Biomedical subjects

Eric R Bittner

Publications and source records attributed to Eric R Bittner.

12 recordsLinked to original sources

PubMed2007

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 semiconducting 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 (C==C 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 method.

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PubMed2006

Exciton regeneration dynamics in model donor-acceptor polymer heterojunctions.

We present a theoretical investigation on various semiconducting materials that exhibit photovoltaic and photoluminecent properties. Our focus is on the relaxation dynamics that occur upon photoexcitation of a couple of type II donor-acceptor heterojunction systems. In addition to the diabatic approach our two-band exciton model employs to study the phonon-assisted relaxations, we adopt the Marcus-Hush semiclassical method to incorporate lattice reorganization. This enables us to look at the state-to-state interconversions from the relaxed excited-state configurations in model polymer blends of poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine) (PFB) with F8BT. Our results stress the significance of vibrational relaxation in the state-to-state relaxation. Furthermore, while a tightly bound charge-transfer state (exciplex) remains the lowest excited state, we show that the regeneration of the optically active lowest excitonic state in TFB:F8BT is possible via the existence of a steady state.

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PubMed2006

Time-convolutionless master equation for mesoscopic electron-phonon systems.

The time-convolutionless master equation for the electronic populations is derived for a generic electron-phonon Hamiltonian. The equation can be used in the regimes where the golden rule approach is not applicable. The equation is applied to study the electronic relaxation in several models with the finite number of normal modes. For such mesoscopic systems the relaxation behavior differs substantially from the simple exponential relaxation. In particular, the equation shows the appearance of the recurrence phenomena on a time scale determined by the slowest mode of the system. The formal results are quite general and can be used for a wide range of physical systems. Numerical results are presented for a two level system coupled to Ohmic and super-Ohmic baths, as well as for a model of charge-transfer dynamics between semiconducting organic polymers.

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PubMed2006

Lattice theory of ultrafast excitonic and charge-transfer dynamics in DNA.

We propose a lattice fermion model suitable for studying the ultrafast photoexcitation dynamics of ordered chains of deoxyribonucleic acid (DNA) polymers. The model includes both parallel (intrachain) and perpendicular (cross-chain) terms as well as diagonal cross-chain terms coupling neighboring bases. The general form of our Hamiltonian is borrowed from lattice fermion models of quantum chromodynamics. The band structure for this model can be determined analytically, and we use this as a basis for computing the singly excited states of the poly(dA)poly(dT) DNA duplex using configuration interaction singles. Parameters for the model are taken from various literature sources and our own ab initio calculations. Results indicate that the excited states consist of a low energy band of dark charge-separated states followed by separate bands of delocalized excitonic states which have weak mixing between the thymidine and adenosine sides of the DNA chain. We then propose a lattice exciton model based upon the transition dipole-dipole couplings between bases and compare the analytical results for the survival probability of an initially localized exciton to exact numerical results. The results herein underscore the competing role of excitonic and charge-transfer dynamics in these systems.

DNA↗

PubMed2006

A self-consistent field quantum hydrodynamic approach for molecular clusters.

We present a novel self-consistent orbital-free method useful for quantum clusters. The method uses a hydrodynamical approach based on the de Broglie-Bohm description of quantum mechanics to satisfy an orbital-free density functional-like Euler-Lagrange equation for the ground state of the system. In addition, we use an information theoretical approach to obtain the optimal density function derived from a series of statistical sample points in terms of density approximates. These are then used to calculate an approximation to the quantum force in the hydrodynamic description. As a demonstration of the utility and flexibility of the approach, we compute the lowest-energy structures for small rare-glass clusters of argon and neon with 4, 5, 13, and 19 atoms. Extension to more complex systems is straightforward.

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PubMed2006

Exactly solvable approximating models for Rabi Hamiltonian dynamics.

The interaction between an atom and a one mode external driving field is an ubiquitous problem in many branches of physics and is often modeled using the Rabi Hamiltonian. In this paper we present a series of analytically solvable Hamiltonians that approximate the Rabi Hamiltonian and compare our results to the Jaynes-Cummings model which neglects the so-called counter-rotating term in the Rabi Hamiltonian. Through a unitary transformation that diagonalizes the Jaynes-Cummings model, we transform the counter-rotating term into separate terms representing several different physical processes. By keeping only certain terms, we can achieve an excellent approximation to the exact dynamics within specified parameter ranges.

Biophysical Phenomena↗

PubMed2005

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, Commun. Math. Phys. 48, 119 (1976)] 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 satisfies 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.

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PubMed2005

Quantum hydrodynamics: capturing a reactive scattering resonance.

The hydrodynamic equations of motion associated with the de Broglie-Bohm formulation of quantum mechanics are solved using a meshless method based upon a moving least-squares approach. An arbitrary Lagrangian-Eulerian frame of reference and a regridding algorithm which adds and deletes computational points are used to maintain a uniform and nearly constant interparticle spacing. The methodology also uses averaged fields to maintain unitary time evolution. The numerical instabilities associated with the formation of nodes in the reflected portion of the wave packet are avoided by adding artificial viscosity to the equations of motion. A new and more robust artificial viscosity algorithm is presented which gives accurate scattering results and is capable of capturing quantum resonances. The methodology is applied to a one-dimensional model chemical reaction that is known to exhibit a quantum resonance. The correlation function approach is used to compute the reactive scattering matrix, reaction probability, and time delay as a function of energy. Excellent agreement is obtained between the scattering results based upon the quantum hydrodynamic approach and those based upon standard quantum mechanics. This is the first clear demonstration of the ability of moving grid approaches to accurately and robustly reproduce resonance structures in a scattering system.

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PubMed2005

Exciton dissociation dynamics in model donor-acceptor polymer heterojunctions. I. Energetics and spectra.

In this paper we consider the essential electronic excited states in parallel chains of semiconducting polymers that are currently being explored for photovoltaic and light-emitting diode applications. In particular, we focus upon various type II donor-acceptor heterojunctions and explore the relation between the exciton binding energy to the band offset in determining the device characteristic of a particular type II heterojunction material. As a general rule, when the exciton binding energy is greater than the band offset at the heterojunction, the exciton will remain the lowest-energy excited state and the junction will make an efficient light-emitting diode. On the other hand, if the offset is greater than the exciton binding energy, either the electron or hole can be transferred from one chain to the other. Here we use a two-band exciton to predict the vibronic absorption and emission spectra of model polymer heterojunctions. Our results underscore the role of vibrational relaxation and suggest that intersystem crossings may play some part in the formation of charge-transfer states following photoexcitation in certain cases.

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PubMed2005

Photoconductivity and current producing states in molecular semiconductors.

We present a methodology for computing photocurrent production in molecular semiconducting molecules. Our model combines a single-configuration interaction picture with the nonequilibrium Green's function approach to compute the current response of a molecular semiconducting wire following excitation. We give detailed analysis of the essential excitonic, charge transfer, and dipole states for poly-(phenylenevinylene) chains of length 32 and 48 repeat units under an electric field bias and use this to develop a reduced dimensional tunneling model which accounts for chain-length and field-dependent behavior. In this paper, we consider the decay of an excited electron/hole state on a molecular wire under bias attached to semiconducting leads at either end. We find that the current produced by the decay of an excitation depends not only upon the lifetime of the state, as given by the imaginary part of its complex eigenvalue, but also upon the net charge on terminal ends of the molecule. We also find that weakly bound electron/hole charge-transfer pairs can decay into the continuum via field induced tunneling and produce a net current whereas excitonic states decay via tunneling but give no net current contribution.

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PubMed2003

Spin-dependent electron-hole capture kinetics in luminescent conjugated polymers.

The recombination of electron-hole pairs injected in extended conjugated systems is modeled as a multistep interconversion relaxation in monoexcited electronic state space, mediated by electron-phonon coupling. The computed ratio of triplet-to-singlet exciton formation times r=tau(T)/tau(S) increases from 0.9 for a model dimer to 2.5 for a 32-unit chain, in good agreement with recent experiments. We rationalize the conjugation-length dependence of r in terms of spin-specific energetics and mutual vibronic coupling of the excited states.

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PubMed2002

Quantum dissipation in unbounded systems.

In recent years trajectory based methodologies have become increasingly popular for evaluating the time evolution of quantum systems. A revival of the de Broglie--Bohm interpretation of quantum mechanics has spawned several such techniques for examining quantum dynamics from a hydrodynamic perspective. Using techniques similar to those found in computational fluid dynamics one can construct the wave function of a quantum system at any time from the trajectories of a discrete ensemble of hydrodynamic fluid elements (Bohm particles) which evolve according to nonclassical equations of motion. Until very recently these schemes have been limited to conservative systems. In this paper, we present our methodology for including the effects of a thermal environment into the hydrodynamic formulation of quantum dynamics. We derive hydrodynamic equations of motion from the Caldeira-Leggett master equation for the reduced density matrix and give a brief overview of our computational scheme that incorporates an adaptive Lagrangian mesh. Our applications focus upon the dissipative dynamics of open unbounded quantum systems. Using both the Wigner phase space representation and the linear entropy, we probe the breakdown of the Markov approximation of the bath dynamics at low temperatures. We suggest a criteria for rationalizing the validity of the Markov approximation in open unbound systems and discuss decoherence, energy relaxation, and quantum/classical correspondence in the context of the Bohmian paths.

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