Split diffraction spots

In diffraction contrast images made in a cluster of split diffraction spots, orientation domains exhibit differences in brightness (i.e., domain contrast), especially for conditions close to 5 = 0 where the intensity variation with, s is steep (Fig, 35). The presence of translation variants is not reflected in the diffraction pattern, except by some diffuse scattering. 

Calculated electron diffraction pattern from the surface of figure (3) for normal incidence at 94 eV. The observed patterns are in good agreement with these calculations for an ordered step array. [After Ellis and Schwoebel( )] The split diffraction spots are characteristic of the pattern from a stepped surface. 

Anatase Precursor Reaction. The earliest BaTi03 nucleation from the hydrothermally treated Ti precursor could not be visually discerned from the aggregated anatase. However, electron diffraction patterns suggest the presence of barium titanate. The HRTEM photomicrograph of a sample extract on ramping prior to attainment of the isothermal temperature illustrates lOnm nuclei in Figure 6. Due to the overlap and (approximately 10°) misorientation of the nuclei (observed by the split diffraction spots... 

Dark field images can be used to determine the shape of a small particle as shown by Yacaman et al. (6-7). When an electron beam enters on a small particle the presence of wedges will produce a splitting on the diffracted spots as shown by Gomez et al (8). A... 

It may be necessary to screen several crystals, with a range of sizes, before choosing one that is best suited for collecting a full dataset. CareM examination of the candidate crystals under a microscope wiU aid in choosing a well-faceted crystal that appears to be single. The initial series of frames that are collected to determine the unit cell should be examined as they are recorded since they contain valuable indicators of crystal quality. While some radial splitting of the diffraction spots (away from the beam stop) due to the presence of both Kal and Ka2... 

Figure 6. HRTEM photomicrograph of the anatase derived BaTi03 nuclei including selected area diffraction pattern. The double arrows indicate a split reflection spot of BaTi03 lattice misfit of the anatase and BaTi03 denoted by an and bt, respectively.

Typically the substrate is not cut exactly on a low-index plane. When the diffraction spots are sharp enough, the spots may be split along the reciprocal lattice rod direction (vertically on the pattern) into two spots when the beam is aligned up or down the staircase of surface steps causing the miscut. The separation of these spots reflects the miscut of the sample surface relative to the low index planes. This is useful to know because miscut is used in some cases to enhance heteroepitaxy. 

Figure 5. Micro-diffraction pattern of square gold particle showing splitting of the spots.

Figure 5a shows the diffraction pattern associated with the clean (100) platinum surface. There are extra diffraction features in addition to those expected for this surface structure from the X-ray unit cell. This surface exhibits a so-called (5x1) surface structure (8). There are two perpendicular domains of this structure and there are 3, , f, and f order spots between the (00) and (10) diffraction beams. The surface structure is not quite as simple as the shorthand notation indicates, as shown by the splitting of the fractional order beams. The surface structure appears to be stable at all temperatures... 

Figure 4 Crossing of ultrashort pulses, (a) The pancake effect is illustrated for two crossed 40 fs pulses with 200 pm spot sizes. These two pulses, which travel from left to right, would have been produced from a single pulse with a beamsplitter. The spatial extent of the overlap area is seen to be only a fraction of the incident beam s spatial extent, (b) Pulses split with diffractive optics and recombined with a two-lens telescope overlap over the entire spatial extent of the beam profile. (Adapted from Ref. 14.)...

Figure 7. a) Image of a crystal exhibiting twinning domains. The twin boundaries (T.8) are white arrowed, b) electron diffraction pattern characterized by a splitting of the spots hhO. 

Fig. 7.3. The [001] diffraction pattern of the Ortho-I phase. Splitting of superstructure spots is caused by orthorhombic twinning (see text). Streaks appear when vacancies order in an irregular stacking of chains, as a precursor to Ortho-II or Ortho-III phases.

Surface steps are revealed because LEED is only sensitive to a few atomic layers, so that interference occurs between beams diffracted from neighbouring terraces due to horizontal and vertical shift. Depending on primary beam energy and direction, and the direction of the diffracted beam, the interference will be constructive or destructive. In the former case, there is no change of spot shape, but if the interference is destructive, spot splitting or broadening occurs. Consequently, it is important to study the variation of spot shape with experimental conditions (especially primary beam energy) to derive the number and distribution of atomic steps. 

Warren et al. [240] have explored some twenty ditferent sulphonates, mostly aromatic, in the preparation of films of polypyrrole as well as poly(3-methylthiophene), from aqueous solution and from acetonitrile solution. Only PPy-/ TS films from an aqueous medium show the splitting of diffraction peaks this is not interpreted further. Rather, the degree of order is estimated from the intensity of the diffraction peak. Benzenesulphonate ranks first in this respect. Dodecylbenzenesulphonate is also effective, and additionally shows a small-angle peak. This suggests that especially alkyl chains are effective in arranging themselves in a domain of hydrophobic character. (Dodecylbenzenesulphonic acid has been found to be an excellent surfactant for polyaniline and to facilitate its processing see Section 6.4.1.) Some of the films give spot patterns in electron diffraction. Warren et al. [240] state, however, that cell data cannot be derived from these. 

The presence of steps on a single crystal surface is readily discernible by LEED. The LEED patterns differ from those expected from crystals with low-index faces in that the diffraction beams are split into doublets. This splitting (Fig. 10) is a function of ordered steps on the surface. The distance between the split beams is inversely related to the distance between the steps, i.e., the terrace width. From the variation of the intensity maximum of the doublet spots with electron energy the step height can be determined. 

Secondary nematic structure was found by Blumstein and co-workers during the x-ray investigation of polyester prepared from 4,4 -hydroxy(2,2 -methyl) azoxybenzene and dodecanedioic acid. The diffraction pattern of the nematic shows the development of enhanced order characteristic of smectic phase (Fig. 16) the first of the meridional arcs splits up into four sharp spots. This phenomenon is incompatible with the classical definition of the nematic phase and suggests an additional order of macromolecules within cybotactic groups for which a structural model is proposed. 

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