The Interaction of X-rays with Matter

All analytical methods that use some part of the electromagnetic spectrum have evolved into many highly specialized ways of extracting information. The interaction of X-rays with matter represents an excellent example of this diversity. In addition to straightforward X-ray absorption, diffraction, and fluorescence, there is a whole host of other techniques that are either directly X-ray-related or come about as a secondary result of X-ray interaction with matter, such as X-ray photoemission spectroscopy (XPS), surface-extended X-ray absorption fine structure (SEXAFS) spectroscopy, Auger electron spectroscopy (AES), and time-resolved X-ray diffraction techniques, to name only a few [1,2]. 

The interaction of X-rays with matter is weak compared to that of electrons. This leads to a large penetration depth of some /xm... 

Since the discovery of x-rays by Roentgen in 1896, this region of the electromagnetic spectrum has been a source of significant contributions to our fundamental knowledge of atomic structure and to our techniques for chemical analysis. By 1927, six Nobel prizes in physics had been awarded for studies on the physics of x-rays and the interaction of x-rays with matter. 

This review aims to provide chemists working on porous materials with a basic understanding of XAS and related methods and to show them where the techniques might be of help in their research. After a short section on the basic physics of the interaction of X-rays with matter (Sect. 2), the basic physical processes and a theoretical description of XAFS are discussed (Sect. 3) in such a way that the method can be understood. Section 4 presents some experimental details and an example of a typical data analysis procedure. The next section is devoted an explanation of the type of information contained in X-ray absorption spectra by using examples from zeoHte chemistry (Sect. 5). Newer developments, especially with regard to time-resolved and in situ studies, as well as related techniques such as electron energy loss spectroscopy (EELS) and anomalous diffraction, are described in Sect. 6. 

Analytically useful x-rays interact almost exclusively with the electrons in matter. For this reason, any discussion of the interaction of x-rays with matter should begin with a description of the interaction of an x-ray photon with a single free electron and proceed to multielectron atoms and thence to multiatom solids. It will be shown that two basic interaction processes dominate (1) the photoelectric effect and (2) x-ray scattering. 

The primary effect of the interaction of X-rays with matter includes the production of high-energy electrons, which are the main agents through which all the effects of X-rays arise. 

Characteristic X-rays arise when an incident electron knocks an electron out of an inner shell of an atom. An electron from an outer shell falls back to fill the place of the ejected electron and, in doing so, emits an X-ray characteristic of the energy difference between the inner and outer shell electrons and so of the atom concerned (Figure 8.5). As the interaction of X-rays with matter is much less than that of electrons, the characteristic X-rays can escape from the whole of the interaction volume. This and the details of the X-ray detectors means that the images formed from characteristic X-rays have the lowest resolution of the techniques discussed here. 

In the X-ray energy region, the complex refractive index n, by which the magnitude of the interaction of X-rays with matter is described, is normally expressed as... 

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