Vibrational control of molecular electron transfer reactions

ABSTRACT

Vibrational motions promote molecular electron transfer (ET) reactions by bringing electron donor and electron acceptor electronic states to fleeting resonance, and by modulating the donor-to-acceptor electronic coupling. The main experimental signature of molecular motion effects on the ET rate is the temperature dependence of the rate, which gives information about the overall free energy activation barrier for the ET reaction. Another approach to probing the vibrational control of ET reactions is to excite specific electron-transfer-active vibrational motions by external infrared (IR) fields. This type of experimental probe is potentially more specific than thermal excitation and recent experiments have shown that molecular ET rates can be perturbed by mode-specific IR driving. We review the theory and experiments of vibrational control of ET rates, and discuss future challenges that need to be tackled in order to achieve the mode-specific tuning of rates.     

Electron spin resonance-scanning tunneling microscopy

Electron spin resonance-scanning tunneling microscopy (ESR-STM) is a rapidly developing surface-science technique that is sensitive to a single spin existing on or nearby a solid surface. The single spin is detected through elevated noise at the Larmor frequency that appears when the single spin participates in the tunneling process between the tip and the surface. In this review, experimental and theoretical works which have been performed up to date on ESR-STM are reviewed. The remaining experimental problems which have to be solved, possible approaches to differentiate between different mechanisms and the future of ESR-STM are discussed.\ PACS: 72.25.Dc Spin polarized transport in semiconductors, 72.70.+m Noise processes and phenomena, 73.20.Hb Impurity and defect levels; energy states of adsorbed species, 73.40.Gk Tunneling, 75.70.Rf Surface magnetism, 75.76.+j Spin transport effects, 76.30.-v Electron paramagnetic resonance and relaxation, 78.47.-p Spectroscopy of solid state dynamics     

Models of plastic depinning of driven disordered systems

Two classes of models of driven disordered systems that exhibit historydependent dynamics are discussed. The first class incorporates local inertia in the dynamics via nonmonotonic stress transfer between adjacent degrees of freedom. The second class allows for proliferation of topological defects due to the interplay of strong disorder and drive. In mean field theory both models exhibit a tricritical point as a function of disorder strength. At weak disorder depinning is continuous and the sliding state is unique. At strong disorder depinning is discontinuous and hysteretic.     

Simulating high energy cascades in metals

Abstract

The processes of radiation damage, from initial defect production to microstructure evolution, occur over a wide spectrum of time and size scales. An understanding of the fundamental aspects of these processes requires a spectrum of theoretical models, each applicable in its own time and distance scales. As elements of this multi-model approach, molecular dynamics and binary collision simulations play complementary roles in the characterization of the primary damage state of high energy collision cascades. Molecular dynamics is needed to describe the individual point defects in the primary damage state with the requisite physical reality. The binary collision approximation is needed to model the gross structure of statistically significant numbers of high energy cascades. Information provided by both models is needed for connecting the defect production in the primary damage state with the appropriate models of defect diffusion and interaction describing the microstructure evolution. Results of binary collision simulations of high energy cascade morphology are reviewed. The energy dependence of freely migrating defect fractions calculated in recent molecular dynamics simulations are compared to results obtained much earlier with a binary collision/annealing simulation approach. The favorable agreement demonstrates the viability of the multi-model approach to defect production in high energy cascades.     

Study of the structural photoinduced dynamics of a solid Kr matrix with an NO impurity

In the present work we studied the immediate medium response to the excitation to the Rydberg state of NO impurity embedded in a solid Kr matrix. The excitation, extended over a large range of the lattice was investigated by classical molecular dynamics simulations. This has been done using Lennard-Jones pair potentials from the literature for the interactions and fitted in this work for the ones, since these last have not been reported in literature. Thus is obtained the first shell response to the excitation of the impurity (approximately the first 2 ps) as well as the response of the continuous shells up to the 10th one. This first response of the first shell is compared to that for similar systems (Ne and Ar matrixes doped with NO). Therefore some theoretical conclusions are drawn. The results indicate the inertial character of the response propagation throughout the surrounding medium and the high degree of nuclear coherence at short times.Received: 27 February 2003, Published online: 30 July 2003PACS: 34.30.+h Intramolecular energy transfer; intramolecular dynamics; dynamics of van der Waals molecules - 02.70.Ns Molecular dynamics and particle methods - 31.70.Ks Molecular solids     

Electron Transfer Between Molecules: Study with Langevin Equations

In this paper, we investigate the electron transfer （ET） in donor-acceptor model. The Langevin equation with random forces is used. The oscillations of the primary states observed in experimental data have been shown with this approach. And other features on the dependence of the rate of ET on temperature, free energy, and reorganization energy have also been clearly shown.     

Evidence for the purely electronic character of primary electron transfer in purple bacteria Rh. Sphaeroides

A quantum-chemical calculation of the excited electronic states of a Rh. Sphaeroides reaction centre was performed. We discovered a new excited electronic state which can participate in electron transfer (ET). The energy gradient calculations showed that photoexcitation activates only high-frequency vibrational modes. This contradicts the widely accepted picture of ET resulting from vibrational wave packet motion. An alternative model is suggested where ET has a purely dissipative character and occurs only due to pigment--protein interaction. With this model, we demonstrate that oscillations in the femtosecond spectra can be caused by the new electronic state and non-Markovian character of dissipative dynamics.     

虚时间分裂算符方法研究溶剂电子转移动力学

在Sumi-Marcus理论中，采用虚时间分裂算符方法研究电子转移动力学．此方法具体应用于计算嗪-1和N,N-二甲基苯胺分子之间的电子转移速率常数．通过计算得到的两种反应物态的平均速率常数和一个长时间的速率常数，揭示了不同的sink函数时的电子转移动力学．在数值模拟过程中还发现了一些新的电子转移特性．     

Effect of oxidation state and coordination with Hg(II) on the electronic spectra of an anthraquinone–rhodamine sensor

ABSTRACT

In this study, a computational examination of the electronic transitions and through-space energy transfer processes lends insight into the experimental electronic spectra of a redox-sensitive rhodamine–anthraquinone dyad. Electronic transitions were calculated using density functional theory (DFT) and time-dependent DFT (TDDFT) based on models optimised from single-crystal X-ray diffraction (XRD) ion positions. DFT calculations were performed on gas-phase models using the Vienna Ab Initio Software Package (VASP) with the functional developed by Perdew, Burke, and Ernzerhof (PBE) on a basis set of plane waves. Using the DFT results, select transitions were evaluated based on a dipole–dipole coupling mechanism to find the Förster resonance energy transfer coupling, the square of which is approximately proportional to the rate of energy transfer between the donor and the acceptor. Electronic transitions during the relaxation of charge carriers are also investigated using nonadiabatic molecular dynamics. In order to investigate the transitions contributing to key peaks in the experimental absorbance spectra, TDDFT calculations were performed in Gaussian 09 with the B3LYP functional on the sensor in solution phase, which is simulated using a polarisable continuum model (PCM). The computed electron transfer process from the excited rhodamine to the quinone correlates better with the experimental data than does the computed Förster resonance energy transfer (FRET) process. This computed electron transfer process is faster than the radiative lifetime of the fluorescent state, which collectively suggests that the charge transfer process quenches fluorescence. These results support the observation that fluorescence is more prominent in the oxidised sensor than in the reduced sensor.     

Configurations and observables in an Ising model with heat flow

We study a two dimensional Ising model between thermostats at different temperatures. By applying the recently introduced KQ dynamics, we show that the system reaches a steady state with coexisting phases transversal to the heat flow. The relevance of such complex states on thermodynamic or geometrical observables is investigated. In particular, we study energy, magnetization and metric properties of interfaces and clusters which, in principle, are sensitive to local features of configurations. With respect to equilibrium states, the presence of the heat flow amplifies the fluctuations of both thermodynamic and geometrical observables in a domain around the critical energy. The dependence of this phenomenon on various parameters (size, thermal gradient, interaction) is discussed also with reference to other possible diffusive models.     

Ultra-fast dynamics of coherent lattice and spin excitations at the Gd(0001) surface

The Gd(0001) surface is investigated by pump–probe experiments using femtosecond laser pulses at 740–860 nm wavelength. Employing optical second-harmonic generation, spin and lattice dynamics are separated through the symmetry of optical field contributions that are even and odd with respect to magnetization reversal. A coherent phonon–magnon mode at a frequency of 3 THz that is excited through the exchange-split surface state is observed in the time domain. A magneto-elastic phonon–magnon interaction based on spin–orbit coupling is weak for Gd and a modulation of the exchange interaction mediated by the lattice vibration is proposed as a microscopic interaction mechanism of this coupled mode. In parallel, electron–electron and electron–phonon interactions and their magnetic counterparts lead to incoherent dynamics of the electron, lattice, and spin subsystems. Variation of the optical wavelength shows that for longer wavelengths up to 860 nm the coherent mode dominates, while for shorter ones (≥740 nm) incoherent contributions prevail. This dependence indicates that selective depopulation of the occupied surface state component drives the coherent excitation. However, temperature-dependent studies show that the oscillation amplitude of even and odd contributions scales with the spin polarization of the surface state, suggesting that the spin dependence of the ion potentials contributes as well. Furthermore, the frequency of the coherent mode presents a blue shift with a delay of 0.17 THz/ps that saturates at the static frequency of the respective bulk phonon. This behavior is a consequence of equilibration of the screened ion potential at the surface subsequent to the intense laser excitation. PACS 78.47.+p; 63.22.+m; 63.20.Ls; 75.30.Ds     

Heavy fermions in transition metals and transition-metal oxides

Heavy-fermion formation in transition metals and transition-metal oxides is reviewed and compared to observations in canonical f-derived heavy-fermion systems. The work focuses on the dynamic susceptibilities which reveal a characteristic temperature and frequency dependence and which can be unambiguously determined via nuclear magnetic resonance and electron-spin resonance measurements as well as via quasielastic neutron-scattering studies. Different routes to heavy-fermion behaviour are discussed, amongst them Kondo systems, frustrated magnets, and electronically correlated systems close to a metal-insulator transition. From a theoretical point of view, utilizing dynamical mean-field theory, we show that dynamic susceptibilities as calculated for the Hubbard model and for the periodic Anderson model look qualitatively rather similar. These different theoretical concepts describe an universal behaviour of the temperature dependent dynamic susceptibility.Received: 15 May 2003, Published online: 9 September 2003PACS: 71.27.+a Strongly correlated electron systems; heavy fermions - 71.30.+h Metal-insulator transitions and other electronic transitions - 76.60.-k Nuclear magnetic resonance and relaxation - 76.30.Kg Rare-earth ions and impurities     

A numerical study of energy transfer mechanisms in materials following irradiation by swift heavy ions

Swift heavy ions interact with electrons in materials and this may yield permanent atomic displacements; the energy transfer mechanisms that bring electronic excitations into atomic motion are not fully understood, and are generally discussed in terms of two theories, viz. Coulomb explosion and heat exchange between excited electrons and atoms, which is limited by electron-phonon coupling. We address this problem for a “generic” material using a semi-classical numerical approach where the dynamics of the evolving electron density is calculated by using molecular dynamics simulations applied to pseudo-electrons. The forces exerted on the nuclei are then used to calculated the trajectories of the nuclei. From the temporal evolution of the atomic kinetic energy, we find that the energy transfer between the electrons and the nuclei can be divided in two parts. First, a Coulomb heating starts the motion of the atoms by giving them a radial speed; this process differs from Coulomb explosion because the atoms are not displaced over interatomic distances. Second, a thermal energy transfer, as described in linear transport theory, takes place. Our study thus confirms the domination of thermal energy exchange mechanisms over Coulomb explosion models.     

Laser polarization effects on K-shell Compton scattering

We study Compton scattering on the ground state of the hydrogen atom in the presence of an intense low-frequency electric field (the laser) of arbitrary polarization, for incident and scattered photons of energies around 15 keV. The adopted formalism is the nonrelativistic one developed by Voitkiv et al. [J. Phys. B: At. Mol. Opt. Phys. 36, 1907 (2003)] and applied by them for a circularly polarized laser. We explore the spectrum and the electron energy distribution in their dependence on the incident photon energy or electric field intensity, for different polarizations.     

Isomerization and dissociation of small (CH <Emphasis Type="Bold">3</Emphasis>CN) <Emphasis Type="Bold">n</Emphasis> molecular clusters: a computational study

The isomerization and evaporation processes in the neutral homogeneous (CH₃CN)_n molecular clusters (n = 2-7) have been investigated using classical molecular dynamics simulations. The evaporation rate constants and the kinetic energy release in the dissociation have been analysed as a function of the cluster size and as a function of the internal energy in the parent cluster. The competition between monomer and dimer ejections has been also carefully studied. All the dynamical properties in these dissociative processes have been discussed in relation to the static properties of the clusters involved in the dissociation and also in relation to the solid-liquid like transition which appears in these homogeneous molecular clusters. Received 19 November 2002 / Received in final form 5 February 2003 Published online 29 April 2003 RID="a" ID="a"e-mail: pascal.parneix@ppm.u-psud.fr RID="b" ID="b"Laboratoire associé à l'université Paris-Sud.     

Dynamics of multi-frequency minority games

The dynamics of minority games with agents trading on different time scales is studied via dynamical mean-field theory. We analyze the case where the agents decision-making process is deterministic and its stochastic generalization with finite heterogeneous learning rates. In each case, we characterize the macroscopic properties of the steady states resulting from different frequency and learning rate distributions and calculate the corresponding phase diagrams. Finally, the different roles played by regular and occasional traders, as well as their impact on the systems global efficiency, are discussed.Received: 4 August 2003, Published online: 22 September 2003PACS: 87.23.Ge Dynamics of social systems - 05.65.+b Self-organized systems - 02.50.Le Decision theory and game theory - 05.10.Gg Stochastic analysis methods     

Intermolecular Processes in Excited Electronic States in the Gas Phase (Review)

A brief review of works on the temperature dependences of the rate constants k_q of the intermolecular processes proceeding in the excited electronic states in the gas phase is given. The dependences k_q(T) for such biomolecular processes as intermolecular vibrational energy transfer in the triplet state vibrational quasi-continuum, triplet-triplet electron excitation energy transfer, and intermolecular photoinduced electron transfer have been compared. The experimental data have shown that in the gas phase for all analyzed intermolecular processes both an increase and a decrease in k_q with increasing temperature (T) is observed, which is not associated with the specific intermolecular interactions leading to the formation of long-lived components. The change in the type of temperature dependence is due to the change in the mechanisms of the radiationless transitions with increasing density of vibrational levels in the final electronic state. The applicability of the known models based on the theory of radiationless transitions for predicting the temperature dependences k_q(T) is discussed. __________ Translated from Zhurnal Prikladnoi Spektros-kopii, Vol. 72, No. 4, pp. 429–439, July–August, 2005.     

Statistical approach of the modulational instability of the discrete self-trapping equation

The discrete self-trapping equation (DST) represents an useful model for several properties of one-dimensional nonlinear molecular crystals. The modulational instability of DST equation is discussed from a statistical point of view, considering the oscillator amplitude as a random variable. A kinetic equation for the two-point correlation function is written down, and its linear stability is studied. Both a Gaussian and a Lorentzian form for the initial unperturbed wave spectrum are discussed. Comparison with the continuum limit (NLS equation) is carried out.Received: 29 May 2003, Published online: 4 August 2003PACS: 63.70.+h Statistical mechanics of lattice vibrations and displacive phase transitions - 05.45.-a Nonlinear dynamics and nonlinear dynamical systems - 05.45.Yv Solitons     

Turbulent thermal convection in a closed domain: viscous boundary layer and mean flow effects

In this paper the effects of viscous boundary layers and mean flow structures on the heat transfer of a flow in a slender cylindrical cell are analysed using the direct numerical simulation of the Navier-Stokes equations with the Boussinesq approximation. Ideal flows are produced by suppressing the viscous boundary layers and by artificially enforcing the flow axisymmetry with the aim of checking some proposed explanations for the Nusselt number dependence on the Rayleigh number. The emerging picture suggests that, in this slender geometry,the presence of the viscous boundary layers does not have appreciable impact on the slope of the Nu vs. Ra relation while a transition of the mean flow is most likely the reason for the slope increase observed around Ra=2 x 10⁹.Received: 7 March 2003, Published online: 22 September 2003PACS: 47.27.Te Convection turbulent flows - 47.32.-y Fluid flow buoyant - 44.25.+f Heat transfer convective