Other Scattering Techniques

Atoms are not rigidly bound to the lattice, but vibrate around their equilibrium positions. If we were able to look at the crystal with a very short observation time, we would see a slightly disordered lattice. Incident electrons see these deviations, and this, for example, is the reason that in LEED the spot intensities of diffracted beams depend on temperature at high temperatures the atoms deviate more from their equilibrium position than at low temperatures, and a considerable number of atoms are not at the equilibrium position necessary for diffraction. Thus, spot intensities are low and the diffuse background high. Similar considerations apply in other scattering techniques, as well as in EXAFS and in Mossbauer spectroscopy. 

The OFC, shown in Figure 8.1 and Figure 8.2, is suitable for measurements on the 0.1-1 s time scale. The surface properties of this cell, in particular the relationship between the surface excess F, surface expansion rate, 0 = (l/r)d(rUs)/dr (r = radial distance = radial surface velocity), and C, depend solely on adsorption kinetics at that particular C and surface excess. The OFC has been designed to provide a large ( 50 cm ), almost flat surface for analysis by ellipsometric, spectroscopic, and other scattering techniques. The cell consists... 

The dependence on target mass makes ion scattering techniques ideal for the study of multielement systems. By increasing the incident ion mass, the energy separation between different elements becomes larger. On the other hand, radiation-induced damage becomes a more important consideration. 

P. R. Watson./. Phys. Chem. Ref. Data. 19, 85, 1990. Compilation of structural data attained by MEIS and other ion scattering techniques. 

The STEM Is Ideally suited for the characterization of these materials, because one Is normally measuring high atomic number elements In low atomic number metal oxide matrices, thus facilitating favorable contrast effects for observation of dispersed metal crystallites due to diffraction and elastic scattering of electrons as a function of Z number. The ability to observe and measure areas 2 nm In size In real time makes analysis of many metal particles relatively rapid and convenient. As with all techniques, limitations are encountered. Information such as metal surface areas, oxidation states of elements, chemical reactivity, etc., are often desired. Consequently, additional Input from other characterization techniques should be sought to complement the STEM data. 

By using NFS, information on both rotational and translational dynamics can be extracted. In many cases, it would be favorable to obtain separate information about either rotational or translational mobility of the sensor molecule. In this respect, two other nuclear scattering techniques using synchrotron radiation are of advantage. Synchrotron radiation-based perturbed angular correlations (SRPAC) yields direct and quantitative evidence for rotational dynamics (see Sect. 9.8). NIS monitors the relative influence of intra- and inter-molecular forces via the vibrational density of states (DOS) which can be influenced by the onset of molecular rotation (see Sect. 9.9.5). 

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