-20 T cm2.

Inelastic Electron Scattering Spectroscopy

Abstract: Publisher Summary Spectroscopy, in one form or another, plays a key role in the evolution of modern physics-optical spectroscopy and atomic physics being the canonical example. One studies the energy dependence of the emission or absorption of some kind of particle or field by a selected sample material. This energy dependence reveals dynamical information about the sample. The most successful kinds of spectroscopy are those in which the particle and sample interact only weakly, because then the dynamical information is characteristic of the unperturbed sample. The chapter provides a comparison between four spectroscopies of solids revealing the range of energies and wave vectors accessible to each. The diagonal line represents optical absorption. It is impossible to independently vary the energy and momentum of a photon, as Maxwell's equations fix one once the other is known. Therefore, any excitation in a solid that is created by absorption of a photon must lie on the line shown. A beam of particles is incident on the sample and scatters through some angle, losing certain energy. If multiple scattering is not a problem, then the energy lost and momentum transferred in the sample are ascribed to the creation of a single excitation. By varying the scattering angle and the energy loss, an entire region in energy-momentum space is studied.

Developments in Extended X-Ray Absorption Fine Structure Applied to Chemical Systems

Abstract: level, is given by the various spectroscopies, spanning the electromagnetic spectrum. In the present review we describe a structure probe based on a spectroscopy (X-ray absorption spectroscopy) but for which structure information can be deduced due to a diffraction-like phenomenon. Specifically, extended X-ray absorption fine structure (EXAFS ) spectroscopy consists of measuring the absorption spectrum of matter for X rays with photon energies (or wavelengths) in the vicinity of the absorption edges that char acterize the elements present in the material. Such spectra show modulation, called fine structure, of the X-ray absorption coefficient and this is present because photoelectrons released from the X ray absorbing atoms backscatter coherently from neighboring attoms. In this sense, EXAFS spectroscopy, although in­ volving X rays, is a structure probe because of an electron scattering

High overtones of C–H stretch in liquid benzene measured by laser optoacoustic spectroscopy

Abstract: We present a first measurement of the absorption profile of the eighth harmonic of the C–H stretch in liquid benzene using a recently developed optoacoustic absorption measurement technique involving a pulsed dye laser and submersed piezoelectric transducer. The absorption maximum occurs at 21 040±7cm−1, with a peak absorption coefficient of (9.2±1.5) ×10−5 cm−1. The full linewidth of the absorption profile is 354±9cm−1. We have previously reported similar measurements for the sixth and seventh harmonics. We have also obtained spectra of lower harmonics by conventional spectrophotometry. We make a comparative survey of the studies of this series of overtone absorptions. With our accurate determination of the nth harmonic absorption for large n, we propose an improved anharmonic formula to fit the experimental peak positions. We also note that the absorption linewidth increases linearly with n in liquid benzene.

Ni2+ coordination in aqueous NiCl2 solutions: Study of the extended x‐ray absorption fine structure

Abstract: Extended x‐ray absorption fine structure measurements are reported for aqueous NiCl2 solutions of molar concentration 2.78 and 3.74. Analysis of the Fourier filtered spectra by a multiparameter fitting procedure reveals a first coordination sphere consisting of six water molecules with an Ni–O bond distance of 2.07 A, and no first sphere Ni–Cl coordination. Measurement of an outer coordination sphere consisting of both Cl− ions and water molecules is also reported. The relationship of these results to previously postulated ordered structures in these solutions is discussed.

Soft X-ray absorption and EXAFS on the K edge of aluminium

Abstract: The authors report EXAFS (extended X-ray absorption fine structure) measurements performed in the 1000-2500 eV range using synchrotron radiation delivered by the ACO storage ring (LURE). The K edge EXAFS spectra of Al in elemental aluminium and alpha alumina have been analysed; it is shown that accurate measurements of the first-neighbour distances are possible even for low-Z atoms. In the case of the metal, the authors emphasise the importance of the screening of the core hole potential in the first 150 eV part of the spectrum.

EXAFS and surface‐EXAFS studies in the soft x‐ray region using electron yield spectroscopy

Abstract: This paper discusses extended x‐ray absorption fine‐structure (EXAFS) measurements using monochromatized synchrotron radiation in the 300–1000‐eV range. EXAFS spectra were obtained by detecting the secondary electron yield from the sample. The electron yield technique is shown to exhibit several unique advantages over conventional absorption measurements. In particular, no thin‐film samples are needed and, more important, measurements of surface phemonema become feasible. Results are presented for the N K‐edge EXAFS in Si3N4 and for the O K‐edge surface EXAFS of an oxygen monolayer on Ni (100).

Surface EXAFS investigation of oxygen chemisorption on GaAs(110)

Abstract: Surface extended x‐ray absorption fine structure measurements (EXAFS) are reported for the initial oxidation stage of the GaAs(110) surface which is characterized by a +2.9 eV binding energy shift of the As 3d photoemission peak. The surface EXAFS measurements employed a variation of the previously used secondary‐electron‐collection technique which significantly increased the surface sensitivity. We find oxygen to be chemisorbed in atomic form. The importance of using phaseshifts derived from model systems rather than using theoretically calculated phaseshifts for distance determinations from low Z atoms is pointed out.