Debye Scherrer ring

For isotropic materials all reflections represent concentric rings (Debye-Scherrer rings) in an image recorded on 2D detector5 if during exposure the detector was... 

The arrangement of helices in the solid and liquid crystalline states of poly(a-phenylethyl isocyanide) were determined by X-ray and electron diffraction. Well-defined diffraction patterns were obtained from oriented films using selected area electron diffraction. Intermolecular and intramolecular patterns were calculated from the five Debye-Scherrer rings. All the reflections were indexed in terms of a pseudo-hexagonal triclinic unit cell, with... 

A polycrystalline thin film does not have any preferred orientation (Figure 6.4 (c)). In such a case, the diffraction from the crystal is not a spot but a so-called Debye-Scherrer ring. Therefore, the sample does not have to be inclined to obtain the diffraction pattern. Conventional 2 0-6 scans move the scattering vector H in the radial direction along the film surface normal. Thus, these scans give sufficient information when the film is polycrystalline. The obtained diffracted intensity must be corrected in terms of the absorption and the Lorentz polarization. These two terms and the obtained diffracted intensity have the following relation ... 

Figure 1. Test micrograph showing the displacement of the unscattered beam (small dots) in the selected area diffraction (SAD) pattern when it occurs in polar coordinates (Philips EM 300). The tilt has been fixed at the 002 Bragg angle for carbon ( 0.3°) and the azimuth changed by small increments. The 000 spot displaces along a practically perfect circle which corresponds to the 002 Debye Scherrer ring. Such a device allows exploration of any position in the SAD pattern, even when neither sharp nor intense hkl reflections are visible. The SAD pattern of an asphaltene heat-treated at 500°C has been superimposed to the test micrograph. Various positions of a 0.13 A aperture are shown.

Figure 1.12 Debye Scherrer rings from an ideal fine grained (a, left) and a grainy (b, right) powder sample.

The use of two-dimensional detectors, where the entire, or a significant part of, the Debye-Scherrer rings are collected will, in addition to improved counting statistics, limit the negative effect of small samples, i.e. non-statistical number of crystallites and, to some extent, texture. Geometric effects of these detectors are described in detail in Chapter 14. 

Fig. 18A, B. USAXS-patterns of A a shear-oriented nematic block copolymer phase B the oriented mesoporous monolith prepared therefrom. The shift of the Debye-Scherrer ring is due to removal of solvent during the sol-gel process [110]...

The scattered electrons strike the MCP as indicated in Figure 7.2 producing a diffraction pattern on the phosphor screen. This pattern is in the form of Debye-Scherrer rings similar to powder diffraction as a result of the orientational and spatial disorder of the trapped cluster ions. This screen is imaged by a charge-coupled device (CCD) camera mounted externally to the UHV chamber. The distance from the trap center to the MCP surface can be varied from ca 8 to 12 cm. The e -beam cross-section and trap aperture limit the detection of scattered electrons to a maximum... 

Electron diffraction photographs of the as-grown unoriented PPV[A] and PPV[B] described above showed that there were no gross structural differences in the two forms. In both cases sharp Debye-Scherrer rings were seen,showing that both types of film have an isotropic distribution of well-defined crystallites. 

For structure solution and refinement, diffraction peak intensities need to be highly reproducible to represent X-ray scattering from the crystal. An ideal powder sample is composed of grains <5 pm in size. If the grains are shaped as needles or plates, steps must be taken to prepare the sample in a fashion that minimizes a preferred orientation of the grains. In this respect, measurements of their Debye-Scherrer ring by means of CCD area detectors or e-scanning with point detectors may be necessary to screen powder samples. 

The discussion of this technique has been kept short because the diffraction spectra are very similar to Debye - Scherrer patterns the method is becoming very important for molecular structure analysis. An electron beam in a high vacuum (0.1-10 Pa) collides with a molecular beam, and the electrons are diffracted by the molecules. The film reveals washed-out Debye-Scherrer rings, from which a radial electron density distribution function for the molecule can be derived. Together with spectroscopic data, the distribution makes it possible to infer the molecular structure 129]. 

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