Stochastic parameters in the analysis of the kinetics of topochemical processes

Application of stochastic parameters to the analysis of the kinetics of topochemical processes is discussed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, Nos. 5–6, pp. 448–453, September–December, 1992.     

Electrochemical and spectroscopic characterization of surface sol-gel processes

(3-Mercaptopropyl)trimethoxysilane (MTS) forms a unique film on a platinum substrate by self-assembly and sol-gel cross-linking. The gelating and drying states of the self-assembled MTS sol-gel films were probed by use of electrochemical and spectroscopic methods. The thiol moiety was the only active group within the sol-gel network. Gold nanoparticles were employed to detect the availability of the thiol group and their interaction further indicated the physicochemical states of the sol-gel inner structure. It was found that the thiol groups in the open porous MTS aerogel matrix were accessible to the gold nanoparticles while thiol groups in the compact MTS xerogel network were not accessible to the gold nanoparticles. The characteristics of the sol-gel matrix change with time because of its own irreversible gelating and drying process. The present work provides direct evidence of gold nanoparticle binding with thiol groups within the sol-gel structures and explains the different permeability of "aerogel" and "xerogel" films of MTS on the basis of electrochemical and spectroscopic results. Two endogenous species, hydrogen peroxide and ascorbic acid, were used to test the permeability of the self-assembled sol-gel film in different states. The MTS xerogel film on the platinum electrode was extremely selective against ascorbic acid while maintaining high sensitivity to hydrogen peroxide in contrast to the relatively high permeability of ascorbic acid in the MTS aerogel film. This study showed the potential of the MTS sol-gel film as a nanoporous material in biosensor development.     

AES and XPS investigations for the topochemical characterization of dope elements on WC powders

Summary Trace elements bias the hard metal production and can deteriorate the technological and mechanical properties of sintered tools. Therefore, within the COST 503 action, the surface enrichment of the dope elements Ca, Si, Al, P on 1.5 m WC milling grade starting powders has been investigated by AES and XPS. AES depth profiles and high resolution AES (HRAES) elemental maps describe the microscopic distribution of traces. XPS spectra are compared with the AES results and contribute to the understanding of the topochemical structure of doped WC-powder grains.     

Thin-film TiO2 electrode surface characterization upon CO2 reduction processes

The effect of electrochemical reduction of CO₂ on the structure and morphology of titanium(IV) oxide thin films was examined after a fixed-potential bulk electrolysis process. Films deposited on ITO (Indium-Tin Oxide) substrates were used as the working electrodes and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF₄]) as solvent and as supporting electrolyte. Grazing incidence X-ray diffraction analysis performed before and after the electrolysis process indicated no microstructural changes of the anatase films. X-ray photoelectron spectroscopy revealed peaks associated with adsorbed carbonate ions at 288 eV and CO₂ species at 293 eV, whereas Ti2p peaks displacements for CO₂-saturated TiO₂/ITO surfaces in [BMIm][BF₄] revealed chemical bonding effects. Auger electron spectroscopy revealed a high carbon content on CO₂-exposed films, and suggested a strong chemisorption of CO₂ and CO₃²⁻ species on the TiO₂/ITO surface in [BMIm][BF₄] solvent system. A significant decrease in carbon content after bulk electrolysis indicated that the CO₂ electroreduction process is not controlled by either diffusion or by adsorption of CO₂ on the TiO₂/ITO electrode surface.     

Theory of laser-simulated surface processes

Theoretical techniques for describing laser-stimulated surface processes in a vacuum and at a gas-surface interface are presented. For adspecies-surface systems, the laser excitation of vibrational degrees of freedom is considered, and quantum-mechanical and classical models and also an “almost first-principles” treatment of the competition between multiphoton absorption and multiphonon relaxation are discussed. The laser excitation of electronic degrees of freedom is considered with respect to surface states of semiconductors and metals, for the predissociation of diatomic adspecies on metal substrates, for ionization, and for resonance fluorescence of a gaseous atom near a metal. In connection with gas-surface interactions, the influence of laser radiation on diffraction patterns and energy transfer in atom-surface scattering is explored. Collisional ionization and ion neutralization in the presence of laser radiation are discussed. The roles of partial pressure and surface coverage in laser-stimulated surface processes are analyzed. Finally, some ideas on surface waves and annealing are presented.     

Mechanisms of surface processes in silicon etching

A unified view on the mechanism allowing one to explain the experimental features governing spontaneous silicon etching by atomic fluorine is presented. Analysis of the phenomenological equation of adsorption shows a significant difference between etching mechanisms at high and low heat of adsorption on the surface being etched. As follows from the parameter estimates, one or another case can be realized under different experimental conditions. At steady-state the etching is argued to be determined only by the processes taking place on the SiF. film surface. To describe the process, it is necessary to understand the mechanism of overcoming the surface barrier for fluorine penetration into the film. At low heat of fluorine adsorption the barrier is overcome by thermal activation. In the opposite case the etching mechanism includes electron tunneling from silicon to adatoms and creation of a surface electric field. The field lowers the high energetic barrier for fluorine penetration. Based on the kinetic equations describing the electronic and atomic processes on the surface, the equation of the field strength is obtained. This equation is analyzed in different limit cases. The observed features are shown to be reproduced at some conditions on the parameters. Definite predictions on the temperature dependence of the etch rate are made.     

Equilibrium fluctuations in the theory of surface processes on microparticles

The question of the role of equilibrium fluctuations in the adsorption theory and kinetics of surface processes occurring on the particles of the nanometer size range is discussed. Differences are put forward that need to be introduced to the fluctuation theory of surface processes on microparticles and that generalize Hill’s approach to describing the thermodynamic properties of small systems. We show the importance of allowing for the discrete character of adsorption centers on the surfaces and their heterogeneity when describing adsorption isotherms and the rates of adsorption processes.     

Electron transfer mechanisms in surface dynamical processes

Some topics concerning on the dynamical electron transfer processes between adparticle and surface are discussed based on recent theoretical studies. They include the band effect on the electron transfer probability, the change from the diabatic to adiabatic behavior seen in the field induced desorption (FID), and the effect of the couplings with the medium degrees of freedom on the electron transfer process. It is elucidated how the competition of the energy parameters, i.e., the band width, the inverse of the scattering time and the interaction energy leads to different features of the electron transfer. Natural crossover of the FID behavior from the diabatic to the adiabatic limit is clarified by the generalized kinematic equation based on the quantum model of the electron transfer. Enhancement of the diabatic behavior by the coupling with the heat bath or sorrounding medium is concluded with the stochastic trajectory method and the time-developing operator method.     

Study of transient thermal surface processes

Summary The method of nanosecond time-resolved photothermal displacement spectroscopy in combination with time-resolved light reflectivity measurements has been used to study aluminum and stainless steel substrates with sputter deposited Au and Sn films. The results show, that this method can be used as a non-contacting surface thermometer. In addition, a strong correlation between signal decay and film thickness could be observed.

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Untersuchung transienter thermischer Oberflächenprozesse

     

The specific heat of surface layers

Summary The well-known relation ofW. Thomson for the latent heat of surface expansion was obtained by applying the second law to a cyclic surface expansion and heating process. A simple derivation of an expression for the excess heat capacity of the surface layer is obtained by applying the first law to the same cyclic process.If we do not include the amount of potential energy converted to heat,A /TdT in the definition of the excess heat capacity, the resultant relation between the specific heat of the surface layers and their excess heat capacity is of the same form as that expressing the latent heat of vaporization in terms of the differential heat capacities of the liquid and vapor phases.Numerical calculation shows that the excess heat capacity for the surface layer of water amounts to 24 percent of the heat capacity of the molecular layer in the bulk liquid phase.     

The role of surface viscoelasticity in slide coating processes

In the slide hopper coating process for simultaneously applying multiple layers of coating liquids to a moving web, surfactants must be added to the photographic emulsion to ensure a stable position of the liquid bridge formed between the lower edge of the slide hopper and the moving web. In slide coating of gelatin solutions without an added surfactant, the liquid bridge becomes instable and begins to oscillate if critical coating conditions are reached. The addition of anionic surfactants stabilizes the liquid bridge against oscillations. The action of the added surfactants is a result of their interaction with gelatin. The degree of binding can be used as a measure of the interaction. The binding of anionic and cationic surfactants to gelatin was investigated using a surfactant-selective electrode. The binding isotherms of the surfactants to an alkali-processed bone gelatin were determined and compared with the surface dilational properties of the gelatin/surfactant adsorption layers. The comparison of surface rheological data obtained by the oscillating bubble method with the results of coating experiments demonstrates that the viscoelastic properties of gelatin/anionic surfactant adsorption layers are of essential importance to the stabilization of the liquid bridge against oscillations. Pure elastic adsorption layers are not able to stabilize the liquid bridge. The mechanism of the stabilizing action is discussed. Received: 30 October 2000/Revised: 12 February 2001/Accepted: 14 February 2001     

固态二乙炔拓扑聚合反应动力学的量子化学研究

利用协同反应模型和EHCO-ASED量子化学方法, 对固态二乙炔的拓扑聚合反应:MDA(Molecular diacetylenes)→PBT(Polybutatrienes)→PDA(Polydiacetylenes)的势能曲线进行了计算, 并对其轨道对称性以及能隙随反应坐标的变化进行了分析; 很好地解决了文献中用Woodward-Hoffmann轨道对称守恒原理对此反应进行分析时所遇到的问题, 指出了此反应是热允许的原因。     

Galvanomagnetic size studies of metallic surface processes

This review gives an account of basic ideas concerning scattering of current carriers at metallic surfaces. It deals with the physical picture and theory of surface-sensitive galvanomagnetic size effects, i.e., transverse magnetic resistance, static skin effect, Sondheimer oscillations, and conduction electron focusing. These processes are employed to study the peculiarities of current carrier surface scattering with regard to the electron–hole transfers. Diffraction of conduction electrons at adsorbed submonolayer lattices is considered. An outlook is given of the application of these phenomena to the study of adsorption and ordering of adsorbed submonolayer films of various symmetries and chemical content, involving, in particular, precursor states. Influence of the diffusion of adsorbed atoms into the substrate on the kinetic size effects is considered in detail.     

A general theory of surface seperation processes

A thermodynamic theory of surface separation has been formulated for mixtures of both inert and chemically interacting substances. The behaviour of the surface separation lines in the vicinity of singular points in concentration diagrams has been investigated. As an example, the behaviour of the foam separation lines in dilute three- and four-component solutions has been considered. The possibility of classification of systems by means of a thermodynamic—topological rule has been exhibited.     

Kinetics of elementary processes at the surface of a solid

A simple model of a multiphonon transition is discussed in relation to a continuous spectrum in the initial state; estimates are given the relative probabilities of volume and surface processes.