bluuebunny/crossref_metadata_2025_split huggingface dataset:
Energy Dissipation in Chemical Reactions on Ultrafast Timescales
Authors: Russell Daniel J. , Paige Mark E. , Harris Charles B. 
Published: July 2012

AbstractA series of picosecond experiments and computer simulations will be presented that test collisional and hydrodynamic models for vibrational relaxation in liquids. The relationships between isolated binary collision models (IBC) and stochastic dynamics will be presented. The appropriateness of IBC theory in describing vibrational relaxation in liquids will also be discussed.
Stochastic Resonance
Authors: Moss Frank 
Published: July 2012

AbstractStochastic Resonance (SR) is a term given to an effect which is manifest in multi stable nonlinear systems driven simultaneously by noise and a weak periodic function, whereby the information flow through the system, in the form of the frequency of the periodic function, is assisted by the noise. For every frequency of the modulation, the information flow is optimum for a specific noise intensity, that is for a specific Kramers transition rate, hence the term resonance. Two physical quantities which characterize the response of such a system have been the objects of a flurry of recent experimental and theoretical activity: the Fourier transform, or auto correlation function of the appropriate state variable, and the probability density of the residence or escape times. The former have been used to obtain the power spectra and hence the signal‐to‐noise ratios of the response, while the latter directly reflect the rates and symmetry properties of the system. Calculation of these quantities pose specific problems for theorists characteristic of non stationary Fokker‐Planck systems. In this paper, I will briefly review the recent activity and include some remarks on the historical foundations of SR.
Effect of Periodic Driving on the Escape in Periodic Potentials
Authors: Jung Peter , Hänggi Peter 
Published: July 2012

AbstractWe consider activation processes in multistable systems exposed to external fluctuations and periodic modulation. The concept of defining escape rates out of a basin of attraction as the ratio of total flux over the basin boundary and the population inside the basin of attraction is generalized for periodically driven systems. Thereby, the escape rate is connected with the Floquet‐spectrum of the time inhomogeneous Fokker‐Planck operator. Our formalism is demonstrated for the particular case of a multistable washboard potential. Numerical results are compared with theoretical results in the limits of small and large driving frequencies, respectively.
Dynamics of Multidimensional Barrier Crossing in the Overdamped Limit
Authors: Carmeli Benny , Mujica Vladimiro , Nitzan Abraham 
Published: July 2012

AbstractTwo methods for numerical solution of multidimensional diffusion problems are presented and applied to the two dimensional barrier crossing problem in the overdamped limit. One of these methods is based on evaluating the smallest non‐vanishing eigenvalue of the Smoluchowski equation, and the other is based on an adaption of Chandler's steady state correlation function approach. Both methods make use of the fast Fourier transform algorithm for solving a transformed version of the Smoluchowski equation. The numerical solutions are compared to results based on the Kramers theory and some observations concerning effects of the dynamics of barrier crossing problems are made.
Interrelations of Different Methods for the Determination of Rates: Flux Over Population, Generalized Reactive Flux, the Lowest Eigenvalue and Its Rayleigh Quotient
Authors: Talkner Peter 
Published: July 2012

AbstractThe ratio of Kramers' current carrying stationary probability density and the equilibrium probability density is utilized as smoothed characteristic function in the generalized reactive flux method recently proposed by Borkovec and Talkner [J. Chem. Phys. 92, 5307 (1990)]. Under a certain condition on the potential, as generalized transition state rate Kramers' phase space diffusion limited rate is obtained. It then represents an upper bound for the true rate. An approximate expression for the plateau value of the generalized reactive flux yields a Rayleigh quotient for the lowest eigenvalue of the considered Fokker Planck process.
Periodic Orbit Approach to the Quantum‐Kramers‐Rate
Authors: Hänggi Peter , Hontscha Waldemar 
Published: July 2012

AbstractThe quantum analog of Kramers reaction rate for a dissipative environment is derived on the basis of a periodic orbit approach for multidimensional tunneling. The resulting reaction rate expression holds at all temperatures, thus covering [in contrast to the imaginary free energy method (“bounce‐” method)] the classical and the quantum regime on the same basis.
Traversal Time in Tunneling
Authors: Landauer Rolf 
Published: July 2012

AbstractTraversal time in tunneling has become a somewhat controversial subject, and this note presents a personal perspective on some of these discussions. There is a widely invoked linear relationship yielding dwell time as a weighted average of traversal time and reflection time. I assert that it is totally without basis. Many authors follow wave packets through barriers. The results of such calculations are hard to correlate with experiment. Stevens has provided a wave packet approach which avoids some of the problems typically faced by such analyses; nevertheless its analytical details are questioned. The significance of complex traversal times, favored by some investigators, is questioned. The relative merit of various possible “clocks” in the analysis of traversal time is discussed, but without definitive conclusions.
Electronic Structure and Bonding in Cyclic B‐S and B‐Se Compounds Studied by Solid State 11B NMR
Authors: Conrady‐Pigorsch R. , Müller‐Warmuth W. , Schwetlik G. , Wienkenhöver M. , Krebs B. 
Published: July 2012

AbstractSolid state 11B (and in one case 10B) NMR spectra of four cyclic boron‐chalcogen compounds have been measured at various frequencies and analysed in terms of nuclear quadrupole and chemical shift interactions. Application of the Townes‐Dailey theory to the quadrupole results, corrected for ionic contributions to the electric field gradient and to the bonding, yields detailed information about the importance of μ‐bonding in the respective materials. The occupation number of the 2pz orbitals of the sp2‐hybridized boron atoms appear to be particularly large for systems containing chalcogen‐chalcogen bridges. From both the chemical point of view and the spectroscopic result, among the six‐ and five‐membered rings, diiodotriselenadiborolane is distinguished by an exceptional position.
On the Stability of Microclusters with Three‐Body Forces
Authors: Yurtsever Ersin , Elmaci Nuran 
Published: July 2012

AbstractA structural analysis of microclusters is carried out on 36 high‐symmetry structures using classical and quantum mechanical minimization methods. For the classical calculations, the total energy is optimized utilizing two‐body potential functions first and then the effects of three‐body forces to these structures are analyzed. The same set of high‐symmetry clusters made of Carbon atoms are studied by a semi‐empirical quantum mechanical method (AM1) and correlations between two approaches are studied. – At large three‐body intensity parameter Z*, both AM1 and static results show that linear conformations are preferred to two‐dimensional structures. Three‐dimensional structures seem to be the least stable ones. But at small Z*, these results are reversed. Overall there is a good agreement between both set of results for Z* = 0.8, suggesting that a qualitative analysis of Carbon clusters can be done with simple potential functions provided that properly weighted three‐body interactions are also employed.
The Dielectric Properties of tert.‐Butanol/Water Mixtures as a Function of Composition
Authors: Kaatze U. , Schumacher A. , Pottel R. 
Published: July 2012

AbstractAt 25°C the complex (dielectric) permittivity of the system tert.‐butanol/water has been measured as a function of frequency v(3 MHz ≦ v ≦ 70 GHz) and mole fraction x (0 ≦ × ≦ 1). Different relaxation functions are fitted to the measured spectra. It is found that within the complete composition range an empirical function is appropriate, which is based on an unsymmetric continuous distribution of relaxation times. Other than expected on grounds of previous light scattering and ultrasonic absorption studies there are neither indications for the formation of microphases nor for special hydrophobic hydration effects. The parameters of the relaxation spectral function are discussed to yield information on the liquid structure and molecular motions of the mixtures. Comparison is made with further aqueous systems which had been measured over a wide composition range, in particular other alcohol/water mixtures but also aqueous solutions of nearly non‐dipolar quinoxaline and of dimethyl sulfoxide.
Precise Measurements of Second Virial Coefficients of Simple Gases and Gas Mixtures in the Temperature Range Below 300 K
Authors: Schramm B. , Elias E. , Kern L. , Natour Gh. , Schmitt A. , Weber Ch. 
Published: July 2012

AbstractA new apparatus for measurements of second virial coefficients of gases or gas mixtures at room temperature is described. It was used to determine virial coefficients of He, H2, N2, CO2, NH3, and the interaction virial coefficient of Ar/NH3. Furthermore, low temperature interaction virial coefficients of Ar/NH3and H2/CO are reported and finally a critical revision of second virial coefficients of He, H2, Ar, N2, and CO at 77.3 K is presented. The virial coefficients are discussed in their relation to new intermolecular potentials.
The Inverse Photopyroelectric Technique for the Measurement of Concentration and Transport Properties in Binary Systems: The Thermal Effusivity of Ethanol‐Water Mixtures
Authors: Chirtoc Mihai , Toşa Valer , Bićanić Dane , Torfs Paul 
Published: July 2012

AbstractA new method and apparatus are described for the determination of concentration in liquid mixtures through the measurement of their thermal effusivity. The inverse photopyroelectric method (IPPE) implies the front illumination of a thin pyroelectric sensor the rear side of which is brought into the intimate thermal contact with the sample. The feasibility of this novel technique is demonstrated with ethanol‐water mixtures. Good agreement between the experiment and theory has been obtained.
UV‐Resonance‐Raman Studies of Protonated Nucleic Acid Bases
Authors: Gfrörer A. , Schnetter M. E. , Wolfrum J. , Greulich K. O. 
Published: July 2012

AbstractUV‐resonance‐Raman‐spectra of protonated adenine, guanine, cytosine and 5‐methyl‐cytosine (mono‐phosphates) were taken with a UV‐spectrometer based on an excimer laser system. Four different wavelengths (220, 230, 260 and 270 nm) were used for resonant excitation of the Raman spectra and most of the spectra were taken both in H2O and in D2O. The very different excitation patterns obtained for different wavelengths, clearly demonstrate that an appropriate choice of the excitation wavelength for resonance enhancement increases the potential of Raman spectroscopy for the detection of protonated bases in complex nucleic acid structures significantly. For adenine the protonation is most clearly seen with an excitation wavelength of 220 nm, whereas for cytosine the most prominent changes occur at 260 nm. Apart from a couple of skeletal modes that undergo both frequency and intensity changes, the spectral region of the exocyclic modes (1550–1700 cm−1) yields additional information with resonant excitation. The enlarged spectral data basis obtained with the variation of the excitation wavelength is used to assign bands of the protonated species to molecular bond structures.
Photochemistry of Colloidal Silver Particles: The Effects of N2O and Adsorbed CN−
Authors: Linnert Thomas , Mulvaney Paul , Henglein Arnim 
Published: July 2012

AbstractColloidal silver particles (∼ 3 nm diameter) are photo‐oxidized in aqueous solution in the presence of nitrous oxide, the quantum yield increasing with decreasing wavelength. Nitrogen is formed simultaneously. The effect is explained by photo‐electron emission from the particles, followed by the capture of the emitted electrons by N2O. In the presence of propanol‐2, the silver particles are not destroyed but catalyse the photo‐decomposition of N2O.—Silver particles carrying adsorbed CN− ions are also photo‐oxidized even in the absence of N2O. At the same time, hydrogen is formed by reduction of the aqueous solvent. The silver particles even dissolve when illuminated at 420 nm, i.e. into the long‐wavelength tail of the plasmon absorption band. The effect is attributed to enhanced photo‐electron emission due to 1) the increased negative potential of the Fermi level in the CN− ‐modified silver particles, 2) the negative overall‐charge of these particles, and 3) rapid hole capture by pre‐complexed surface atoms.—For a long time, metals have been thought to possess practically no photochemistry. The observations made here show that metals, when present as a fine dispersion, are demonstrably photoactive.
The Static Dielectric Constant of the Water‐Benzene Mixture System to 400°C and 2800 bar
Authors: Deul R. , Franck E. U. 
Published: July 2012

AbstractThe water‐benzene mixture system at high temperature and high density is considered as a model system to study the dielectric properties of polar‐nonpolar binary mixtures. The static dielectric constant or permittivity ε and the density have been measured at 300, 350 and 400°C between 100 and 2800 bar for 7 different water‐benzene mixtures covering the whole range between pure water and pure benzene. Apparatus and procedure are described. An autoclave contains a cell closed with a flexible bellows. Inside the cell is a cylindrical condenser, filled with a sample mixture, separated from the pressurizing fluid. Thus dielectric and density measurements can be performed simultaneously with one sample.—Extensive tables with data are given. The uncertainty is believed to be below ±1% for most data. At 400°C and 2000 bar ε(χw = 1.0) is 19.40 and decreases to 9.67 (χw = 0.9) and to 3.68 (χw = 0.5). For pure benzene the value is 2.1. Several continuum theories are examined and discussed to represent the data. The best result was obtained with the Landau‐Lifshitz‐Looyenga relation. It needs dielectric constants of the pure partners and the mixture densities but requires no adjustable parameters. On the basis of the present investigation this relation is recommended also to estimate dielectric constants for dense, supercritical binary systems of water with nonpolar gases.
Pressure Effect on the Radiationless Deactivation of Singlet Oxygen by 1,4‐Diazobicyclo [2.2.2] octane in Six Different Solvents
Authors: Seikel K. , Brauer H.‐D. 
Published: July 2012

AbstractThe rate constantkqof quenching of singlet oxygen (1O2) by 1,4‐diazobicyclo [2.2.2] octane (DABCO) has been determined in six solvents in dependence of pressure using time resolved phosphorescence techniques. Experimental activation volumes vary between −19.0 ml·mol−1(chloroform) ≤ ΔVobs# ≤ −10.0 ml·mol−1(benzonitrile). The data are analyzed by means of a kinetic scheme assuming a preequilibrium of1O2and DABCO to form a singlet contact complex1C. The data are consistent with the assumption that the deactivation of1O2occurs via a charge transfer process between the collision partners of C converting1C to3C, which subsequently splits into the deactivation products. The statistical model developed by Yoshimura and Nakahara is used to calculate the reaction volume ΔVcof the preequilibrium. ΔVcand the activation volume ΔVCT# of the actual deactivation of1O2by DABCO contribute practically in equal parts to ΔVobs.# ΔVCT.# values strongly depend on the solvent polarity. On the basis of Kirkwood's electrostriction model the actual activation volume (ΔCT#)° without electrostriction effect and the dipole moment of the transition state μCT* have been estimated to: (ΔVCT#)° = −4.3 ml·mol−1and μCT# = 7.8 Debye, respectively.
Apparently “Universal” Dielectric Behaviour of Electrolytic Solutions
Authors: Adams J. , Aust A. , Brockmann U. , Klockgeter K. , Schütz G. , Steffen A. , Stockhausen M. 
Published: July 2012

AbstractThe real part of conductivity was measured for a number of salt solutions in different polar (nonaqueous) solvents over a broad frequency range up to 72 GHz at 20°C. The slope of the double log representation of conductivity versus frequency decreases with increasing salt concentration and solution viscosity. An extrapolation to infinite viscosity yields slope values similar to those found with vitreous systems which are claimed to exhibit “universal” behaviour.
Nuclear Spin Relaxation. Nuclear Spin Relaxation and Electrical Conductivity in Lithium Germanate Glasses
Authors: Kanert O. , Kloke M. , Küchler R. , Rückstein S. , Jain H. 
Published: July 2012

Abstract7Li nuclear spin relaxation (NSR) in the laboratory frame (T1) as well as in the rotating frame (T1ϱ) and electrical conductivity measurements were performed in (1 – x) GeO2 · xLi2O glass series, where x = 0.001 to 0.36, between 3 K and the glass transition temperature (≃ 700 K). Below about 250 K the NSR data can be interpreted consistently in the framework of thermally activated low‐frequency excitations of disordered modes intrinsic to the glassy state of matter. The modes are described by asymmetric double well potential (ADWP) configurations with suitable densities of states. Above about 250 K the NSR rates increase exponentially with rising temperature due to diffusive jumps of the Li ions. Deduction of corresponding transport properties, however, from such data by means of standard NSR theories turns out to be difficult. Models are discussed to overcome these difficulties by relating NSR with electrical conductivity. The results are found to correspond best with Funke's model of a coupled motion of the ions caused by Coulomb interaction.
Reorientational Correlation Functions of Simple Supercooled Liquids as Revealed by NMR Studies
Authors: Rössler E. , Börner K. , Tauchert J. , Taupitz M. , Pöschl M. 
Published: July 2012

Abstract31P,1H and2H NMR investigations on supercooled simple organic liquids are presented. Combining measurements of spin‐lattice relaxation, spin‐spin relaxation and stimulated echo decays the reorientational correlation functionF2(t) of the second Legendre polynomial is probed over a wide temperature interval. Correlation times in the range of 10−11– 1s are covered, and the scaling property of the correlation functionF2(t) corresponding to the α‐relaxation, i.e., the main relaxation in the supercooled liquid, is checked. The functionF2(t) can be described equally well by a Cole‐Davidson distribution of correlation times or by a Kohlrausch‐Williams‐Watts function (10−8< τ < 1s). NearTgin the highly viscous regime the correlation times become independent from the molecular size. At similar temperatures the bifurcation of a secondary process (β‐process) is observed. Combining results from neutron scattering and NMR the completeF2(t) has to be described by a three‐step function near the glass transition temperatureTgand by a two‐step function at temperatures well aboveTg. The transition atTgis accompanied by the onset of non‐exponential spin‐lattice relaxation and by a discontinuous change of the temperature coefficient of the relaxation.
Flash CIDNP Study of Photosensitized Electron Transfer Reactions of Methoxybenzenes
Authors: Goez Martin , Eckert Gerd 
Published: July 2012

AbstractThe photoreactions between 9,10‐anthraquinone and the donors C6H6–n (OMe)n, where n = 1 …3, in acetonitrile‐d3 have been investigated by time resolved measurements of chemically induced dynamic nuclear polarization (“flash CIDNP”). The activation parameters for the degenerate electron transfer between five of the donors and their radical cations have been determined. For these, the same isokinetic relationship as in the previously studied case of para‐substituted N,N‐dimethylanilines was found to hold. Theoretical rate constants for the self exchange processes were calculated from AM 1 data by a model based on the Marcus theory. Their agreement with the experimental values was satisfactory over the whole range of investigated temperatures (237 K … 298 K).
Slowing Down of Electrolytic Conductance in the Critical Region of Partially Miscible Ternary Liquid Mixtures
Authors: Flindt S. , Steiger A. , Becker F. 
Published: July 2012

AbstractThe electrolytic conductance and the capacitance of 6 ternary liquid systems of the type carboxylic acid + tertiary amine + hydrocarbon are investigated in the neighbourhood of critical solution points (C.P.) A stirred conductance cell is used which yields information not only from the one‐phase region but also from a small section along the rectilinear diameter in the two‐phase region. The plots of conductance resp. capacitance vs. temperature exhibit a fall‐off by 5 to 20 per cent in the critical region which is virtually symmetric with respect to the C.P. Evaluation of the data in the fall‐off region, the width of which – depending on the chemical nature of the system – ranges from ±0.035 to ±0.45 K, yields critical exponents ∂ between 0.53 and 0.67. This suggests that the fall‐off of conductance resp. capacitance results mainly from the critical slowing down of the transport phenomena by the divergence of the correlation length ζ.
A Large Bathochromic Shift from the Solution to the Solid State in 1,4‐Diketo‐3,6‐Diphenyl‐Pyrrolo‐[3,4‐c]‐Pyrrole
Authors: Mizuguchi Jin , Wooden Gary 
Published: July 2012

AbstractA large bathochromic shift (about 1400 cm−1) from the solution to the solid state in the title compound has been investigated from the standpoint of the change in electron density on the nitrogen atom caused by deprotonation or by intermolecular hydrogen bonding. The electronic state of the diketopyrrolopyrrole chromophore is found to be most sensitively affected by an environment of proton acceptors which interact with the NH group. Mono and di‐deprotonation in solution brings about large bathochromic displacements of 2650 and 3400 cm−1, respectively. The deprotonation leads to increased electron density on the nitrogen atom and, as a consequence, more overall electron density in the chromophore which contributes to the bathochromic shift. Similar increase in electron density is also operative in the solid state through the intermolecular hydrogen bonding between the NH of one molecule and the oxygen of another. The contribution of the hydrogen bond to the large shift is estimated to be about 1000 cm−1 in the solid state.
Quantum Sized Semiconductor Particles in Solution and in Modified Layers
Authors: Weller Horst 
Published: July 2012

AbstractThe preparation of highly monodisperse, quantum‐sized CdS particles is described. In these samples a band splitting into discrete exciton‐like states is observed.—Quantum‐sized CdS and PbS particles prepared in situ on highly porous, polycrystalline TiO2 electrodes work as highly efficient sensitizers. Photocurrent yields of more than 70% are reported. In the case of PbS it is shown that an efficient electron transfer is only possible with particles in the size quantization regime.