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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. 5918-5934 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Dynamic percolation theory is used to obtain the tracer diffusion coefficient in binary mixtures of "noninteracting'' lattice gas (with only the blocking interactions, i.e., double occupancy of a lattice site is forbidden) within the effective medium approximation (EMA). Our approach is based on regarding the background particles as a changing random environment. The result is expressed in terms of two fluctuation time parameters which we attempt to determine self-consistently. We compare two possible choices for these parameters which are consistent with our former results for the single component system. The resulting tracer diffusion coefficient for both choices compares well with numerical simulations whenever single bond EMA is expected to be reliable. Comparison is also made with the theoretical results of Sato and Kikuchi [Phys. Rev. B 28, 648 (1983)] and discrepancies between both theories are discussed.
    Type of Medium: Electronic Resource
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  • 2
    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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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Dynamic percolation theory is adapted to obtain diffusion coefficients for particles with blocking interactions on incomplete lattices, within an effective medium approximation (EMA). The substrate lattices have static bond disorder. The motion of a tracer particle among identical background particles is regarded as particle motion in a fluctuating random environment superimposed on the statically disordered lattice; the fluctuations results from the motion of the background particles. Several schemes for incorporating the effect of the background particles are discussed, all relating their motion in different ways to the macroscopic diffusion. Comparisons with Monte Carlo simulations are performed for two-dimensional simple square and three-dimensional simple cubic lattices. In the range where single bond EMA is thought to be reliable, good agreement with the simulation is achieved.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 1292-1293 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 2242-2256 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics and spectra of negatively charged water clusters, containing a single excess electron, are investigated. In our calculations the atomic water constituents of the clusters are treated classically while the excess electron is described quantum mechanically using the fast Fourier transform algorithm to solve the Schrödinger equation. Information about ground and excited electronic states corresponding to the equilibrium, finite temperature, ground-state ensemble configurations can be obtained by solving for these states for given nuclear configurations generated via quantum mechanical path-integral molecular dynamics simulations. As an alternative, more efficient way, we introduce the adiabatic simulation method which consists of propagating the nuclei in real time while concurrently annealing the electronic wave functions to their correct values corresponding to the instantaneous, dynamically generated nuclear configurations. The resulting trajectories can be used for analyzing nuclear motion in the ground electronic state as well as for calculating energy distributions for the ground and excited electronic states and the (vertical) excitation line shape. We study the cluster size effect on these quantities, and in particular, by comparing results for(H2O)−64 and (H2O)−128, we conclude that the vertical ionization potential increases while the vertical excitation energy to the bound excited state decreases for larger cluster sizes. For the smallest negatively charged water cluster (H2O)−2, where adiabatic separation of electronic and nuclear motion does not hold, we simulate the time evolution in the TDSCF approximation. The dynamics reveals the close correlation between the electronic binding energy and the cluster dipole, and provides information on intramolecular and intermolecular vibrational motion. Comparison of vibrational density of states evaluated from the nuclear trajectories of the negatively charged and the neutral dimer shows that most of the modes associated with intermolecular motions shift to the red upon electron attachment (a few modes, possibly those associated directly with the magnitude of the total molecular dipole, shift to the blue).
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3516-3523 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper, we present an electrostatic theory of the effect of small atomic cluster environment on molecular oscillator strengths and spectra. The molecular dipole is represented by a classical oscillating charge distribution and the cluster atoms by spherical polarizable particles. From our general theoretical results, we calculate the effect of cluster size and geometry on the molecular radiative lifetime and the spectral shift. These properties exhibit a considerable sensitivity to the cluster and molecular geometry, i.e., the cluster size and structure, the molecule–atom(s) distance(s), the spatial size of the molecular transition dipole, and molecular orientation within the cluster. The reduced plot of the relative change in the oscillator strength vs the relative frequency shift seems to have universal character and is useful in analyzing and predicting experimental trends. Available experimental results are consistent with the theoretical predictions.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 3557-3564 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theory that is able to account for electrostatic effects in microscopic situations is formulated in terms of the path integral method. The theory relates the solution of the Poisson equation to the propagator of the diffusion equation. Applications are made to some typical problems of interest, such as the solvation energy of an ion in a solution and to the electrical properties of a diffuse surface.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 2734-2749 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The non-Markovian theory of activated rate processes developed by Carmeli and Nitzan is applied to investigate unimolecular reactions in condensed phases with particular emphasis on the molecular size (number of internal degrees of freedom) dependence of the effect of solvent friction on the reaction rate. The model consists of one reaction coordinate coupled to n−1 nonreactive modes. The molecule solvent interaction is treated within the context of the generalized Langevin equation. The reaction dynamics may be roughly described as two consecutive processes: the well (energy diffusion) dynamics where it is assumed that fast intramolecular vibrational relaxation and slower overall molecular energy diffusion dominate the process, and the barrier dynamics where it is assumed that the motion along the reaction coordinate is only weakly coupled to the nonreactive modes. This model leads to a result for the reaction rate which, as in the one-dimensional case, is obtained as the inverse of the sum of two times: the barrier crossing time and the energy diffusion time. The latter is very sensitive to molecular size and becomes extremely short for large molecules. Correspondingly, the Kramers turnover region is predicted to occur for low molecular weight solvent in the high pressure gas phase, as was found in recent experiments. For higher viscosities the rate is dominated by the barrier crossing time with a large (larger for larger molecules) transition state rate plateau and with a falloff for high viscosities. Recent interesting results by Straub et al. which have pointed out the dominance of spatial diffusion in the well for extremely high viscosities (overdamped well motion) are argued to be irrelevant for most molecular situations.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 3915-3927 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A framework for estimating heating and expected temperature rise in current carrying molecular junctions is described. Our approach is based on applying the Redfield approximation to a tight binding model for the molecular bridge supplemented by coupling to a phonon bath. This model, used previously to study thermal relaxation effects on electron transfer and conduction in molecular junctions, is extended and used to evaluate the fraction of available energy, i.e., of the potential drop, that is released as heat on the molecular bridge. Classical heat conduction theory is then applied to estimate the expected temperature rise. For a reasonable choice of molecular parameters and for junctions carrying currents in the nA range, we find the temperature rise to be a modest few degrees. It is argued, however, that using classical theory to describe heat transport away from the junction may underestimate the heating effect. © 2002 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 9205-9208 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The traversal time for tunneling is a measure of the time during which the transmitted particle can be affected by interactions localized in the barrier. The Büttiker–Landauer approach, which estimates this time by imposing an internal clock on the system, has been applied so far for relatively simple one-dimensional models. Here we apply this approach to estimate the traversal time for electron tunneling through a realistic three-dimensional model of a water layer. Observed structure in the energy dependence of times computed reflects the existence of transient tunneling resonances associated with instantaneous water structures. © 2001 American Institute of Physics.
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