Diffraction by domains

Fig. 2—Diffraction by domains. A nematic orientation that is originally planar usually results in a well-ordered domain structure that behaves like a phase grating, and a diffraction pattern consisting of a series of spots is obtained (top). When the orientation is originally perpendicular, clusters of domains may give rise to diffraction spots or rings depending on the relative diameter of the probe (w) and the dimensions of the domain clusters.

When scattering or diffraction phenomena are studied, however, the geometry becomes particularly important, both because of the rich nature of the optical processes and because of the more varied ways in which such processes are applied to display devices. In the case of diffraction by domains, different results may be obtained depending on the original orientation of the fluid and the relation between the beam probe size and the domain cluster size. 

Fig. 12— Voltage dependence of diffraction by domains. The intensity distribution of the diffraction rings (for a single wavelength) with increasing dc voltage demonstrates a constant intensity profile even at 60 volts where the diffraction angle is nearly 50 . No dynamic scattering occurs. ...

Usually, at least several hundreds of fringes are measured and characterized. Taking the classical example of X-ray diffraction, coherent domains are defined by the stacks of polyaromatic layers (Figure 2). The coherent domains are distinguished from the single layers and their relative... 

Since the magnetic interaction vector q is known, it is possible to deduce the magnetic form factor f( ). Although possible when using unpolarized neutrons, its measurement is much more precise using polarized neutron diffraction by a (single domain) ferromagne-... 

Frequency-domain BSS. In this mode, the spectrum of the diffracted probe light is obtained by using a Fabry-Perot interferometer. Light is diffracted by incoherent thermal phonons and the scattering wavevector is determined by the detection angle, which can be accurately fixed by limiting the collection aperture. 

The introduction recounts the history of the emphasis on X-ray diffraction by crystals since the discovery of X-rays. The book is then divided irrto two parts. The first part focuses on the description of the basic theoretical concepts, the irrstrumerttation arrd the presentation of traditional methods for data processing and the irrterpretation of the results. The second part is devoted to a more specific domain which is the quantitative study of the rrricrostmcture by X-ray diffraction. 

In principle, we can distinguish (for surfactant self-assemblies in general) between a microstructure in which either oil or water forms discrete domains (droplets, micelles) and one in which both form domains that extend over macroscopic distances (Fig. 7a). It appears that there are few techniques that can distinguish between the two principal cases uni- and bicontinuous. The first technique to prove bicontinuity was self-diffusion studies in which oil and water diffusion were monitored over macroscopic distances [35]. It appears that for most surfactant systems, microemulsions can be found where both oil and water diffusion are uninhibited and are only moderately reduced compared to the neat liquids. Quantitative agreement between experimental self-diffusion behavior and Scriven s suggestion of zero mean curvature surfactant monolayers has been demonstrated [36]. Independent experimental proof of bicontinuity has been obtained by cryo-electron microscopy, and neutron diffraction by contrast variation has demonstrated a low mean curvature surfactant film under balanced conditions. The bicontinuous microemulsion structure (Fig. 7b) has attracted considerable interest and has stimulated theoretical work strongly. 

X-ray and electron diffraction studies of unoriented polymers allow the calculation of some interplanar distances in the crystalline domains. Diffraction by oriented fibers gives more information, since the axis of the fiber usually parallels one of the crystallographic axes. Both the conformation of single chains and the arrangement of the chains in the crystal lattice can be deduced, although the methods of analysis are neither simple nor unambiguous. 

Surface reconstructions have been observed by STM in many systems, and the teclmique has, indeed, been used to confmn the missing row structure in the 1 x 2 reconstruction of Au(l 10) [28]. As the temperature was increased within 10 K of the transition to the disordered 1 1 phase (700 K), a drastic reduction in domain size to -20-40 A (i.e. less than the coherence width of LEED) was observed. In this way, the STM has been used to help explain and extend many observations previously made by diffraction methods. 

The HIV-l protease is a remarkable viral imitation of mammalian aspartic proteases It is a dimer of identical subunits that mimics the two-lobed monomeric structure of pepsin and other aspartic proteases. The HIV-l protease subunits are 99-residue polypeptides that are homologous with the individual domains of the monomeric proteases. Structures determined by X-ray diffraction studies reveal that the active site of HIV-l protease is formed at the interface of the homodimer and consists of two aspartate residues, designated Asp and Asp one contributed by each subunit (Figure 16.29). In the homodimer, the active site is covered by two identical flaps, one from each subunit, in contrast to the monomeric aspartic proteases, which possess only a single active-site flap. 

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