Ions, diffracting power

It is not essential to measure the absolute intensities experimentally Hargreaves (see Lipson and Cochran, 1953) pointed out that by making use of the knowledge that the difference between the absolute structure amplitudes of corresponding reflections of the two crystals must be constant (and equal to the difference between the diffracting powers of the two ions), two sets of relative structure amplitudes can be put on the same scale. We know that the relations between absolute structure amplitudes must be as on the left-hand side of Fig. 210, where the... 

X-Ray diffraction from single crystals is the most direct and powerful experimental tool available to determine molecular structures and intermolecular interactions at atomic resolution. Monochromatic CuKa radiation of wavelength (X) 1.5418 A is commonly used to collect the X-ray intensities diffracted by the electrons in the crystal. The structure amplitudes, whose squares are the intensities of the reflections, coupled with their appropriate phases, are the basic ingredients to locate atomic positions. Because phases cannot be experimentally recorded, the phase problem has to be resolved by one of the well-known techniques the heavy-atom method, the direct method, anomalous dispersion, and isomorphous replacement.1 Once approximate phases of some strong reflections are obtained, the electron-density maps computed by Fourier summation, which requires both amplitudes and phases, lead to a partial solution of the crystal structure. Phases based on this initial structure can be used to include previously omitted reflections so that in a couple of trials, the entire structure is traced at a high resolution. Difference Fourier maps at this stage are helpful to locate ions and solvent molecules. Subsequent refinement of the crystal structure by well-known least-squares methods ensures reliable atomic coordinates and thermal parameters. 

The mby fluorescence emission is induced by laser excitation and can be revealed through a monochromator and a CCD detector. The wavelength and the power of the laser excitation are not restrictive at low pressure, and even few milliwatts of the 647.1-nm excitation line of a Kr ion laser can induce an easily detectable fluorescence emission. Any lower wavelength can be used as well. Typical exciting laser fines used are the 488- and 514.5-nm emissions of an Ar ion laser. Things are more complicated at pressures of 100 GPa, where the mby signal decreases in intensity and the two components are unresolved [235, 248-251]. Recently, it has been demonstrated by means of x-ray diffraction that... 

When X-rays are passed through a crystal of sodium chloride, for example, you get a pattern of spots called a diffraction pattern (Figure 3.15b). This pattern can be recorded on photographic film and used to work out how the ions or atoms are arranged in the crystal. Crystals give particular diffraction patterns depending on their structure, and this makes X-ray diffraction a particularly powerful technique in the investigation of crystal structures. 

Figure 10. Experimental set-up for Raman spectroscopy with the evanescent wave. For excitation, an argon ion laser was used ( 488 nm, output power 1.5 W cw). The material of the reflection plate is fused silica or super dense flint glass. Monochromator Ebert mount Jarrell-Ash 30 cm focal length diffraction gratings 150 or 600 g/mm.

Ignoring for the present ionization and contraction of the lattice, the structure of cesium chloride may be considered similar to that of cesium metal, but with the cesium atoms removed from the body centers and chloride ions inserted. In the diffraction pattern for cesium chloride, the 100 reflections, 111 reflections, and other reflections absent from the pattern for cesium metal are present but are weak. Wave interference similar to that occuring for cesium metal must occur, but here interference is not complete. The planes of chloride ions are not as strong reflectors or scatterers as the planes of cesium ions (the scattering power of an atom rises sharply with atomic number). Thus, interference in cesium... 

In most of the reactions discussed the active entity of the zeolite catalysts is introduced via ion exchange. Thus a knowledge of the possible siting of cations is a prerequisite for an understanding of the location and nature of the active sites in zeolites. In this respect the periodicity of the internal surface of the zeolites provides an almost unique opportunity to study the surface composition in considerable detail using powerful analytical methods such as X-ray diffraction. 

The iast part of our treatment of the central aspects of the chapter concerned molecular dynamics. We showed the power of molecular dynamics simulations in ionic solutions and what excellent agreement can be obtained between, say, the distribution function of water molecules around an ion calculated from molecular dynamics simulation and that measured by neutron diffraction. 

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