2,208 research outputs found
An effective Hamiltonian approach for Donor-Bridge-Acceptor electronic transitions: Exploring the role of bath memory
We present here a formally exact model for electronic transitions between an
initial (donor) and final (acceptor) states linked by an intermediate (bridge)
state. Our model incorporates a common set of vibrational modes that are
coupled to the donor, bridge, and acceptor states and serves as a dissipative
bath that destroys quantum coherence between the donor and acceptor. Taking the
memory time of the bath as a free parameter, we calculate transition rates for
a heuristic 3-state/2 mode Hamiltonian system parameterized to represent the
energetics and couplings in a typical organic photovoltaic system. Our results
indicate that if the memory time of the bath is of the order of 10-100 fs, a
two-state kinetic (i.e., incoherent hopping) model will grossly underestimate
overall transition rate.Comment: 9 pages, 2 figure
Decoherent Histories and Non-adiabatic Quantum Molecular Dynamics
The role of quantum coherence loss in mixed quantum-classical dynamical
systems is explored in the context of the theory of quantum decoherence
introduced recently by Bittner and Rossky. (J. Chem. Phys. {\bf 103}, 8130
(1995)). This theory, which is based upon the consistent histories
interpretation of quantum mechanics, introduces decoherence in the quantum
subsystem by carefully considering the relevant time and length scales over
which one must consider the effects of phase interference between alternative
histories of the classical subsystem. Such alternative histories are an
integral part of any quantum-classical computational scheme which employ
transitions between discrete quantum states; consequently, the coherences
between alternative histories have a profound effect on the transition
probability between quantum states. In this paper, we review the Bittner-Rossky
theory and detail a computational algorithm suitable for large-scale quantum
molecular dynamics simulations which implements this theory. Application of the
algorithm towards the relaxation of a photoexcited aqueous electron compare
well to previous estimates of the excited state survival time as well as to the
experimental measurements.Comment: 22 pages, 3 figure
Exciton dissociation at donor-acceptor polymer heterojunctions: quantum nonadiabatic dynamics and effective-mode analysis
The quantum-dynamical mechanism of photoinduced subpicosecond exciton
dissociation and the concomitant formation of a charge-separated state at a
TFB:F8BT polymer heterojunction is elucidated. The analysis is based upon a
two-state vibronic coupling Hamiltonian including an explicit 24-mode
representation of a phonon bath comprising high-frequency (CC stretch) and
low-frequency (torsional) modes. The initial relaxation behavior is
characterized by coherent oscillations, along with the decay through an
extended nonadiabatic coupling region. This region is located in the vicinity
of a conical intersection hypersurface. A central ingredient of the analysis is
a novel effective mode representation, which highlights the role of the
low-frequency modes in the nonadiabatic dynamics. Quantum dynamical simulations
were carried out using the multiconfiguration time-dependent Hartree (MCTDH)
method
Quantum Mechanics with Trajectories: Quantum Trajectories and Adaptive Grids
Although the foundations of the hydrodynamical formulation of quantum
mechanics were laid over 50 years ago, it has only been within the past few
years that viable computational implementations have been developed. One
approach to solving the hydrodynamic equations uses quantum trajectories as the
computational tool. The trajectory equations of motion are described and
methods for implementation are discussed, including fitting of the fields to
gaussian clusters.Comment: Prepared for CiSE, Computing in Science and Engineering IEEE/AIP
special issue on computational chemistr
Phonon-driven ultrafast exciton dissociation at donor-acceptor polymer heterojunctions
A quantum-dynamical analysis of phonon-driven exciton dissociation at polymer
heterojunctions is presented, using a hierarchical electron-phonon model
parameterized for three electronic states and 24 vibrational modes. Two
interfering decay pathways are identified: a direct charge separation, and an
indirect pathway via an intermediate bridge state. Both pathways depend
critically on the dynamical interplay of high-frequency C=C stretch modes and
low-frequency ring-torsional modes. The ultrafast, highly non-equilibrium
dynamics is consistent with time-resolved spectroscopic observations
Quantum transport in chains with noisy off-diagonal couplings
We present a model for conductivity and energy diffusion in a linear chain
described by a quadratic Hamiltonian with Gaussian noise. We show that when the
correlation matrix is diagonal, the noise-averaged Liouville-von Neumann
equation governing the time-evolution of the system reduces to the Lindblad
equation with Hermitian Lindblad operators. We show that the noise-averaged
density matrix for the system expectation values of the energy density and the
number density satisfy discrete versions of the heat and diffusion equations.
Transport coefficients are given in terms of model Hamiltonian parameters. We
discuss conditions on the Hamiltonian under which the noise-averaged
expectation value of the total energy remains constant. For chains placed
between two heat reservoirs, the gradient of the energy density along the chain
is linear.Comment: 6 pages, to appear in J. Chem. Phy
Resonance lamp absorption measurement of OH number density and temperature in expansion tube scramjet engine tests
In this paper, we report results of hydroxyl radical and static temperature measurements performed in the General Applied Science Laboratories-NASA HYPULSE expansion tube facility using the microwave resonance lamp absorption technique. Data were obtained as part of a series of hydrogen/air and hydrogen/oxygen combustion tests at stagnation enthalpies corresponding to Mach 17 flight speeds. Data from a representative injector configuration is compared to a full Navier-Stokes CFD solution
Flow establishment in a generic scramjet combustor
The establishment of a quasi-steady flow in a generic scramjet combustor was studied for the case of a time varying inflow to the combustor. Such transient flow is characteristic of the reflected shock tunnel and expansion tube test facilities. Several numerical simulations of hypervelocity flow through a straight duct combustor with either a side wall step fuel injector or a centrally located strut injector are presented. Comparisons were made between impulsively started but otherwise constant flow conditions (typical of the expansion tube or tailored operations of the reflected shock tunnel) and the relaxing flow produced by the 'undertailored' operations of the reflected shock tunnel. Generally the inviscid flow features, such as the shock pattern and pressure distribution, were unaffected by the time varying inlet conditions and approached steady state in approx. the times indicated by experimental correlations. However, viscous features, such as heat transfer and skin friction, were altered by the relaxing inlet flow conditions
Ising spins coupled to a four-dimensional discrete Regge skeleton
Regge calculus is a powerful method to approximate a continuous manifold by a
simplicial lattice, keeping the connectivities of the underlying lattice fixed
and taking the edge lengths as degrees of freedom. The discrete Regge model
employed in this work limits the choice of the link lengths to a finite number.
To get more precise insight into the behavior of the four-dimensional discrete
Regge model, we coupled spins to the fluctuating manifolds. We examined the
phase transition of the spin system and the associated critical exponents. The
results are obtained from finite-size scaling analyses of Monte Carlo
simulations. We find consistency with the mean-field theory of the Ising model
on a static four-dimensional lattice.Comment: 19 pages, 7 figure
The extensive nature of group quality
We consider groups of interacting nodes engaged in an activity as many-body,
complex systems and analyse their cooperative behaviour from a mean-field point
of view. We show that inter-nodal interactions rather than accumulated
individual node strengths dominate the quality of group activity, and give rise
to phenomena akin to phase transitions, where the extensive relationship
between group quality and quantity reduces. The theory is tested using
empirical data on quantity and quality of scientific research groups, for which
critical masses are determined.Comment: 6 pages, 6 figures containing 13 plots. Very minor changes to
coincide with published versio
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