Centered dark-field

Experimentally, diffraction contrast imaging is done by first tilting the crystal to a two-beam condition and obtaining a selected-area electron diffraction pattern. A BE image can be obtained by placing the objective aperture to the transmitted beam. To obtain a DE image, one can place the objective aperture to the diffracted beam and form a DE image. In order not to move the objective aperture and switch between BE and DE modes, one needs to use a centered dark-field (CDE) method. 

The Dark-Field symmetry is observed inside a hkl diffracted disk (often characterized by its diffraction vector g) which is exactly in Bragg orientation. This situation occurs when the hkl Bragg line goes through the center of its hkl diffracted disk. 

Plates containing varying numbers of silver atoms per unit area were immersed for 5 minutes in a hydroquinone-silver nitrate physical developer which gave no spontaneous deposition of silver within 30 minutes. The number of developed centers per unit area wras counted under dark field illumination at a magnification of 500 X. 

Polarization-based contrast (211 in which the specimen is illuminated normally with linearly polarized light and the analyzer is replaced by a small disc of Polaroid in the center of the objective back focal plane. This allows any intermediate contrast condition between bright field and dark field to be selected, and appears to be untried in the context of liquid crystals. 

Figure 5 shows an example from [20] obtained by this method. The bright field disk and six surrounding 220 dark field disks still contain enough HOLZ lines. Thus, the central disk shows the BF symmetry 3 m. The exact 220 Bragg positions exist at the centers of the corresponding DF disks. Each of these disks shows the DF mu symmetry due to a horizontal two-fold axis. A pair of the 220 and 220 disks, which exhibits the g DF symmetry shows the translational symmetry or 2ji. The disks, as a whole, form the whole-pattern symmetry 3m... 

The condenser shortld also be strain free, and the NA should be near the highest objective NA to be used. The condenser should be centerable in its mormtinorderto aid in bringing the optical system into perfect concerrtricity. Often, the sample may reqrrire a diflererrt type of illumination, such as phase contrast or dark field. Provision should be made for simple changeover to diflererrt condensers, which must be used for these types of trarrsrrritted illumination techniques. 

The recent experimental developments on dark and photoinduced ET reactions give support to the previous speculations on the relevance of these interfaces in such fields as catalysis and solar energy conversion. These disciplines have been, and still are centered on processes at solid solution interfaces. However, particular applications require molecu-larly defined interfaces, where reactants exhibit different solubility properties. In this section, we shall consider some of these systems and the advances reported so far. 

Figure 7.11 Field ion microscopy image of a platinum tip. As Pt has the fee structure, the fourfold symmetry of the picture implies that the center corresponds to a (100) facet. Dark areas near the four comers are (111) facets (from Muller [26]). 

Silica makes up 12.6 mass-% of the Earth s crust as crystalline and amorphous forms. It was found that both modifications show similar main luminescence bands, namely a blue one centered at 450 nm ascribed to which substitutes for Si, red centered at 650 nm linked with non-bridge O, and dark-red at 700-730 nm linked with Fe. Time-resolved luminescence of hydrous volcanic glasses with different colors and different Fe, Mn, and H2O contents were measured and interpreted (Zotov et al. 2002). The blue band with a short decay time of 40 ns was connected with T2( D)- Ai ( S) and Ai C G)- Ai ( S) ligand field transitions of Fe " ", the green band with a decay time of approximately 250 ps with a Ti( G)- Ai( S) transition in tetrahedrally coordinated Mn ", while the red band with a much longer decay time of several ms with T1 (4G)- Ai( S) transitions in tetrahedrally coordinated Fe ". We detected Fe " " in the time-resolved luminescence spectrum of black obsidian glass (Fig. 4.43d). 

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