Objectives diffraction pattern

Figure 3.14 Calculated sensitivity as a function of radial distance from the center of an objective diffraction pattern, comparing Schwarzschild objective used in a nonconfocal (solid line) and confocal (dash-dot line) configuration. The plateau still occurs near 5 [im distance for both cases, but the enclosed sensitivity has been increased to more than 80%. Also note how the confocal case approaches 100% more quickly as a function of radial distance.

The otiier type of noncrystalline solid was discovered in the 1980s in certain rapidly cooled alloy systems. D Shechtman and coworkers [15] observed electron diffraction patterns with sharp spots with fivefold rotational synnnetry, a syimnetry that had been, until that time, assumed to be impossible. It is easy to show that it is impossible to fill two- or tliree-dimensional space with identical objects that have rotational symmetries of orders other than two, tliree, four or six, and it had been assumed that the long-range periodicity necessary to produce a diffraction pattern with sharp spots could only exist in materials made by the stacking of identical unit cells. The materials that produced these diffraction patterns, but clearly could not be crystals, became known as quasicrystals. 

We have thus far discussed the diffraction patterns produced by x-rays, neutrons and electrons incident on materials of various kinds. The experimentally interesting problem is, of course, the inverse one given an observed diffraction pattern, what can we infer about the stmctirre of the object that produced it Diffraction patterns depend on the Fourier transfonn of a density distribution, but computing the inverse Fourier transfomi in order to detemiine the density distribution is difficult for two reasons. First, as can be seen from equation (B 1.8.1), the Fourier transfonn is... 

Collectively, the post-specimen lenses serve one of two purposes they magnify either the diffraction pattern from the sample produced at the back focal plane of the objective lens or they magnify the image produced at the image plane of the objective lens. These optical planes are illustrated in the elearon ray diagram in... 

A TEM provides the means to obtain a diffraction pattern from a small specimen area. This diffraction pattern is obtained in diffraction mode, where the post-specimen lenses are set to examine the information in the transmitted signal at the back focal plane of the objective lens. 

Sample B provided platinum crystallites that were analyzed by both EDS and MAED. MAED of several 3 nm crystallites shows a wide variation of orientations with respect to the electron beam, however, many of the patterns match (111) and (110) orientations. An example of the MAED patterns observed Is shown In Figure 2. The diffraction pattern was made with a 25 pm objective aperture at a camera length of 2 m. 

Electron Microscopy can be used for resolution of smaller objects the practical limit of resolution being a few angstrom units. Electron Microscopy has been used in the study of the morphology of crystalline polymers. The usual techniques of replication, heavy-metal shadowing, and solvent etching are widely used. The direct observation of thin specimens, like polymer single crystals, is also possible and permits the observation of the electron-diffraction pattern of some specimen area, which is invaluable for... 

Electron diffraction patterns are usually produced with transmission electron microscopes. These instruments are composed of several magnetic lenses. The main lens is the objective lens, which, in addition to forming the first magnified image of the specimen, also produces the first diffraction pattern. This original pattern is then magnified by the other lenses of the microscope so as to produce the final diffraction patterns on the screen or on a camera. 

Electron diffraction takes place as early as when the incident electrons interact with the specimen. The objective lens acts as a diffractometer to make the electron diffraction pattern observable at its back-focal plane. Electron diffraction is not affected by the CTF of the objective lens so that the resolution limit of the diffraction pattern is much higher than that in the image. T q)ically the EDP contains reflections up to resolution of 1 A or... 

In optics and spectroscopy, resolution is often limited by diffraction. To a good approximation, the spread function may appear as a single-slit diffraction pattern (Section II). If equal-intensity objects (spectral lines) are placed close to one another so that the first zero of one sine-squared diffraction pattern is superimposed on the peak of the adjacent pattern, they are said to be separated by the Rayleigh distance (Strong, 1958). This separation gives rise to a 19% dip between the peaks of the superimposed patterns. 

Figure 2.5 Operational principles of a transmission electron microscope. S, specimen OL, objective lens BF, back focal plane Imj, intermediate image IL, intermediate lens Im2, second intermediate image PL, projector lens FIm, final image DP, diffraction pattern.

Fig. 166. An optical diffractometer. A, light source B, pinhole C and D, lenses E, optically flat mirror the diffraction pattern of an object placed at O is seen in plane F. (Taylor, Hinde, and Lipson, 1961.)...

We performed a series of theoretical studies on pump-probe diffraction patterns with a twofold objective the first aim is to evaluate the effect of electronic and vibrational excitation on electron diffraction patterns, compared to that of structural rearrangements that are the primary goal for observation in structural dynamics measurements. Secondly, we wish to explore to what extent electronic and vibrational probability density distributions are observable using the pump-probe electron diffraction methodology. Previously we have discussed the effect of electronic excitation in atomic systems,[3] and the observability of vibrational excitation in diatomic and triatomic systems.[4,5] We have now extended this work to the 8-atomic molecule s-tetrazine (C2H2N4). 

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