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Quantum furling and unfurling are inelastic transitions between localized and delocalized electronic states. We predict scenarios where these processes govern charge transport through donor-bridge-acceptor molecular junctions. Like in the case of ballistic transport, the resulting currents are nearly independent of the molecular bridge length. However, currents involving quantum furling and unfurling processes can be controlled by the coupling to vibrations in the intra-molecular and the extra-molecular environment, which can be experimentally tuned. Our study is based on rate equations for exchange of energy (bosons) and particles (fermions) between the molecular bridge and its environment. An efficient algorithm is introduced for a compact representation of the relevant rate equations, which utilizes the redundancies in the rate matrix and the sparsity of the creation and annihilation operators in the molecular Fock space. [ABSTRACT FROM AUTHOR]

We explore the transport of fermions through a quantum conductor in the presence of contact vibrations. The latter are coupled to charge transfer between the fermion reservoirs and the conductor but remain inert to the charging state of the conductor itself. We derive explicit expressions for charge transfer rates into and out of the conductor which extend the scope of rate theories of inelastic transport to the presence of contact vibrations. Implementing the theory to a simple model with a uniform vibronic coupling at different contact orbitals, we demonstrate and analyze the effect of such vibrations on the charge current. Asymmetry between contact vibrations at the two reservoirs is shown to induce a pronounced current rectification, especially in the limit of floppy (low frequencies) contacts. At high frequencies, vibrational quantization is shown to suppress the effect, in accord with the increasing contact rigidity. This quantum result requires corrections beyond the classical theory of charge hopping. [ABSTRACT FROM AUTHOR]

It has been known for several decades that the electric current through tunneling junctions is affected by irradiation. In particular, photon-assisted currents by asymmetric irradiation of the two leads was demonstrated and studied extensively in tunneling junctions of different compositions and for different radiation wavelengths. In this work, this phenomenon is revisited in the context of single molecule junctions. Restricting the theoretical discussion to adiabatic periodic driving of one lead with respect to the other within a non-interacting electron formulation, the main features of specific molecules are encoded in the discrete electronic energy levels. The detailed level structure of the molecule is shown to yield new effects in the presence of asymmetric driving of the leads. In particular, when the field-free tunneling process is dominated by a single electronic level, the electric current can be suppressed to zero or flow against the direction of an applied static bias. In the presence of a second electronic level, a directional photo-electric effect is predicted, where not only the magnitude but also the direction of the steady state electric current through the tunneling junction can be changed by a monotonous increase of the field intensity. These effects are analyzed and explained by outlying the relevant theory, using analytic expressions in the wide-band limit, as well as numerical simulations beyond this limit. [ABSTRACT FROM AUTHOR]

Biological electron transfer (ET) is one of the most studied biochemical processes due to its immense importance in biology. For many years, biological ET was explained using the classical incoherent transport mechanism, i.e. sequential hopping. One of the relatively recent major observations in this field is long-range extracellular ET (EET), where some bacteria were shown to mediate electrons for extremely long distances on the micrometer length scales across individual nanowires. This fascinating finding has resulted in several suggested mechanisms that might explain this intriguing EET. More recently, the structure of a conductive G. sulfurreducens nanowire has been solved, which showed a highly ordered quasi-1D wire of a hexaheme cytochrome protein, named OmcS. Based on this new structure, we suggest here several electron diffusion models for EET, involving either purely hopping or several degrees of mixed hopping and coherent transport, in which the coherent part is due to a local rigidification of the protein structure, associated with a decrease in the local reorganization energy. The effect is demonstrated for two closely packed heme sites as well as for longer chains containing up to several dozens porphyrins. We show that the pure hopping model probably cannot explain the reported conductivity values of the G. sulfurreducens nanowire using conventional values of reorganization energy and electronic coupling. On the other hand, we show that for a wide range of the latter energy values, the mixed hopping-coherent model results in superior electron diffusion compared to the pure hopping model, and especially for long-range coherent transport, involving multiple porphyrin sites. [ABSTRACT FROM AUTHOR]

We highlight a new manifestation of quantum interference in transport through single-molecule junctions, owing to different charge transport pathways from the molecule through the contacts into the three-dimensional electrode. Such contact interference can be identified with characteristic features of the current–voltage curve, as dramatic as closing an otherwise open transport channel. Based on analysis of the necessary conditions for observation of this phenomenon, we conclude that this effect should be detectable in relatively small molecules, where a molecular site shares coupling to two electrodes, and an electrode surface site shares coupling to different molecular sites. The importance of the surface-molecule connectivity scheme implies that commonly used reduced models which couple different molecular sites to effectively independent electrodes, fail to account for this effect and must be corrected in order to properly capture contact interference. [ABSTRACT FROM AUTHOR]

Within a generic model, we discuss the possibility of coherent control of charge fluxes in unbiased molecular junctions. The control is induced by resonances between the Rabi frequency due to a pumping laser field and internal characteristic frequencies of pre-designed molecular donor-bridge-acceptor complexes. Two models are considered: a coherently controlled molecular charge pump and a molecular switch. The study generalizes previous consideration of light induced current [M. Galperin and A. Nitzan, Phys. Rev. Lett. 95, 206802 (2005)] and of a molecular electron pump [R. Volkovich and U. Peskin, Phys. Rev. B 83, 033403 (2011)] and accounts for the coherently driven charge transport in an unbiased molecular junction with symmetric coupling to leads. Numerical examples demonstrate the feasibility of the control mechanism for realistic junctions parameters. [ABSTRACT FROM AUTHOR]

Defining the conditions for coherent site-directed transport from an electron donor to a specific acceptor through tunneling barriers in a network of multiple donor/acceptors sites is an important step toward controlling electronic processes in molecular networks. The required analysis is most challenging since the entire network in essentially involved in coherent transport. In this work we introduce an efficient approach for formulating an effective donor/acceptor coupling in terms of the entire network parameters. The approach is based on implementation of Feshbach projection operators to map the entire network Hamiltonian onto a subspace defined by two specific donor and acceptor sites. This nonperturbative approach enables to define regimes of network parameters in which the effective donor-acceptor coupling is optimal. This is demonstrated numerically for simple models of molecular networks. [ABSTRACT FROM AUTHOR]

Possible mechanisms for charge-transport-induced dissociation in donor-bridge-acceptor complexes are studied. Two mechanisms for dissociation at the molecular bridge are captured within a simple model of an anharmonic bridge vibration coupled nonlinearly to an electronic degree of freedom. A direct mechanism is associated with vibronic excitations to the nuclear continuum and an alternative dissociation mechanism involves intermediate quasibound vibrational states (Feshbach resonances). The two different mechanisms of charge-transport-induced dissociation are analyzed and their interplay as a function of the system parameters is examined. A parameter regime is suggested where the phenomenon should be experimentally accessible. [ABSTRACT FROM AUTHOR]

The Landauer-type formulation of current through a molecular junction with electronic-nuclear coupling introduced by Troisi et al. [J. Chem. Phys. 118, 6072 (2003)] is generalized to account for the dependence of the molecule-leads coupling terms on the nuclear coordinates. Although this electronic-nuclear coupling is external to the molecule there is no need to extend the molecular subspace when projection operators are employed for calculations of the current through the junction. A test case of a conductor with vibrating contacts to the leads is studied numerically. It is demonstrated that contact vibrations lead to inelastic contributions to the current and to characteristic features in the I-V curve and its derivatives, similar to the ones observed for internal (molecular) electronic-nuclear coupling. [ABSTRACT FROM AUTHOR]

A model for electron transfer in donor-bridge-acceptor complexes with electronic coupling to nuclear bridge modes is studied using the Redfield formulation. We demonstrate that the transport mechanism through the molecular bridge is controlled by the location of the electronic-nuclear coupling term along the bridge. As the electronic-nuclear coupling term is shifted from the donor/acceptor-bridge contact sites into the bridge, the mechanism changes from kinetic transport (incoherent, thermally activated, and bridge-length independent) to coherent tunneling oscillations. This study joins earlier works aiming to explore the factors which control the mechanism of electronic transport through molecular bridges and molecular wires. [ABSTRACT FROM AUTHOR]

The effect of electronic-nuclear coupling on electronic transport through a complex molecular network is studied. Electronic tunneling dynamics in a network of N donor/acceptor sites, connected by molecular bridges, is shown to be controlled by electronic-nuclear coupling at the bridges. Particularly, electronic coupling to an accepting nuclear mode at the contact site between the donor and the rest of the network is shown to affect the tunneling path selection to specific acceptors. In the “deep” tunneling regime, the network is mapped onto an N-level system using a recursive perturbation expansion, enabling analytical treatment of the electronic dynamics. The analytic formulation is applied for two model systems, demonstrating site-directed tunneling by electronic-nuclear coupling. Numerical simulations suggest that this phenomenon is not limited to the deep tunneling regime. [ABSTRACT FROM AUTHOR]

Effects of anharmonic bridge vibrations on electronic tunneling in donor-bridge-acceptor complexes are studied using a model of anharmonic bridge vibration coupled nonlinearly to an electronic degree of freedom. An anharmonicity parameter is introduced, enabling to reproduce the standard harmonic model with linear coupling as a limiting case. The frequency of electronic tunneling oscillations between the donor and acceptor sites is shown to be sensitive to the nuclear anharmonicity, where stretching and compression modes have an opposite effect on the electronic frequency. This phenomenon, that cannot be accounted for within the harmonic approximation, is analyzed and explained. [ABSTRACT FROM AUTHOR]

Electronic tunneling in a complex molecular network of N(>2) donor/acceptor sites, connected by molecular bridges, is analyzed. The “deep” tunneling dynamics is formulated using a recursive perturbation expansion, yielding a McConnell-type reduced N-level model Hamiltonian. Applications to models of molecular junctions demonstrate that the donor-bridge contact parameters can be tuned in order to control the tunneling dynamics and particularly to direct the tunneling pathway to either one of the various acceptors. [ABSTRACT FROM AUTHOR]

Deep electronic tunneling through molecular barriers in donor-bridge-acceptor complexes is studied using an analytically solvable model. The effective tunneling matrix element is formulated as a sum over vibronic tunneling pathways. For a symmetric system the frequency of tunneling oscillations is shown to increase with the strength of electronic-nuclear coupling at the bridge, the number of electronic-nuclear coupling sites, or the frequency of a bridge vibration. Acceleration by several orders of magnitude is demonstrated within the range of realistic molecular parameters. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Details the theoretical investigation of field induced switching in a disordered electron wave coupler. System in which quantum transport was demonstrated; Tunneling and its control in a 1D double well; Two-dimensional model for dual electron wave coupler with static defect.

Studies the mechanism of electrons tunneling through a narrow water barrier between Pt(100) metal surfaces. Transmission probability for an incoming electron; Structural origin of the tunneling resonances; Time delay for the trapped electrons; Possible role played by structural cavities in the water structure.

Proposes the use of temperature-dependent Schrodinger equation for the study of quantum mechanical system-bath dynamics. Application of a time-dependent self consistent field (TDSCF) approximation for a system-bath wave function; Effects of TDSCF approximation; Analysis on the quantum mechanical system-bath correlations.

A numerical scheme is suggested for accurate large-scale quantum dynamics simulations. The time-dependent Schrödinger equation with finite time-dependent interaction terms is replaced by an inhomogeneous equation with imaginary boundary operators applied along the time axis. This equation is solved globally for a finite time interval using recent Krylov subspace-based iterative methods that are accelerated by a Fourier grid preconditioner. The same scheme is applied also to time-independent reactive-scattering calculations with absorbing boundary operators where the operation of the Green’s function is carried out by solving an inhomogeneous time-independent equation. The scheme is economic in terms of both memory requirement and computation time. It is especially favorable when high grid densities are required, e.g., for representation of highly oscillatory fields or high-energy wave functions. Illustrative applications are given for representative models of bound and dissociative systems driven by time-dependent pulsed fields, and for time-independent calculations of the cumulative reaction probability for the generic reaction H+H2 at high collision energies. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Final state-selected spectra in unimolecular decomposition are obtained by a random matrix version of Feshbach’s optical model. The number of final states which are independently coupled to the molecular quasibound states is identified with the number of states at the dividing surface of transition state theory (TST). The coupling of the transition state to the molecular complex is modeled via a universal random matrix effective Hamiltonian which is characterized by its resonance eigenstates and provides the correct average unimolecular decay rate. The transition from nonoverlapping resonances which are associated with isolated Lorentzian spectral peaks, to overlapping resonances, associated with more complex spectra, is characterized in terms of deviations from a χ2-like distribution of the resonance widths and the approach to a random phase-distribution of the resonance scattering amplitudes. The evolution of the system from a tight transition state to reaction products is treated explicitly as a scattering process where specific dynamics can be incorporated. Comparisons with recently measured final state-selected spectra and rotational distributions for the unimolecular reaction of NO2 show that the present model provides a useful new approach for understanding and interpreting experimental results which are dominated by overlapping resonances. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new approach is introduced for computing probabilities of molecular transitions in time-dependent fields. The method is based on the stationary (t,t’) representation of the Schrödinger equation and is shown to be equivalent to infinite order time-dependent perturbation theory. Bound-to-bound (i.e., photoexcitation) and bound-to-continuum (i.e., photoreaction) transitions are regarded as reactive collisions with the ‘‘time coordinate’’ as the reaction coordinate in an extended Hilbert space. A numerical method based on imposing absorbing boundary conditions for the time coordinate in a discrete variable representation framework is introduced. A single operation of the Green’s operator provides all the state-specific transition probabilities as well as partial state-resolved (inclusive) reaction probabilities. Illustrative numerical applications are given for model systems. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Unimolecular decay processes are studied in the regime of overlapping resonances with the goal of elucidating how unimolecular reaction rates depend on resonances widths (the imaginary part of the Siegert eigenvalues). As illustrated analytically for one-dimensional models and numerically for a more general random matrix version of Feshbach’s optical model, transition state theory (TST, Rice–Ramsperger–Kassel–Marcus) provides the correct average unimolecular decay rate whether the resonances are overlapping or not. For all studied cases, the explicit ‘‘universal’’ dependence of the TST average rate on the average resonance width (for a given energy, or an energy interval) is that of a saturation curve: in the regime of nonoverlapping resonances (i.e., weak coupling) the standard relation ‘‘unimolecular decay rate=resonance width /h’’ holds, but as the resonance overlap increases (strong coupling) the rate saturates, becoming practically independent of the average resonance width in the strong overlapping limit. On the basis of these conclusions, a discussion of what has been or can be measured in experiments of unimolecular decay that relates to the average decay rate and to the resonance widths is given. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using the (t,t’) method as introduced in Ref. [J. Chem. Phys. 99, 4590 (1993)] computational techniques which originally were developed for time independent Hamiltonians can be used for propagating an initial state for explicitly time dependent Hamiltonians. The present paper presents a time dependent integrator of the Schrödinger equation based on a Chebychev expansion, of the operator U(x,t’,t0→t), and the Fourier pseudospectral method for calculating spatial derivatives [(∂2/∂x2),(∂/∂t’)]. Illustrative numerical examples for harmonic and Morse oscillators interacting with CW and short pulsed laser fields are given. [ABSTRACT FROM AUTHOR]

A new approach is introduced for the calculation of multiphoton partial decay rates in ionizing and dissociative processes. The partial decay rates (partial resonance widths) are calculated by carrying out asymptotic analysis of a complex scaled resonance quasienergy solution of the time-dependent Schrödinger equation obtained when the acceleration, momentum, reduced momentum, or length gauges are used. The partial width analysis is made only after applying the proper complex-scaled transformations on the resonance solution. The recently developed (t,t’) algorithm [J. Chem. Phys. 99, 4590 (1990)] significantly reduces the computational effort when the Hamiltonian is represented in the momentum gauge or in the length gauge. The new approach is applied to a model Hamiltonian which describes the multiphoton detachment of Cl- in the ArF laser field. [ABSTRACT FROM AUTHOR]

A new powerful computational method is introduced for the solution of the time dependent Schrödinger equation with time-dependent Hamiltonians (not necessarily time-periodic). The method is based on the use of the Floquet-type operator in an extended Hilbert space, which was introduced by H. Sambe [Phys. Rev. A 7, 2203 (1973)] for time periodic Hamiltonians, and was extended by J. Howland [Math Ann. 207, 315 (1974)] for general time dependent Hamiltonians. The new proposed computational algorithm avoids the need to introduce the time ordering operator when the time-dependent Schrödinger equation is integrated. Therefore it enables one to obtain the solution of the time-dependent Schrödinger equation by using computational techniques that were originally developed for cases where the Hamiltonian is time independent. A time-independent expression for state-to-state transition probabilities is derived by using the analytical time dependence of the time evolution operator in the generalized Hilbert space. Illustrative numerical examples for complex scaled time periodic model Hamiltonians are given. [ABSTRACT FROM AUTHOR]

The complex coordinate scattering theory for the calculation of T-matrix elements, as was introduced by Engdahl, Moiseyev, and Maniv [J. Chem. Phys. 94, 1636 (1991)] and by Peskin and Moiseyev [J. Chem. Phys. 96, 2347 (1992)], is shown to satisfy the complex version of the Kohn variational principle introduced by Nuttall and Cohen [Phys. Rev. 188, 1542 (1969)]. This theory and the related S-matrix version of the Kohn variational principle, developed by Zhang, Chu, and Miller [J. Chem. Phys. 88, 6233, (1988)] are combined to formulate a generalized variational basis set approach for quantum scattering calculations. In this approach the Kohn variational procedure to optimize the linear parameters in the T matrix is followed by an optimization of the complex nonlinear parameters. This enables the application of the complex coordinate analytical continuation of the T matrix to the calculation of scattering probability amplitudes for long range potentials. Illustrating numerical applications to short and long range potentials are given. [ABSTRACT FROM AUTHOR]

The complex coordinate scattering theory is reformulated and applied to He/Cu(115) scattering. The weakly asymmetric corrugated surface leads to strong dependence of the diffraction intensities on the direction of the incident He beam. The calculated transition probabilities are in excellent agreement with the experimental values measured by Perreau and Lapujoulade. We show that additional information about the atom/surface interaction potential can be obtained if another experiment would be carried out when the Cu crystal will be rotated by 180°. [ABSTRACT FROM AUTHOR]

The method developed by Engdahl–Moiseyev–Maniv [J. Chem. Phys. 94, 1636 (1991)] for computing the scattering intensities of atomic beams from periodically corrugated solid surface is presented in more general form for scattering of molecules from solid surfaces. The method is numerically exact. By complex scaling of the Hamiltonian the full Green operator is calculated, using techniques that were originally developed for bound states. There is no need to impose specific boundary conditions on the scattering states. The method is used to calculate the rotationally inelastic transition intensities vs normal incident beam energy for HD scattering from Ag(111) and Pt(111) surfaces. Our results are in a very good agreement with theoretical scattering transition probabilities previously obtained by Whaley and Light and by Schinke. [ABSTRACT FROM AUTHOR]

The problem of time-dependent molecule-leads coupling in single-molecule junctions is acute for the structure, stability and functionality of these systems during non-equilibrium transport. Time-dependence of molecule-lead coupling operators may rise when they are perturbed directly, or formally (indirectly), when time-dependent perturbation to the molecule or to the leads are mapped onto the coupling operators by unitary transformations. In this work a reduced density matrix formulation is outlined for the study of transport in the presence of time-dependent couplings. The formulation assumes weak molecule-lead coupling, but accounts for many body interactions within the molecule, and for strong time-dependent field. For band limited or periodic fields the formulation leads to kinetic equations which map the time-dependent transport problem onto a static transport problem with multiple leads, thus providing convenient working equations as well as a useful tool for interpreting experiments on field driven single molecule junctions. Applications to concrete models are demonstrated. [ABSTRACT FROM AUTHOR]

Vibrational energy transfer driven by anharmonicity is the major mechanism of energy dissipation in polyatomic molecules and in molecules embedded in condensed phase environment. Energy transfer pathways are sensitive to the particular intra-molecular structure as well as to specific interactions between the molecule and its environment, and their identification is a challenging many-body problem. This work introduces a theoretical approach which enables to identify the dominant pathways for specified initial excitations, by screening the different possible relaxation channels. For each channel, the many-body Hamiltonian is mapped onto a respective all-vibrational Spin-Boson Hamiltonian, expressed in terms of the harmonic frequencies and the anharmonic coupling parameters obtained from the electronic structure of the molecule in its environment. A focus is given on the formulation of the relaxation rates when different limits of perturbation theory apply. In these cases the proposed Spin-Boson Screening approach becomes especially powerful. [ABSTRACT FROM AUTHOR]

Presents a response to a comment on the article 'On the Relation between Unimolecular Reaction Rates and Overlapping Resonances,' by Peskin et al published at the 'Journal of Chemical Physics' vol 106, no 4810. Optical model and the trapping effect; Physical interpretation of the rate saturation; Broadening of the distribution of resonance widths.

Understanding current-induced bond rupture in single-molecule junctions is both of fundamental interest and a prerequisite for the design of molecular junctions, which are stable at higher-bias voltages. In this work, we use a fully quantum mechanical method based on the hierarchical quantum master equation approach to analyze the dissociation mechanisms in molecular junctions. Considering a wide range of transport regimes, from off-resonant to resonant, non-adiabatic to adiabatic transport, and weak to strong vibronic coupling, our systematic study identifies three dissociation mechanisms. In the weak and intermediate vibronic coupling regime, the dominant dissociation mechanism is stepwise vibrational ladder climbing. For strong vibronic coupling, dissociation is induced via multi-quantum vibrational excitations triggered either by a single electronic transition at high bias voltages or by multiple electronic transitions at low biases. Furthermore, the influence of vibrational relaxation on the dissociation dynamics is analyzed and strategies for improving the stability of molecular junctions are discussed. [ABSTRACT FROM AUTHOR]

We consider the effects of dephasing on field-induced coherent charge and energy transport in molecular junctions. Within generic models we show that dephasing controls the relative intensities of energy and charge fluxes, and that the dependence of the energy flux on the dephasing rate is nonmonotonic. We further demonstrate the possibility for laser-controlled charge-energy separation in multiterminal molecular junctions, a prerequisite for engineering low-heating stable nanoscale devices. [ABSTRACT FROM AUTHOR]

Quantum transport in a fluctuating medium is often described using phenomenological time-dependent model Hamiltonians. Fitting the spectrum of the time-dependent forces onto a collection of oscillators, medium fluctuations can be mapped onto an interaction with a harmonic bath. In this framework, classical as well as quantum mechanical schemes can be applied to estimate averaged transport rates. Considering a generic two state model, a classical approach which relates a transfer rate to each fluctuation, and the total rate to an ensemble average, is shown to reproduce a standard rate expression. This expression turns out to be the high temperature limit of a quantum averaging scheme, which deviates from the classical result at finite temperature. The quantum averaging over fluctuating parameters is proposed here as an efficient and general approach to estimate the validity of quantum-classical simulations which treat quantum transport within an ensemble of molecular configurations generated by classical molecular dynamics. [ABSTRACT FROM AUTHOR]

The possibility of using single-molecule junctions as electron pumps for energy conversion and storage is considered. It is argued that the small dimensions of these systems allow the use of unique intramolecular quantum coherences in order to pump electrons between two leads and to overcome relaxation processes which tend to suppress the pumping efficiency. In particular, we demonstrate that a selective transient excitation of one chromophore in a bichromophoric donor-bridge-acceptor molecular junction model yields currents which transfer charge (electrons and holes) unevenly to the two leads in the absence of a bias potential. The utility of this mechanism for charge pumping in steady-state conditions is proposed. [ABSTRACT FROM AUTHOR]

Abstract: A time-dependent approach for the calculation of steady-state currents through molecular junctions is presented. Using the close analogy between electronic currents and thermal reactive scattering rates, algorithms and methodologies originally developed for calculations of thermal reaction rates are utilized for electronic transport calculations. Resonant transport is considered in detail, in view of the dominant contribution of resonance states to the current in molecular junctions. The problem of long-time delays due to population of long-lived resonances is a challenge for most time-dependent calculation of steady-state currents, involving an infinite time-limit. A solution to this problem is presented by reformulating a flux-averaging method for the molecular junction scenario. The new formulation is implemented for commonly used simple tight binding models of molecular junctions, demonstrating its numerical advantages. [Copyright &y& Elsevier]

The flux averaging method for time‐dependent calculations of resonant tunneling probabilities is generalized for the case of multichannel scattering by analyzing a generic model for resonant tunneling in a molecular junction with N terminals (N open scattering channels). The transmission probability between any two channels is expressed as a weighted average of time integrals over probability fluxes through different spatial contours. In the case of resonance scattering the transmission probability is shown to converge within a (short) direct‐scattering timescale, circumventing the problem of slow convergence of each one of the time integrals due to population of a long‐lived resonance state. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 [ABSTRACT FROM AUTHOR]

Long-range coherent electron transfer between a donor and an acceptor is often assisted by intermediate molecular bridge, via the superexchange tunneling mechanism. The effect of electronic–nuclear coupling intensity on the tunneling rate and mechanism is analyzed using a generalized spin-boson model, in which the two level system, representing the donor and the acceptor is coupled to a dissipative nuclear bath only indirectly, via additional N bridge sites. A Langevin–Schroedinger equation, based on a mean field approximation, is applied in order to study the corresponding many-body dynamics, and the results are supported by numerically exact calculations for a single nuclear bridge mode. At zero temperature and when the electron tunneling is slower than the nuclear motion, the main effect of electronic–nuclear coupling is the dissipation of electronic energy at the bridge into nuclear vibrations. At small coupling intensities, the electronic tunneling rate increases due to this dissipative mechanism, but as the coupling intensity increases the tunneling into the acceptor is suppressed and efficient dissipation leads to electronic trapping (solvation) at the bridge. This analysis agrees with numerous experimental and theoretical studies, emphasizing the importance of the nuclear bridge conformation and the bridge flexibility in controlling the electron transfer rate in donor–bridge–acceptor systems. [Copyright &y& Elsevier]

In this paper we propose to use intramolecular electron and electronic energy transfer as a way of connecting between logical operations that are implemented on different molecules. We have already shown that the rate of through-bond-mediated intramolecular singlet–singlet electronic energy transfer (EET) in certain rigid bichromophoric molecules is five orders of magnitude higher than that observed in flexibly linked systems containing the same donor–acceptor chromophores, for which through space EET via exchange interaction is dominant. This is the first direct evidence that the rigidity of the interchromophore bridge is significant for promoting through-bond bridge-mediated intramolecular EET. Preliminary results of a theoretical model for explaining this striking observation are discussed. [Copyright &y& Elsevier]

A new method is introduced for the calculation of transmission probabilities in resonant tunneling. Probability fluxes at the entrance and at the exit scattering channels are integrated as functions of time, and a weighted average of them `on the fly'' provides the infinite-time limit of the transmission probability. The convergence is obtained within the direct-scattering timescale, regardless of time-delays due to population of long-lived resonances. Therefore, the propagation time may be shortened by orders of magnitude. The method is derived and demonstrated for resonant tunneling through different double-barrier structures, in one dimension and in three dimensions. [Copyright &y& Elsevier]

A quantum mechanical treatment of the photofragment angular momentum polarization following photodissociation of diatomic molecules is presented. This treatment extends that of Siebbeles et al. [J. Chem. Phys. 100, 3610 (1994)] by considering photodissociation of a molecule whose angular momentum is polarized in the laboratory frame, and also treats properly the angular momentum coupling between the two photofragments. The formalism treats coherent excitation of dissociative surfaces and the consequences of nonadiabatic coupling between surfaces. The possibility of exploiting the parent molecule polarization in order to control the photofragment polarization when both parallel- and perpendicular-type dissociations are active is discussed. An example is given in which significant control over the molecular frame polarization of the fragments is achieved following photolysis of a parent molecule prepared by an electric dipole transition, which may be of use in the study of photoinitiated bimolecular reactions. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Resonant tunneling of electrons through a dissipative barrier is studied within a generalized spin-boson model. The model accounts for a nonlinear inelastic coupling between the electron and harmonic (bath) nuclear modes inside the barrier. In particular, we study the case in which the nuclear frequencies are similar to the decay rate of the electronic resonance states, as may appear in electron transport through narrow molecular barriers. Two cases are considered. In the case of a single responsive mode numerically exact wave-packet dynamics shows that the inelastic coupling increases the time delay of the tunneling electron in the barrier and the decay process out of the barrier is characterized by nonexponential steps. In the case of a multimode (Ohmic) bath an approximate treatment of the dynamics, based on temperature dependent nonlinear Langevin-Schro¨dinger equations, implies that the trapped electron wavepacket is fully solvated in the barrier due to irreversible dissipation at low temperatures. An interpretation of the solvation process in terms of the mean-field approximation is given and discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The dynamics of electron tunneling through water layers embedded between two metal plates is studied by electron wave-packet simulations. The tunneling flux is shown to increase by orders of magnitude due to resonances when the thermal motion of the water nuclei is "frozen" and transient molecular nanocavities dominate the tunneling mechanism. This enhancement is observed even when the energy width of the wave-packet is larger than the resonance width, and the transmission probability does not show resonance peaks as a function of the impact electron energy. The wave-packet simulations are based on a parallel solution of the multidimensional time-dependent Schro¨dinger equation, in which the N-dimensional Hilbert space is distributed into subspaces associated with an N-dimensional hypercube of processors. The propagated wave function is fully distributed at all times and the computation rate can increase linearly with the number of processors. The significant advantage of the present algorithm over serial algorithms is in the ability to increase the size of the propagated wave-functions without increasing the computation time by adding more processors. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The study of chemical reactions in environments under nonequilibrium conditions has been of interest recently in a variety of contexts, including current-induced reactions in molecular junctions and scanning tunneling microscopy experiments. In this work, we outline a fully quantum mechanical, numerically exact approach to describe chemical reaction rates in such nonequilibrium situations. The approach is based on an extension of the flux correlation function formalism to nonequilibrium conditions and uses a mixed real and imaginary time hierarchical equations of motion approach for the calculation of rate constants. As a specific example, we investigate current-induced intramolecular proton transfer reactions in a molecular junction for different applied bias voltages and molecule–lead coupling strengths. [ABSTRACT FROM AUTHOR]

A quantum sampling algorithm for the interpolation of diabatic potential energy matrices by the Grow method is introduced. The new procedure benefits from penetration of the wave packet into classically forbidden regions, and the accurate quantum mechanical description of nonadiabatic transitions. The increased complexity associated with running quantum dynamics is reduced by using approximate low order expansions of the nuclear wave function within a Multi-configuration time-dependent Hartree scheme during the Grow process. The sampling algorithm is formulated and applied for three representative test cases, demonstrating the recovery of analytic potentials by the interpolated ones, and the convergence of a dynamic observable. [ABSTRACT FROM AUTHOR]

The ability to control electronic tunneling in complex molecular networks of multiple donor/acceptor sites is studied theoretically. Our past analysis, demonstrating the phenomenon of site-directed transport, was limited to the coherent tunneling regime. In this work we consider electronic coupling to a dissipative molecular environment including the effect of decoherence. The nuclear modes are classified into two categories. The first kind corresponds to the internal molecular modes, which are coupled to the electronic propagation along the molecular bridges. The second kind corresponds to the external solvent modes, which are coupled to the electronic transport between different segments of the molecular network. The electronic dynamics is simulated within the effective single electron picture in the framework of the tight binding approximation. The nuclear degrees of freedom are represented as harmonic modes and the electronic-nuclear coupling is treated within the time-dependent Redfield approximation. Our results demonstrate that site-directed tunneling prevails in the presence of dissipation, provided that the decoherence time is longer than the time period for tunneling oscillations (e.g., at low temperatures). Moreover, it is demonstrated that the strength of electronic coupling to the external nuclear modes (the solvent reorganization energy) controls the coherent intramolecular tunneling dynamics at short times and may be utilized for the experimental control of site-directed tunneling in a complex network. [ABSTRACT FROM AUTHOR]

Colloidal quantum dots (CQDs) arefree-standing nanostructureswith chemically tunable electronic properties. This combination ofproperties offers intriguing new possibilities for nanoelectromechanicaldevices that were not explored yet. In this work, we consider a newscanning tunneling microscopy setup for measuring ligand-mediatedeffective interdot forces and for inducing motion of individual CQDswithin an array. Theoretical analysis of a double quantum dot structurewithin this setup reveals for the first time voltage-induced interdotrecoil and dissociation with pronounced changes in the current. Consideringrealistic microscopic parameters, our approach enables correlatingthe onset of mechanical motion under bias voltage with the effectiveligand-mediated binding forces. [ABSTRACT FROM AUTHOR]

The flux averaging method which was introduced in order to calculate transmission probabilities in resonant tunneling processes is reformulated for calculations of thermal rate constants for reactions with long-lived intermediate states, associated with quantum resonances. The rate constant is expressed as a time-dependent weighted average of time integrals over different flux–flux correlation functions. This exact expression for the rate is shown to converge to its asymptotic value within the ‘direct’ dynamics timescale, regardless of the population of long-lived resonance states. The method is derived and demonstrated for symmetric and non-symmetric resonance-supporting double-barrier potentials. [Copyright &y& Elsevier]

The fundamental biological process of electron transfer (ET) takes place across proteins with common ET pathways of several nanometers. Recent discoveries push this limit and show long-range extracellular ET over several micrometers. Here, we aim in deciphering how protein-bound intramolecular cofactors can facilitate such long-range ET. In contrast to natural systems, our proteinbased platform enables us to modulate important factors associated with ET in a facile manner, such as the type of the cofactor and its quantity within the protein. We choose here the biologically relevant protoporphyrin molecule as the electron mediator. Unlike natural systems having only Fe-containing protoporphyrins, i.e., heme, as electron mediators, we use here porphyrins with different metal centers, or lacking a metal center. We show that the metal redox center has no role in ET and that ET is mediated solely by the conjugated backbone of the molecule. We further discuss several ET mechanisms, accounting to our observations with possible contribution of coherent processes. Our findings contribute to our understanding of the participation of heme molecules in long-range biological ET. [ABSTRACT FROM AUTHOR]

In a nanoscale molecular junction at finite bias voltage, the intramolecular distribution of vibrational energy can strongly deviate from the thermal equilibrium distribution and specific vibrational modes can be selectively excited in a controllable way, regardless of the corresponding mode frequency. This is demonstrated for generic models of asymmetric molecular junctions with localized electronic states, employing a master equation as well as a nonequilibrium Green’s function approach. It is shown that the applied bias voltage controls the excitation of specific vibrational modes by tuning the efficiency of vibrational cooling processes due to energy exchange with the leads. [ABSTRACT FROM AUTHOR]