X rays from synchrotrons

An experimental technique that is useful for structure studies of biological macromolecules and other cry stals with large unit cells uses neither the broad, white , spectrum characteristic of Laue methods nor a sharp, monochromatic spectrum, but rather a spectral band with AA-/A, 20%. Because of its relation to the Laue method, this technique is called quasi-Laue. It was believed for many years that the Laue method was not useful for structure studies because reflections of different orders would be superposed on the same point of a film or an image plate. It was realized recently, however, that, if there is a definite minimum wavelength in the spectral band, more than 80% of all reflections would contain only a single order. Quasi-Laue methods are now used with both neutrons and x-rays, particularly x-rays from synchrotron sources, which give an intense, white spectrum. 

Tunable VUV and soft X-rays from synchrotron light sources are ideally suited to study surface, near surface (thin film) and interface phenomena. These photons can access many useful core levels of many elements (site specific), they usually do not penetrate very deep and they produce electrons with very short escape depths (surface sensitivity). On the other hand, hard X-rays with great penetration power are ideally suited for all kinds of imaging, such as tomography of rocks and soils. 

For analytical purposes. X-rays are generated in four ways (I) by bombardment of a metal target with a beam of high-energy electrons. (2) by exposure of a substance to a primary beam of X-rays to generate a secondary beam of X-ray fluorescence, (3) by use of a radioactive source whose decay process results in X-ray emission, and (4) from a synchrotron radiation source. Only a few laboratories in the United States have facilities to produce X-rays from synchrotron radiation. For this reason, wc will consider only the first three sourcc.s. 

With the availability of intense tunable radiation in the range firom ultraviolet to hard X-rays from synchrotrons, powerful new experimental techniques have been developed to probe the structural and electronic properties of solids and surfaces. In particular, angle-resolved photoemission gives information about the electronic properties in the valence bands of solids while core level spectroscopy provides an element-specific spectroscopic tool. 

Three forms of radiation commonly used are soft X-rays (from synchrotrons), electron beams, and gamma radiation, say from Co °. The first two can be used to make patterns in thin films since they are unidirectional. An electron beam of, say, 50 keV can be directed in much the same manner of operating a scanning electron microscope. Poly(methyl methacrylate) has been used as a thin film on silicon in order to produce patterns with line widths in the tens of nanometers (see Section 13.5). The scissioned (low-molecular-weight) polymer dissolves so much faster than the original material that the final pattern is relatively undistorted. The production of a pattern using unidirectional X-rays requires exposure of a similar film through a mask with opaque lines. 

Unlike for synchrotron radiation, the maximum iatensity of x-rays from an x-ray tube is limited by how fast heat can be removed from the target to prevent its melting. In a conventional sealed tube, the target is stationary, relatively small, and must be continually cooled with water. In a rotating anode tube, the target is larger and is continually rotated so that the heat can be distributed over a larger surface. With such a tube the amount of heat, and hence. 

Bierck, B. R., Wells, S. A., and Dick, R. I. (1988) "Compressible Cake Filtration Monitoring Cake Formations Using X-Rays from a Synchrotron Source," Water Pollution Control Federation Journal, Vol. 60, No. 5, 645-650... 

Other techniques utilize various types of radiation for the investigation of polymer surfaces (Fig. 2). X-ray photoelectron spectroscopy (XPS) has been known in surface analysis for approximately 23 years and is widely applied for the analysis of the chemical composition of polymer surfaces. It is more commonly referred to as electron spectroscopy for chemical analysis (ESCA) [22]. It is a very widespread technique for surface analysis since a wide range of information can be obtained. The surface is exposed to monochromatic X-rays from e.g. a rotating anode generator or a synchrotron source and the energy spectrum of electrons emitted... 

Nuclear absorption of incident X-rays (from the synchrotron beam) occurs elastically, provided their energy, y, coincides precisely with the energy of the nuclear transition, Eq, of the Mossbauer isotope (elastic or zero-phonon peak at = E m Fig. 9.34). Nuclear absorption may also proceed inelasticaUy, by creation or annihilation of a phonon. This process causes inelastic sidebands in the energy spectrum around the central elastic peak (Fig. 9.34) and is termed nuclear inelastic scattering (NIS). 

Crystal structure determinations from very small samples have become possible due to the high intensities of the X-rays from a synchrotron. Very high pressures can be exerted on a small sample situated between two anvils made from diamond. In this way, our knowledge of the behavior of matter under high pressures has been widened considerably. Under pressure the elements of the fifth and sixth main groups exhibit rather unusual structures. A synopsis of the structures that occur is given in Fig. 11.9. 

Extended X-ray absorption fine structure (EXAFS) A technique for observing the local structure around a metal centre, using X-rays from a synchrotron source. The atom of interest absorbs photons at a characteristic wavelength and the emitted electrons, undergoing constructive or destructive interference as they are scattered by the surrounding atoms, modulate the absorption spectrum. The modulation frequency corresponds directly to the distance of the surrounding atoms while the amplitude is related to the type and number of atoms. In particular, bond lengths and coordination numbers may be derived. 

HREM), TEM results of aerinite are compared with X-ray and synchrotron powder diffraction data. Six selected area electron diffraction (SAED) patterns and two HREM images from Pb5MoOg single crystals are used to solve their structure. The unit cell parameters of these crystals confirm the known powder diffraction data. 

A Laue X-ray diffraction pattern from a protein crystal. A stationary crystal is irradiated with very intense white, multiwavelength X rays from a synchrotron source. The diffraction pattern is rich in information. A single 0.1 ms X-ray pulse may provide a pattern with enough information to determine a three-dimensional structure. 

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