Resonant coupling of bound excitons with LO phonons in ZnO: excitonic polaron states and Fano interference

We report on a photoluminescence observation of robust excitonic polarons due to resonant coupling of exciton and longitudinal optical (LO) phonon as well as Fano-type interference in high quality ZnO crystal. At low enough temperatures, resonant coupling of excitons and LO phonons leads to not only traditional Stokes lines (SLs) but also up to second-order anti-Stokes lines (ASLs) besides the zero-phonon line (ZPL). The SLs and ASLs are found to be not mirror symmetric with respect to the ZPL, strongly suggesting that they are from different coupling states of exciton and phonons. Besides these spectral features showing the quasiparticle properties of exciton-phonon coupling system, the first-order SL is found to exhibit characteristic Fano lineshape, caused by quantum interference between the LO components of excitonic polarons and the continuous phonon bath. These findings lead to a new insight into fundamental effects of exciton-phonon interactions.     

Exciton-phonon self-trapping: A continuous transition

An exciton-phonon system of a linear aggregate is investigated in the intermediate-coupling regime. For sufficiently large intermolecular coupling. the variational approach predicts a discontinuous transition from a mobile exciton at weak exciton-phonon coupling to an immobile exciton at strong coupling. Fully converged numerical calculations exhibit strong deviations from the variational approach at intermediate exciton-phonon coupling strength and do not show a discontinuous transition. In the strong-coupling regime, results from the variational approach (which are closely related to the small polaron theory) deviate considerably from numerically converged calculations and an appropriate perturbation approximation.     

Dynamics of a Holstein polaron with off-diagonal coupling

Dynamics of a one-dimensional Holstein polaron with off-diagonal exciton-phonon coupling is studied by employing the Dirac-Frenkel time-dependent variational principle. The trial state used is the Davydov D(2) Ansatz with two sets of variational parameters, one for each constituting particle in the linearly coupled exciton-phonon system. Validity of the approach is carefully checked by quantifying how faithfully the trial state follows the Schro?dinger equation. A close examination of variational outputs reveals fine details of polaron dynamics and intricacies of dynamic exciton-phonon correlations. In the absence of diagonal coupling, the change in the polaron effective mass hinges on the sign of the transfer integral due to the antisymmetric nature of the off-diagonal coupling. The role of the off-diagonal coupling switches from being an agent of transport at moderate coupling strengths to that of localization at large coupling strengths. Increasing the phonon bandwidth leads to a reduced polaron effective mass at the zone center and an overall lowering of the polaron band.     

A model calculation on the transport of excitation energy in a molecular crystal

We report a model calculation of the transport of a local (site) excitation in a doped molecular crystal containing one impurity. We do not consider the impurity as a direct trap for electronic excitations (zero trap depth) but assume that exciton-phonon interaction is exclusively given by the coupling of excitons with the vibrational displacement of the impurity. The dynamical problem is solved by using a time-dependent effective potential consisting of equilibrium average exciton-phonon interaction and fluctuations around this average. Two correlation functions are computed using the slow phonon limit and assuming that the temperature of the system is 300 K. Transmission of the excitation energy over a distance of eight spacings takes place, electronically, within a few picoseconds. With the exciton-phonon interaction switched on, calculated correlation functions diminish very rapidly with increasing time, indicating that an irreversible transfer of excitonic energy to the thermal bath takes place. Thus transmission of the excitation energy over such a distance (and without a high rate of trapping) is not an efficient process.     

Interpolyelectrolyte complexes: a single-molecule insight

Polyelectrolyte (PE) complexes (PECs) between long polycation poly(methacryloyloxyethyl dimethylbenzylammonium chloride) and short polyanion polystyrene sulfonic acid adsorbed onto mica were studied by atomic force microscopy. If one component is taken in excess, then a rapid coupling of the oppositely charged polyions first leads to the formation of nonequilibrium structures when collapsed PEC particles coexist with unreacted PEs molecules. The equilibrium PEC particles possess micelle-like core-shell morphology if the short polyion is taken in excess. When long PE is given in excess, equilibrium PECs are stabilized by wrapping the long polyion around hydrophobic segments of the PEC. We propose that transformations of initially formed nonequilibrium aggregates proceed through slow reactions (addition or/and substitution) of primary complexes with unreacted PEs chains, which finally leads to equilibrium PECs with optimized morphology. As expected, the mixing of oppositely charged PEs in a near-stoichiometric ratio leads to highly aggregated water-insoluble PECs.     

Resonant nonequilibrium temperatures

In this paper, we investigate nonequilibrium temperatures in a two-state system driven to a nonequilibrium steady state by the action of an oscillatory field. The nonequilibrium temperature is determined by coupling a small cavity or probe to the nonequilibrium system and studying the fluctuating noise in the cavity, as has been proposed in the context of glassy systems. We show the presence of resonant effects in the nonequilibrium temperature and discuss the existence of a constitutive steady-state equation in such nonequilibrium conditions. We propose this simple model as an excellent system to carry out experimental measurements of nonequilibrium temperatures. This may help to better understand the physical meaning of such an elusive concept.     

Coherent Exciton-Phonon Coupling in CdSe/ZnS Nanocrystals Studied by Two-Dimensional Electronic Spectroscopy

Coherent exciton-phonon coupling in CdSe/ZnS nanocrystals have been investigated by temperature-dependent two-dimensional electronic spectroscopy (2DES) measurements. Benefiting from the ability of 2DES to dissect assembles in nanocrystal films, we have clearly identified experimental evidences of coherent coupling between exciton and phonon in CdSe/ZnS nanocrystals. In time domain, 2DES signals of excitonic transitions beat at a frequency resonant to a longitudinal optical phonon mode; in energy domain, phonon side bands are distinct at both Stokes and anti-Stokes sides. When temperature increases, phonon-induced exciton dephasing is observed with dramatic broadening of homogeneous linewidth. The results suggest exciton-phonon coupling is essential in elucidating the quantum dynamics of excitonic transitions in semiconductor nanocrystals.     

CD spectra of polynuclear complexes of diimine ligands: theoretical and experimental evidence for the importance of internuclear exciton coupling

We have recently reported on dinuclear complexes Lambda,Lambda-[Co(2)L(2)Cl(2)]CoCl(4) of two novel chiral ligands (1a and 1b) which contain pyridyl-imine chelate groups (Telfer, S. G.; Sato, T.; Kuroda, R. Chem. Commun. 2003, 1064-1065). The absolute configuration of the cobalt(II) centers was unambiguously assigned by X-ray crystallography. However, the sign of the exciton couplets in their CD spectra was opposite to that expected on the basis of the stereochemistry of the metal centers. We present a rationalization of these anomalous spectra in terms of an "internuclear" exciton coupling model which takes into account the coupling of chromophores located on different metal centers. We have performed a series of semiempirical (ZINDO) calculations which provide quantitative support to this model. These findings show that the absolute configuration of the metal centers in a polynuclear complex may be incorrectly assigned on the basis of CD data if internuclear coupling effects are not taken into consideration. We summarize the CD spectral data of number of other chiral polynuclear complexes from the literature, including dinuclear complexes bridged by the 2,2'-bipyrimidine ligand, complexes of the HAT ligand, and dinuclear triple-stranded helicates. The amplitude of the CD spectra of many of these complexes is not additive with the number of chromophores. These anomalous spectra can be accounted for by taking internuclear coupling effects into consideration.     

The influence of thermostats and manostats on reverse nonequilibrium molecular dynamics calculations of fluid viscosities

Reverse nonequilibrium molecular dynamics to calculate the shear viscosity of Lennard-Jones liquids was extended to simulations at constant number of particles, constant volume, and constant pressure using a Berendsen thermostat and a Berendsen manostat. Using additional systems such as water and hexane, we also report on the performance of shear viscosity calculations of systems with electrostatic and nontrivial intramolecular interactions when a manostat is applied. We compare the shear viscosities of simulations using no coupling, only temperature coupling, and temperature and pressure coupling and characterize discrepancies, where observed. From this, we deduce guidelines for when and how manostats can be usefully applied in reverse nonequilibrium simulations.     

Triplet excition emissions of octafluoronaphthalene crystalline complexes with naphthalene and durene

The phosphorescence spectra of naphthalene-octafluoronaphthalene and durene-octafluoronaphthalene at 4.2 K reveal a localized exciton emission from octafluoronaphthalene, a weak intercomponent triplet-states quasi-resonance and a weak exciton-phonon coupling.     

Statistical mechanical theory for steady state systems. V. Nonequilibrium probability density

The phase space probability density for steady heat flow is given. This generalizes the Boltzmann distribution to a nonequilibrium system. The expression includes the nonequilibrium partition function, which is a generating function for statistical averages and which can be related to a nonequilibrium free energy. The probability density is shown to give the Green-Kubo formula in the linear regime. A Monte Carlo algorithm is developed based upon a Metropolis sampling of the probability distribution using an umbrella weight. The nonequilibrium simulation scheme is shown to be much more efficient for the thermal conductivity of a Lennard-Jones fluid than the Green-Kubo equilibrium fluctuation method. The theory for heat flow is generalized to give the generic nonequilibrium probability densities for hydrodynamic transport, for time-dependent mechanical work, and for nonequilibrium quantum statistical mechanics.     

Electronic energy transfer in mixed organic solids: Anderson localization or delocalization?

A simple Anderson transition model, ignoring guest clusterization, excitation lifetime, sensor concentration, exciton-phonon coupling and thermalization, appears to be incompatible with the critical concentrations observed for triplet exciton transport in several ternary crystal systems. Dynamic percolation, involving hopping or tunneling through long-range clusters, remains our suggested model.     

Characterization and determination of poly(vinylpyrrolidone) by complexation with an anionic azo-dye and nonequilibrium capillary electrophoresis

Using capillary zone electrophoresis in nonequilibrium conditions, the complexes of poly(vinylpyrrolidone) (PVP) with anionic azo-dyes dissociate following a first-order kinetics. Two peaks due to the remaining PVP–dye complexes and the equilibrium concentration of the free dye, plus an exponential region due to the dye liberated by the complexes during the electrophoretic run, are obtained. This behaviour was closely similar to that described in the literature for protein–probe and DNA–protein mixtures, upon application of the technique known as nonequilibrium capillary electrophoresis of equilibrium mixtures or NECEEM. Using Congo Red and Acid Blue 113, information about the maximal stoichiometry and average stability of the PVP–dye complexes was obtained. The procedure was also useful to predict the average molecular mass of PVP and to determine PVP in cleaning products and pharmaceutical preparations. By using an appropriate probe, the procedure should be also useful to characterize and determine many other synthetic or natural nonionic polymers, and to study polymer–probe interactions.     

Transport of matter and energy in a mesoscopic thermo-hydrodynamic approach

We derive a thermo-hydrodynamic theory for particles and energy flow, based on a nonequilibrium grand-canonical ensemble formalism. The time-dependent kinetic coefficients are explicitly given in terms of microscopic mechanical quantities. The time evolution equations describing the coupled flow of energy and particles are derived. The second-rank tensorial fluxes of current of energy and particles present in the nonequilibrium ensemble are nondiagonal. We obtain a generalized Fick's Law, which presents the effect of the energy flow on the particle diffusion equation.     

Real-time monitoring of protein complexes reveals their quaternary organization and dynamics

The dynamics of protein complexes are crucial for their function yet are challenging to study. Here, we present a nanoelectrospray (nESI) mass spectrometry (MS) approach capable of simultaneously providing structural and dynamical information for protein complexes. We investigate the properties of two small heat shock proteins (sHSPs) and find that these proteins exist as dodecamers composed of dimeric building blocks. Moreover, we show that these proteins exchange dimers on the timescale of minutes, with the rate of exchange being strongly temperature dependent. Because these proteins are expressed in the same cellular compartment, we anticipate that this dynamical behavior is crucial to their function in vivo. Furthermore, we propose that the approach used here is applicable to a range of nonequilibrium systems and is capable of providing both structural and dynamical information necessary for functional genomics.     

Phonon mediation of solid state photoreaction: Photodimerization of o -methoxy cinnamic acid

The solid state photodimerization reaction ofo-methoxy cinnamic acid is shown to be mediated by a lattice phonon. The phonon participation, in this case, is through a mode softening and not through strong exciton-phonon coupling as is generally observed. Raman phonon spectroscopy suggests that the reaction is heterogeneous. Infrared spectroscopy has been used to study the internal vibrations of the reactant and the product. Partly presented at the International Laser Science Conference II, 1986 held at Seattle, USA.     

Determining affinity-selected ligands and estimating binding affinities by online size exclusion chromatography/liquid chromatography-mass spectrometry

Size exclusion chromatography (SEC) isolation of affinity-selected ligands combined with reverse phase liquid chromatography-mass spectrometry (LC-MS) is an effective means for identifying members of mixtures which form tightly bound noncovalent complexes with target proteins. A potential liability of the approach is that the SEC isolation is carried out under nonequilibrium conditions favoring protein/ligand complex dissociation. At long SEC isolation times and/or for complexes with fast off-rates the extent of dissociation can jeopardize the ability to detect the affinity-selected components. Additionally, equilibrium binding affinities cannot be exactly determined from the measured distribution of isolated ligands. We present here an online SEC/LC-MS system for determining affinity-selected members of active mixtures which reduces this liability. A kinetic model of the SEC isolation process is developed to determine the practical limits for the application of the method and to extrapolate equilibrium binding affinities from the nonequilibrium data. The utility of online SEC/LC-MS for identifying affinity-selected ligands and for estimating binding affinities is demonstrated for a small molecule mixture of compounds with known binding affinities and for a simple combinatorial mixture.     

Nonlinear electroosmosis in hierarchical monolithic structures

We studied the dependence of electroosmotic flow (EOF) velocity and separation efficiency for neutral analytes in 100 microm ID capillary monoliths on a variation of the mobile phase ionic strength and applied electrical field strength, i.e., we covered a range for the concentration of Tris buffer from 10(-5) to 10(-2) M and applied electrical field strengths up to 10(5) V/m. The silica-based monoliths are hierarchically structured having intraskeleton mesopores and interskeleton macropores. While a linear dependence of the average EOF velocity on applied field strength could be observed with 5 x 10(-3) M Tris (turning slightly nonlinear at a higher concentration due to thermal effects), this dependence becomes systematically nonlinear as the Tris concentration is reduced towards 10(-4) M. Increased velocities by more than 50% compared to those expected from linear behavior are realized at 10(5) V/m. Concomitantly, as the Tris concentration is reduced from 10(-3) to 10(-4) M, we notice an improvement in plate heights by a factor of more than 2 (they approach 2 microm for ethylbenzoate). We complementary analyzed the onset of the nonlinear EOF dynamics in a hierarchical monolith and the significantly reduced axial dispersion in view of nonequilibrium electrokinetic effects which may develop in porous media due to the presence of ion-permselective regions, e.g., the mesoporous monolith skeleton. In this respect, a decreasing mobile phase ionic strength favors the formation of nonequilibrium concentration polarization in strong electrical fields, and a coupling of the electrostatics and hydrodynamics then may explain nonlinear EOF velocities and increasing separation efficiencies depending on the Tris concentration and applied field strength.     

Resonant energy transfer assisted by off-diagonal coupling

Dynamics of resonant energy transfer of a single excitation in a molecular dimer system are studied in the simultaneous presence of diagonal and off-diagonal exciton-phonon coupling. It is found that, at given temperatures, the off-diagonal coupling can enhance both the coherence of the resonant energy transfer and the net quantity of energy transferred from an initially excited monomer to the other. Also studied is the dynamics of entanglement between the dimer system and the phonon bath as measured by the von Neumann entanglement entropy, and the inter-monomer entanglement dynamics for the excitonic system.