Contrast dynamic scattering

The prerequisite for an experimental test of a molecular model by quasi-elastic neutron scattering is the calculation of the dynamic structure factors resulting from it. As outlined in Section 2 two different correlation functions may be determined by means of neutron scattering. In the case of coherent scattering, all partial waves emanating from different scattering centers are capable of interference the Fourier transform of the pair-correlation function is measured Eq. (4a). In contrast, incoherent scattering, where the interferences from partial waves of different scatterers are destructive, measures the self-correlation function [Eq. (4b)]. 

These uncertain atoms remain to be verified by a careful structure refinement. For a structure refinement, as many reflections as possible should be included. The phases are not needed at the refinement stage, but if possible complete 3D data out to 1 A resolution should be used. Strong and weak reflections are equally important. Such data can be obtained by electron diffraction, which is not affected by the contrast transfer function of the electron microscope, but suffers from dynamical scattering. The higher the accuracy of the amplitudes, the more accurate will the atomic positions become. 

A variant of the zero average contrast method has been applied on a solution of a symmetric diblock copolymer of dPS and hPS in benzene [331]. The dynamic scattering of multicomponent solutions in the framework of the RPA approximation [324] yields the sum of two decay modes, which are represented by exponentials valid in the short time limit. For a symmetric diblock the results for the observable scattering intensity yields conditions for the cancellation of either of these modes. In particular the zero average contrast condition, i.e. a solvent scattering length density that equals the average of both... 

The model given by the dynamic equations (51), (52) and the free energy functional (16) has also been used to calculate the dynamic scattering intensity in bulk contrast [137]. Related models have been employed to determine the dynamic scattering intensity in film contrast [132,138,139],... 

We should also note that the visualization of defects by a nematic liquid crystal, based on the flexoelectric effect in a spatially nonuniform field, is significantly more efiicient than in the dynamic scattering mode [81, 82], because of the higher contrast, the lower visualization voltages (3 to 4 times), and the lower leakage current (about 10 times) [84-86]. 

A Sussmann found that the useful lifetime of a d.c. driven cell is inversely proportional to the current density in the cell [83]. On the other hand the electrical conductivity of the liquid crystal determines the contrast of a display based on an electrohydrodynamic effect such as dynamic scattering. The conductivity can therefore not be decreased at will in favor of longer display lifetime. For optimum contrast the conductivity is about 10 (i2 cm)" to 10 (f2 cm)" ... 

The specific electrical conductivity of pure Azoxy-compounds lies between 10 (12 cm)" and 10 ( 2 cm) h Various dopants are added to the Azoxy-compound to influence the conductivity and orientation. The effect of these additives on the variation with time of the electrical conductivity, of the switching times, and of contrast can be measured. It has been found that after roughly 500 hours of operation for instance the cell conductance when measuring the d.c. current flowing through the cell decreases by one to two powers of ten. As subsequent a.c. measurements revealed, this phenomenon results from the formation of double layers with lower electrical conductivity near the electrodes. These double layers disturb the ion injection from the electrode. As a result of low current densities the dynamic scattering disappears almost entirely. 

The relative magnitude of the coherent and incoherent structure factors mainly depends on the nature of the nuclei. For almost all the atoms bcoh inc of the principal exceptions is the H atom for which bcoh = -3.74 fm, bine = 25.22 fm. Because of that the incoherent signal of protons is 50 times larger than the coherent one. For structural analysis, i.e. for the determination of the equilibrium positions of the sample atoms, it is preferable to work with deuterated samples. In contrast dynamical measurements of individual guests require a highly incoherent scatterer and hydrogen is the best candidate. 

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