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1986 | Buch

X-Rays and Electrons Kapitel lesen Erstes Kapitel lesen

EXAFS: Basic Principles and Data Analysis

verfasst von: Dr. Boon K. Teo

Verlag: Springer Berlin Heidelberg

Buchreihe : Inorganic Chemistry Concepts

Enthalten in: Professional Book Archive

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Über dieses Buch

The phenomenon of Extended X-Ray Absorption Fine Structure (EXAFS) has been known for some time and was first treated theoretically by Kronig in the 1930s. Recent developments, initiated by Sayers, Stern, and Lytle in the early 1970s, have led to the recognition of the structural content of this technique. At the same time, the availability of synchrotron radiation has greatly improved both the acquisition and the quality of the EXAFS data over those obtainable from conventional X-ray sources. Such developments have established EXAFS as a powerful tool for structure studies. EXAFS has been successfully applied to a wide range of significant scientific and technological systems in many diverse fields such as inorganic chemistry, biochemistry, catalysis, material sciences, etc. It is extremely useful for systems where single-crystal diffraction techniques are not readily applicable (e.g., gas, liquid, solution, amorphous and polycrystalline solids, surfaces, polymer, etc.). Despite the fact that the EXAFS technique and applications have matured tremendously over the past decade or so, no introductory textbook exists. EXAFS: Basic Principles and Data Analysis represents my modest attempt to fill such a gap. In this book, I aim to introduce the subject matter to the novice and to help alleviate the confusion in EXAFS data analysis, which, although becoming more and more routine, is still a rather tricky endeavor and may, at times, discourage the beginners.

Inhaltsverzeichnis

Frontmatter
Chapter 1. X-Rays and Electrons
Abstract
X-rays are electromagnetic radiations which lie between ultraviolet light and gamma rays in the electromagnetic spectrum. X-rays are characterized by the relatively short wavelengths of 0. 01Å to 100Å, with hard X-rays on one end and soft X-rays on the other. They are conventionally produced by either the conversion of the kinetic energy of charged particles into radiation or the excitation of atoms in a target upon which fast moving electrons impinge. The former produces a continuous spectrum of X-rays whereas the latter gives rises to characteristic lines of nearly monochromatic X-rays.
Boon K. Teo
Chapter 2. Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy
Abstract
Extended X-ray absorption fine structure (EXAFS) refers to the oscillatory variation of the X-ray absorption as a function of photon energy beyond an absorption edge. The absorption, normally expressed in terms of absorption coefficient (μ), can be determined from a measurement of the attenuation of X-rays upon their passage through a material. When the X-ray photon energy (E) is tuned to the binding energy of some core level of an atom in the material, an abrupt increase in the absorption coefficient, known as the absorption edge, occurs. For isolated atoms, the absorption coefficient decreases monotonically as a function of energy beyond the edge. For atoms either in a molecule or embedded in a condensed phase, the variation of absorption coefficient at energies above the absorption edge displays a fine structure called EXAFS. Such fine structure may extend up to 1000 eV above the absorption edge and may have an amplitude of up to a few tenths (normally 1–20%) of the edge jump.
Boon K. Teo
Chapter 3. EXAFS Parameters
Abstract
Before describing in detail the various aspects of EXAFS theory and data analysis techniques, it is instructive to first discuss, qualitatively and graphically, the various variables (or parameters) and functions and their effects and correlations.
Boon K. Teo
Chapter 4. Theory of EXAFS
Abstract
Even though the basic physical explanation of EXAFS was provided by Kronig as being due to modification of the final state of the photoelectron by the crystal (Kronig, 1931) or, in the case of gaseous molecules, by atoms surrounding the excited atom (Kronig, 1932), a great deal of confusion still existed when this field was reviewed by Azaroff in 1963. The question arises as to whether a long-range order theory formulated in terms of Bloch waves (Kronig, 1931) or a short-range order theory in terms of scattering by neighboring atoms (Kronig, 1932, Hartree, et al. 1934, Shiraiwa, et al. 1958) is more appropriate. A major source of the confusion was that quantitative comparison between theory and experiment was difficult if not impossible at the time.
Boon K. Teo
Chapter 5. Improvement of EXAFS Theory
Abstract
The single-electron single-scattering theory of EXAFS described in the previous chapter forms the basis of data analysis in EXAFS spectroscopy. In many cases, all it requires is parameterization of the relevant variables or functions. These variables or functions can then be determined from model compounds or from theoretical calculations. For some specialized applications or in some particular systems, however, this phenomenological theory is inadequate and modification or generalization of the theory is required. In this chapter, possible breakdowns of the single scattering theory of EXAFS will be discussed:
1.
Phase transferability problem caused by uncertainty in energy threshold;
 
2.
Amplitude transferability problem caused by inelastic scattering processes at (a) the central atom and (b) the neighboring environment;
 
3.
Smearing effects caused by static and thermal disorders;
 
4.
Multiple scattering effects caused by intervening atoms.
 
Boon K. Teo
Chapter 6. Data Analysis in Practice
Abstract
The first step in EXAFS data analysis is to convert the experimentally measured total absorption data μ(E) in energy (E) space to the interference function χ(k) in photoelectron momentum (k) space. This data reduction procedure involves background removal, conversion of E to k, normalization, μ0 correction, weighting scheme, etc. In order to extract structural information from χ(k), an appropriate function of χ(k) in terms of the structural parameters must be chosen (or developed in some cases) as described in the previous chapters. The choice of these phenomenological expressions, often quite equivalent or similar, depends largely on the system of interest and the data analysis technique to be employed. Though vary widely in form, there are basically two major approaches to EXAFS data analysis: the Fourier transform (FT) and the curve fitting (CF) techniques. Each of these techniques has its own advantages and disadvantages. A compromise of these two methods is Fourier filtering (FF) followed by curve fitting which is the most commonly used technique. We shall describe these as well as other methods in some detail.
Boon K. Teo
Chapter 7. Theoretical Amplitude and Phase Functions
Abstract
It is clear from previous chapters that structural determinations by EXAFS spectroscopy rely heavily on our knowledge concerning the amplitude and phase functions. In other words, both the Fourier transform and the curve fitting techniques commonly used in EXAFS data analysis require a detailed knowledge of the amplitude F(k) and phase ϕ(k) functions for the determination of chemical information such as coordination number N, Debye-Waller factor σ, and interatomic distance r through the assumptions of amplitude and phase transferabilities.
Boon K. Teo
Chapter 8. Multiple Scattering and Bond Angle Determination
Abstract
While EXAFS spectroscopy can provide structural information about the local environment of the absorbing atoms in terms of radial distribution functions (distances), no direct method of determining angular information is available; except, perhaps, for elaborate measurements on single crystals utilizing polarized X-rays. Furthermore, the very same advantageous characteristics of EXAFS (short-range, single-scattering) are also its serious limitations: distance determinations out to only ca 4Å. The situation, however, changes dramatically when atoms (including the X-ray absorbing atom and its neighbors) are arranged in a linear or nearly collinear fashion. In such cases, EXAFS contributions from neighboring atoms as far as 8Å can be observed. For these systems, both the amplitude and the phase of the EXAFS of a more distant neighbor are significantly affected by the intervening atom(s). In particular, the amplitude is greatly enhanced and is therefore commonly called “focusing” effect. The short-range single-scattering theory of EXAFS fails in these situations and one must take into account multiple scattering processes involving the intervening atoms.
Boon K. Teo
Backmatter
Metadaten
Titel
EXAFS: Basic Principles and Data Analysis
verfasst von
Dr. Boon K. Teo
Copyright-Jahr
1986
Verlag
Springer Berlin Heidelberg
Electronic ISBN
978-3-642-50031-2
Print ISBN
978-3-642-50033-6
DOI
https://doi.org/10.1007/978-3-642-50031-2