Quasi-continuous surfaces

Quasi-continuous surface regime, appearing when dp/Is 2> 1 ... 

Because of its major significance, deposition at quasi-continuous surfaces has been investigated extensively in terms of the RSA model. Most results concern hard spherical particle deposition at planar interfaces of infinite extension [1, 5, 44-48]. However, there exist also results for polydisperse spherical particles [52] and for anisotropic hard particles of a convex shape like squares [53], rectangles (cylinders), spherocylinders (disk rectangles) and ellipses (spheroids) [2]. Results are also available for particles interacting via the short-range repulsive potential stemming from the electric double layers [5, 13, 43, 44, 48]. 

The above theoretical data obtained for quasi-continuous surfaces can be used as reference states for the interpretation of particle deposition at random site surfaces, as discussed below. 

Figure 11.8 shows typical results, obtained as the dependence of the initial deposition probability (expressed in terms of the reduced mass transfer rate kc/fec) negatively charged polystyrene latex particles (averaged diameter 0.9 Jim) on the coverage of spherically shaped sites (positively charged polystyrene latex particles 0.45 (tm in diameter, deposited on abate mica surface) [72]. As can be seen, the deposition probability (reduced initial transfer rate) increased abmptly with the site coverage and attained maximum values pertinent to quasi-continuous surfaces... 

In both cases, because of restrictions imposed on the excitation process (e.g. optical selection rules), the initially excited state is not an exact eigenstate of the molecular Hamiltonian (see below). At the same time, if the molecule is large enough, this initially prepared zero-order excited state is embedded in a bath of a very large number of other states. Interaction between these zero-order states results from residual molecular interactions such as corrections to the Bom Oppenheimer approximation in the first example and anharmonic corrections to nuclear potential surfaces in the second. These exist even in the absence of interactions with other molecules, giving rise to relaxation even in isolated (large) molecules. The quasi-continuous manifolds of states are sometimes referred to as molecular heat baths. The fact that these states are initially not populated implies that these baths are at zero temperature. 

Eisert, R. and Levsen, K., Development of a prototype system for quasi-continuous analysis of organic contaminants in surface or sewage water based on inline coupling of solid-phase microextraction to gas chromatography, J. Chromatogr. A, 737, 59-65, 1996. 

At the next stage of high-dose implantation, starting from Fq > 10 ion/cm, the existing MNPs coalesce to form MNP aggregates or quasi-continuous films in the surface layer (Figure 8.1). For example, the irradiation of epoxy resin by 49-keV cobalt ions at higher-than-threshold doses favors the formation of thin... 

Because there are several parameters that should be taken into account to describe the reaction kinetics of reactions in the form of powder mixture, the analysis could be quite complicated. As a result, various assumptions have been made to simplify the analysis models, in order to derive appropriate kinetic equations. For isothermal reactions, it is generally assumed that the particles of reactant A are equal-sized spheres, which are embedded in a quasi-continuous medimn of reactant B, so that the reaction product is formed coherently and uniformly on the surface of the A particles [49, 50]. In this case, the volume of the unreacted components at lime t is given by ... 

The basic RSA simulation scheme is applicable not only for the quasi-continuous deposition regime but also for deposition at surface features of various shapes and dimensions. 

A big step forward came with the discovery that bombardment of a liquid target surface by abeam of fast atoms caused continuous desorption of ions that were characteristic of the liquid. Where this liquid consisted of a sample substance dissolved in a solvent of low volatility (a matrix), both positive and negative molecular or quasi-molecular ions characteristic of the sample were produced. The process quickly became known by the acronym FAB (fast-atom bombardment) and for its then-fabulous results on substances that had hitherto proved intractable. Later, it was found that a primary incident beam of fast ions could be used instead, and a more generally descriptive term, LSIMS (liquid secondary ion mass spectrometry) has come into use. However, note that purists still regard and refer to both FAB and LSIMS as simply facets of the original SIMS. In practice, any of the acronyms can be used, but FAB and LSIMS are more descriptive when referring to the primary atom or ion beam. 

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