bluuebunny/crossref_metadata_2025 huggingface dataset:
Diagnosing transport and mixing using a tracer-based coordinate system
Authors: Shuckburgh Emily , Haynes Peter
Published: October 2003

The advection-diffusion equation for the concentration of a tracer may be transformed into a pure diffusion equation by using the area inside concentration contours as a coordinate. The corresponding effective diffusivity depends on the geometry of the tracer field, which is determined by the underlying flow. Recent studies have used effective diffusivity, calculated from a suitable tracer, as a qualitative indicator of the transport and mixing properties of a given flow. Here we show that the effective diffusivity may further be used as a quantitative diagnostic of transport and mixing. We use a family of incompressible two-dimensional time-periodic flows as a test-bench and compare the calculated effective diffusivity with other diagnostics. The results demonstrate how the effective diffusivity accurately captures the location and character of barrier and mixing regions. We also show that the effective diffusivity parameterizes the transport of particles relative to the tracer-based coordinates. These results support the use of effective diffusivity as a quantifier of transport and mixing, and thereby strengthen the conclusions of previous qualitative studies.
Application of a VUV Fourier transform spectrometer and synchrotron radiation source to measurements of. VI. The ε(0,0) band of NO
Authors: Cheung A. S-C. , Wong A. L. , Lo D. H-Y. , Leung K. W-S. , Yoshino K. , Thorne A. P. , Murray J. E. , Imajo T. , Ito K. , Matsui T.
Published: October 2003

The ε(0,0) (D 2Σ+–X 2Πr) band of NO has been recorded by using a vacuum ultraviolet Fourier transform spectrometer from Imperial College, London, with synchrotron radiation at the Photon Factory, KEK, Japan, as a continuum light source. Analysis of the ε(0,0) band provides accurate rotational line positions and term values as well as the photoabsorption cross sections. Molecular constants of the v=0 level of the D 2Σ state have been determined as T0=53 291.10±0.10 cm−1, B0=1.991 07±0.000 05 cm−1, and D0=(6.6±0.1)×10−6 cm−1. Accurate rotational line strengths have also been obtained and the sum of the line strengths for all rotational lines is determined as 2.18×10−15 cm2 cm−1. The band oscillator strength of the ε(0,0) band is determined to be (2.47±0.12)×10−3.
Selective detection of isomers with photoionization mass spectrometry for studies of hydrocarbon flame chemistry
Authors: Cool Terrill A. , Nakajima Koichi , Mostefaoui Toufik A. , Qi Fei , McIlroy Andrew , Westmoreland Phillip R. , Law Matthew E. , Poisson Lionel , Peterka Darcy S. , Ahmed Musahid
Published: October 2003

We report the first use of synchrotron radiation, continuously tunable from 8 to 15 eV, for flame-sampling photoionization mass spectrometry (PIMS). Synchrotron radiation offers important advantages over the use of pulsed vacuum ultraviolet lasers for PIMS; these include superior signal-to-noise, soft ionization, and access to photon energies outside the limited tuning ranges of current VUV laser sources. Near-threshold photoionization efficiency measurements were used to determine the absolute concentrations of the allene and propyne isomers of C3H4 in low-pressure laminar ethylene–oxygen and benzene–oxygen flames. Similar measurements of the isomeric composition of C2H4O species in a fuel-rich ethylene–oxygen flame revealed the presence of substantial concentrations of ethenol (vinyl alcohol) and acetaldehyde. Ethenol has not been previously detected in hydrocarbon flames. Absolute photoionization cross sections were measured for ethylene, allene, propyne, and acetaldehyde, using propene as a calibration standard. PIE curves are presented for several additional reaction intermediates prominent in hydrocarbon flames.
Reactions of phenylium ions C6(H,D)5+ with D2
Authors: Ascenzi D. , Bassi D. , Franceschi P. , Tosi P. , Di Stefano M. , Rosi M. , Sgamellotti A.
Published: October 2003

The reaction of phenylium and d-phenylium ions with molecular deuterium has been investigated both experimentally and theoretically. Theoretical calculations have been carried out at the B3LYP/6-31G* level of theory. The key features of the potential energy surfaces and all the relevant thermochemical parameters have been calculated. These results are in good agreement with previous theoretical studies concerning the unimolecular dissociation of benzenium ions. Theoretical calculations provide a clear insight on the reaction mechanisms for the system under study. Experimental studies have been carried out by means of a guided beam tandem mass spectrometer. We have measured the reaction cross section for four different reaction channels, as a function of the collision energy, in the range from thermal energies up to about 1 eV. The bimolecular reactivity of C6H5+ is dominated by the substitution of one or two hydrogen atoms by deuterium, leading to the formation of C6H4D+ and C6H3D2+ products. The reaction cross section is nearly the same for the two reaction channels. This fact is interpreted as evidence of a statistical behavior, as expected by assuming a fast shift of hydrogen (deuterium) within the aromatic ring. Another very weak reaction channel leading to the formation of the benzene ion has been measured. The cross section for such a process is lower than about 10−2 Å2 over the entire energy range explored in our experiment. A structure observed at collision energies around about 0.1 eV has been attributed to the reaction of phenylium ions in their triplet state. When the density of the gas in the scattering cell is increased, the formation of collisionally stabilized benzenium ions is observed.
Hybrid quantum chemical studies for the methanol formation reaction assisted by the proton transfer mechanism in supercritical water: CH3Cl+nH2O→CH3OH+HCl+(n−1)H2O
Authors: Hori T. , Takahashi H. , Nitta T.
Published: October 2003

The proton transfer along the chain of hydrogen bonds is involved in many chemical reactions in aqueous solution and known to play a decisive role. We have performed the hybrid quantum chemical simulations for the methanol formation reaction catalyzed by the proton transfer mechanism [CH3Cl+nH2O→CH3OH+HCl+(n−1)H2O, n=3] in supercritical water (SCW) to investigate the role of water solvent on the reaction. In the simulation, the electronic state of the chemically active solutes (CH3Cl+3H2O) has been determined quantum mechanically, while the static water solvent has been represented by a classical model. The activation free energy for the water–catalytic reaction in SCW has been found to be 9.6 kcal/mol, which is much lower than that in the gas phase (29.2 kcal/mol). The fractional charge analysis has revealed that the notable charge separation in the solute complex takes place at the transition state (TS) and the resulting huge dipole gives rise to the considerable stabilization of the TS as compared to the reactant. It has been shown that the reaction assisted by the proton transfer mechanism is energetically much favored than the ionic SN2 reaction (CH3Cl+OH−→CH3OH+Cl−, 18.8 kcal/mol). The present calculations suggest that the proton migrations through the chain of hydrogen bonds can be regarded as a probable candidate responsible for the anomalous reactivities observed in SCW.
Experimental realization of a fetching algorithm in a 7-qubit NMR spin Liouville space computer
Authors: Long Gui Lu , Xiao Li
Published: October 2003

Using a 7-qubit spin Liouville space computer, we have demonstrated successfully a fetching algorithm that finds marked items using only a single query. The essential idea is the operation of quantum computers in classical parallel. We gave the detailed pulse sequence for coherent control of the 7 qubits. A generalization of the pulse sequences for arbitrary qubit number is also provided.
Σ
3

−
and Π3 states of GeC and GeSi: The problematic dissociation energy of GeC
Authors: Sari Levent , Yamaguchi Yukio , Schaefer Henry F.
Published: October 2003

The Σ3 − and Π3 states for the GeC and GeSi diatomics have been investigated at the highly correlated coupled-cluster levels of theory. Large basis sets [including TZ3P(2d,2f)+2diff, cc-pVQZ, and aug-cc-pVQZ] were employed in order to predict reliable values for the experimentally unknown spectroscopic properties. The ground states were confirmed to be the Π3 state for GeC and the Σ3 − state for GeSi. Advanced theoretical treatments such as inclusion of core-valence correlation, scalar relativity, and complete basis set extrapolations have been performed to determine accurate energetic properties. The dissociation energies (D0) of 91.4 kcal/mol and 72.9 kcal/mol have been predicted for the X 3Π state of GeC and X 3Σ− state of GeSi, respectively. It is observed that the theoretical value of 72.9 kcal/mol for GeSi is in very good agreement with the experimental value of 71±5 kcal/mol. However, the predicted dissociation energy for the X 3Π state of GeC is found to be 18 kcal/mol smaller than the mass spectrometric measurements. Both the X 3Π and A 3Σ− state potential energy surfaces for GeC have been examined by the full valence configuration interaction technique to search for a possible transition state that might explain this discrepancy. However, it has been found that both states smoothly dissociate to the ground P3 states of carbon and germanium without any transition state. Several theoretical and experimental aspects related to the determination of the dissociation energy of GeC have been discussed. It is concluded that the theoretically determined value of 91.4 kcal/mol should be more reliable than the reported experimental values.
Forward–backward semiclassical dynamics for quantum fluids using pair propagators: Application to liquid para-hydrogen
Authors: Nakayama Akira , Makri Nancy
Published: October 2003

Forward–backward semiclassical dynamics (FBSD) methods are emerging as a practical way of simulating dynamical processes in large quantum systems. In this paper we develop a pair-product approximation to the coherent state density. This form is accurate at low temperatures, enhancing significantly the convergence of Monte Carlo methods and thus allowing the simulation of quantum fluids. The scheme is applied to the calculation of velocity autocorrelation function of liquid para-hydrogen at several thermodynamic state points (between T=14 K and T=25 K). The results of the forward–backward semiclassical method with the pair-product approximation to the coherent state density exhibit good agreement with experimental measurements and other theoretical calculations. These calculations demonstrate that the FBSD method, in conjunction with an accurate representation of the coherent state density, allows an accurate description of dynamical processes in condensed phase systems at low temperatures where quantum mechanical effects play a significant role.
Fundamental limits on two-photon absorption cross sections
Authors: Kuzyk Mark G.
Published: October 2003

We apply sum rules to calculate the maximum limit of the resonant two-photon absorption cross section. Comparison of the theory with experimental data from the literature shows that no molecule ever measured exceeds this limit. Molecules with more π electrons have larger two-photon cross sections; but, the two-photon cross sections fall further below the fundamental limit as the number of electrons increases—implying that the additional electrons are not participating efficiently. The sum rules can aid in developing structure property relationships by providing metrics that lead to a deeper understanding of the available data.
Densification effects on structural relaxation of polyolefins
Authors: Clark Jason , Maranas Janna K.
Published: October 2003

We investigate the effect of densification on intramolecular and intermolecular chain packing and structural relaxation of four saturated hydrocarbon polymers: poly(ethylene propylene) (PEP), poly(ethylene butene), atactic polypropylene, and head-to-head polypropylene. Within this series, the local chain architecture, specifically the frequency and size of pendant groups, varies. Density changes of ρ0±5%, where ρ0 is the ambient density, are considered at a temperature of 400 K, which is well above the glass transition temperatures of all four polymers. Intramolecular chain packing remains nearly unchanged over this density range, and intermolecular packing shows a tendency towards closer and more efficient packing at higher densities. The effect of density on dynamics depends on the spatial scale considered. Self-intermediate scattering functions, as measured in incoherent quasielastic neutron scattering experiments, are calculated for spatial scales between 2.5 and 20 Å. At length scales less than the closest intermolecular spacing, densification has little effect on mobility. The largest effects are observed in the vicinity of the nearest chain spacing distance, where mobility is decreased and dynamic heterogeneity, as evidenced by the stretching parameter in stretched exponential fits, is increased with densification. At length scales approaching Rg, both effects level off and are slightly less than at spatial scales of interchain packing. The most flexible of the four materials, PEP, is least affected by density changes.
Analysis of the contributions of three-body potentials in the equation of state of 4He
Authors: Ujevic Sebastian , Vitiello S. A.
Published: October 2003

The effect of three-body interatomic contributions in the equation of state of He4 are investigated. A recent two-body potential together with the Cohen and Murrell [Chem. Phys. Lett. 260, 371 (1996)] three-body potential are applied to describe bulk helium. The triple-dipole dispersion and exchange energies are evaluated subjected only to statistical uncertainties. An extension of the diffusion Monte Carlo method is applied in order to compute very small energies differences. The results show how the three-body contributions affect the ground-state energy, the equilibrium, melting and freezing densities.
Selecting the information content of two-dimensional Raman spectra in liquids
Authors: Ma Ao , Stratt Richard M.
Published: October 2003

The wealth of information carried by the two-dimensional Raman spectra of liquids comes with a price. The signal arises from a mixture of two entirely different mechanisms, each of which reveals its own perspective on intermolecular dynamics. In this paper we analyze the dynamical origins and consequences of both of these mechanisms. By applying an instantaneous-normal-mode formalism to the two-dimensional Raman spectrum of a solution of CS2 dissolved in Xe, we find discernable differences in the specific molecular degrees of freedom and basic symmetries that contribute to each mechanism. We then show how these differences can be exploited to separate these mechanisms experimentally. In particular, we point out how it should be possible to use two-dimensional Raman spectroscopy to measure the explicitly anharmonic contributions to liquid dynamics without obscuring the results by simultaneously measuring the nonlinear coupling of the polarizability to that dynamics.
Experimental and theoretical investigation of the c 1Π–a 1Δ transition of NH/D–Ne
Authors: Kerenskaya Galina , Schnupf Udo , Heaven Michael C.
Published: October 2003

A study of NH/D–Ne was undertaken to investigate the structure of this complex and examine the ability of high-level theoretical methods to predict its properties. The c 1Π–a 1Δ transition was characterized using laser induced fluorescence measurements. Spectra recorded in the vicinity of the monomer show groups of complex features associated with the monomer P(2), Q(2), and R(2) lines. The present study focused on the low-energy bands associated with P(2). Results from theoretical calculations were used to guide the interpretation of the spectra. Two-dimensional potential energy surfaces were calculated using second-order multireference perturbation theory with large correlation consistent basis sets. The potential surfaces were used to predict the rovibronic structure of the c–a system. Calculated rovibronic energy level patterns could be recognized in the spectra but quantitative discrepancies were found. For the a and c states the ab initio potentials were found to be too shallow, and for the c state the equilibrium intermolecular separation was too short. These errors are attributed to incomplete recovery of the dynamical correlation energy.
Pinchoff and satellite formation in compound viscous threads
Authors: Craster R. V. , Matar O. K. , Papageorgiou D. T.
Published: October 2003

The breakup of viscous compound threads in the presence of insoluble surfactant at both interfaces is investigated. We use asymptotic methods in the limit of long axisymmetric waves to derive a coupled system of five one-dimensional (1-D) partial differential equations governing the evolution of the outer and inner interfaces, the surfactant concentrations there, and the leading order axial velocity component in the jet. The linear, and nonlinear, stability of these equations is then investigated for a wide range of outer to inner viscosity ratio, m, outer to inner surface tension ratio, γ, the ratio of initial outer to inner radii, α, initial surfactant concentrations at the outer and inner interfaces, Γ10 and Γ20, surfactant activities, β1 and β2, and the Schmidt numbers, Sc1 and Sc2, defined as the ratio of the kinematic viscosity to the surfactant surface diffusion coefficient. We also show that if Sc1=Sc2, these results are recovered via solution of 1-D evolution equations governing the dynamics of an effective single surfactant covered thread, which are obtained through appropriate rescalings; these rescalings are detailed herein.
On the applicability of the step function nonradiative lifetime model for diffusion controlled reactions
Authors: Litniewski Marek , Gorecki Jerzy
Published: October 2003

We derive an approximate expression for the time-dependent reaction rate coefficient, k(t), of the Smoluchowski equation for the step function nonradiative lifetime (SFNL) model in the case of structureless liquid (i.e., if there are no spatial correlations between molecules of reactants). The SFNL model assumes that a reaction occurs with equal probability for reactants at distances between r0 and r1. The accuracy of the obtained analytical formula for k(t) is absolutely sufficient for practical applications like the interpretation of experiments on fluorescence quenching. A molecular dynamics has shown that the SFNL model much better describes the simulation results than the Smoluchowski–Collins–Kimball model if the distance between r1 and r0 cannot be neglected.
Study of supercritical krypton by small-angle neutron scattering: Analysis of the cross-over region
Authors: Bonetti M. , Calmettes P. , Bervillier C.
Published: October 2003

Small-angle neutron scattering spectra of supercritical krypton have been measured in the vicinity of the critical point along the critical isochore and at off-critical densities. The reduced temperature, τ̃=(T−Tc)/Tc, and density, ρ̃=(ρ−ρc)/ρc, were in the ranges 10−3<τ̃<9.5×10−2 and −0.263<ρ̃<+0.246, respectively. T and ρ are the temperature and the density, and Tc and ρc their critical values, respectively. A fit of a parametric representation of the structure factor to the spectra measured along the critical isochore allows the amplitudes of the correlation length and of the isothermal compressibility to be determined accurately. Within the studied range of wave-number transfers, q, the amplitude of a q3 term corresponding to the r−6 dependence of both the two-body van der Waals and the three-body Axilrod–Teller potentials can be determined. However the amplitude of the density dependent three-body Axilrod–Teller interaction cannot be inferred from the spectra measured at off-critical densities.
Comment on “The cohesive energetics of solid cesium chloride” [J. Chem. Phys. 118, 2308 (2003)]
Authors: Aguado Andrés
Published: October 2003

In a recent paper [J. Chem. Phys. 118, 2308 (2003)], Pyper has concluded that the ab initio Perturbed Ion (AIPI) model [Comput. Phys. Commun. 103, 287 (1997)] does not provide useful insights into the physical origins of short-range repulsive forces in ionic crystals. In this comment, I show that Pyper’s conclusion is wrong.
State-resolved dissociation dynamics of glyoxal near the threshold for formation of fragment HCO
Authors: Chen Ming-Wei , Lee Sheng Jui , Chen I-Chia
Published: October 2003

Fluorescence excitation spectra for transition Ã 1Au–X̃ 1Ag of trans-glyoxal in a supersonic jet were recorded with laser excitation in a wavelength region near 395 nm. With resolution 0.04 cm−1, most lines in these spectra are resolved and are assigned rotationally; of six bands assigned, three have c-type rotational structure, and another three have types a/b hybrid, a and b. Fluorescence decays with quantum beats, resulting from coherent excitation of S1 and T1 states, are observed for most rotational levels in this region. In spectra transformed to the frequency domain for these decays, the widths are larger than those obtained at low excitation energy. The widths from nonzero frequency lines, reflecting mostly the triplet character, indicate that the triplet state is dissociating. Fluorescence spectra of fragment HCO confirm that the dissociation channel correlated to the triplet surface of glyoxal involves formation of radical products. From an abrupt decrease of lifetime of the triplet state and onset of disappearance of fluorescence of glyoxal, the threshold for formation of fragment HCO from trans-glyoxal is determined to be 394.4 nm. With data for heat of formation this threshold yields an exit barrier 2.5±0.5 kcal/mol. Signal of HCO for wavelengths greater than the threshold position up to 400.5 nm is observed and is assigned to arise from the S0 surface. Rotationally state-resolved appearance rates of HCO for a vibrational state near the dissociation threshold are measured. Relative to lifetimes obtained from eigenstates in the quantum-beat data, the appearance time of product HCO reflecting the overall depletion of glyoxal is on average longer. Some observed gateway states with enhanced yields of HCO are considered to result from strong singlet–triplet interaction.
Intermolecular forces and fixed-node diffusion Monte Carlo: A brute force test of accuracies for He2 and He–LiH
Authors: Mella Massimo , Anderson James B.
Published: October 2003

The accuracy of the fixed-node approximation and diffusion Monte Carlo method in computing the interaction energy of van der Waals systems was investigated. Tests were carried out by simulating the electronic structures of He2 and He–LiH. These two systems were chosen as representative of two fundamentally different interactions, namely the weak dispersion forces in He2 and the dipole/induced–dipole interaction in He–LiH. The results for both systems are in excellent agreement with “state of the art” calculations, thereby indicating a high accuracy for the fixed-node approximation. Also, our interaction energies for He–LiH indicate that the coupled cluster single double triple method [Taylor and Hinde, J. Chem. Phys. 111, 973 (1999)] gives an accurate prediction of the interaction potential for that system.
Fractal clustering of inertial particles in random flows
Authors: Bec Jérémie
Published: October 2003

It is shown that preferential concentrations of inertial (finite-size) particle suspensions in turbulent flows follow from the dissipative nature of their dynamics. In phase space, particle trajectories converge toward a dynamical fractal attractor. Below a critical Stokes number (non-dimensional viscous friction time), the projection on position space is a dynamical fractal cluster; above this number, particles are space filling. Numerical simulations and semi-heuristic theory illustrating such effects are presented for a simple model of inertial particle dynamics.
Surfactant-stabilized structures in confined liquids
Authors: Overduin S. D. , Patey G. N.
Published: October 2003

Grand canonical Monte Carlo simulations are used to investigate the effect of surfactant on binary Lennard-Jones mixtures confined between planar, chemically patterned surfaces. Near bulk demixing coexistence, confined binary mixtures form liquid “bridges” joining specifically patterned surface areas. The length of these bridges is restricted by the unfavorable liquid–liquid interfacial tension, and in the present paper we show that this constraint can be significantly reduced by adding surfactant to the system. This leads to very extended liquid bridges and to other structures not found in the simple two-component case. We give a qualitative discussion of the surface-surfactant -induced liquid structures and examine in detail the associated forces acting between the plates.
The rotational motion and electronic relaxation of the Gd(III) aqua complex in water revisited through a full proton relaxivity study of a probe solute
Authors: Fries P. H. , Ferrante G. , Belorizky E. , Rast S.
Published: October 2003

Recent advances in the design of fast field cycling (FFC) relaxometers make it now possible to explore the nuclear magnetic relaxation dispersion (NMRD) of semidilute nuclei with short relaxation times. The paramagnetic relaxation rate enhancement of the protons of the tetramethylammonium (CH3)4N+ cation due to the intermolecular magnetic dipolar coupling with the electronic spin S=7/2 of [Gd(D2O)8]3+ in heavy water has been measured between 10 kHz and 800 MHz by combining FFC and standard relaxation techniques. In order to interpret the complete paramagnetic NMRD profile, particularly in the low field region, two previously neglected features are taken into account: (i) The evolution beyond the Redfield limit of the electronic relaxation of the spin S is obtained from accurate Monte Carlo simulations. (ii) The time fluctuation of the static zero field splitting (ZFS) is attributed not only to the usual global Brownian rotational diffusion of the complex, but also to the rearrangement of the water molecules in the first hydration shell of the Gd3+ ion via 90° pseudorotations [Th. Kowall et al., J. Phys. Chem. 99, 13078 (1995)]. To calculate the longitudinal electronic relaxation function G∥(t) of the Gd3+ ion, its static and transient ZFS parameters in the aqua complex as well as the correlation times of the Brownian rotation and vibrations of this complex are needed. We use the values of these parameters derived from an independent multiple frequency and temperature study of the full electronic paramagnetic resonance spectra of Gd3+ in light water H2O, for magnetic fields where the Redfield limit applies. The predicted NMRD profile is in excellent global agreement with experiment over the whole proton frequency range, especially if the correlation times governing the rotational dynamics of the aqua complex are slightly increased to account for the higher viscosity of D2O with respect to H2O.
Immiscible front evolution in randomly heterogeneous porous media
Authors: Tartakovsky Alexandre M. , Neuman Shlomo P. , Lenhard Robert J.
Published: October 2003

The evolution of a sharp interface between two immiscible fluids in a randomly heterogeneous porous medium is investigated analytically using a stochastic moment approach. The displacing fluid is taken to be at constant saturation and to have a much larger viscosity than does the displaced fluid, which is therefore effectively static. Capillary pressure at the interface is related to porosity and permeability via the Leverett J-function. Whereas porosity is spatially uniform, permeability forms a spatially correlated random field. Displacement is governed by stochastic integro-differential equations defined over a three-dimensional domain bounded by a random interface. The equations are expanded and averaged in probability space to yield leading order recursive equations governing the ensemble mean and variance of interface position, rate of propagation and pressure gradient within the displacing fluid. Solutions are obtained for one-dimensional head- and flux-driven displacements in statistically homogeneous media and found to compare well with numerical Monte Carlo simulations. The manner in which medium heterogeneity affects the mean pressure gradient is indicative of how it impacts the stability of the mean interface. Capillary pressure at the interface is found to have a potentially important effect on its mean dynamics and stability.
Canonical perturbation theory versus Born–Oppenheimer-type separation of motions: The vibrational dynamics of C3
Authors: Robert Julien , Joyeux Marc
Published: October 2003

It is shown that the canonical perturbation procedure we recently developed for floppy molecules [M. Joyeux and D. Sugny, Can. J. Phys. 80, 1459 (2002)] gives precise results when several pseudopotential energy surfaces cross, while in this case a Born–Oppenheimer-type separation of motions totally fails. As an example, it is demonstrated that the vibrational degrees of freedom of C3 remain decoupled up to high energies, although a previous study based on a Born–Oppenheimer scheme erroneously concluded there were “heavy interactions” between some pairs of surfaces.
Improved leap-size selection for accelerated stochastic simulation
Authors: Gillespie Daniel T. , Petzold Linda R.
Published: October 2003

In numerically simulating the time evolution of a well-stirred chemically reacting system, the recently introduced “tau-leaping” procedure attempts to accelerate the exact stochastic simulation algorithm by using a special Poisson approximation to leap over sequences of noncritical reaction events. Presented here is an improved procedure for determining the maximum leap size for a specified degree of accuracy.
Conductivity of molten sodium chloride in an alternating electric field
Authors: Petravic Janka , Delhommelle Jérôme
Published: October 2003

We study the properties of molten sodium chloride in alternating electric fields of two amplitudes and for a large range of frequencies using nonequilibrium molecular dynamics simulations, and compare the responses with two different methods of temperature control to the predictions of linear response theory. We find that the considerable nonlinearity in the resulting current density observed at low frequencies can be explained by the characteristics of the nonlinear response to constant fields. We also comment on the differences in the dissipation mechanisms and the entropy change with two thermostats.
A new approach to turbulent transport of a mean scalar
Authors: Blackman Eric G. , Field George B.
Published: October 2003

We develop a simple mean field approach to the transport of a passive scalar for which the fundamental equation is a second order differential equation in the transported quantity, not a first order equation. Triple correlations are included, as they must be for any realistic description of turbulence. No correlation time enters the theory, only an eddy turnover time. The approach is simpler than standard approaches which incorporate triple correlations, but more realistic than Gaussian or short correlation time closures which do not. A similar approach has proven useful in magnetohydrodynamics.
Complex formation in binary propionic acid-triethylamine mixtures: A dielectric relaxation and titration study
Authors: Orzechowski K. , Pajdowska M. , Fuchs K. , Kaatze U.
Published: October 2003

Complex (dielectric) permittivity spectra of propionic acid-triethylamine mixtures have been measured using four different frequency domain methods to cover the frequency range from 1 MHz to 40 GHz. At 20 °C spectra have been determined at eight mole fractions xb of base between 0.11 and 0.8. For one mixture (xb=0.3) measurements have been performed at ten temperatures between −20 °C and 25 °C. Additionally, the extrapolated low frequency (static) permittivity of the propionic acid triethylamine system has been obtained at 0⩽xb⩽1 from titration measurements in the frequency range between 1 MHz and 1 GHz. The spectra exhibit a dielectric relaxation in the microwave region, subject to a small unsymmetric relaxation time distribution, and an extrapolated high frequency permittivity ε(∞) which noticeably exceeds the squared optical refractive index nD2. The microwave relaxation is discussed in terms of acid–base complexes in the liquid, including proton transfer complexes. The dielectric spectra do not evidence a clear stoichiometric composition of the complex structures. The gap between ε(∞) and nD2 is assigned to significant contributions from atomic polarization and the dielectric dispersion due to the reorientational motions of dipolar acid and base molecules.
Quasielastic neutron scattering characterization of the relaxation processes in a room temperature ionic liquid
Authors: Triolo Alessandro , Russina Olga , Arrighi Valeria , Juranyi Fanni , Janssen Stefan , Gordon Charles M.
Published: October 2003

We report the first quasielastic neutron scattering measurements on a room temperature ionic liquid: 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6]. Data were collected using a medium resolution spectrometer as a function of temperature in the range 250–320 K. The data unequivocally indicate the existence of two different relaxation processes: a fast, localized motion occurring in the subpicosecond range and a slower process spanning the subnanosecond regime. These results provide experimental support to recently published molecular dynamics simulations. Evidence for slower, unresolved dynamics (under the present experimental conditions) is also obtained. Both temperature and momentum transfer dependence of the intermediate incoherent dynamic structure factor were investigated, after Fourier transformation into the temporal domain. The fast process shows no appreciable Q- and T-dependence. On the other hand the slow process shows evidence of a complex, non-Debye and non-Arrhenius behavior.
A class of steady solutions to two-dimensional free convection
Authors: Toh Sadayoshi , Matsumoto Takeshi
Published: October 2003

We obtained steady solutions to the two-dimensional Boussinesq approximation equations without a mean temperature gradient. This system is referred to as free convection in this paper. Under an external flow described by the stream function Ψ=−Ayf(x), steady solutions are found. They are kept steady by the balance between the strain of Ψ and the diffusion. In this sense, they are similar to the Burgers vortex layer solution. Two examples other than f(x)=x are shown to have steady solutions. We discuss the relation between these solutions and long-lived fine scale coherent structures observed in direct numerical simulations of two-dimensional free convection turbulence.
On the frontal collision of two round jets in water
Authors: Voropayev S. I. , Afanasyev Y. D. , Korabel V. N. , Filippov I. A.
Published: October 2003

In our experiment two laminar round jets collide in water forming a zero-momentum toroidal vortex and this flow is modeled theoretically. First, the linearized time dependent basic solution for the starting round jet is derived in a straightforward manner. Then a superposition of these solutions is used to model the frontal collision of two round jets. The resulting flow patterns are calculated and compared with the experiments. The comparison shows good qualitative agreement.
Two-dimensional potential theory for translating and rotating solids
Authors: Minotti F. O.
Published: October 2003

Two-dimensional inviscid theory is adapted to evaluate easily the flow, hydrodynamic force and moment of the force on a solid that translates and rotates with rather arbitrary imposed velocities. As an example a moving thin Kutta–Joukowski airfoil is considered.
Surface-plasmon-enhanced light scattering from microscopic spheres
Authors: Jory M. J. , Cann P. S. , Sambles J. R. , Perkins E. A.
Published: October 2003

The enhanced light scattering from microscopic latex spheres placed in the optical field associated with a surface-plasmon resonance is explored. Spheres of 200 nm diameter are placed on an optically thin gold film that supports the surface-plasmon and the scattered intensity is then measured as a function of scattering angle. This is compared to the scattering profiles obtained from spheres placed on a bare glass substrate. In both cases, the experimental data are compared to theory. This system is of interest in the field of optical biosensing.
Evidence of ferroelastic–ferroelastic phase transition in BiMoxW1−xO6 compounds
Authors: Jiménez R. , Castro A. , Jiménez B.
Published: October 2003

Through mechanoelastic, thermal expansion, and dielectric measurements as a function of the temperature on ceramics of BiMoxW1−xO6 (x=0, 0.10, and 0.25) solid solutions, anomalies in the corresponding parameters are found below the ferroparaelectric phase transition temperature. One of these anomalies, that corresponding to the temperature close to 940 K, contains mechanoelastic features that strongly suggest that we are dealing with a ferroelastic–ferroelastic phase transition. The behavior of thermal expansion and dielectric constant suggests that this phase transition scarcely affects the macroscopic ferroelectric properties in that temperature range. The origin of the proposed ferroelastic–ferroelastic phase transition could be attributed to the tilting of BO6 octahedra.
Study of negative-bias temperature-instability-induced defects using first-principle approach
Authors: Soon Jia Mei , Loh Kian Ping , Tan Shyue Seng , Chen T. P. , Teo W. Y. , Chan L.
Published: October 2003

In this letter, we report the study of negative-bias temperature-instability (NBTI)-induced defect using first-principle calculations. From our calculations, we found that the NBTI effect leads to an overall decrease in frontier molecular orbital energy gap at the interface. We studied the interface structure at an atomistic level and proposed an explanation for this phenomenon using molecular orbital theory and band theory. In addition, weakening of bond strength of Si–O at the defect site is observed. It is found that upon formation of a defect, an energy state is created inside the band gap of SiO2. These results improve our understanding of the consequences associated with the NBTI effect, and the self-propagating nature of the NBTI effect.
Quantitative measurement of the ferromagnetic resonance signal by force detection
Authors: Naletov V. V. , Charbois V. , Klein O. , Fermon C.
Published: October 2003

A magnetic resonance force microscope is used to measure ΔMz, the change of longitudinal magnetization associated with the resonant absorption of microwave power in a ferromagnetic crystal. It is demonstrated that quantitative measurements of ΔMz can be obtained by analyzing the dipolar force exerted between a disk sample and a cylindrical probe magnet aligned and magnetized along their axis. Finite size effects are found to be important when probing the resonance in a micron-sized sample.
Discharge comparison of nonequilibrium atmospheric pressure Ar/O2 and He/O2 plasma jets
Authors: Wang S. , Schulz-von der Gathen V. , Döbele H. F.
Published: October 2003

A plasma jet has been developed that operates using rf power and produces a stable homogeneous discharge at atmospheric pressure. Discharge characteristics for the gas mixture of He/O2 and Ar/O2 were studied. The temperature distribution at the exit of the nozzle of Ar/O2 discharge is about 100 K higher than that for the He/O2 discharge—identified by an increased temperature—extends much farther downstream for Ar/O2 discharge in contrast to the He/O2 discharge—250 mm as compared to 75 mm. Optical emission spectra for both discharges in the jet effluent shows only fractions of the peak intensities of Ar, He, and O atomic lines compared with spectra taken looking obliquely inside the jet volume. The spatial profile of excited atomic oxygen at λ=777 nm (4S0 3p–3s) taken side on to the flow direction outside the nozzle shows good correlation to the temperature distribution for both discharges. These results indicate that the Ar/O2 discharge has better energy transfer efficiency than the He/O2 discharge. Study of the electrical properties of the two discharges shows that the two discharges exhibit a different capacitive nature and the voltage wave form for He/O2 advances that of the Ar/O2 discharge by 16°, which means there is more ohmic component in the Ar/O2 discharge.
Response to “Comment on ‘The cohesive energetics of cesium chloride [J. Chem. Phys. 118, 2308 (2003)]’ ”
Authors: Pyper N. C.
Published: October 2003

The comment by A. Aguado on the article J. Chem. Phys. 118, 2308 (2003) presents new computations with the ab initio perturbed ion (AIPI) method and concludes that this approach provides more insight into ionic crystal cohesion than using the relativistic integrals program. However, uncertainties in the AIPI energies prevent variational arguments from determining the relative merits of the two methods. There is therefore no evidence either from the original article or elsewhere to justify the claims in the comment.
Exploratory numerical experiments on the differences between genuine and “passive” turbulence
Authors: Tsinober A. , Galanti B.
Published: October 2003

We report a selection of results of numerical experiments aiming to identify and study differences between genuine and passive turbulence with the emphasis on qualitative aspects. The main concern is with two issues: (i) The difference between production rate of vorticity and that of a solenoidal passive vector and a gradient of passive scalar with the same diffusivity, and (ii) qualitative differences in the amplification of disturbances in genuine turbulence and passive objects for statistically stationary turbulent flow. One of the differences between the evolution of vorticity and passive vectors is the consequence of nonconservation of energy of a passive vector manifested in positive production of the energy of its vector potential. The main source of differences in the amplification of disturbances is due to the nonlinearity of the equations for vorticity (and strain) resulting also in differences in the linearized equations for the disturbances of vorticity and passive vectors.
Photovoltaic cells made from conjugated polymers infiltrated into mesoporous titania
Authors: Coakley Kevin M. , McGehee Michael D.
Published: October 2003

We have made photovoltaic cells by infiltrating the conjugated polymer regioregular poly(3-hexylthiophene) into films of mesoporous titania, which are self-assembled using a structure directing block copolymer. The mesoporous titania films were chosen because they have pores with a diameter slightly less than 10 nm, which is the exciton diffusion length in many conjugated polymers, and because they provide continuous pathways for electrons to travel to an electrode after electron transfer has occurred. The photovoltaic cells have an external quantum efficiency of 10% and a 1.5% power conversion efficiency under monochromatic 514 nm light. Experiments that vary the amount of polymer in the titania films suggest that the performance of the cells is limited by poor hole transport in the polymer.
Role of nanometer-scale quasicrystals in improving the mechanical behavior of Ti-based bulk metallic glasses
Authors: Kim Yu Chan , Na Jong Hyun , Park Jin Man , Kim Do Hyang , Lee Jin Kyu , Kim Won Tae
Published: October 2003

The effect of the precipitation of nanosized quasicrystals on the mechanical properties of Ti–Zr–Cu–Ni–Be bulk metallic glasses (BMG) has been investigated. The Ti40Zr29Cu8Ni7Be16 BMG crystallizes by forming a few nanometer-size quasicrystals in the amorphous matrix, enabling the fabrication of quasicrystal-reinforced BMG matrix composites. Simultaneous improvement of strength and ductility can be obtained when 3–5-nm-size quasicrystals are isolated and homogeneously distributed in an amorphous matrix. The fracture strength and global strain, respectively, increase from 1921 MPa and 5.1% for as-cast BMG to 2084 MPa and 6.2% for partially crystallized BMG with the volume fraction of ∼7% quasicrystals. These improvements may be attributed to the structural similarity between quasicrystalline and amorphous phases. Stable low-energy interface between two phases may act as a source for multiple-shear-band formation.
Sensitive elemental analysis of aqueous colloids by laser-induced plasma spectroscopy
Authors: Pu X. Y. , Ma W. Y. , Cheung N. H.
Published: October 2003

Highly sensitive elemental analysis of lead carbonate colloids was demonstrated by two-pulse laser-induced plasma spectroscopy. The first laser pulse created a vapor plume with the particulates concentrated in space because of their slower propagation. They were then ablated by an ArF laser pulse that efficiently atomized and excited the lead analyte. The lead emissions were much enhanced, while the background continuum interference was minimized. The detection limit for lead was shown to be 14.2 ppb, compared with 13 ppm achieved by conventional laser-induced breakdown spectroscopy of lead ions in water and 210 ppb for lead aerosols.
Electronic structure of the contact between carbon nanotube and metal electrodes
Authors: Dag S. , Gülseren O. , Ciraci S. , Yildirim T.
Published: October 2003

Our first-principles study of the contact between a semiconducting single-walled carbon nanotube (s-SWNT) and metal electrodes shows that the electronic structure and potential depend strongly on the type of metal. The s-SWNT is weakly side-bonded to the gold surface with minute charge rearrangement and remains semiconducting. A finite potential barrier forms at the contact region. In contrast, the molybdenum surface forms strong bonds, resulting in significant charge transfer and metallicity at the contact. The radial deformation of the tube lowers the potential barrier at the contact and increases the state density at the Fermi level.
Switching dynamics in poly(vinylidene fluoride) and copolymers
Authors: von Seggern Heinz , Leal Ferreira Guilherme F.
Published: October 2003

Theoretical aspects of the field dependent switching dynamics of poly(vinylidene fluoride) and its copolymers have been investigated based on the experimental data of Furukawa [T. Furukawa, Phase Transitions 18, 143 (1989)]. Starting from published theories of the polarization built up for different field strengths and its comparison to experimental results, some characteristic features are observable which are not explainable by present theories. Therefore a model is proposed which includes the electrical properties induced by the two-phase system of poly(vinylidene fluoride) that consists of a ferroelectric and a nonferroelectric phase and a depolarization factor that takes into account the shape of dipolar regimes. In addition, material homogeneities have to be incorporated by means of hysteresis in order to describe the observed reduction of total polarization for small applied electric field strength.
Magnetic properties of carbon-doped FePt nanogranular films
Authors: Perumal A. , Ko Hyun-Seok , Shin Sung-Chul
Published: October 2003

The room-temperature magnetic properties and magnetization reversal process of ordered (FePt)1−xCx thin films, prepared by a cosputtering technique on MgO (100) substrates held at 400 °C substrate temperature, have been studied. Perpendicular magnetic anisotropy was obtained for the films with carbon concentrations up to 25 vol % and coercivity enhancement was observed with small carbon-doped FePt thin films. A clear spin-reorientation transition from perpendicular to the in-plane direction was observed with an increase in carbon concentration. The observed magnetic properties are intercorrelated to microstructural changes that occur as a function of the carbon doping.