Stochastic simulation of catalytic surface reactions in the fast diffusion limit

The master equation of a lattice gas reaction tracks the probability of visiting all spatial configurations. The large number of unique spatial configurations on a lattice renders master equation simulations infeasible for even small lattices. In this work, a reduced master equation is derived for the probability distribution of the coverages in the infinite diffusion limit. This derivation justifies the widely used assumption that the adlayer is in equilibrium for the current coverages and temperature when all reactants are highly mobile. Given the reduced master equation, two novel and efficient simulation methods of lattice gas reactions in the infinite diffusion limit are derived. The first method involves solving the reduced master equation directly for small lattices, which is intractable in configuration space. The second method involves reducing the master equation further in the large lattice limit to a set of differential equations that tracks only the species coverages. Solution of the reduced master equation and differential equations requires information that can be obtained through short, diffusion-only kinetic Monte Carlo simulation runs at each coverage. These simulations need to be run only once because the data can be stored and used for simulations with any set of kinetic parameters, gas-phase concentrations, and initial conditions. An idealized CO oxidation reaction mechanism with strong lateral interactions is used as an example system for demonstrating the reduced master equation and deterministic simulation techniques.     

Liquid-vapor density profiles from equilibrium limit of diffusion equation for interacting particles

Liquid-vapor density profiles are derived from the equilibrium limit of diffusion equation for interacting particles. These profiles are in good agreement with classical hyperbolic tangent relation. For simple Lennard-Jones fluids, predicted density distributions agree with computer simulation data, but have a slightly sharper transition zone. For alkali metals with Lennard-Jones-like potentials, the new equations predict a very good average distribution with quite satisfactory agreement with Monte Carlo simulation results. For liquid metals and water surfaces, accurate interfacial profile predictions also can be achieved by using effective two-body potential data instead of Lennard-Jones parameters.     

Spatial neutron flux distributions around A 14 MeV neutron generator

The neutron flux distribution in the vicinity of 30, 20 and 10 mm diameter targets is measured by irradiating concentric ring-type iron monitors at different distances from the target and counting the induced⁵⁶Mn activity. Considering the many uncertainties, satisfactory agreement was found between theory and experiment. Aspirant of the N.F.W.O.     

Computing stationary distributions in equilibrium and nonequilibrium systems with forward flux sampling

We present a method for computing stationary distributions for activated processes in equilibrium and nonequilibrium systems using forward flux sampling. In this method, the stationary distributions are obtained directly from the rate constant calculations for the forward and backward reactions; there is no need to perform separate calculations for the stationary distribution and the rate constant. We apply the method to the nonequilibrium rare event problem proposed by Maier and Stein, to nucleation in a 2-dimensional Ising system, and to the flipping of a genetic switch.     

Coherent control of indirect photofragmentation in the weak-field limit: control of transient fragment distributions

We demonstrate theoretically that laser-induced coherent quantum interference control of asymptotic states of dissociating molecules is possible--even in the (one-photon) weak-field limit starting from a single vibrational eigenstate--when resonances are in play. This is illustrated for the NaI molecule, where it is shown that the probability of observing atomic fragments as well as the distribution of their relative momenta can be changed by a phase modulated pulse with a fixed bandwidth. This type of control is restricted to finite times during the indirect fragmentation.     

Classical differential cross sections and rotational energy transfer distributions for realistic ion—molecule potentials in the CPST limit

Classical differential cross sections, rotational energy transfer distributions at specified scattering angles and the first moments of the rotational energy transfer distributions are calculated for two ion—molecule systems: K⁺ ?CSCl and Li⁺ ?CO. The deflection angles and change in angular momentum are calculated using classical perturbation scattering theory (CPST). Monte Carlo techniques are then used to calculate the orientation averaged total differential cross sections and the rotational energy transfer distributions. Results are compared with experiment and agreement is found to be satisfactory. These two systems represent two extremes in anisotropy. For Li⁺ ?CO a strong classical rainbow peak is still seen in the differential cross section, while in the K⁺ ?CSCl system the rainbow is complete quenched. In the rotational energy transfer distributions of both systems, rotational rainbow peaks are clearly observed. The calculations also predict a leveling off of the first moment of the rotational energy transfer distribution at high angles, corresponding to the transition to repulsive scattering. On the basis of these results some comments are made on the nature of classical rainbow scattering for anisotropic systems.     

Reorientational correlation functions and memory functions in the J-diffusion limit of the extended rotational diffusion model

New methods for calculating free rotor memory functions and reorientational correlation functions with the J-diffusion model are presented. The methods are much more efficient than previous methods, and have applications in other areas.     

Measuring the unusually slow ionic diffusion in polyaniline via study of yolk-shell nanostructures

We demonstrate a unique capability in partially oxidizing the oligoaniline shell on gold nanoparticles to polyaniline. Because of the solubility difference, the unreacted inner shell section can be selectively dissolved by 2-propanol, giving yolk-shell nanostructures and, thus, making it possible for assessing the oxidized section. The ionic diffusion through the polymer shell is found to be the rate-determining step in the overall process. Conservative estimates show that the diffusion coefficient of AuCl(4)(-) is at least 700 times slower than that of the typical rate values in traditional studies. It is most likely caused by the lack of micropores in the polymer structures. Such mircopores are hard to avoid in preparing polymer membranes by casting or drying of polymers dissolved in organic solvents. We can rule out the presence of irregular pores on the basis of the uniformly oxidized shell section. With the nanoscale shells, the system is sensitive enough to detect minute changes in the shell or small differences among the individual nanoparticles. Even with a small increase in porosity, for example, when the polyaniline shell is swollen using small amounts of DMF (3%, 5%, or 10% in aqueous solutions), the diffusion coefficient of AuCl(4)(-) increases to 4, 11, and 17 times, respectively. Thus, our study demonstrates a new methodology for studying the diffusion of ions in hydrophobic polymers.     

Comparison of flux density distributions from massive or ring-shaped isotopic neutron sources

A comparison has been made between flux density distributions from massive and ring-shaped cylindrical isotopic neutron sources. A considerable gain in direct fast neutron flux is obtained for the latter geometry as well as a neat separation of fast and thermal flux density maxima along the axis of the source. Applications of these favourable properties are discussed.     

Influence of diffusion on pore size distributions determined by xenon porometry

Xenon porometry is a new method for characterization of porous materials. In this method, the material is immersed in a medium, and its properties are studied by means of 129Xe NMR spectra of xenon dissolved in the sample. The method is particularly suitable for the determination of pore size distribution of the material, since the spectra display two signals whose chemical shift is dependent on the pore size. A prerequisite for an accurate determination is the fact that the diffusion of xenon between different pores is slow enough. The diffusion is studied in this work using two-dimensional exchange spectroscopy (2-D EXSY). The spectra measured as a function of the mixing time imply that the exchange is really slow as compared with the NMR time scale, and therefore the distribution of the resonance frequencies indeed represents the pore size distribution.     

Measuring diffusion coefficients of gaseous propane in heavy oil at elevated temperatures

Journal of Thermal Analysis and Calorimetry - Molecular diffusion is an important phenomenon for solvent transport during vapor extraction and hot solvent injection into heavy oil reservoirs....     

Interpretation of electron diffusion coefficient in organic and inorganic semiconductors with broad distributions of states

The carrier transport properties in nanocrystalline semiconductors and organic materials play a key role for modern organic/inorganic devices such as dye-sensitized (DSC) and organic solar cells, organic and hybrid light-emitting diodes (OLEDs), organic field-effect transistors, and electrochemical sensors and displays. Carrier transport in these materials usually occurs by transitions in a broad distribution of localized states. As a result the transport is dominated by thermal activation to a band of extended states (multiple trapping), or if these do not exist, by hopping via localized states. We provide a general view of the physical interpretation of the variations of carrier transport coefficients (diffusion coefficient and mobility) with respect to the carrier concentration, or Fermi level, examining in detail models for carrier transport in nanocrystalline semiconductors and organic materials with the following distributions: single and two-level systems, exponential and Gaussian density of states. We treat both the multiple trapping models and the hopping model in the transport energy approximation. The analysis is simplified by thermodynamic properties: the chemical capacitance, C(mu), and the thermodynamic factor, chi(n), that allow us to derive many properties of the chemical diffusion coefficient, D(n), used in Fick's law. The formulation of the generalized Einstein relation for the mobility to diffusion ratio shows that the carrier mobility is proportional to the jump diffusion coefficient, D(J), that is derived from single particle random walk. Characteristic experimental data for nanocrystalline TiO(2) in DSC and electrochemically doped conducting polymers are discussed in the light of these models.     

Measuring the ionic flux of an electrochemically actuated conducting polymer using modified scanning ion conductance microscopy

We propose a modification of a scanning ion conductance microscope suitable for probing an electrode in an operating electrochemical cell. We demonstrate its use by measuring salt concentration variations near a conducting polymer electrode as the polymer is electrochemically oxidized and reduced. The electrochemical control circuit is opened to isolate the working electrode, at a frequency sufficiently high that the electrode capacitance maintains the electrode potential. The local solution conductivity variations are detected through the probe current during the open-circuit time. We demonstrate two-stage ion exchange during oxidation and reduction of poly(3,4-ethylenedioxythiophene) films that develops strongly with repeated cycling and is correlated with actuation changes. Spatial composition variations of the film, caused by redox current distribution over the surface, and electromigration to the probe tip, causing local solution composition changes, have clear and characteristic effects on the measured transients.     

Measuring the diffusion of noble gases through a porous medium using prompt gamma activation analysis

Detection of anthropogenic noble gas isotopes in the atmosphere is an important indication that a below ground nuclear-test has taken place. Diffusion plays a critical role in the transport of these gases through the geological media to the surface where they can be detected. Better techniques are need with which to study the diffusion of noble gases through porous systems. Here we demonstrate the suitability of using prompt gamma activation analysis to measure the time dependent concentration of argon as a result of its diffusion through a porous medium that is saturated with nitrogen at atmospheric pressure. The experiments were conducted in a 1 m long tube, 10 cm diameter, and packed with fine SiO₂ sand. Prompt gamma activation analysis was used to measure the concentration of argon within the experimental system as a function of time.     

Increasing the speed limit for hole transport in DNA

Transport of positive charge or holes in DNA occurs via a thermally activated multi-step hopping mechanism. The fastest hopping rates reported to date are those for repeating poly(purine) sequences in which hopping occurs via a random walk mechanism with rate constants of k(hop) = 4.3 × 10(9) s(-1) for poly(dG) and 1.2 × 10(9) s(-1) for poly(dA). We report here the dynamics of charge separation in DNA conjugates possessing repeating 7-deazaadenine (dzA) sequences. These data provide an estimated value of k(hop) = 4.2 × 10(10) s(-1) for poly(dzA), an order of magnitude faster than for poly(dG).     

An analysis of semi-finite linear diffusion in the long-time limit: A clarification of the response to periodic sinusoidal and square-wave perturbations

Low-frequency peturbation of the surface concentration in a system exhibiting semi-finite diffusion can be modeled by an equivalent circuit comprising a resistor (R_s) and capacitor (C_s) connected in series. Mathematical analyses of other workers (supported by our own computer simulations) of sinusoidal and square-wave perturbations show that the values of R_s and C_s, though frequency independent (in the low frequency domain), depend upon the nature of the perturbation. This apparent paradox is resolved by noting that even a low-frequency square wave has high frequency components which modify the composition of the system; a sine wave is pure frequency.