Traveling waves and nonequilibrium stationary patterns in two-component reactive langmuir monolayers

A simple kinetic model of a two-component phase-separating Langmuir monolayer with a chemical reaction is proposed. Its analysis and numerical simulations show that nonequilibrium periodic stationary structures and patterns of traveling stripes can spontaneously develop. The nonequilibrium phase diagram of this system is constructed and the properties of the patterns are discussed.     

Computer Simulation of Nonequilibrium Growth of Crystals in a Two-Dimensional Medium with a Phase-Separating Impurity

Two-dimensional nonequilibrium growth of crystals (quasistable faceted and dendritic) in the presence of a phase separating impurity is studied by computer simulation. It is shown that there is a gradual modification in this system from quasistable faceted growth to the formation of dendrites when the impurity concentration increases. If there is dendritic growth in the presence of a phase-separating impurity, the cyclic changes in the morphology, expressed through the periodic occurrence of tertiary branches of a dendrite, are observed when the phase-separating impurity concentration is raised. This behavior of the morphology is considered as a reentrant nonequilibrium phase transition.     

Compositional dependence of positron annihilation in the Li2OSiO2 glass system

Positron lifetimes are studied in the phase-separating Li₂OSiO₂ glass system as a function of molar composition. The intensity of the longest lifetime is found to be proportional to the amount of the silica-rich phase. The crystallization of the lithia-rich phase affects mainly the intensity of the intermediate lifetime.     

State-to-state dissociation–recombination and chemical exchange rate coefficients in excited diatomic gas flows

In this paper, simple analytical state-to-state rate coefficients for the dissociation–recombination and chemical exchange reactions are presented on the basis of kinetic theory in nonequilibrium excited diatomic gases. They take into account the excited vibrational and electronic states of the chemical species and are expressed according to the preferential character of the chemical reactions. Evolution of these rate coefficients varying according to the translational temperature, bringing into play molecules CO and C₂, are discussed.     

Mass spectrometry of high pressure arcs in air and SF6

For the development of SF₆ pufferbreakers it is important to gain insight into the chemical composition of the SF₆ gas during the high-current period as well as close to current zero. For this reason a mass-spectrometric sampling system is presented which allows dynamic measurements of chemical reactions occurring in high-pressure arcs. Results of mass-spectroscopic investigations on a dc and a pulsed arc are presented.     

Chemical and catalytic effects of ion implantation

Energetic particles are used for inducing chemical reactions as well as for modifying the properties of materials with regard to their bulk and surface chemical behavior. The effects are partly caused by radiation damage or phase intermixing, partly by the chemical properties of the individual bombarding particles. In this contribution a survey of relevant applications of these techniques is presented:

1) Chemical reactions of implanted and recoil atoms and their use for syntheses, doping and labeling of compounds.

2) The formation of thin films by decomposing chemical compounds with ion beams.

3) Catalytic effects on substrates treated by sputtering or ion implantation. Recent results with nonmetallic substrates are reviewed. Mainly hydrogenation reactions at a solid/gas interface or redox reactions at an electrified solid/liquid interface are mentioned.

The present status and future prospects of these kinds of investigations will be discussed.     

超高速分子摄影术——飞秒泵浦-探测方法在分子超快动力学研究中的应用

飞秒泵浦-探测技术是一种可以在原子运动时间尺度上实时观测化学反应的有力手段.利用飞秒泵浦-探测技术,可以实时观测化学反应过渡态,在分子层次上了解基元化学反应的过程和机理,从而深入地了解化学反应的本质和历程.文章介绍了飞秒泵浦-探测技术的基本原理以及在分子超快动力学研究中的应用,并结合作者所在研究小组的工作,展示了基于该技术的几种典型的飞秒光谱方法在分子超快动力学研究领域的主要成果.最后,展望了该技术的发展前景和方向.     

A microfluidic pumping mechanism driven by non-equilibrium osmotic effects

A mechanism is presented which drives a fluid flow using two chemically reacting molecular species and osmotic effects. For concreteness the mechanism is discussed in the context of a tube which at each end has a capping membrane which is permeable to the fluid but impermeable to the two molecular species. The chemical reactions occur at sites embedded in the capping membrane. Labeling the two chemical species A and B, at one end the reactions split each molecule of species B into two molecules of species A. On the other end two molecules of species A are fused together to form a single molecule of species B. A mathematical model of the solute diffusion, fluid flow, and osmotic effects is presented and used to describe the non-equilibrium steady-state flow rate generated. Theoretical and computational results are given for how the flow rate depends on the relative diffusivities of the solute species and the geometry of the system. An interesting feature of the pump is that for the same fixed chemical reactions at the tube ends, fluid flows can be driven in either direction through the tube, with the direction depending on the relative diffusivities of the solute species. The theoretical results are compared with three-dimensional numerical simulations of the pump.     

Organic chemistry at interfaces studied by optical second-harmonic and IR-vis sum-frequency generation

Three selected systems are presented which demonstrate the versatility of second-order nonlinear techniques for interface analysis and thin film monitoring. With second-harmonic generation processes at buried interfaces can be monitored in real time as is shown by thiol adsorption on polycrystalline gold from solution. The time dependent second-harmonic signal shows a strong wavelength dependence due to resonances. In situ monitoring of thin-film formation by second-harmonic generation can differentiate between the reaction at the interface and the subsequent film growth. The chemical vapor deposition of polyamic acid is presented as example. Hydrogen adsorption on diamond C(111) is analysed by sum-frequency generation. It is demonstrated that sum-frequency generation has the sensitivity and specificity to trace chemical reactions at surfaces and to infer on the orientation of chemical bonds.     

On the mechanism of effective chemical reactions with turbulent mixing of reactants and finite rate of molecular reactions

A generalization of the theory of chemical transformation processes under turbulent mixing of reactants and arbitrary values of the rate of molecular reactions is presented that was previously developed for the variant of an instantaneous reaction [13]. The use of the features of instantaneous reactions when considering the general case, namely, the introduction of the concept of effective reaction for the reactant volumes and writing a closing conservation equation for these volumes, became possible due to the partition of the whole amount of reactants into “active” and “passive” classes; the reactants of the first class are not mixed and react by the mechanism of instantaneous reactions, while the reactants of the second class approach each other only through molecular diffusion, and therefore their contribution to the reaction process can be neglected. The physical mechanism of reaction for the limit regime of an ideal mixing reactor (IMR) is revealed and described. Although formally the reaction rate in this regime depends on the concentration of passive fractions of the reactants, according to the theory presented, the true (hidden) mechanism of the reaction is associated only with the reaction of the active fractions of the reactants with vanishingly small concentration in the volume of the reactor. It is shown that the rate constant of fast chemical reactions can be evaluated when the mixing intensity of reactants is much less than that needed to reach the mixing conditions in an IMR.     

Stereodynamics of chemical reactions: quasi-classical,quantum and mixed quantum-classical theories

In this review, some benchmark works by Han and coworkers on the stereodynamics of typical chemical reactions, triatomic reactions H + D₂, Cl + H₂ and O + H₂ and polyatomic reaction Cl+CH₄/CD₄, are presented by using the quasi-classical, quantum and mixed quantum-classical methods. The product alignment and orientation in these A+BC model reactions are discussed in detail. We have also compared our theoretical results with experimental measurements and demonstrated that our theoretical results are in good agreement with the experimental results. Quasi-classical trajectory (QCT) method ignores some quantum effects like the tunneling effect and zero-point energy. The quantum method will be very time-consuming. Moreover, the mixed quantum-classical method can take into account some quantum effects and hence is expected to be applicable to large systems and widely used in chemical stereodynamics studies.     

Correlations in stochastic theories of chemical reactions

A comprehensive study of correlations in linear and nonlinear chemical reactions is presented using coupled chemical and diffusion master equations. As a consequence of including correlations in linear reactions the approach to the steady-state Poisson distribution from an initial non-Poissonian distribution is given by a power law rather than the exponential predicted by neglecting correlations. In nonlinear reactions we show that a steadystate Poisson distribution is achieved in small volumes, whereas in large volumes a non-Poissonian distribution is built up via the correlation. The spatial correlation function is calculated for two examples, one which exhibits an instability, the other which exhibits a second-order phase transition, and correlation length and correlation time are calculated and shown to become infinite as the critical point is approached. The critical exponents are found to be classical.     

Surface order large deviations of phase interfaces for the continuum Widom-Rowlinson model in high density limit

We establish a surface order large deviation principle characterising, in the phase coexistence region, the exponential decay rates for the probabilities of macroscopic fluctuations of phase-separating interfaces for the continuum Widom-Rowlinson binary gas, with the thermodynamic and high fugacity limits taken simultaneously. The large deviation rate function is given by an isotropic surface energy functional and hence it attains its minimum for balls which are the most favourable shapes of ‘droplets’ of dominated phase within the ‘ocean’ of dominating phase.     

Reduction of round-off errors in chemical kinetics

In this paper, a modification of chemical kinetics is introduced in order to reduce round-off errors in stiff source terms. Fast chemical processes close to partial equilibrium are often composed of different contributions that cancel each other out (for example forward and backward reactions). These contributions tend to be very large compared to their sum. Thus, computer evaluation of the chemical source term can lead to serious round-off errors due to cancellation. This can cause severe convergence problems within Newton's method when simulating the process with implicit integration methods and thus lead to additional step size reduction. Here, a method is presented that reduces the round-off error by making a slight change to the model. Furthermore, this change is smooth and does not change the dimension, so that it is also possible to apply the method locally when solving partial differential equations. Numerical examples presented in this paper verify the approach.     

Identification of minority compounds in natural ilmenites by X-ray absorption spectroscopy

Natural ilmenites are used all over the world as raw materials in white pigment (TiO₂) production. Besides the FeTiO₃ in the raw material many other compounds are present. The chemical compounds based on the minority elements influence quality of the final product and are difficult to identify. The knowledge about chemical bonding of the minor elements enables to properly adjust chemical reactions during production processes and to improve quality of the final product. In this paper the X-ray absorption spectroscopy (XAS) identification of the chemical compounds formed by Mg, Mn and Cr in natural ilmenites originating from different places is presented.     

Development of the reaction time accelerating molecular dynamics method for simulation of chemical reaction

We present a novel and efficient method to integrate chemical reactions into molecular dynamics to simulate chemical reaction systems. We have dubbed this method RTAMD, an acronym for reaction time accelerating molecular dynamics. The methodology we propose here requires no more than the knowledge of the empirical intermolecular potentials for the species at play as well as the elementary reaction path among them. Bond formation during the simulation is performed by changing the inter-atomic potentials from those of the non-bonded species to those of the bonded ones, and a reaction is deemed to occur by the distance separating the bond forming atoms. In this way the energy barrier for a reaction is no longer considered; the estimation of the reaction rate, however, is possible by introducing the principles of the transition state theory. The simplicity of the present scheme to simulate chemical reactions enables it to be used in large-scale MD simulations involving a large number of simultaneous chemical reactions and to evaluate kinetic parameters. In this paper, the basic theory of the method is presented and application to simple equiatomic reaction system where the reaction rates were estimated was illustrated.     

Intensification of chemical reactions in aqueous solutions upon irradiation with an accelerated electron beam

The results from investigating the effect of radiation exposure on the rate of chemical reactions in acid aqueous solutions of Fe²⁺, which are widely used in hydrometallurgy of nonferrous metals to extract the target components into solution from ores, are presented. The potential of the applied method is demonstrated.     

Method of solving the inverse problem of chemical kinetics for catalytic reactions in which each step involves main reactants

A new method is suggested for solving the inverse problem of chemical kinetics. This method provides means to derive, from unsteady-state experimental data, the rate constants of catalytic reactions in which each step involves at least one main reactant. Examples of applying this method to two- and three-step reactions are presented.     

ϑ-Sensors: A new concept for advanced solid-state ionic gas sensors

A new principle of solid state electrochemical sensors based on the kinetics of controlled chemical reactions of the gas with the electroactive species of a solid electrolyte is presented and demonstrated for the measurement of CO₂ partial pressures. The reaction may be for many gases modified by the formation of intermediate product phases.A periodic voltage or current signal is applied to the electrolyte. The current corresponds to alternating chemical reactions at the electrodes in contact with the gas. The new concept avoids the often complicated application of reference electrodes and shows characteristic signals for different types of gases and allows in this way to overcome problems of cross-sensitivity. The experimental results show high accuracy with an approximately linear relationship between the current and the CO₂ partial pressure and also show extraordinarily fast response.     

Experimental studies of phase transitions in isolated metal clusters

The reactions of isolated, neutral transition metal clusters with small molecules are used to probe cluster structure and to identify changes in structure with cluster size. Examples are presented of reactivity, adsorbate uptake, and product composition studies. The general conclusion is that transition metal clusters seem to have structure (are “solid”) under typical experimental conditions, and that their structure, i.e., the way the atoms pack, can change many times in the growth sequence from small clusters to bulk metal. These phase changes are often accompanied by dramatic changes in both chemical and physical properties. Evidence is presented for the existence of isomers of certain cluster sizes for some metals. In a few cases, the chemical evidence can be used to propose possible cluster structure; this is illustrated for iron and nickel clusters.