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  • 1
    Electronic Resource
    Electronic Resource
    Random growth statistics of long-chain single molecule poly-(p-phenylene vinylene) (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 9585-9593 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Structures of poly-(p-phenylene vinylene) (PPV) were generated using a random growth algorithm. The algorithm assumes that the configuration of a part of the polymer (a few monomers length) can be sampled from the configuration of a PPV trimer. The probabilities of the configurations of the trimer are taken as the Boltzmann weight of the energies. We constructed several types of polymers with different numbers of cis-defects which were added to the polymer either uniformly or randomly distributed within the entire polymer. Polymer characteristics, such as conjugation length, end-to-end distance, and radius of gyration, were also calculated. The trends of these characteristics were found to be inversely proportional with the number of cis-defects in the polymer. Although average conjugation lengths are generally independent of the distribution of cis-defects, the morphology of the polymer is dependent on cis-defect distribution. This suggests that conformational disorder rather than cis-defect density is the determining factor in exciton localization and diffusion in these systems. Finally, we derive a simple model similar to the Ising model that relates the energy needed to break conjugation to the average conjugation length. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1413975
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  • 2
    Electronic Resource
    Electronic Resource
    Integrating the quantum Hamilton–Jacobi equations by wavefront expansion and phase space analysis (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 8888-8897 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we report upon our computational methodology for numerically integrating the quantum Hamilton–Jacobi equations using hydrodynamic trajectories. Our method builds upon the moving least squares method developed by Lopreore and Wyatt [Phys. Rev. Lett. 82, 5190 (1999)] in which Lagrangian fluid elements representing probability volume elements of the wave function evolve under Newtonian equations of motion which include a nonlocal quantum force. This quantum force, which depends upon the third derivative of the quantum density, ρ, can vary rapidly in x and become singular in the presence of nodal points. Here, we present a new approach for performing quantum trajectory calculations which does not involve calculating the quantum force directly, but uses the wavefront to calculate the velocity field using mv=(bold del)S, where S/(h-dash-bar) is the argument of the wave function ψ. Additional numerical stability is gained by performing local gauge transformations to remove oscillatory components of the wave function. Finally, we use a dynamical Rayleigh–Ritz approach to derive ancillary equations-of-motion for the spatial derivatives of ρ, S, and v. The methodologies described herein dramatically improve the long time stability and accuracy of the quantum trajectory approach even in the presence of nodes. The method is applied to both barrier crossing and tunneling systems. We also compare our results to semiclassical based descriptions of barrier tunneling. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1319987
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  • 3
    Electronic Resource
    Electronic Resource
    A quantum molecular dynamics study of exciton self-trapping in conjugated polymers: Temperature dependence and spectroscopy (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 7684-7692 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We examine the dynamics of exciton self-trapping in conjugated polymer systems using mixed quantum-classical molecular dynamics. The model treats the exciton as a two-dimensional quantum mechanical wave function representing a particle/hole quasiparticle interacting with a classical vibrational lattice [M. N. Kobrak and E. R. Bittner, J. Chem. Phys. 112, 5399 (2000)]. We show that the dynamics are influenced strongly by thermal disorder in the lattice, and that there is a dramatic change in the self-trapping mechanism as temperature increases. At low temperatures, the rate of localization is limited by the time required for the vibrational lattice to respond to the creation of the particle–hole pair, while at higher temperatures thermal disorder permits localization on time scales limited primarily by electronic response. We simulate the time-resolved fluorescence spectrum for the model system, and compare the temperature dependence of the spectrum to recent time-resolved fluorescence upconversion studies on polydiacetylene derivatives. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481379
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  • 4
    Electronic Resource
    Electronic Resource
    Quantum relaxation dynamics using Bohmian trajectories (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 6309-6316 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a new Bohmian trajectory based treatment of quantum dynamics suitable for dissipative systems. Writing the density matrix in complex-polar form, we derive and define quantum equations of motion for Liouville-space trajectories for a generalized system coupled to a dissipative environment. Our theory includes a vector potential which mixes forward and backwards propagating components and pulls coherence amplitude away from the diagonal region of the density matrix. Quantum effects enter via a double quantum potential, Q(x,y), which is a measure of the local curvature of the density amplitude. We discuss how decoherence can be thought of as a balancing between localization brought on by contact with a thermal environment which increases the local curvature of the density matrix and delocalization due to the internal pressure of the quantum force which seeks to minimize the local curvature. The quantum trajectories are then used to propagate an adaptive Lagrangian grid which carries the density matrix, ρ(x,y), and the action, A(x,y), thereby providing a complete hydrodynamiclike description of the dynamics. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1394747
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  • 5
    Electronic Resource
    Electronic Resource
    Erratum: "A dynamic model for exciton self-trapping in conjugated polymers. II. Implementation" [J. Chem. Phys. 112, 5410 (2000)] (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 891-891 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481866
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  • 6
    Electronic Resource
    Electronic Resource
    A dynamic model for exciton self-trapping in conjugated polymers. I. Theory (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 5399-5409 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this article we present a time-dependent quantum/classical model for the dynamics of excitons in photoexcited conjugated polymer systems. Within this model, the excitation is treated quantum mechanically as a fully correlated electron/hole pair that interacts self-consistently with the vibrational motions of the polymer lattice. Spin and spatial symmetry considerations allow us to segregate singlet and triplet components into odd and even parity manifolds upon exchange of coordinates. We adapt the parameters used in various semiempirical models to produce a Hamiltonian that is continuous in the two-dimensional space and integrate the coupled equations of motion for the exciton wave function and the lattice. Ths approach includes the electronic correlations necessary to reproduce excitonic behavior and allows the study of both singlet and triplet exciton states. In this article, we use the approach to study the structure and formation of a self-trapped exciton at T=0 K starting from an initially free state. Within our model, the net stabilization of the singlet exciton upon localization is 238 cm−1 indicating that self-trapped exciton states in these systems are weakly bound relative to a free exciton. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481109
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  • 7
    Electronic Resource
    Electronic Resource
    A dynamic model for exciton self-trapping in conjugated polymers. II. Implementation (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 5410-5419 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We examine the electronic and vibrational dynamics of a model conjugated polymer using a particle–hole treatment for electronic excitation described in Ref. . We observe the transition from a delocalized free exciton state to a self-trapped exciton, and compare the characteristics of the process of localization to those predicted by existing theories. We find that the reaction path to self-trapping involves a well-defined intermediate state, complicating the process of cooling for the self-trapped exciton. We also find that high-energy excitons do not couple strongly to the lattice, and therefore do not self-trap. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481126
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  • 8
    Electronic Resource
    Electronic Resource
    Franck–Condon spectra and electron-libration coupling in para-polyphenyls (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 5863-5870 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Proceeding from quantum-chemical potential energy surfaces, we compute the absorption and fluorescence spectra of conventional and ladder-type para-phenylene oligomers (OPP and OLPP) with up to 7 benzene rings. Electronically excited states are addressed by means of extended configuration interaction within a standard molecular all-valence-electron semiempirical Hamiltonian. Adiabatic excitation energies, interstate distortions and normal modes are used to compute Franck–Condon band shapes with rigorous consideration of vibrational structure. Theoretical spectra agree with the experiment and rationalize the striking disparities in the linear optical response of OPP and OLPP. Whereas electron–phonon coupling in OLPP is essentially restricted to the carbon–carbon bond-stretching modes, photoexcitation, and emission processes in OPP are followed by significant relaxations in ring-torsional degrees of freedom. The broadening of spectra of OPP, especially pronounced in absorption, and the large Stokes shift between absorption and emission are traced back to the strong coupling of electronic excitations and low-frequency libration motions. The results highlight the importance of ring-torsional flexibility in conjugated polymers. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1351853
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  • 9
    Electronic Resource
    Electronic Resource
    Quantum tunneling dynamics using hydrodynamic trajectories (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 9703-9710 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we compute quantum trajectories arising from Bohm's causal description of quantum mechanics. Our computational methodology is based upon a finite-element moving least-squares method (MWLS) presented recently by Wyatt and co-workers [Lopreore and Wyatt, Phys. Rev. Lett. 82, 5190 (1999)]. This method treats the "particles" in the quantum Hamilton–Jacobi equation as Lagrangian fluid elements that carry the phase, S, and density, ρ, required to reconstruct the quantum wave function. Here, we compare results obtained via the MWLS procedure to exact results obtained either analytically or by numerical solution of the time-dependent Schrödinger equation. Two systems are considered: first, dynamics in a harmonic well and second, tunneling dynamics in a double well potential. In the case of tunneling in the double well potential, the quantum potential acts to lower the barrier, separating the right- and left-hand sides of the well, permitting trajectories to pass from one side to another. However, as probability density passes from one side to the other, the effective barrier begins to rise and eventually will segregate trajectories in one side from the other. We note that the MWLS trajectories exhibited long time stability in the purely harmonic cases. However, this stability was not evident in the barrier crossing dynamics. Comparisons to exact trajectories obtained via wave packet calculations indicate that the MWLS trajectories tend to underestimate the effects of constructive and destructive interference effects. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481607
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  • 10
    Electronic Resource
    Electronic Resource
    Many-body effects and resonances in universal quantum sticking of cold atoms to surfaces (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 2614-2621 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The role of shape resonances and many-body effects on universal quantum sticking of ultracold atoms onto solid surfaces is examined analytically and computationally using an exactly solvable representation of the Dyson equation. We derive the self-energy renormalization of the transition amplitude between an ultracold scattering atom and the bound states on the surface in order to elucidate the role of virtual phonon exchanges in the limiting behavior of the sticking probability. We demonstrate that, to first order in the interactions for finite ranged atom–surface potentials, virtual phonons can only rescale the strength of the atom–surface coupling and do not rescale the range of the coupling. Thus, universal sticking behavior at ultralow energies is to be expected for all finite ranged potentials. We demonstrate that the onset of the universal sticking behavior depends greatly on the position of the shape resonance of the renormalized potential and for sufficiently low energy shape resonances, deviations from the universal s(E)∝(square root of)E can occur near these energies. We believe that this accounts for many of the low energy sticking trends observed in the scattering of submillikelvin H atoms from superfluid 4He films. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468692
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