Static local structure

Structurally Dyuamic CA the only generalizations mentioned so far were generalizations of either the rules or state space. Another intriguing possibility is to allow for the lattice C itself to become a full participant in the dynamical evolution of the system, much as the classically static physical space-time arena becomes a bona-fide dynamic element in general relativity. The idea is to study the behavior of systems evolving according to both value and local structure rules ... 

It has been proposed recently [28] that static friction may result from the molecules of a third medium, such as adsorbed monolayers or liquid lubricant confined between the surfaces. The confined molecules can easily adjust or rearrange themselves to form localized structures that are conformal to both adjacent surfaces, so that they stay at the energy minimum. A finite lateral force is required to initiate motion because the energy barrier created by the substrate-medium system has to be overcome, which gives rise to a static friction depending on the interfacial substances. The model is consistent with the results of computer simulations [29], meanwhile it successfully explains the sensitivity of friction to surface film or contamination. 

Given the character of the water-water interaction, particularly its strength, directionality and saturability, it is tempting to formulate a lattice model, or a cell model, of the liquid. In such models, local structure is the most important of the factors determining equilibrium properties. This structure appears when the molecular motion is defined relative to the vertices of a virtual lattice that spans the volume occupied by the liquid. In general, the translational motion of a molecule is either suppressed completely (static lattice model), or confined to the interior of a small region defined by repulsive interactions with surrounding molecules (cell model). Clearly, the nature of these models is such that they describe best those properties which are structure determined, and describe poorly those properties which, in some sense, depend on the breakdown of positional and orientational correlations between molecules. 

Standard static and dynamic light scattering methods assume that there is very little multiple scattering by the particles, that is, the dispersion has to be sufficiently dilute so that the photons are scattered only once as they pass through the sample. Is there a way to look inside a dispersion that is cloudy or milky, such as a foam, and to extract information on the local structure and its kinetics and relaxation Or, is it possible to tailor a dispersion so that... 

XAS can be used in several different ways to determine local structural information about catalysts in reactive atmospheres. This structural information may be static or dynamic it may be geometric or electronic. The depth of information that can be ascertained is often dependent upon the type of catalyst, for example, supported metal nanoclusters versus bulk or surface oxides. It may also be controlled by some property of the catalyst, for example, the concentration of the element in the catalyst that is being investigated. In this section a few examples are provided to highlight the importance and relevance of XAFS in catalyst characterization. The examples are focused on (1) structural information characterizing samples in reactive atmospheres, (2) transformation of one species to another, (3) oxidation state determination, (4) determination of supported metal cluster size and shape, and (5) electronic structure. These examples illustrate the type of information that can be learned about the catalyst from XAFS spectroscopy. 

A number of important questions, however, still remain open. The first is which ion displacements Pb or Nb/Mg/Sc are responsible for the formation of polar regions What is the nature of non-polar matrix into which polar clusters are assumed to be embedded Are the polar clusters and the non-polar matrix dynamic or static entities Another still open important problem in PMN relaxor is related to microscopic inhomogeneities in the site occupancy of the Mg2-1" and Nb5+ cations and their role in the formation of the polar regions. An existence of a 1 1 Mg/Nb ordered microregions is still discussed in literature. Since the 1 1 Sc/ Nb chemical order in PbSci/2Nbi/203 really exists, a comparative study of the high-temperature local structure of these two relaxors is of interest. 

This review deals with the applications of photolurainescence techniques to the study of solid surfaces in relation to their properties in adsorption, catalysis, and photocatalysis, After a short introduction, the review presents the basic principles of photolumines-cence spectrosajpy in relation to the definitions of fluorescence and phosphorescence. Next, we discuss the practical aspects of static and dynamic photoluminescence with emphasis on the spectral parameters used to identify the photoluminescent sites. In Section IV, which is the core of the review, we discuss the identification of the surface sites and the following coordination chemistry of ions at the surface of alkaline-earth and zirconium oxides, energy and electron transfer processes, photoluminesccncc and local structure of grafted vanadium oxide, and photoluniinescence of various oxide-... 

The local structure around zinc and manganese atoms in as-synthesised sample of MnZnAPO-34 was also characterised by means of EXAFS. This technique provides a description of the short-range order of selected atomic species in terms of the number of neighbours, distances, and thermal and static disorder within a range of those distances. 

The structure, however, is not static but is subject to thermally driven fluctuations. The local structure changes continuously as a function of time due to orientational and translational molecular motions. The time scale of these motions may range from nanoseconds up to several hundred years. The structure of the amorphous state as well as its time-dependent fluctuations can be analysed by various scattering techniques, such as X-ray, neutron, electron and light scattering. 

The collective structure of aqueous IL solutions was studied by means of MD simulations [101]. Various concentrations of [C4mim][BF4] and TIP3P water were simulated at the very same size of the simulation box. For the analysis, the ternary system cation/anion/water was subdivided into binary networks. The local structure of each of these six networks and the mutual orientation of the network constituting partners were studied. Furthermore, the collective structure of the whole samples was characterized by the contribution of each species to the static dielectric constant e(co= 0) and to the Kirkwood factor [101]. 

If the difference in atomic number between the absorber element and the backscattering element is >10 and if only one kind of element backscatters, EXAFS spectra can be analyzed readily to provide local structural data on adsorbed species. However, because the electron mean free path, thermal and static disorder parameters (Debye-Waller factors), and coordination number for an absorber environment cannot be determined a priori with sufficient accuracy, EXAFS data for suitable reference compounds of known molecular structure must be used to help interpret the EXAFS spectrum for an interfaeial region. 

Depending on the strength of the electron-phonon coupling, one may observe the formation of a polaron or an exclmer. The formation of a polaron does not lead to the loss of the identity of the monomer in the excited state, but simply the excitation is localized by local lattice-deformation. The excimer formation requires a severe distortion oi the local structure which leads to an excited state dimer. It may also be pointed out that the polaron mechanism is a purely dynamic effect which can occur even in a defect-free lattice. In contrast, the excimer formation can occur either by a dynamic effect due to strong electron-phonon coupling or by a static effect due to sites deformed by the presence of defects. 

Deformation such as drawing, compression, annealing, strain, creep and stress relaxation of polymers including fibers may produce quite different orientational behavior, the results of which can be examined with solid-state NMR from both the static and dynamic viewpoints. The accurate model produced on the basis of atomic resolution of the local structure and the local dynamics can be built up in order to interpret the mechanical properties of polymers and the deformation mechanisms. 

Figure 2.13 Schematic elastic scattering curve of a spherical colloid (e.g. a microemulsion droplet) in solution. As a rule of thumb q 1 is an approximate measure for the spatial resolution of the used scattering experiment. At low values of q (i.e. in the Cuinier region of the scattering curve) the overall size and shape of the particles as well as correlations between different particles can be monitored (typically by static and dynamic light scattering). At high values q, the internal structure of the particles, i.e. the local structure of the interfacial film is resolved (e.g. by neutron or X-ray small angle scattering and neutron spin-echo spectroscopy (NSE)).

From static dielectric constants measured at different temperatures in a set of nonpolar solvents, it seems possible to determine relative, if not absolute values of the unwinding function (T) which should be closely related to Flory s expansion coefficient x(T). With polar solvents, by calculating the factor g", it may be possible to find evidence for a disturbance of the local structure due to the presence of the large solute molecule. Finally, a closer study of relaxation phenomena may give an opportunity to separate the energy of activation into two parts, one of which could be related to the solvent-polymer interactions. -... 

X-ray absorption is a powerful technique to obtain local electronic and structural properties and has been widely used to characterize catalysts (Yokoyama 1995). This technique involves the determination of the electronic state through X-ray absorption near edge structure (XANES) analysis and of the local structure through the extended X-ray absorption fine structure (EXAFS). The data can be collected not only in a static but also in dynamic state (in-situ conditions) allowing one to study the catalyst during pretreatment and catalytic reaction. 

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