Conductivity of water-in-oil microemulsions: Fluctuations from the charge generation-recombination equilibrium

A theory for conductivity fluctuations in water-in-oil microemulsions is presented. The treatment is based on the charge fluctuations model proposed by Eicke et al., Hall, and Halle. The portion of current fluctuations due to fluctuations in the number of charge carriers is attributed to the charge generation-recombination equilibrium: two neutral drops positive drop+negative drop. The power spectrum of the fluctuations is a Lorentzian determined by the forward and reverse rates of this equilibrium. Implications of the charge fluctuation model for experimental studies of water-in-oil microemulsions are suggested.     

Frequency spectra of transport properties in lonic liquids: contribution of charge fluctuation modes

Abstract

The influence of charge fluctuation modes on the frequency spectra for shear viscosity, self-diffusion and thermal diffusion is discussed. Both the one-component plasma, and modifications which are anticipated in real ionic liquids to the charge fluctuation modes, are considered. The latter treatment utilizes a simple model into which is incorporated plasmon dispersion and plasmon damping, the damping being both at zero wave number k and at order k²

The main emphasis of the paper is on non-analytic behaviour with frequency, and the modifications which occur for damped charge fluctuation modes. Because of the dominant influence of mass fluctuation modes at very long time, which are not considered quantitatively here, the theory discussed should have relevance at intermediate times, such as are treated in present molecular dynamical calculations. Contact is made with such computer simulation of molten BeF₂ and a model of molten salts     

Charge carrier dynamics in phonon-induced fluctuation systems from time-dependent wavepacket diffusion approach

A time-dependent wavepacket diffusion method is proposed to deal with charge transport in organic crystals. The electron-phonon interactions in both site energies and electronic couplings are incorporated by the time-dependent fluctuations which are generated from the corresponding spectral density functions. The numerical demonstrations reveal that the present approach predicts the consistent charge carrier dynamics with the rigorous quantum approaches. In addition, the diffusion coefficients obtained from the Marcus formula are well reproduced at the weak electronic coupling and high temperature limits. It is also found that the charge mobility feature of the crossover from the band-like to the hopping-type cannot be predicted from the fluctuations induced by the linear electron-phonon interactions with an Ohmic spectral density; however, it indeed appears as the electronic coupling fluctuation exponentially depends on the nuclear coordinates. Finally, it should be noted that although the present approach neglects the imaginary fluctuation, it essentially incorporates the coherent motion of the charge carrier and quantum effect of the phonon motion with a broad regime of the fluctuations for symmetric systems. Besides, the approach can easily be applied to systems having thousands of sites, which allows one to investigate charge transport in nanoscale organic crystals.     

Structure of Partly Quenched Molten Copper Chloride

Abstract

The structural modifications induced in a model of molten CuCl by quenching the chlorine component into a microporous disordered matrix are evaluated using the hypernetted-chain closure in Ornstein-Zernike relations for the pair distribution functions in random systems. Aside from obvious changes in the behaviour of long-wavelength density fluctuations, the main effect of partial quenching is an enhanced delocalization of the Cu⁺ ions. The model suggests that the ionic mobility in a superionic glass is enhanced relative to the melt at the same temperature and density. Only very minor quantitative differences are found in the structural functions when the replica Onstein-Zernike relations derived by Given and Stell for a partly quenched system are simplified to those given earlier by Madden and Glandt.     

Molecular dynamics study of screening at ionic surfaces

Molecular dynamics simulations of NaCl fluid are used to understand the behavior of ionic fluid to screen the field generated by charges on the ionic crystal surfaces in absence of any external electric field. The NaCl fluid in the strongly coupled regime (corresponding to the melt) in contact with the charged octopolar (111) NaCl surface shows that the spatial correlations decay in an oscillatory manner, with a screening length lambdaQ given by the envelope of the damped oscillations. By contrast to the Debye-Huckel theory, in the strongly coupled regime, lambdaQ increases with increasing coupling strength (also seen in bulk ionic simulations). The NaCl fluid confined between neutral (100) NaCl surfaces also shows weak oscillatory charge decay near the surface. Similar oscillatory exponential decay was seen when the NaCl fluid was confined between two analytically smooth neutral walls. The origin of these oscillations was due to the difference in ion sizes. NaCl fluid confined between neutral octopolar (110) and dipolar (110) surface show stronger density oscillations than (100) surface but comparatively very weak charge oscillations. This paper shows that the strength of the charges on the crystal surfaces is enough to induce a characteristic spatial distribution of charges in the contacting fluid and the extent of distribution depends on the type of surface.     

The landau-placzek ratio for multicomponent fluids

Abstract

Under the assumption that the coupling between the sound modes and modes associated with heat and mass diffusion can be neglected, an expression for the Landau-Placzek ratio for multicomponent fluids is derived using thermodynamic fluctuation theory. Applications of the general formula to ternary systems are discussed briefly     

Alternating charge densities,peierls distortion,and charge-conjugation symmetry in correlated one-dimensional diatomic systems

The full-optimized-APSG approach based on the MC SCF technique is developed and applied to study ground-state properties of one-dimensional correlated systems. The effects of electron–electron interactions and bond relaxation are considered for the conjugated diatomic polymer; charge distribution and bond relaxation are calculated for the N = 50 chain within a wide range of site energy and e–e integral modulation involving the case of alternancy symmetry for diatomic systems. With relation to the results obtained, the problem of the neutral–ionic transition in mixed-stack crystals is discussed. © 1994 John Wiley & Sons, Inc.     

Invited Lecture. Fluctuation absorption of sound in smectics

Abstract

A comparison is made of the low-frequency dynamics of smectics A and smectics C. It is shown that in both cases fluctuations of smectic layer displacements bring about contributions to the bulk viscosity coefficients (diverging as ω^?1) and consequently lead to anomalous sound attenuation. However, the coefficients of these fluctuation contributions differ greatly between smectics A and smectics C. The reason for this is a strong coupling of the orientational mode in smectics C to layer displacements (the so-called undulation mode). The results of this work make it possible to give a complete interpretation of the experimental data on sound absorption in smectics.     

Alternant systems and their properties

Neutral, ionic, and complete alternant systems are studied using the alternancy symmetry adapted (ASA ) approach. This approach is based on an explicit construction of ASA operators that are up to the sign invariant with respect to the particle-hole symmetry transformation. These operators serve as building blocks of alternant systems, and they determine their characteristic properties. All Hamiltonians describing neutral alternant systems are explicitly constructed. Up to some minor restrictions, all Hamiltonians describing ionic and complete alternant systems are also explicitly constructed. Inversely, given a Hamiltonian ? in a second quantization notation, one can easily check whether or not this Hamiltonian describes a neutral (ionic, complete) alternant system. All linear properties characteristic to neutral (ionic, complete) alternant systems are obtained. In particular, all one- and two-particle properties are derived in an explicit form. The properties obtained substantially generalize “classical” properties of alternant systems such as, in the case of neutral alternant systems, uniform charge density distribution, vanishing bond orders between atomic sites of the same parity, and alternancy selection rules for the electric dipole transitions.     

Sound velocity dispersion in room temperature ionic liquids studied using the transient grating method

Sound velocity is determined by the transient grating method in a range from 10(6) to 10(10) Hz in three room temperature ionic liquids, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-butyl-3-methylimidazolium hexafluorophosphate, and N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide. In all room temperature ionic liquids studied, the sound velocity increased with increasing frequency. The cause of this change is posited to be structural relaxation in the room temperature ionic liquids. Frequency dependence of the sound velocity is not reproduced by a simple Debye relaxation model. The sound velocity dispersion relation in 1-butyl-3-methylimidazolium hexafluorophosphate matches a Cole-Davidson function with parameters determined by a dielectric relaxation [C. Daguenet et al., J. Phys. Chem. B 110, 12682 (2006)], indicating that structural and reorientational relaxations are strongly coupled. Conversely, the sound velocity dispersions of the other two ionic liquids measured do not match those measured for dielectric relaxation, implying that structural relaxation is much faster than the reorientational relaxation. This difference is discussed in relation to the motilities of anions and cations.     

Charge reduction in electrosprays: slender nanojets as intermediates

Molecular dynamics simulations were used to study charge reduction in electrosprayed liquids through the formation of slender nanojet intermediates. The dynamics of shape relaxation and disintegration were followed as a function of charge in cylindrical water nanojets containing protonated diglycine molecules. Depending on the overall charge, simulations showed three basic scenarios for nanojet evolution. Moderately charged nanojets reduced to spheres, whereas nanojets charged close to the Rayleigh limit divided into two offspring droplets. Due to the large Coulomb interaction between ions, highly charged nanojets suffered repeated fission until the resulting droplets were charged below the Rayleigh limit. We demonstrated the role of surface fluctuations and Maxwell stress distributions in the disintegration process. The relaxation dynamics of the moderately charged systems to spherical geometry followed a damped oscillator behavior. Compared to neutral water jets, the presence of charges in subcritical nanojets resulted in a stiffer system with longer relaxation times to spherical geometry. Interparticle forces acting between the separating offspring droplets in nanojet breakup were also determined. Due to the increased role of fluctuations in nanojets, the Rayleigh limit was shown to overestimate the maximum charge on stable systems indicating higher nanodroplet production efficiency than one would expect from macroscopic theories alone.     

Density oscillations in a model of water and other similar liquids

It is suggested that the dynamics of liquid water have a component consisting of O^?2z (oxygen) anions and H^+z (hydrogen) cations, where z is a (small) reduced effective electron charge. Such a model may apply to other similar liquids. The eigenmodes of density oscillations are derived for such a two-species ionic plasma, included the sound waves, and the dielectric function is calculated. The plasmons may contribute to the elementary excitations in a model introduced recently for the thermodynamics of liquids. It is shown that the sound anomaly in water can be understood on the basis of this model. The results are generalised to an asymmetric short-range interaction between the ionic species as well as to a multi-component plasma, and the structure factor is calculated.     

Exploring ultrafast H-atom elimination versus photofragmentation pathways in pyrazole following 200 nm excitation

The role of ultraviolet photoresistance in many biomolecules (e.g., DNA bases and amino acids) has been extensively researched in recent years. This behavior has largely been attributed to the participation of dissociative (1)πσ* states localized along X-H (X ═ N, O) bonds, which facilitate an efficient means for rapid nonradiative relaxation back to the electronic ground state via conical intersections or ultrafast H-atom elimination. One such species known to exhibit this characteristic photochemistry is the UV chromophore imidazole, a subunit in the biomolecules adenine and histidine. However, the (1)πσ* driven photochemistry of its structural isomer pyrazole has received much less attention, both experimentally and theoretically. Here, we probe the ultrafast excited state dynamics occurring in pyrazole following photoexcitation at 200 nm (6.2 eV) using two experimental methodologies. The first uses time-resolved velocity map ion imaging to investigate the ultrafast H-atom elimination dynamics following direct excitation to the lowest energy (1)πσ* state (1(1)A" ← X(1)A'). These results yield a bimodal distribution of eliminated H-atoms, situated at low and high kinetic energies, the latter of which we attribute to (1)πσ* mediated N-H fission. The time constants extracted for the low and high energy features are ~120 and <50 fs, respectively. We also investigate the role of ring deformation relaxation pathways from the first optically bright (1)ππ* state (2(1)A' ← X(1)A'), by performing time-resolved ion yield measurements. These results are modeled using a (1)ππ* → ring deformation → photofragmentation mechanism (a model based on comparison with theoretical calculations on the structural isomer imidazole) and all photofragments possess appearance time constants of <160 fs. A comparison between time-resolved velocity map ion imaging and time-resolved ion yield measurements suggest that (1)πσ* driven N-H fission gives rise to the dominant kinetic photoproducts, re-enforcing the important role (1)πσ* states have in the excited state dynamics of biological chromophores and related aromatic heterocycles.     

Following the excited state relaxation dynamics of indole and 5-hydroxyindole using time-resolved photoelectron spectroscopy

Time-resolved photoelectron spectroscopy was used to obtain new information about the dynamics of electronic relaxation in gas-phase indole and 5-hydroxyindole following UV excitation with femtosecond laser pulses centred at 249 nm and 273 nm. Our analysis of the data was supported by ab initio calculations at the coupled cluster and complete-active-space self-consistent-field levels. The optically bright (1)L(a) and (1)L(b) electronic states of (1)ππ? character and spectroscopically dark and dissociative (1)πσ? states were all found to play a role in the overall relaxation process. In both molecules we conclude that the initially excited (1)L(a) state decays non-adiabatically on a sub 100 fs timescale via two competing pathways, populating either the subsequently long-lived (1)L(b) state or the (1)πσ? state localised along the N-H coordinate, which exhibits a lifetime on the order of 1 ps. In the case of 5-hydroxyindole, we conclude that the (1)πσ? state localised along the O-H coordinate plays little or no role in the relaxation dynamics at the two excitation wavelengths studied.     

Polarization relaxation in an ionic liquid confined between electrified walls

The response of a room temperature molten salt to an external electric field when it is confined to a nanoslit is studied by molecular dynamics simulations. The fluid is confined between two parallel and oppositely charged walls, emulating two electrified solid-liquid interfaces. Attention is focused on structural, electrostatic, and dynamical properties, which are compared with those of the nonpolarized fluid. It is found that the relaxation of the electrostatic potential, after switching the electric field off, occurs in two stages. A first, subpicosecond process accounts for 80% of the decay and is followed by a second subdiffusive process with a time constant of 8 ps. Diffusion is not involved in the relaxation, which is mostly driven by small anion translations. The relaxation of the polarization in the confined system is discussed in terms of the spectrum of charge density fluctuations in the bulk.     

Geldart B 类颗粒气固流化床内的压力波动特性(英文)

采用多通道压力采集系统测量了Geldart B类颗粒（树脂）矩形流化床(2.000m×0.300m×0.025m)内的压力波动,探索了流化床内的压力波动特征;同时采用标准方差、自相关和互相关函数分析了表观气速和静床高度对压力波动、压力波速度和压力波主频的影响。结果表明,气泡行为（如：气泡的形成、发展、聚并和破碎）是影响流化床内压力波动的主要因素;密相和稀相界面处的压力波动幅值主要由气泡崩塌决定;压力波在流化床内进行传播,并且具有明显的周期性特征;此外,压力波动、压力波速度和压力波主频均与表观气速和静床高度密切相关。     

Unraveling ultrafast dynamics in photoexcited aniline

A combination of ultrafast time-resolved velocity map imaging (TR-VMI) methods and complete active space self-consistent field (CASSCF) ab initio calculations are implemented to investigate the electronic excited-state dynamics in aniline (aminobenzene), with a perspective for modeling (1)πσ* mediated dynamics along the amino moiety in the purine derived DNA bases. This synergy between experiment and theory has enabled a comprehensive picture of the photochemical pathways/conical intersections (CIs), which govern the dynamics in aniline, to be established over a wide range of excitation wavelengths. TR-VMI studies following excitation to the lowest-lying (1)ππ* state (1(1)ππ*) with a broadband femtosecond laser pulse, centered at wavelengths longer than 250 nm (4.97 eV), do not generate any measurable signature for (1)πσ* driven N-H bond fission on the amino group. Between wavelengths of 250 and >240 nm (<5.17 eV), coupling from 1(1)ππ* onto the (1)πσ* state at a 1(1)ππ*/(1)πσ* CI facilitates ultrafast nonadiabatic N-H bond fission through a (1)πσ*/S(0) CI in <1 ps, a notion supported by CASSCF results. For excitation to the higher lying 2(1)ππ* state, calculations reveal a near barrierless pathway for CI coupling between the 2(1)ππ* and 1(1)ππ* states, enabling the excited-state population to evolve through a rapid sequential 2(1)ππ* → 1(1)ππ* → (1)πσ* → N-H fission mechanism, which we observe to take place in 155 ± 30 fs at 240 nm. We also postulate that an analogous cascade of CI couplings facilitates N-H bond scission along the (1)πσ* state in 170 ± 20 fs, following 200 nm (6.21 eV) excitation to the 3(1)ππ* surface. Particularly illuminating is the fact that a number of the CASSCF calculated CI geometries in aniline bear an exceptional resemblance with previously calculated CIs and potential energy profiles along the amino moiety in guanine, strongly suggesting that the results here may act as an excellent grounding for better understanding (1)πσ* driven dynamics in this ubiquitous genetic building block.     

Fluctuation theorem for nonequilibrium reactions

A fluctuation theorem is derived for stochastic nonequilibrium reactions ruled by the chemical master equation. The theorem is expressed in terms of the generating and large-deviation functions characterizing the fluctuations of a quantity which measures the loss of detailed balance out of thermodynamic equilibrium. The relationship to entropy production is established and discussed. The fluctuation theorem is verified in the Schl?gl model of far-from-equilibrium bistability.     

Effect of side-chain length on charge mobilities in neutral poly(3-alkylthiophene)s: Determination from dielectric measurement

For neutral poly(3-alkylthiophene)s (P3ATs), the electric modulus formalism of dielectric relaxation measurement together with the use of nonexponential decay function can be applied to describe the behavior of conductivity relaxation and carrier transport. The charge mobilities of neutral P3ATs calculated from conductivity relaxation with the use of the defect-diffusion model are in agreement with the data from field-effect transistor measurement. The temperature dependence of charge mobility exhibits a local maximum right after the end of glass transition region, which can be attributed to the transition of soft conformons in the disordered phase to localized conformons. The charge mobilities and activation energies of mobilities for P3ATs are dependent on conjugation length and volume fraction of conducting units. © 1994 John Wiley & Sons, Inc.     

Highly asymmetric electrolytes in the primitive model: Hypernetted chain solution in arbitrary spatial dimensions

The pair‐correlation functions for fluid ionic mixtures in arbitrary spatial dimensions are computed in hypernetted chain (HNC) approximation. In the primitive model (PM), all ions are approximated as nonoverlapping hyperspheres with Coulomb interactions. Our spectral HNC solver is based on a Fourier‐Bessel transform introduced by Talman (J. Comput. Phys. 1978, 29, 35), with logarithmically spaced computational grids. Numeric efficiency for arbitrary spatial dimensions is a commonly exploited virtue of this transform method. Here, we highlight another advantage of logarithmic grids, consisting in efficient sampling of pair‐correlation functions for highly asymmetric ionic mixtures. For three‐dimensional fluids, ion size and charge‐ratios larger than 1000 can be treated, corresponding to hitherto computationally not accessed micrometer‐sized colloidal spheres in 1‐1 electrolyte. Effective colloidal charge numbers are extracted from our PM results. For moderately large ion size and charge‐asymmetries, we present molecular dynamics simulation results that agree well with the approximate HNC pair correlations. © 2013 Wiley Periodicals, Inc.