Diffusion halo

TEM offers two methods of specimen observation, diffraction mode and image mode. In diffraction mode, an electron diffraction pattern is obtained on the fluorescent screen, originating from the sample area illuminated by the electron beam. The diffraction pattern is entirely equivalent to an X-ray diffraction pattern a single crystal will produce a spot pattern on the screen, a polycrystal will produce a powder or ring pattern (assuming the illuminated area includes a sufficient quantity of crystallites), and a glassy or amorphous material will produce a series of diffuse halos. 

In the first case, there is only partial instead of complete long-range three-dimensional order. Fiber spectrum features are diffuse haloes (besides sharp reflections) on the layer lines. 

The first step in interstellar chemistry is the production of diatomic molecules, notably molecular hydrogen. Observations of atomic hydrogen in dense clouds show that this species cannot be detected except in a diffuse halo surrounding the cloud, so that an efficient conversion of H into H2 is necessary. In the gas phase this might be accomplished by the radiative association reaction,... 

Heavy but diffuse halos of dark matter are considered, an initial mass of 108Mq is assumed (Mb/Mb = 10). 

Crystalline forms presenting large amounts of disorder of the kind (ii) or (iii) are generally called mesomorphic modifications (Section 3.6), in analogy with the ordered liquids (smectic and nematic). In these cases the lack of periodicities in one or two dimensions (e.g., along the chain axes or along the directions normal to the chain axes) prevents the definition of a unit cell. Typical features in the X-ray diffraction patterns of mesomorphic forms are diffuse halos on the equator or on the layer lines depending on the kind of disorder present. 

Mesomorphic forms characterized by conformationally ordered polymer chains packed in lattices with different kinds of lateral disorder have been described for various isotactic and syndiotactic polymers. For instance, for iPP,706 sPP,201 sPS,202 syndiotactic poly(p-methylstyrene) (sPPMS),203 and syndiotactic poly(m -methylstyrene),204 mesomorphic forms have been found. In all of these cases the X-ray fiber diffraction patterns show diffraction confined in well-defined layer lines, indicating order in the conformation of the chains, but broad reflections and diffuse haloes on the equator and on the other layer lines, indicating the presence of disorder in the arrangement of the chain axes as well as the absence of long-range lateral correlations between the chains. 

The low angle scattering was generally featureless. However, on some occasions — more commonly at high concentrations — a very diffuse halo which intensified with time could be seen forming on the white observation surface. 

Figure 6 Singly occupied molecular orbital (SOMO) of a propeller-like trimer radical anion of acetonitrile obtained using density functional theory. The structure was immersed in a polarizable dielectric continuum with the properties of liquid acetonitrile. Several surfaces (on the right) and midplane cuts (on the left) are shown. The SOMO has a diffuse halo that envelops the whole cluster within this halo, there is a more compact kernel that has nodes at the cavity center and on the molecules.

Polymers with little structural symmetry and with bulky pendant groups, such as atactic polystyrene (PS), are usually amorphous. Amorphous polymers have no long-range order, and their x-ray diffraction patterns are diffuse halos rather than the sharp peaks which are characteristic of crystalline polymers. 

Similarly with low-molecular nematics is manifested in that the nematic polymers may form equally well schlieren texture, typical for low-molecular nematics (Fig. 18a) (polymers B.3.3-B.3.4, Table 9). The enthalpy of transition from LC state to isotropic melt is also close to that for low-molecular nematics. At the same time, there also exist definite structural differences. X-ray patterns of the same polymers, even in unoriented state, display certain elements of structural ordering in the arrangement of side branches (a weak diffuse halo at small angles), which could indicate a sibotactic nematic type of ordering. These differences are most distinct for oriented polymer films. As an example Fig. 18b, c, present X-ray patterns of unoriented and oriented samples of one and the same nematic polymer 121 l24. In fact two sharp small angle... 

Important data on the structure of the films were obtained in an analysis of electron diffraction patterns recorded directly in the transmission electron microscope. In all cases, the diffraction patterns had the form of diffuse halos, which indicate that nanoparticles are in the amorphous state [30]. The fact that the nanoparticles are amorphous is in all probability due to the exceedingly fast cooling of nanometer drops after the expansion of the plasma cloud. Estimates of the cooling rate of nanodrops at the instant of their hardening give values of up to 107K/sec. 

The small angle scattering resolution is determined by the closest approach to the primary beam (Fig. 27). This is hmited by the aperture slits which produce a diffuse halo. The guard slits are only used to limit the diffuse scattering without cutting into the primary beam. For a given size of the aperture slits — — the size of the... 

The resolution obtained in practiee is smaller as the calculations assume a linear extrapolation of the diffuse halo towards zero which is not true. It can be increased, however, by deereasing Sj (Eq. 3-6) with a corresponding loss in intensity. F2 cannot be increased indefinitively for architectural reasons. Thus for two SAXS-cameras operated by EMBL at DORIS, the resolution lies in the range 1(X)0 to 15(X) A. 

The wide angle x-ray diffraction pattern of undeformed corneum exhibits diffuse halos at 4.6 A and 9.8 A common to proteins (Figure 4). The lack of the 5.1-A reflection characteristic of alpha-keratin structures in undeformed comeum suggests that the protein is considerably less oriented and perhaps of a lower alpha content than wool. This is supported by the fact that the 5.1-A reflection begins to appear in samples of comeum which were hydrated and stretched to 100% or more (Figure 6) and allowed to dry in the extended state. The increased orientation of the lipid reflections in the stretched sample demonstrates further their association with the orienting protein fibrils. 

Since the X-ray diffraction pattern of every crystalline form of a compound is imique, the technique is widely used for the identification and characterization of solid phases. XRD is the technique of choice to identify different polymorphic forms of a compound (Fig. 1). It can also be used to identify the solvated and unsolvated (anhydrous) forms of a compound, provided their lattice structures are different. The technique can also reveal differences in the crystallinity of compounds. The XRD pattern of an amorphous (noncrystalline) compound will consist of one or more broad diffuse halos (Fig. 2A). ... 

Some evidence for ionic, rather than covalent, bonding can be gathered from electron density distribution maps determined by X-ray diffraction (Chapter 3). Ions are observed as essentially spherical, highly concentrated distributions of electron density on the lattice points, with a small diffuse halo around the outside containing few electrons between the ions. This contrasts markedly with covalent bonding, where the shared electrons between the atoms generate a much higher electron density between the extremes. 

Fig. 3.7. (c) Corresponding SAD pattern showing the characteristic diffuse halo of the amorphous phase besides reflections of unreacted, crystalline Ni and Zr... 

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