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  • 1990-1994  (19)
  • 1950-1954  (1)
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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8187-8195 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The properties of excess hydrated electrons in liquid water, at room temperature, are studied via coupled quantum-classical simulations. In these simulations, the system evolves dynamically on the adiabatic potential energy surface with the electron maintained in the ground state throughout the process. The diffusion constant of the hydrated electron under field-free conditions is found to be the same as that obtained, via the Nernst–Townsend–Einstein relation, from the electron mobility simulated for a system under an electric field of 3.2×106 V/cm, acting on the electron. For larger electric fields, the electron mobility is found to be field dependent. The mode of migration of the excess electron is polaronic in nature and the influence of the intramolecular degrees of freedom of the water molecules on the hydrated electron transport properties is investigated. It is shown that the electron diffusion constant obtained in simulations under field-free conditions with rigid-water molecules [D0e=(3.7±0.7)×10−5 cm2/s] is larger than that obtained from simulations where a flexible-water model potential is employed D0e=(1.9±0.4)×10−5] cm2/s] and smaller than the experimental estimated value obtained from conductivity measurements (4.9×10−5 cm2/s). The difference between the diffusion constants calculated for the two models is correlated with a marked enhancement of the probability of reversal of the direction of motion of the migrating electron in flexible water. The self-diffusion constant of water using the rigid-molecules model [Ds=(3.6±0.4)×10−5 cm2/s] is also larger than that found for the flexible-water molecule model Ds=(2.3±0.2)×10−5] cm2/s], with the latter in agreement with the experimental value (Ds=2.3×10−5 cm2/s). Structural and dynamical aspects of hydrated electron transport are discussed.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 550-560 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Filtration using large pressures is an effective method for removing liquids from a flocculated suspension and creating a high volume fraction filtercake. Recent experimental work exhibits phenomena that are unexplained by previous calculations with nonlinear models. These models are modified and now predict the region of clear liquid and the high concentration of the filtercake observed in filtration at large pressure. The governing equations are based on the assumption that, at sufficiently high volume fractions, a network forms through the aggregation of flocs and possesses a compressive yield stress Py(φ) that depends only on the local volume fraction φ.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1495-1509 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Pressure filtration is an important method for removing liquids from a suspension. Previous work used linear models or applied to stable suspensions. Nonlinear models for flocculated suspensions are studied here. The equations governing the consolidation of flocculated suspensions under the influence of an applied pressure are based on the assumption that when the volume fraction is high enough, the network formed from the aggregation of flocs possesses a compressive yield stress Py(φ) that is a function of local volume fraction φ only. There are two modes of operation of the pressure filter—the fluid flux or the applied pressure is specified—and both of these are studied. The resulting nonlinear partial differential equations involve the time-dependent piston position, and in the case of the suspension being initially unnetworked, another internal moving boundary below which the suspension is networked. The small time behavior of these systems is obtained with an asymptotic method. In general, at later times, the solution can only be found numerically and an algorithm for doing this is discussed. The important parameters and properties of the filter cake are described. The results suggest various ways of controlling the filtration process, which may be useful in the manufacture of ceramics.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The properties of small neutral and positively charged sodium clusters and the fragmentation dynamics of Na++4 are investigated using a simulation technique which combines classical molecular dynamics on the electronic Born–Oppenheimer ground-state potential surface with electronic structure calculations via the local spin-density functional method. Results for the optimal energies and structures of Nan and Na+n (n≤4) are in quantitative agreement with previous studies and experimental data. Fission of Na++4 on its ground state Born–Oppenheimer potential-energy surface, following sudden ionization of selected configurations of an Na+4 (or Na4) cluster, whose vibrational energy content corresponds to 300 K, is found to occur on a picosecond time scale. The preferred fission channel is found to be Na+3+Na+, with an interfragment relative translational kinetic energy of ∼2 eV, and a vibrationally excited Na+3. The dynamics of the fragmentation process is analyzed.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6535-6542 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The initial stages of the evolution of an electron injected into bulk water (at 300 K) and into thin water films (1–4 monolayers) adsorbed on a Pt(111) substrate at 50 K are investigated. It is shown that for electrons injected into bulk water with an initial translational kinetic energy between 1.54 and 6.18 eV (i.e., subexcitation energies), the electron momentum time-correlation function 〈pˆ(0)pˆ(t)〉, decays to zero on a time scale of less than 1 fs, reflecting strong backscattering of the electron by the water molecules. On this time scale the electron propagation in the medium is dominated by elastic processes. Furthermore, during this initial stage the system is well represented by a static aqueous medium. Transmission of electrons injected into thin films of adsorbed water is also dominated by elastic scattering. The dependence of the electron transmission probability on the film thickness and the initial injection energy are in accord with recent experimental results of photoinjected electrons into adsorbed water films.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6226-6238 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Variational calculation based on a continuum dielectric model, and numerical simulations based on the RWK2-M water potential and on a pseudopotential for the electron–water interaction, are used to evaluate excitation energies and optical spectra for bound interior states of an excess electron in water clusters and in bulk water. Additionally, optical data for surface states are obtained from numerical simulations. The simulation approach uses adiabatic dynamics based on the quantum-classical time-dependent self-consistent field (TDSCF) approximation and the fast-Fourier transform (FFT) algorithm for solving the Schrödinger equation. Both approaches predict very weak or no cluster size dependence of the excitation spectrum for clusters that support interior solvated electron states. For an electron attached to the cluster in a surface localization mode, bound excited states exist for most nuclear configurations of clusters down to (H2O)−18, and the corresponding excitation energy is strongly shifted to the red relative to that associated with stable internal states in larger clusters. Binding and excitation energies associated with surface states are about half the value of these quantities for interior states. The present variational continuum dielectric theory is in relatively good agreement with the simulation results on the size dependence of the relative stability of interior states. However, it strongly underestimates the vertical excitation energy of the solvated electron. It is suggested that optical spectroscopy of excess electrons in water clusters could serve as a sensitive probe of the transition from surface to interior localization modes as the number of water molecules in the cluster is increased.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 738-747 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The question of the existence of helical waves in fluid flow through a circular pipe is discussed. In the case of pipe flow forced only by an axial pressure gradient, direct simulations have failed to find any self-sustaining helical waves. The effect of residual swirl in the inlet flow is investigated, by performing linear stability calculations and direct simulations of a forced basic flow. The results are qualitatively similar to that of flow in a rotating pipe, in which stable helical waves are known to exist. These results suggest that residual rotation may be an important mechanism in triggering laminar-turbulent transition in circular pipe flow.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 1937-1949 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Structural and dynamical properties of n-hexadecane films confined between two crystalline solid substrates are investigated using molecular-dynamics simulations for a range of molecule-to-solid interaction strengths, external loads, and film thicknesses. The confined films are infinite in extent in one of the directions parallel to the solid surfaces and finite in the other direction exposing a liquid-to-vacuum interface (i.e., semidroplet configuration). At sufficiently high loads, whose magnitude depends upon the molecule-to-surface interaction strength, layering, i.e., oscillations in the film density in the direction normal to the boundary, occurs accompanied by an enhancement of surface-parallel orientational ordering of the molecules in the interfacial region. In addition, preferential interfacial adsorption of chain ends, as well as molecular conformations enhancing bridging of the two solid surfaces, are observed. The dynamics of thinning, from a three- to a two-layer film, exhibits a marked dependence on the molecule-to-surface interaction strength. At a critical load the relatively strong adsorption system exhibits a sudden collapse to two layers, resulting in lateral stress accumulation, and subsequent spreading via a cooperative mechanism. On the other hand, thinning of the weakly adsorbed film is gradual, with no stress accumulation, accompanied by spreading occurring via diffusion and adsorption of molecules at the liquid-to-vacuum interface.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 1365-1377 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The energetics, structure, and stability of a dielectron solvated in an internal cavity in water clusters, (H2O)−2n, at 300 K are investigated using coupled quantum-classical molecular-dynamics simulations. In these calculations the ground state of the dielectron is calculated concurrently with the atomic configurations using the local-spin-density functional method, and the nuclear degrees of freedom evolve classically on the Born–Oppenheimer potential-energy surface. For n=64 and 128 the internal single-cavity dielectron state is unstable, while for n=256 (as well as in bulk water) it is energetically stable, fluctuating between a compact spherical configuration (e2c) and an elongated ellipsoidal dumbbell-shaped one (e2d). Transitions between these two states of the dielectron are accompanied by structural and orientational transformations of the surrounding water molecules. The induced molecular orientational order is enhanced and is of longer range in (H2O)−2256 than is the case for a solvated single excess electron. By extrapolating our results to the bulk limit we conclude that a spin-paired dielectron state in bulk water, at 300 K, is a stable species relative to two single separated hydrated electrons.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 4997-5013 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new class of cluster-catalyzed reactions is proposed and investigated using extensive molecular-dynamics simulations. These reactions involve the collision between reactants which are embedded in inert gas clusters, whose role is that of a local heat bath (i.e., extended third body). The concept is demonstrated for the reactions [Na4Cl3]+Arq+Cl− (for q=12 and 32) and [Na14Cl12]+2Ar30+Cl− for several initial temperatures, relative translational energies between the reactants, and impact parameters. For associative reactions, i.e., Cl− attachment accompanied by the release of Ar atoms, the dynamics of the reactions, the dependencies of the isomeric structures of the product alkali-halide clusters, and of reaction pathways and branching ratios on reactant size and reaction conditions are systematically studied.
    Type of Medium: Electronic Resource
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