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  • 1
    Electronic Resource
    Electronic Resource
    Cluster diffusivity: Structure, correlation, and scaling (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 9018-9025 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have investigated the diffusion of clusters on a triangular lattice using Monte Carlo simulations. A cluster is defined as a two-dimensional collection of particles which are connected to each other, either directly or indirectly through other particles in the cluster, by nearest-neighbor bonds. Each particle is allowed to hop, with probability αδb/2/(α−δb/2+αδb/2), to a vacant nearest-neighbor site with the constraint that the hop does not break the cluster. The change in the number of bonds is given by δb. The equilibrium clusters are correlated animals with structure controlled by the parameter α. We show that the diffusion coefficient of a cluster can be decomposed into two factors. One is a measure of the weighted length of the "active'' perimeter and the other is a measure of the correlation between pairs of steps taken by the cluster during its walk. The perimeter measure is asymptotically proportional to cluster size N, as anticipated for ramified animals, but it crosses over to N1/2 dependence for smaller compact clusters with α〉1. Our focus is on the accurate determination of the size and structure dependence of the correlation factor, which is more sensitive to statistical fluctuations. As a result, we describe the scaling of the cluster diffusion coefficient with cluster size.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459708
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  • 2
    Electronic Resource
    Electronic Resource
    Inequivalent models of irreversible dimer filling: "Transition state'' dependence (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8397-8398 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Irreversible adsorption of diatomics on crystalline surfaces is sometimes modeled as random dimer filling of adjacent pairs of sites on a lattice. We note that this process can be implemented in two distinct ways: (i) randomly pick adjacent pairs of sites, jj', and fill jj' only if both are empty (horizontal transition state); or (ii) randomly pick a single site, j, and if j and at least one neighbor are empty, then fill j and a randomly chosen empty neighbor (vertical transition state). Here it is instructive to consider processes which also include competitive random monomer filling of single sites. We find that although saturation (partial) coverages differ little between the models for pure dimer filling, there is a significant difference for comparable monomer and dimer filling rates. We present exact results for saturation coverage behavior for a linear lattice, and estimates for a square lattice. Ramifications for simple models of CO oxidation on surfaces are indicated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459273
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  • 3
    Electronic Resource
    Electronic Resource
    Nonequilibrium percolative c(2×2) ordering: Oxygen on Pd(100) (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 3038-3048 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Dissociative adsorption of oxygen on certain (100) metal surfaces has been modeled as random dimer adsorption onto diagonally adjacent empty sites of a square lattice subject to the additional constraint that all six neighboring sites must be empty (the 8-site model). Here we adapt this model to analyze the nonequilibrium c(2×2) ordering recently observed for oxygen on Pd(100) at coverages up to saturation (〉1/4 monolayer), under conditions of low temperature and high pressure where effects of diffusive mobility can be ignored. We do, however, propose that adsorption could be followed immediately by short range transient mobility to dissipate excess energy. We first show how exact master equations for this model can be used to obtain analytic expressions for various local quantities of interest: the probability of an empty 8-site configuration (which determines the sticking coefficient), the c(2×2) island edge or domain boundary densities, etc. They also provide a characterization of, e.g., the asymptotic decay of spatial correlations. Near-percolating (percolative) c(2×2) ordering is readily observed in computer simulations of the saturation state. Through a simple extension of the physical model, we provide a framework for analysis of the large scale characteristics of this ordering via correlated polychromatic percolation theory. Corresponding scaling relations and some real space renormalization group analysis are described. Simulation results for average sizes, the effective dimension, and perimeter length to size ratios, of c(2×2) islands, are also presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453040
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  • 4
    Electronic Resource
    Electronic Resource
    Analytic observations for the d=1+ 1 bridge site (or single-step) deposition model (1991)
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 32 (1991), S. 2918-2922 
    Details
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: Some exact results for a reversible version of the d=1+1 bridge site (or single-step) deposition model are presented. Exact steady-state properties are determined directly for finite systems with various mean slopes. These show explicitly how the asymptotic growth velocity and fluctuations are quenched as the slope approaches its maximum allowed value. Next, exact hierarchial equations for the dynamics are presented. For the special case of "equilibrium growth,'' these are analyzed exactly at the pair-correlation level directly for an infinite system. This provided further insight into asymptotic scaling behavior. Finally, the above hierarchy is compared with one generated from a discrete form of the Kardar–Parisi–Zhang equations. Some differences are described.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.529085
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  • 5
    Electronic Resource
    Electronic Resource
    Irreversible immobile random adsorption of dimers, trimers, ... on 2D lattices (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 2795-2810 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Models where pairs, triples, or larger (typically connected) sets of sites on a 2D lattice "fill'' irreversibly (described here as dimer, trimer, ... filling or adsorption), either randomly or cooperatively, are required to describe many surface adsorption and reaction processes. Since filling is assumed to be irreversible and immobile (species are "frozen'' once adsorbed), even the stationary, saturation state, which is nontrivial since the lattice cannot fill completely, is not in equilibrium. The kinetics and statistics of these processes are naturally described by recasting the master equations in hierarchic form for probabilities of subconfigurations of empty sites. These hierarchies are infinite for the infinite lattices considered here, but approximate solutions can be obtained by implementing truncation procedures. Those used here exploit a shielding property of suitable walls of empty sites peculiar to irreversible filling processes. Accurate results, including saturation coverage estimates, are presented for random filling of dimers, and trimers of different shapes, on various infinite 2D lattices, and for square tetramers on an infinite square lattice.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448279
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  • 6
    Electronic Resource
    Electronic Resource
    Energy sudden dissociative collisions: Structure and applications of factorization relations (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 1855-1865 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In the energy sudden (ES) approximation for nonreactive molecular collisions, there exist factorization relations by which an arbitrary T-matrix element can be predicted as a spectroscopic linear combination of those out of some other, input state. These were first discovered for ground state input but this restriction was later removed. This general form of the spectroscopic factorization relations is straightforwardly extended here to ES dissociative collisions. One finds that in predicting dissociation amplitudes out of some state, it is necessary to use input data out of a higher (energy) bound state. Thus ground state factorization relations cannot be used. The structure of two natural forms of the factorization relation coefficients (equivalent by virtue of ES consistency conditions among T-matrix elements out of a single state) are analyzed in detail for a collinear atom-truncated square-well diatomic oscillator system. Relevance of these results to the prediction of (dissociative) state specific vibrational enhancement/inhibition is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448369
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  • 7
    Electronic Resource
    Electronic Resource
    General factorization relations and consistency conditions in the sudden approximation via infinite matrix inversion (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 1637-1647 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Local, i.e., multiplicative, operators satisfy well-known linear factorization relations wherein matrix elements (between states associated with a complete set of wave functions) can be obtained as a linear combination of those out of the ground state (the input data). Analytic derivation of factorization relations for general state input data results in singular integral expressions for the coefficients, which can, however, be regularized using consistency conditions between matrix elements out of a single (nonground) state. Similar results hold for suitable "symmetry class'' averaged matrix elements where the symmetry class projection operators are "complete.'' In several cases where the wave functions or projection operators incorporate orthogonal polynomial dependence, we show that the ground state factorization relations have a simplified structure allowing an alternative derivation of the general factorization relations via an infinite matrix inversion procedure. This form is shown to have some advantages over previous versions. In addition, this matrix inversion procedure obtains all consistency conditions (which is not always the case from regularization of singular integrals).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449401
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  • 8
    Electronic Resource
    Electronic Resource
    Femtosecond studies of interparticle electron transfer in a coupled CdS–TiO2 colloidal system (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6222-6225 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the direct femtosecond measurements of the electron transfer process from CdS to TiO2 particles in a coupled colloidal system. The electrons initially created in the conduction band and quickly trapped at the liquid–solid interface in aqueous CdS colloids are found to be transferred to the colloidal TiO2 particles with a time constant of 2 ps, resulting in significantly slower electron–hole recombination. These coupled semiconductor colloids provide a potentially useful system for achieving efficient charge separation, which is important for many chemical reactions involving heterogeneous electron transfer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468376
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  • 9
    Electronic Resource
    Electronic Resource
    Lattice-gas model mimicking the NO+CO reaction on Pt(100) (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3234-3242 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Various NO-reduction reactions on Pt(100) exhibit similar dynamical behavior, presumably due to an empty site requirement for NO dissociation. This motivates analysis of a lattice-gas model which incorporates this feature, and which here is chosen to mimic the NO+CO reaction on Pt(100): both reactants adsorb at single empty sites, NO instantaneously dissociates given an adjacent empty site (and nitrogen adatoms thus formed are immediately removed), and adjacent CO and O instantaneously react. We also include desorption of adsorbed NO and CO, but no adspecies diffusion. At lower temperatures where desorption is absent, we show that poisoning occurs with the reaction rate decreasing as RCO2 ∼ e−kt, where k(approximately-greater-than)0 except for equal reactant adsorption rates. The introduction of desorption produces reactive steady states, and (in different regimes) nonequilibrium poisoning transitions, critical points, and transitions to bistability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467570
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  • 10
    Electronic Resource
    Electronic Resource
    Two-Hilbert space formulations of the quantum statistical mechanics of reactive fluids: Dimer formation and decay (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 5991-6003 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A two-Hilbert space formalism is first used to develop a general class of representations for the quantum mechanics of N-particle reactive systems. Here the N-particle Hilbert space HN is supplemented by a larger arrangement channel space CN of vectors with Hilbert space valued components for each N-particle clustering, and an injection mapping of HN vectors into "physical'' CN vectors. Such representations, for which components of the latter vectors carry an appropriate physical clustering interpretation, provide a rigorous and flexible basis for describing the statistical mechanics of reactive fluids, where atoms and molecules are treated on an equal footing (the molecular picture). Corresponding equilibrium multispecies fugacity or virial expansions follow immediately. Here we focus on analysis of the (previously derived) arrangement channel BBGKY hierarchy for a system where recombination and dissociation, as well as exchange reactions, occur. This formulation (coupled with a corresponding scattering theory) automatically suggests a reactive Boltzmann ansatz which incorporates (standard) noninteracting asymptotic dynamics only for two-molecule nonreactive and reactive exchange collisions. In contrast, e.g., with three molecule recombination, two-molecule dynamics for all three pairs is included (as required for a description of recombination via gradual stabilization of metastables). Finally we compare the resulting reduced form of appropriate channel space hierarchy equations, for a process involving dimer formation and decay, with the corresponding kinetic equations of Lowry and Snider.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451513
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