Reflectivity monochromators

Monochromatization by Bragg reflection, see Bragg reflection Monochromators, relation of wavelength to diffraction angle for common, table, 318-327... 

The divergence of a beam originating from a Bartels four-reflection monochromator is typically 12 arcseconds, or 2.7 thousandths of a degree. Note that, in the case of parabolic multi-layer monochromator, the divergence is 10 times that. 

A four-reflection monochromator comprised of plane single crystals is placed between the source and the sample (see Figure 2.36). The beam irradiates the sample, which is placed on a three axes sample holder. The sample holder makes it possible to orient the normal to any family of ctystal planes in the diffraction plane defined by the axis of the incident beam and the directions of the diffracted beams. Since the beam is strictly parallel, it is not necessary to inclnde a slit between the sample and the monochromator. The intensity of the diffracted beams is meastrred by using a detector which moves along a circle, centered on the sample or with a curved position sensitive detector. This last featrrre makes this system a distant relative of the Debye-Scherrer diffractometers, for which the sample is, by definition, the center of the detection circle. 

Figure 3.7. Calculation of the instrumental resolution function of a diffractometer equipped with a four-reflection monochromator and a curved position sensitive deteetor...

Fig. 1.15 High resolution XRD schematics (a) and typical instrument configuration (b). In (a) the probed volume can be decreased by reducing the primary beam angular divergence A(co) (done in (a) by the use of a two-reflection monochromator) and reducing the detector acceptance A(20) (done in (b) by using an analyzer crystal). The high resolution configuration in (b) uses a two-(shown) or four-reflection monochromator. For overview scans the analyzer crystal can be removed and only the detector with a narrow slit is used in the secondary optics. High-speed line detectors can also be used (without the analyzer crystal)...

Figure 89. All-polymer FLC prototype presented by Idemitsu Kosan Co., Ltd., in 1995. This reflective monochrome display is 0.5 mm thick, has a 1 400 multiplex ratio, and a size of 20 cmx32 cm (400 x 640 pixels each 0.5 mm x 0.5 mm), corresponding to a diagonal of 15 in (37.5 cm).

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. 

The construction of a typical monochromator is shown in Figure 10.12. Radiation from the source enters the monochromator through an entrance slit. The radiation is collected by a collimating mirror, which reflects a parallel beam of radiation to a diffraction grating. The diffraction grating is an optically reflecting surface with... 

Jin x-ray monochromator. A monochromator is a large single crystal (usually graphite) that is oriented so that a very iatense reflection is directed toward the sample. AH wavelengths are absorbed by the monochromator except a small range of wavelengths used for the diffraction experiment. Usually only the characteristic radiation is used if an x-ray tube is the x-ray source. 

Laue Method for Macromolecule X-Ray Diffraction. As indicated above it is possible to determine the stmctures of macromolecules from x-ray diffraction however, it normally takes a relatively long period of data collection time (even at synchrotrons) to collect all of the data. A new technique, the Laue method, can be used to collect all of the data in a fraction of a second. Instead of using monochromated x-rays, a wide spectmm of incident x-rays is used. In this case, all of the reflections that ate diffracted on to an area detector are recorded at just one setting of the detector and the crystal. By collecting many complete data sets over a short period of time, the Laue method can be used to foUow the reaction of an enzyme with its substrate. This technique caimot be used with conventional x-ray sources. 

Light from an appropriate light source (a xenon arc or a halogen or tun ten lamp) passes through a monochromator (probe monochromator). The exit intensity at wavelength "k, IqCK), is focused onto the sample by means of a lens (or mirror). Tbe reflected light is collected by a second lens (mirror) and focused onto an appropriate detector (photomultiplier, photodiode, etc.). For simplicity, the two lenses (mirrors) are not shown in Figure 2. For modulated transmission the detector is placed behind the sample. 

In the early work of Bewick and Robinson (1975), a simple monochromator system was used. This is called a dispersive spectrometer. In the experiment the electrode potential was modulated between two potentials, one where the adsorbed species was present and the other where it was absent. Because of the thin electrolyte layer, the modulation frequency is limited to a few hertz. This technique is referred to as electrochemically modulated infrared reflectance spectroscopy (EMIRS). The main problem with this technique is that data acquisition time is long. So it is possible for changes to occur on the electrode surface. 

The typical experimental setup (here the experiment established at beamline G3/ HASYLAB [12] is shown) is outlined in Figure 5. The white synchrotron radiation is monochromatized by a double crystal monochromator using the Ge (311) reflection... 

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