Laue mode

The other type of x-ray source is an electron syncluotron, which produces an extremely intense, highly polarized and, in the direction perpendicular to the plane of polarization, highly collimated beam. The energy spectrum is continuous up to a maximum that depends on the energy of the accelerated electrons, so that x-rays for diffraction experiments must either be reflected from a monochromator crystal or used in the Laue mode. Whereas diffraction instruments using vacuum tubes as the source are available in many institutions worldwide, there are syncluotron x-ray facilities only in a few major research institutions. There are syncluotron facilities in the United States, the United Kingdom, France, Genuany and Japan. 

Structural information on electrodes can also be obtained as a function of potential by X-ray using monochromatic synchrotron radiation and similar transmission cells to those used for XAS (Figure 7B). Transmission cells (Laue mode) are best suited to adsorbates, while reflection (Bragg) cells are used for thicker films. Laboratory rotating anode sources can also be used, and here the instrument usually demands a reflection cell geometry. Stable systems such as solid-state cells and batteries give better... 

Enzyme-catalyzed reactions may be analyzed from structural studies of the active site of the enzyme and its perturbation and mode of binding of substrates and inhibitors. These analyses may also be aided by structure determinations of enzymes in which appropriate functional side chains have been mutated. Chemical reactions may be followed by observing changes in the diffraction pattern as a function of time. This may be seen in solids via powder diffraction, or in biological macromolecules by use of Laue photographs (in which large amounts of data are measured in a small amount of time). The availability of synchrotron radiation has greatly facilitated this. 

Fleischmann and co-workers have carried out a number of in-situ x-ray diffraction studies at roughened electrodes.Their experiments are different from those previously discussed in that instead of a synchrotron source they employ a conventional x-ray tube ( 1.5 kW) with a position-sensitive proportional counter for the measurement of the diffracted intensity. They have carried out experiments in both transmission (Laue) and reflection (Bragg) modes. 

This uses a multipole wiggler and will have operational modes for focussed Laue diffraction work and monochromatic experiments. The small source sizes should allow an equivalently small focal spot from a grazing incidence mirror system. Exposure times in the microsecond range for a macromolecular crystal should be feasible. Depending on the current achieved in single bunch mode it may be possible, at least for smaller unit cell sizes, to record a Laue pattern from one of the single bunches with an intrinsic time resolution therefore of the bunch width. (Feasibility experiments of this kind have been conducted at CHESS but on an undulator (Szebenyi et al 1989).)... 

For X-ray diffraction experiments, two basic designs are possible The Bragg (or reflection) and the Laue (or transmission) mode. In electrochemical investigations, the former is better suited for studies of adsorbates or of other features parallel to the electrode surface, whereas the second mode is suitable for thick films or layers. In both cases, a cell window as transparent as possible for X-rays with sufficient stability towards this radiation is needed. Thin polymer foils (Mylar or Melinex ) are most commonly used. A typical design of a cell of the Laue type as depicted in Fig. 6.1 shows the X-ray passing through two polymer film windows and the electrolyte solution. 

X-ray crystal monochromators may be operated in the reflection mode (Bragg case) or in the transmission mode (Laue case). The condition for diffraction to occur is given by the familiar Bragg s law... 

A Laue pattern which is shown in Fig. 7.2 is symmetrical arrangement of diffraction spots on the flat film either in the transmission mode or in back reflection mode. The spots when connected generate either an ellipse or a hyperbola. These spots originate due to Bragg s reflection from sets of intersecting planes which are parallel to an axis known as zone axis (Fig. 7.3). Planes having common zone axis generate symmetrical spots, and in most of the cases they show a center of symmetry. 

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