Synchrotron X-ray Sources

Brief description of new possibilities for surface investigations with higlily brilliant synchrotron x-ray sources. 

X-radiation can also be induced by high energy (several Me proton beams from ion accelerators. Such particle-induced x-ray emission (PIXE) (284) is useful for thin samples and particulates, having detection Hmits of g. Intense synchrotron x-ray sources have found appHcations in... 

The first—and central— point in this discussion concerns the pulsed X-ray source. The shortest time scales involved when chemical bonds are formed or broken are of the order of a few femtoseconds. An ideal X-ray source should thus be capable of providing pulses of this duration. Unfortunately, generating them represents a heavy technological problem. The best one can do at present is to use a pulsed synchrotron X-ray source (Fig. 2). Electrons are rapidly circulating in its storage ring at speeds close to the speed of light. X-rays are spontaneously... 

Figure 2. Synchrotron X-ray source (schematic). The electrons execute circular motions in the storage ring and emit intense X-rays along the tangent of the orbit. This radiation is enhanced by undulator magnets that are often placed inside the vacuum vessel for enhanced performance. The storage ring has a number of straight sections for undulators and wigglers (not shown).

Unsubstituted, unsolvated complexes have been observed for all metals, but their low solubility prevents the growth of quality single crystals. This problem was recently overcome with data collected on a powder sample on a synchrotron X-ray source followed by Rietveld refinement.17-19 Not surprisingly, the structures display polymeric chains with alternating metal and Cp units. 

Time-Resolved Crystallography. Time-resolved crystallography (TC) uses an intense synchrotron X-ray source and Laue data collection techniques to greatly reduce crystallographic exposure times. Normal time resolution for X-ray... 

With the development of intense synchrotron X-ray sources, it has become possible to use X-ray absorption spectroscopy to structurally characterize the metal clusters in metalloproteins. A wide variety of systems have been studied in this manner in the last ten years. The present article reviews the nature of X-ray absorption spwtroscopy and the information which can be obtain fix)m its study. The strengths and Kmitations of the technique are discussed with reference to the recent literature. 

Wikoff, W. R., Schildkamp, W. and Johnson, J. E. (1997). Increased resolution data from a large unit ceU crystal collected at a third-generation synchrotron X-ray source. Acta Crystallogr. D 56, 890-893. 

Time-resolved x-ray crystallography (TC) is a more recent advanced application of x-ray crystallography. It uses an intense synchrotron x-ray source and data collection methods to reduce crystallographic exposure times. This allows multiple exposures to be taken over time at near-physiological, crystalline conditions to determine the structures of intermediates. A typical problem with this method is that the existence of the intermediates is brief, resulting in difficulty in interpreting the resulting electron density maps. 

In photoelectron diffraction experiments monoenergetic photons excite electrons from a particular atomic core level. Angular momentum is conserved, so the emitted electron wave-function is a spherical wave centered on the source atom, with angular momentum components / 1, where / is the angular momentum of the core level. If the incident photon beam is polarized, the orientation of the emitted electron wave-function can be controlled. These electrons then propagate through the surface and are detected and analyzed as in LEED experiments. A synchrotron x-ray source normally produces the intense beams of variable energy polarized photons needed for photoelectron diffraction. 

Finally, we must consider the recent availability of synchrotron x-ray sources for industrial use. The high resolution now made possible by these sources produces zeolite XRD powder patterns with features perhaps calculated, but never observed before. Figure 14 shows the enhanced resolution from a synchrotron experiment versus that obtained from a conventional one. Here the XRD patterns of ZSM-5 clearly demonstrate the enhanced resolution and wealth of diffraction information that can be obtained at such sources as the... 

Detectors used for time resolved non-crystalline diffraction experiments on a synchrotron X-ray source should have most if not all of the following characteristics ... 

PIXE (Johansson and Campbell, 1988) and SRIXE (Jones and Gordon, 1989) have similar imaging capabilities and detection limits but both suffer from the drawback that they rely on major pieces of hardware, an accelerator in the PIXE experiment and a synchrotron X-ray source for SRIXE. 

Figure 3.36. SEMs of dry-developed images generated by the process shown in Figure 3.35 a) J.0- xm images printed with a Perkin-Elmer M500, UV-2 mode (b) 0.75-p.m images printed with a Perkin-Elmer M500, UV-2 mode and (c) 0.5- xm images proximity printed with a synchrotron X-ray source. (Reproduced with permission from reference 127. Copyright 1985 Society of...

Stopped or continuous flow reaction methods can be utilized with synchrotron-based SAXS measurements to determine reaction kinetics, including protein folding, nanoparticle growth, breakdown, and aggregation. Using a synchrotron X-ray source, the SAXS spectrum can be collected with a one or two dimensional detector with good quality in periods as short as 0.001 s. 

Furthermore, as discussed above (see section 3.6), the easy selection of the wavelength is only feasible when using a synchrotron x-ray source. 

Figure 4.26. Powder diffraction pattern collected using a synchrotron x-ray source. (Data courtesy of Dr. M.J. Kramer.)...

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