Scattering contrast mechanism

The second major contrast mechanism is extinction contrast. Here the distortion of the lattice arotmd a defect gives rise to a different scattering power from that of the surrotmding matrix. In all cases, it arises from a breakdown or change of the dynamical diffraction in the perfect ciystal. In classical structure analysis, the name extinction was used to describe the observation that the integrated intensity was less than that predicted by the kinematical theoiy. 

Figure 14.1. Schematic diagram showing the principle of image formation and diffraction in the transmission electron microscope. The incident beam/o illuminates the specimen. Scattered and unscattered electrons are collected by the objective lens and foeused back to form first an electron diffraction pattern and then an image. For a 2D or 3D crystal, the electron-diffraetion pattern would show a lattice of spots, eaeh of whose intensity is a small fraetion of that of the incident beam. In praetiee, an in-focus image has no eontrast, so images are recorded with the objeetive lens slightly defocused to take advantage of the out-of-focus phase-contrast mechanism.

The large peak around the primary beam energy results from Rutherford scattering and this process increases with increasing atomic number z. therefore the number of BSE coming out of the specimen reflects the average z value of the material this is the important contrast mechanism for the backscattered electrons (see also section 3.2). 

The three commonly encountered contrast mechanisms in TEM imaging are (i) mass-thickness contrast which occurs due to greater absorption or scattering of incident electrons from denser or thicker parts of the specimen (ii) diffraction contrast where crystalline regions of different orientation exhibit different contrast due to the orientational dependence of Bragg diffraction and (iii) phase contrast where phase-shifted waves from the undiffracted and diffracted beams are allowed to interfere and generate lattice fringes. 

Dynamic secondary ion mass qrectrometry (DSIMS) and Rutherford back scattering (RBS) are techniques that can provide information about composition profiles in polymer films. Both techniques provide elemental sensitivity, but neither will provide chemical bonding information as NEXAFS does, since both techniques rely on mass differences as a contrast mechanism. The depth profile is obtained on the basis of elemental conq>osition profiles in the film. In... 

The theoretical model of CARS signal generation from an arbitrary Raman scatterer by tightly focused Gaussian beams in collinear configuration was described by Cheng et al. [4]. In the general formulation derived from the NLO wave equation, the coherent radiation field is calculated from a three-dimensional (3D) object. This provides a quantitative description of the contrast mechanisms of CARS. 

If infrared absorption or Raman scattering is used as the contrast mechanism, vibrational spectra of samples can be obtained. The combination of the nanoscale spatial resolution of a scanned probe with the chemical specificity of vibrational spectroscopy allows in situ mapping of chemical functional groups with subwavelength spatial resolution. Figure 12 is a shear force image of a thin polystyrene film along with a representative near-field spectrum of the... 

At the time the experiments were perfomied (1984), this discrepancy between theory and experiment was attributed to quantum mechanical resonances drat led to enhanced reaction probability in the FlF(u = 3) chaimel for high impact parameter collisions. Flowever, since 1984, several new potential energy surfaces using a combination of ab initio calculations and empirical corrections were developed in which the bend potential near the barrier was found to be very flat or even non-collinear [49, M], in contrast to the Muckennan V surface. In 1988, Sato [ ] showed that classical trajectory calculations on a surface with a bent transition-state geometry produced angular distributions in which the FIF(u = 3) product was peaked at 0 = 0°, while the FIF(u = 2) product was predominantly scattered into the backward hemisphere (0 > 90°), thereby qualitatively reproducing the most important features in figure A3.7.5. 

The im< e mode produces an image of the illuminated sample area, as in Figure 2. The imj e can contain contrast brought about by several mechanisms mass contrast, due to spatial separations between distinct atomic constituents thickness contrast, due to nonuniformity in sample thickness diffraction contrast, which in the case of crystalline materials results from scattering of the incident electron wave by structural defects and phase contrast (see discussion later in this article). Alternating between imj e and diffraction mode on a TEM involves nothing more than the flick of a switch. The reasons for this simplicity are buried in the intricate electron optics technology that makes the practice of TEM possible. 

However, in contrast to the production know-how , the scientific knowledge on the details of phase equilibria, kinetics, mechanisms, catalysis and mass-transport phenomena involved in polycondensation is rather unsatisfactory. Thus, engineering calculations are based on limited scientific fundamentals. Only a few high-quality papers on the details of esterification and transesterification in PET synthesis have been published in the last 45 years. The kinetic data available in the public domain are scattered over a wide range, and for some aspects the publications even offer contradicting data. 

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