bluuebunny/crossref_metadata_2025 huggingface dataset:
The utility of higher order derivatives in constructing molecular potential energy surfaces by interpolation
Authors: Jordan Meredith J. T. , Thompson Keiran C. , Collins Michael A.
Published: July 2002

In this paper we evaluate the use of higher order derivatives in the construction of an interpolated potential energy surface for the OH+H2→H2O+H reaction. The surface construction involves interpolating between local Taylor expansions about a set of known data points. We examine the use of first, second, third, and fourth order Taylor expansions in the interpolation scheme. The convergence of the various interpolated surfaces is evaluated in terms of the probability of reaction. We conclude that first order Taylor expansions (and by implication zeroth order expansions) are not suitable for constructing potential energy surfaces for reactive systems. We also conclude that it is inefficient to use fourth order derivatives. The factors differentiating between second and third order Taylor expansions are less clear. Although third order surfaces require substantially fewer data points to converge than second order surfaces, this faster convergence does not offset the large cost incurred in calculating numerical third derivatives. We therefore conclude that, without an efficient means for calculating analytic third derivatives, second order derivatives provide the most cost-effective means of constructing a global potential energy surface by interpolation.
Phase control of absorption in large polyatomic molecules
Authors: Wang Xuebin , Bersohn Richard , Takahashi Kenshi , Kawasaki Masahiro , Kim Hong Lae
Published: July 2002

The phenomenon of interference of the amplitude for absorption of one photon of frequency 3ω and the amplitude for absorption of three photons of frequency ω was theoretically predicted by Shapiro, Hepburn, and Brumer. The interference was demonstrated experimentally by varying the relative phase between the tripled frequency photon and three photons with the fundamental frequency by the groups of Elliott and Gordon in atoms and small molecules. In order to see how general this phenomenon is, five compounds were studied, ammonia, trimethylamine, triethylamine, cyclooctatetraene, and 1,1-dimethylhydrazine. CH3I was used as the tripling gas for light in the range 604–600 nm. Interference was observed in all cases. The last four compounds have low ionization potentials and interference was observed between a 3+1 and a 1+1 ionization process with a maximum modulation of 22%. NH3 with a higher ionization potential requires absorption of 3+2 or 1+2 photons and exhibits a maximum modulation of 33%. We conclude that molecular size is no obstacle and that as long as a molecule has sufficiently strong absorption at the tripled frequency, and sufficient vapor pressure, and the laser fundamental beam is very strong, phase control of interference is observable.
Post-Hartree–Fock study on Ar–HCO+ and He–HCO+ complexes: A critical examination of experimental data
Authors: Nowek Andrzej , Leszczyński Jerzy
Published: July 2002

The second order Mo/ller–Plesset (MP2) to the fourth order many-body perturbation theory including single, double, triple, and quadruple substitutions (MP4(SDTQ)), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], quadratic configuration interaction with single and double substitutions (QCISD), and QCISD including noniterative triples contributions [QCISD(T)] ab initio correlated levels of theory have been employed in studies on molecular geometry and stability of the Ar–HCO+ and He–HCO+ complexes. Triple-zeta split-valence (6-311G) and correlation consistent (cc-pVTZ) basis sets augmented with diffuse and polarization functions were used. At applied levels of theory the predicted interaction energies (corrected for the basis set superposition error and zero-point vibrational energy) for Ar–HCO+ range from −3.21 kcal/mol (QCISD/6-311G(2df,2pd) to −4.21 kcal/mol (MP4(SDTQ)/aug-cc-pVTZ), and for He–HCO+ vary from −0.12 kcal/mol (−42 cm−1, MP2/cc-pVTZ) to −0.37 kcal/mol (−130 cm−1, CCSD(T)/aug-cc-pVTZ//CCSD/aug-cc-pVTZ). The optimized Ar–H distance (2.1392 Å at QCISD/cc-pVTZ and 2.1222 Å at MP2/aug-cc-pVTZ) agrees very well with the experimentally determined (2.13 Å) value, whereas when step-by-step higher level methods and basis sets are applied, the predicted He–H distance (1.9156 Å at QCISD(T)/aug-cc-pVTZ dramatically retreats from the experimental value of 2.00 Å.
Absolute HCO concentration measurements in methane/air flame using intracavity laser spectroscopy
Authors: Lozovsky Vladimir A. , Cheskis Sergey , Kachanov Alexander , Stoeckel Frédréic
Published: July 2002

Intracavity laser absorption spectroscopy was used to measure the absorption spectra of a premixed, flat methane/air flame at a total pressure of 30 Torr. The spectra were measured in the spectral range of 16 000–16 300 cm−1. A flat flame burner was placed inside the cavity of a broadband dye laser pumped by a cw argon-ion laser. The spectrum of the laser output was measured by a high resolution spectrograph (with a spectral resolution of 0.003 nm). The spectrum of HCO radicals (Ã 2A′′←X̃ 2A transition) was measured with a high signal-to-noise ratio at different positions above the burner, providing the first quantitative measurement of the absolute concentrations of the HCO radical in flames. The linewidths of the individual rotational lines in the spectrum can be closely fitted by the equation Γ=X+ZN2(N+1)2, where X=0.37±0.03 cm−1 and Z=(8±0.5)10−6. The rotational temperature of the HCO radicals was evaluated from the spectra, but the error and the data scatter are relatively high since the lines with a high rotational quantum number N are strongly superimposed with lines from different branches. The “hot band,” which can be assigned to the transition (0,0,1)–(0,9,1), was observed in spectra measured at high temperature. The value ν3″=1859 cm−1 is evaluated from the position of this “hot band.” The concentration profile of the HCO radical has a maximum value of about 1.2×1013 molecules/cm3 which is in reasonable agreement with computer simulation results, when the uncertainties of the absorption cross section and of the rate constants for HCO reactions are taken into account. The relatively strong lines of the CH2 radical spectrum (the b̃ 1B1←ã 1A1 transition) were also recorded in the studied wavelength range. The spectra of these two radicals can be measured simultaneously which is advantageous in combustion diagnostics.
Long-lived interchain contacts in polymer melts
Authors: Szamel Grzegorz , Wang Tian
Published: July 2002

We will study time dependence of interchain contacts in entangled and unentangled polymer melts. We find that “clusters” of contacts in entangled melts persist in time up to at least 0.2 of the longest relaxation time. The clusters of contacts in un-entangled melts disappear and reappear on at least two orders of magnitude shorter time scale. We also examine time-dependent correlation functions that try to quantify the properties of contacts.
Path integral method by means of generalized coherent states and its numerical approach to molecular systems. I. Ensemble average of total energy
Authors: Nagao Hidemi , Shigeta Yasuteru , Kawabe Hiroyuki , Kawakami Takashi , Nishikawa Kiyoshi , Yamaguchi Kizashi
Published: July 2002

A path integral formulation by the generalized coherent states of the Thouless parameterization is presented to apply to molecular systems. Explicit expressions of the ensemble average of the physical property including the spin space is also derived. In order to compute the ensemble average, a novel calculating scheme is shown. The Monte Carlo method by the metropolis algorithm is used, and in order to avoid the negative sign problem, a reweighting method is attempted. Simple examples are examined to demonstrate performances of calculations. We discussed in relation to the total energy and the number of the division for the partition function or the temperature.
Time-dependent density functional theory and the kinetics of lattice gas systems in contact with a wall
Authors: Fischer H. P. , Reinhard J. , Dieterich W. , Gouyet J.-F. , Maass P. , Majhofer A. , Reinel D.
Published: July 2002

We develop an improved mean-field theory which allows us to describe the diffusive dynamics near phase transformations in condensed systems. Starting from a master equation for a stochastic lattice gas we obtain evolution equations on the single-particle level, whose stationary solutions in principle are consistent with the exact equilibrium statistics. Our method, which generalizes an approach proposed earlier, is based on a combination of a local equilibrium assumption and the lattice version of classical density functional theory. In the continuum limit, which is worked out for attractive interactions, generalized Cahn–Hilliard-type equations are recovered. Microscopic kinetic coefficients can be identified, which in general depend on the instantaneous local correlations in the nonequilibrium state. Moreover we study semi-infinite systems interacting with a planar wall and derive the appropriate boundary conditions to be imposed on the continuum equations. Applications to problems of the kinetics of phase changes influenced by a near wall are pointed out.
Quantum chemical study of simple positronic systems using explicitly correlated Gaussian functions – PsH and PsLi+
Authors: Strasburger K. , Chojnacki H.
Published: July 2002

The electronic structure of positronium hydride has been studied using explicitly correlated Gaussian functions. The resulting energy constitutes new upper bound to the exact nonrelativistic energy of PsH within the Born–Oppenheimer approximation. The two photon annihilation rate was computed using the optimized wave function. Preliminary results for the positron bonded with the lithium atom indicate the stability of this system against the dissociation into Li+ cation and Ps atom.
Liouville’s theorems, Gibbs’ entropy, and multifractal distributions for nonequilibrium steady states
Authors: Hoover Wm. G.
Published: July 2002

Liouville’s best-known theorem, ḟ({q,p},t)=0, describes the incompressible flow of phase-space probability density, f({q,p},t). This incompressible-flow theorem follows directly from Hamilton’s equations of motion. It applies to simulations of isolated systems composed of interacting particles, whether or not the particles are confined by a box potential. Provided that the particle–particle and particle–box collisions are sufficiently mixing, the long-time-averaged value 〈f〉 approaches, in a “coarse-grained” sense, Gibbs’ equilibrium microcanonical probability density, feq, from which all equilibrium properties follow, according to Gibbs’ statistical mechanics. All these ideas can be extended to many-body simulations of deterministic open systems with nonequilibrium boundary conditions incorporating heat transfer. Then Liouville’s compressible phase-space-flow theorem—in the original ḟ≠0 form—applies. I illustrate and contrast Liouville’s two theorems for two simple nonequilibrium systems, in each case considering both stationary and time-dependent cases. Gibbs’ distributions for incompressible (equilibrium) flows are typically smooth. Surprisingly, the long-time-averaged phase-space distributions of nonequilibrium compressible-flow systems are instead singular and “multifractal.” The nonequilibrium analog of Gibbs’ entropy, S≡−k〈lnf〉, diverges, to −∞, in such a case. Gibbs’ classic remedy for such entropy errors was to “coarse-grain” the probability density—by averaging over finite cells of dimensions ∏Δq Δp. Such a coarse graining is effective for isolated systems approaching equilibrium, and leads to a unique entropy. Coarse graining is not as useful for deterministic open systems, constrained so as to describe stationary nonequilibrium states. Such systems have a Gibbs’ entropy which depends, logarithmically, upon the grain size. The two Liouville’s theorems, their applications to Gibbs’ entropy, and to the grain-size dependence of that entropy, are clearly illustrated here with simple example problems.
Photodissociation of CH stretch overtone excited CH3Cl and CHD2Cl(vCH=5): Cl spin–orbit branching and atomic fragment yields
Authors: Lambert H. Mark , Dagdigian Paul J.
Published: July 2002

The photodissociation dynamics of CH3Cl and CHD2Cl excited through their respective fourth C–H stretch overtone transitions have been investigated by detection of ground state Cl(2P3/2), spin–orbit excited Cl(2P1/2), H, and D atomic fragments by 2+1 resonance enhanced multiphoton ionization in a time-of-flight mass spectrometer. The yield of atomic Cl fragments from the vibrationally mediated photodissociation of CHD2Cl is significantly less than for CH3Cl, more than can be accounted for from differences in the cross sections for overtone excitation. This suggests that state mixing leads to a greater amplitude of the wave function of the vibrationally excited level along the dissociation coordinate in CH3Cl than in CHD2Cl. The observed differences in the Cl spin–orbit branching in the photolysis of ground state versus vibrationally excited molecules are ascribed to differences in the nuclear dissociation dynamics, rather than access to other excited electronic states. The relative yields of H and D atoms from the photodissociation of ground and vibrationally excited CHD2Cl are also reported and discussed.
The intermolecular potential of He–OCS
Authors: Higgins Kelly , Klemperer William
Published: July 2002

The dynamics of molecules trapped inside or bound to the surface of liquid helium droplets is a most exciting new development. As an aid toward the understanding of the spectrum of OCS in liquid helium, we report the intermolecular potential between He and OCS studied by ab initio calculations and high-resolution microwave spectroscopy. The potential is found to have three minima, corresponding to a T-shaped configuration for the global minimum and secondary minima at either end of the OCS molecule. The three lowest calculated bound states are loosely localized in each of the minima, with the ground state being T-shaped. Ten rotational transitions of the ground state are observed in the frequency region 1.5–45.0 GHz. Comparison of theory with experiment shows good agreement. The agreement improves substantially if the calculated He–OCS intermolecular potential is made uniformly 10% deeper and the He–OCS separation is reduced by 0.05 Å.
Monte Carlo simulation for the potential of mean force between ionic colloids in solutions of asymmetric salts
Authors: Wu J. Z. , Bratko D. , Blanch H. W. , Prausnitz J. M.
Published: July 2002

A new technique for Monte Carlo sampling of the hard-sphere collision force has been applied to study the interaction between a pair of spherical macroions in primitive-model electrolyte solutions with valences 1:2, 2:1, and 2:2. Macroions of the same charge can attract each other in the presence of divalent counterions, in analogy with earlier observations for planar and cylindrical geometries. The attraction is most significant at intermediate counterion concentrations. In contrast to the entropic depletion force between neutral particles, attraction between macroions is of energetic origin. The entropic contribution to the potential of mean force is generally repulsive at conditions corresponding to aqueous colloids with or without salt. For systems with divalent counterions, the potentials of mean force predicted by mean-field approximations like the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory or the Sogami–Ise (SI) theory are qualitatively different from those observed in the simulations. However, for systems with monovalent counterions, predictions of DLVO theory are in fair agreement with simulation results.
Temporal scaling analysis: Viscoelastic properties of star polymers
Authors: Phillies George D. J.
Published: July 2002

A generalized temporal scaling ansatz for the frequency dependence of the loss and storage moduli and for the shear dependence of the viscosity is tested against studies on entangled solutions of star polymers in good and theta solvents. At lower frequencies or shear rates, the ansatz calls for an exponential or stretched-exponential form [e.g., G0 exp(−αων)] for G″(ω)/ω and G′(ω)/ω2, and correspondingly in κ for η(κ). At higher frequencies, the ansatz indicates that each of these quantities has a power-law dependence on its primary variable. The predicted forms are in excellent agreement with literature data on solutions of poly-α-methylstyrene, polybutadiene, polystyrene, and polyisoprene stars. A power-law correlation α∼G02/3 is observed between the zero-frequency, zero-shear modulus G0 and the low-frequency or low-shear decay constant α of the stretched exponential, the same power-law line describing both star and linear polymers in good solvents.
The structure of liquid ethanol: A neutron diffraction and molecular dynamics study
Authors: Benmore Chris J. , Loh Yen L.
Published: July 2002

A comprehensive neutron scattering study has been performed using selective hydrogen–deuterium substitution on the hydroxyl, methyl, and methylene hydrogen atoms in pure ethanol at room temperature. In this experiment we have directly measured 10 structure factors of the possible 21 partial structure factors that exist for liquid ethanol. The data has been used to obtain details on the conformation of the liquid ethanol molecule in the liquid state. The intermolecular structure were compared to molecular dynamics simulations using the Jorgensen potential for four- and nine-site liquid ethanol molecules. All ten neutron structure factors are in good agreement with the simulation results, although there are some small differences in the structural detail. The neutron structure factors show that hydrogen bonding dominates the structure of liquid ethanol at room temperature, which is consistent with the existence of winding chains predicted by the simulation.
Sum frequency generation orientation analysis of molecular ammonia on the surface of concentrated solutions
Authors: Simonelli Danielle , Shultz Mary Jane
Published: July 2002

Ammonia–water complexes have been detected with sum frequency generation (SFG) at the liquid/vapor interface of concentrated ammonia solutions (0.3x NH3, x=bulk mole fraction). SFG spectra taken with the ssp polarization combination (s-polarized sum frequency signal, s-polarized visible light, p-polarized infrared beam) are dominated by the N–H symmetric stretch (ν1) at 3312 cm−1 and a weaker deformation mode (2ν4) at 3200 cm−1. The dangling (free) OH peak due to water at 3700 cm−1 is suppressed at this concentration, indicating that water molecules are complexed through hydrogen bonds to ammonia at the interface. The polarization dependence of ν1 indicates that the NH3 C3 molecular axis is tilted with respect to the surface normal by θ⩽38°. Variation of the antisymmetric stretch, ν3, sum frequency signal intensity with polarization restricts the lower limit for surface ammonia tilt angles such that 25°⩽θ⩽38°. Based on this orientation, the structure of the ammonia–water complex is discussed.
Ultrafast charge separation and exciplex formation induced by strong interaction between electron donor and acceptor at short distances
Authors: Iwai Shinichiro , Murata Shigeo , Katoh Ryuzi , Tachiya M. , Kikuchi Koichi , Takahashi Yasutake
Published: July 2002

We have measured the decay of fluorescence from the electron acceptors (A) dissolved in donor (D) solvent by femtosecond up-conversion spectroscopy with a time resolution of 75 fs. The measurements have been made for several acceptors (9-cyanoanthracene, CA; 9,10-dicyanoanthracene, DCA; and 1,2,9,10-tetracyanoanthracene, TCA) in the donor solvent (N,N-dimethylaniline, DMA, and aniline, ANL). The decay times obtained are between 150 fs (CA–ANL) and 280 fs (TCA–DMA). Observation of the decay of acceptor fluorescence and the concurrent rise of fluorescence from the exciplex state indicates that the excited acceptor (A*) state directly relaxes to the exciplex state. Charge separation rates (kCS) determined from the decay of the acceptor fluorescence lie between 3.6×1012–6.7×1012/s for six combinations of donors and acceptors. Very weak energy gap (ΔE) dependence of kCS was observed in the region of ΔE of 0.36–1.47 eV. This relation between kCS and ΔE is quite different from the charge recombination rate (kCR) vs ΔE relation in the charge transfer (CT) complexes, which has been reported by Mataga et al. The present result can be explained by a model which takes into account the interaction between the zeroth-order D⋅A* and D+⋅A− states. Ultrafast charge separation would occur from the D⋅A* to exciplex state on an adiabatic potential curve without barrier along the coordinate of the intermolecular stretching vibration. This is considered to be induced by the strong interaction between the donor and acceptor at short distances.
Inertial stochastic dynamics. II. Influence of inertia on slow kinetic processes of supercoiled DNA
Authors: Beard Daniel A. , Schlick Tamar
Published: July 2002

We apply our new algorithms presented in the companion paper (LTID: long-time-step inertial dynamics, IBD: inertial Brownian dynamics) for mass-dependent Langevin dynamics (LD) with hydrodynamics, as well as the standard Brownian dynamical (BD) propagator, to study the thermal fluctuations of supercoiled DNA minicircles. Our DNA model accounts for twisting, bending, and salt-screened electrostatic interactions. Though inertial relaxation times are on the order of picoseconds, much slower kinetic processes are affected by the Brownian (noninertial) approximation typically employed. By comparing results of LTID and IBD to those generated using the standard (BD) algorithm, we find that the equilibrium fluctuations in writhing number, Wr, and radius of gyration, Rg, are influenced by mass-dependent terms. The autocorrelation functions for these quantities differ between the BD simulations and the inertial LD simulations by as much as 10%. In contrast, when the nonequilibrium process of relaxation from a perturbed state is examined, all methods (inertial and diffusive) yield similar results with no detectable statistical differences between the mean folding pathways. Thus, while the evolution of an ensemble toward equilibrium is equally well described by the inertial and the noninertial methods, thermal fluctuations are influenced by inertia. Examination of such equilibrium fluctuations in a biologically relevant macroscopic property—namely the two-site intermolecular distance—reveals mass-dependent behavior: The rate of juxtaposition of linearly distant sites along a 1500-base pair DNA plasmid, occurring over time scales of milliseconds and longer, is increased by about 8% when results from IBD are compared to those from BD. Since inertial modes that decay on the picosecond time scale in the absence of thermal forces exert an influence on slower fluctuations in macroscopic properties, we advocate that IBD be used for generating long-time trajectories of supercoiled DNA systems. IBD is a practical alternative since it requires modest computational overhead with respect to the standard BD method.
Determination of acceptor distribution from fluorescence resonance energy transfer: Theory and simulation
Authors: Rolinski Olaf J. , Birch David J. S.
Published: July 2002

A new method for determining the donor–acceptor distribution function in fluorescence resonance energy transfer (FRET) systems is presented. The approach is based on time-resolved fluorescence experiments with nanosecond resolution. Potential applications of this method include: determination of the morphology of porous materials (e.g., polymers, sol-gels, resins, etc.), monitoring processes occurring on the nanometer scale including biomolecules labeled with the donor/acceptor species, and FRET sensors based on competitive binding. In this paper a theoretical derivation of the method is presented and the method is tested in a series of numerical simulations. The experimental conditions regarding this approach are discussed and its applicability to real measurement systems is demonstrated.
New equation for the light and neutron scattering intensity of fluid mixtures
Authors: Vrij Agienus
Published: July 2002

We derive a new expression for the scattering intensity of fluid mixtures, where one of the components (1) plays an exceptional role. The expression contains two contributions: one from “decorated” particles of component 1—with a modified scattering amplitude—and another one from the sum of the p−1 remaining components. This sum term contains solely direct spatial correlations of the p−1 remaining components, as expressed by their direct correlation functions. The “decoration” of particles 1 is due to the spatial restructuring of the p−1 remaining particle components induced by the presence of particles of component 1. For scattering vector zero (K=0), the decoration can be described in thermodynamic terms as an adsorption of the p−1 kinds of particle species by the particles 1. Our new formula is illustrated with several two-component systems of hard spheres for which the structural functions are (approximately) known. They mimic fluid mixtures of metals and mixtures of colloidal particles. Some calculations are shown for (semi)-dilute solutions of component 1 in a concentrated fluid of particles 2. The results suggest that “decorated” amplitudes and the partial structure factor of component 1 may be obtained, when one substracts simply the scattering of pure component 2 from the total scattering of the mixture. Our formula might be useful for analyzing neutron and x-ray scattering experiments of mixtures of fluid metals with specific attraction forces between the different components and for small-angle scattering of mixtures of colloidal particles.
The gauge field propagator for QED and linearized QCD in inhomogeneous media
Authors: Bickeböller M. , Goldflam R. , Wilets L.
Published: July 2002

The gauge field propagator is calculated in configuration space using an expansion in spherical harmonics for the scalar Green function and in vector spherical harmonics for the tensor Green function. We work in the Coulomb gauge where the scalar Green function is instantaneous and the frequency-dependent tensor Green function is transverse. Explicit expressions are obtained to solve for the propagators in the presence of inhomogeneous dielectric media.
Vector coherent state theory and its application to the orthogonal groups
Authors: Rowe D. J. , Le Blanc R. , Hecht K. T.
Published: July 2002

Vector coherent state theory is developed and presented in a form that explicitly exhibits its general applicability to the ladder representations of all semisimple Lie groups and their Lie algebras. It is shown that, in a suitable basis, the vector coherent state inner product can be inferred algebraically, by K-matrix theory, and changed to a simpler Bargmann inner product thereby facilitating the explicit calculation of the matrix representaions of Lie algebras. Applications are made to the even and odd orthogonal Lie algebras.
Local realizations of kinematical groups with a constant electromagnetic field. I. The relativistic case
Authors: Negro Javier , del Olmo Mariano A.
Published: July 2002

This paper is devoted to the study of the description of elementary physical systems interacting with an external constant electromagnetic field and the construction of their differential wave equations from a group-theoretical point of view. In this context certain local realizations of the Poincaré group are studied. The linearization of this problem is carried out by building the associated representation group that turns out to be the well-known Maxwell group. In this way the usual method (concerning local realizations) that has been employed in studying free systems to the interacting case is extended.
On the local equivalence in a unidimensional world
Authors: Negro Javier , del Olmo Mariano A.
Published: July 2002

The physical meaning of some semiunitary irreducible realizations of the unidimensional Poincaré and Galilei groups, including time inversion, which are characteristic of a unidimensional world is investigated. It is shown that, in order to achieve this purpose, the local equivalence is the most appropriate, and it does not coincide with an equivalence of the global kind. In this study use has been made of the theory of locally operating realizations developed by Hoogland, while dealing with the nontrivial factor systems the representation group technics were used.
Canonical quantization of photons in a Rindler wedge
Authors: Moretti Valter
Published: July 2002

Photons and thermal photons are studied in the Rindler wedge employing Feynman’s gauge and canonical quantization. A Gupta–Bleuler-like formalism is explicitly implemented. Nonthermal Wightman functions and related (Euclidean and Lorentzian) Green functions are explicitly calculated and their complex time analytic structure is carefully analyzed using the Fulling–Ruijsenaars master function. The invariance of the advanced minus retarded fundamental solution is checked and a Ward identity discussed. It is suggested that the KMS condition can be implemented to define thermal states also dealing with unphysical photons. Following this way, thermal Wightman functions and related (Euclidean and Lorentzian) Green functions are built up. Their analytic structure is carefully examined employing a thermal master function as in the nonthermal case and other corresponding properties are discussed. Some subtleties arising dealing with unphysical photons in the presence of the Rindler conical singularity are pointed out. In particular, a one-parameter family of thermal Wightman and Schwinger functions with the same physical content is proved to exist due to a remaining (nontrivial) static gauge ambiguity. A photon version of the Bisognano–Wichmann theorem is investigated in the case of photons propagating in the Rindler Wedge employing Wightman functions. In spite of the found ambiguity in defining Rindler Green functions, the coincidence of (β=2π)-Rindler Wightman functions and Minkowski Wightman functions is proved dealing with test functions related to physical photons and Lorentz photons.
The twisted Heisenberg algebra Uh,w(ℋ(4))
Authors: Abdesselam Boucif
Published: July 2002

A two parametric deformation of the enveloping Heisenberg algebra ℋ(4) that appears as a combination of the standard and a nonstandard quantization given by Ballesteros and Herranz is defined and proved to be Ribbon Hopf algebra. The universal ℛ matrix and its associated quantum group are constructed. A new solution of the Braid group is obtained. The contribution of these parameters in invariants of links and the Wess–Zumino–Witten (WZW) model are analyzed. General results for twisted Ribbon Hopf algebra are derived.
Macroscopic models for semiconductor heterostructures
Authors: Degond P. , Schmeiser C.
Published: July 2002

The semiconductor Boltzmann equation is considered within a material with a space-dependent band characteristic. Its fluid limit under strong elastic collisions leads to the so-called spherical harmonics expansion (SHE) model. For this model, the influence of smoothly space-dependent or abruptly discontinuous band characteristics is analyzed. For the latter case which occurs at heterojunction interfaces, kinetic boundary layers are analyzed and higher order transmission conditions are derived. In a further step, transmission conditions through heterojunctions for the energy-transport and drift-diffusion models (which are obtained as moments of the SHE model) are constructed.
Suppression of Rayleigh–Taylor and bulk convective instabilities in imploding plasma liners and pinches
Authors: Bud’ko A. B. , Liberman M. A. , Velikovich A. L. , Felber F. S.
Published: July 2002

Linear growth of instabilities during implosion of annular and columnar plasma pinches having sharp outer boundaries is analyzed. The dominant modes of the Rayleigh–Taylor instability differ in time, radius, and growth rate from the dominant modes of the bulk convective instability. The spectra of growth rates of perturbations localized azimuthally or axially suggest explanations for filamentation and stratification of plasmas. Localized Rayleigh–Taylor modes grow fastest initially in an implosion. These modes can be suppressed completely, however, by a relatively modest axial magnetic field, which produces magnetic shear in a Z pinch. Similarly, bulk convective modes can be substantially reduced by magnetic shear. The ‘‘window of stability’’ produced by magnetic shear occurs for an axial field magnitude about equal to the azimuthal. Properly shaped current pulses in Z pinches with axial fields may therefore produce stable implosions to high compression ratios, in agreement with recent experiments. The most dangerous Rayleigh–Taylor modes for the symmetry of an annular pinch of initial aspect ratio R0/δ0 are those with wavenumber k∼R0/δ20, where R0 is the initial pinch radius. To suppress pinch instabilities optimally, both the liner thickness and axial magnetic field should be chosen properly.
Correlations of heat and momentum transport in the TFTR tokamak
Authors: Scott S. D. , Arunasalam V. , Barnes C. W. , Bell M. G. , Bitter M. , Boivin R. , Bretz N. L. , Budny R. , Bush C. E. , Cavallo A. , Chu T. K. , Cohen S. A. , Colestock P. , Davis S. L. , Dimock D. L. , Dylla H. F. , Efthimion P. C. , Erhrardt A. B. , Fonck R. J. , Fredrickson E. , Furth H. P. , Goldston R. J. , Greene G. , Grek B. , Grisham L. R. , Hammett G. , Hawryluk R. J. , Hendel H. W. , Hill K. W. , Hinnov E. , Hoffman D. J. , Hosea J. , Howell R. B. , Hsuan H. , Hulse R. A. , Jaehnig K. P. , Janos A. C. , Jassby D. , Jobes F. , Johnson D. W. , Johnson L. C. , Kaita R. , Kieras-Phillips C. , Kilpatrick S. J. , LaMarche P. H. , LeBlanc B. , Little R. , Manos D. M. , Mansfield D. K. , Mazzucato E. , McCarthy M. P. , McCune D. C. , McGuire K. , McNeill D. H. , Meade D. M. , Medley S. S. , Mikkelsen D. R. , Motley R. , Mueller D. , Murphy J. A. , Nagayama Y. , Nazakian R. , Owens D. K. , Park H. , Ramsey A. T. , Redi M. H. , Roquemore A. L. , Rutherford P. H. , Schilling G. , Schivell J. , Schmidt G. L. , Stevens J. , Stratton B. C. , Stodiek W. , Synakowski E. J. , Tang W. M. , Taylor G. , Timberlake J. R. , Towner H. H. , Ulrickson M. , von Goeler S. , Wieland R. , Williams M. , Wilson J. R. , Wong K.-L. , Yoshikawa S. , Young K. M. , Zarnstorff M. C. , Zweben S. J.
Published: July 2002

Measurements of the toroidal rotation speed vφ(r) driven by neutral beam injection in tokamak plasmas and, in particular, simultaneous profile measurements of vφ, Ti, Te, and ne, have provided new insights into the nature of anomalous transport in tokamaks. Low-recycling plasmas heated with unidirectional neutral beam injection exhibit a strong correlation among the local diffusivities, χφ≊χi>χe. Recent measurements have confirmed similar behavior in broad-density L-mode plasmas. These results are consistent with the conjecture that electrostatic turbulence is the dominant transport mechanism in the tokamak fusion test reactor tokamak (TFTR) [Phys. Rev. Lett. 58, 1004 (1987)], and are inconsistent with predictions both from test-particle models of strong magnetic turbulence and from ripple transport. Toroidal rotation speed measurements in peaked-density TFTR ‘‘supershots’’ with partially unbalanced beam injection indicate that momentum transport decreases as the density profile becomes more peaked. In high-temperature, peaked-density plasmas the observed gradient scale length parameter ηtoti=d ln Ti/d ln ne correlates reasonably well with predictions of the threshold for exciting ion-temperature-gradient-driven turbulence (ITGDT), as would be expected for plasmas at marginal stability with respect to this strong transport mechanism. In L-mode plasmas where ITGDT is expected to be too weak to enforce marginal stability, ηtoti exceeds this threshold considerably. However, preliminary experiments have failed to observe a significant increase in ion heat transport when ηtoti was rapidly forced above ηc (the threshold for exciting ITGDT) using a perturbative particle source, as would have been expected for a plasma at marginal stability.
Three-dimensional simulation of ∇
T
i-driven turbulence and transport
Authors: Drake J. F. , Guzdar P. N. , Dimits A.
Published: July 2002

The anomalous transport of ion thermal energy in a heated plasma slab confined by a uniform, straight magnetic field Bz has been investigated in three-dimensional (3-D) fluid simulations. Convection flows are driven unstable by ∇Ti and nonlinearly develop into narrow streams which carry cold edge plasma into the hot center. The convective flows undergo a sharp transition from laminar to turbulent behavior as the thermal energy confined in the slab is increased beyond a critical level. This transition is reminiscent of similar behavior in Rayleigh–Bénard convection in heated fluids. The convective thermal transport increases sharply as this turbulence threshold is exceeded. The structure of the flow patterns and associated transport also depend strongly on the physical dimensions of the confined plasma (Lx,Ly,Lz) =(a,2πa,2πR) compared with the ion Larmor radius ρi. For α=a2/Rρi>1, the dominant flows have kyρi ∼a/R and produce an anomalous cross field thermal transport χ⊥i that scales as via2/R . In the opposite limit α=a2/Rρi <1, dominant flows have kya∼1 and the transport is given by χ⊥i∼ρivi, the Bohm scaling. Close to marginal stability, the transport is greatly reduced. These simulations imply that ion thermal transport in any straight field system such as a tandem mirror or a stellarator with weak shear and nearly rational fields will be strongly anomalous if the ∇Ti threshold for instability is exceeded.
Particle acceleration near X-type magnetic neutral lines
Authors: Deeg Hans-Jörg , Borovsky Joseph E. , Duric Nebojsa
Published: July 2002

The behavior of charged particles near X-type magnetic neutral lines is investigated. The magnetic field is taken to be hyperbolic and time stationary, with a uniform electric field perpendicular to the magnetic field. The general properties of the orbits of noninteracting particles in that field geometry are examined. Approximate analytic solutions to the orbit equations are derived by considering a magnetic-insulation picture where there is a hole in the magnetic insulation around the neutral line. Scaling laws for the dependence of the energy gain on strengths of the magnetic and electric fields are derived. The kinetic-energy distribution function for accelerated particles is derived for particles flowing past the neutral line. These analytic derivations are supported by computer simulations, in which the relativistic equations of motion for single test particles are numerically solved and the orbits and kinetic-energy gains of the particles are obtained. A number of simulations were performed and one parameter at a time was varied. Applications of the results to magnetic-neutral-line regions in space physics and astrophysics are possible.
The role of plasma in advanced accelerators*
Authors: Wurtele Jonathan S.
Published: July 2002

The role of plasma in advanced accelerators is reviewed with emphasis on three significant areas of research: plasma guiding of beams in accelerators, plasma focusing of beams in high-energy linear colliders, and plasma acceleration of beams.
Convective generation of a vortex street in plasma shear flow
Authors: Yamamoto Takashi , Yamashita M. Andrew
Published: July 2002

By numerical calculation, it is shown that vortices are sequentially generated on the background charged layer forming parallel shear flows in a two-dimensional low beta incompressible plasma when the charged layer is locally disturbed by a small amount of additional charge. Numerical experiments are performed in a rectangular domain with 0<x<Lx and −Ly/2<y<Ly/2 and the background charged layers are distributed to form the plasma flow U in the x direction with a velocity shear of ∂U/∂y≠0. The boundaries at x=0 and Lx are to supply plasmas identical to the background plasma initially loaded inside the domain. The plasma particles reaching the boundaries are freely allowed to leave the experimental domain. A vortex (named the mother vortex) first develops the local charge disturbance initially given, and subsequently new vortices (named the daughter vortices) are generated on the charged layer. This can be thought of as a result of propagation of the initial disturbance: A relative plasma flow as seen from the mother vortex can carry downstream the disturbances originating from the vortex, which can trigger growth of the daughter vortex. The numerical experiments in our nonperiodic system show that the intervals between the vortices are relatively close to the wavelength of the most unstable mode predicted by the linear analysis for periodic vortices. It is also shown that the temporal growth rate γ of the vortex in its early growing period is comparable to the corresponding linear growth rate of the unstable periodic vortices with the same wavelength.
Application of nonlinear dynamical invariants in a single electromagnetic wave to the study of the Alfvén-ion-cyclotron instability
Authors: Otani Niels F.
Published: July 2002

Dynamical invariants are derived for particles moving in a single, circularly polarized electromagnetic wave of arbitrary time dependence propagating parallel to a uniform background magnetic field. The invariant associated with helical symmetry is shown to restrict the particle motion to a very narrow region of velocity space. Features of the slow time-scale motion of fixed points associated with the existence of a fourth adiabatic invariant are described for the case of a slowly varying wave. Characteristics of the particle motion thus derived are applied to the analysis of 1d–3v simulations of the saturation of the Alfvén-ion-cyclotron (AIC) instability for a single wave. In particular, an explanation is offered for the appearance of a sharp edge in the velocity distribution function observed in the simulation.
The enstrophy equation budget of bounded turbulent shear flows
Authors: Gorski Joseph J. , Wallace James M. , Bernard Peter S.
Published: July 2002

The turbulent enstrophy equation budget for a fully developed two-dimensional channel flow computed from a direct numerical simulation is compared to that for a flat plate boundary layer flow obtained experimentally. A close agreement is found between the comparable terms in the enstrophy equation suggesting that there probably is common behavior for the enstrophy budget in wall bounded flows.
Investigation of characteristic scales in variable density turbulent jets using a second-order model
Authors: Ruffin E. , Schiestel R. , Anselmet F. , Amielh M. , Fulachier L.
Published: July 2002

This paper is concerned with the investigation of characteristic turbulent scales in circular and plane turbulent free jets with large density variations. The numerical study has been performed with a second-order turbulence model to confirm analytical relations about density and Reynolds number effects on dynamic and scalar turbulence scales derived from requirements of pseudo-self-preservation. The present results explain and quantify trends previously reported from visualizations of variable density turbulent jets performed by Pitts (NBSIR Report No. 86-3340, U.S. Department of Commerce, 1986). It is clearly shown that the parameter relevant for the evolution of these scales is the momentum flux Mj. In addition, the influence of this quantity on small scales is found to be larger than that on large scales.
A method for computing Stokes flow interactions among spherical objects and its application to suspensions of drops and porous particles
Authors: Mo Guobiao , Sangani Ashok S.
Published: July 2002

A method for computing Stokes flow interactions in suspensions of spherical objects is described in detail and applied to the suspensions of porous particles, drops, and bubbles to determine their hydrodynamic transport coefficients.
Analytical and phenomenological studies of rotating turbulence
Authors: Mahalov Alex , Zhou Ye
Published: July 2002

A framework, which combines mathematical analysis, closure theory, and phenomenological treatment, is developed to study the spectral transfer process in turbulent flows that are subject to rotation. First, we outline a mathematical procedure that is particularly appropriate for problems with two disparate time scales. The approach that is based on the Green’s method leads to the Poincaré velocity variables and the Poincaré transformation when applied to rotating turbulence. The effects of the rotation are now conveniently included in the momentum equation as the modifications to the convolution of nonlinear term. The Poincaré transformed equations are used to obtain a time-dependent Taylor–Proudman theorem valid in the asymptotic limit when the nondimensional parameter μ≡Ωt→∞ (Ω is the rotation rate and t is the time). The ‘‘split’’ of the energy transfer in both direct and inverse directions is established. Second, we apply the Eddy-Damped-Quasinormal-Markovian (EDQNM) closure to the Poincaré transformed Euler/ Navier–Stokes equations. This closure leads to expressions for the spectral energy transfer. In particular, a unique triple velocity decorrelation time is derived with an explicit dependence on the rotation rate. This provides an important input for applying the phenomenological treatment of Zhou [Phys. Fluids 7, 2092 (1995)]. In order to characterize the relative strength of rotation, another nondimensional number, a spectral Rossby number, which is defined as the ratio of rotation, and turbulence time scales, is introduced. Finally, the energy spectrum and the spectral eddy viscosity are deduced.
Oscillatory Marangoni convection in cylindrical liquid bridges
Authors: Savino R. , Monti R.
Published: July 2002

Oscillatory Marangoni convection in silicon–oil liquid bridges, sustained by two circular coaxial disks with prescribed time-dependent temperature profiles and bounded by cylindrical free surfaces, is investigated by direct three-dimensional (3-D) and time-dependent simulation of the model equations, using finite difference methods explicit in time and a staggered spatial mesh in cylindrical coordinates. It is shown that, for low enough values of the dimensionless rate of ramping, the time-dependent nature of the boundary conditions becomes unimportant and the computed critical Marangoni numbers approach the values obtained with steady stability analyses. For typical microgravity experiments, involving unsteady boundary conditions, the computed critical Marangoni numbers and the oscillation frequencies agree with available experimental data of sounding rockets and Spacelab experiments. The 3-D thermo-fluid-dynamic oscillatory regime structures are depicted, discussed, and compared with previous experimental and theoretical analyses, providing physical explanations of the onset of instability and coherent pictures of the flow organization when oscillatory conditions are established. Immediately after the onset of instability, the oscillatory flow can be described by a standing wave and a pulsating temperature distribution. When the oscillatory disturbances become large, the azimuthal velocity causes the rotation of ‘‘temperature spots’’ along the free surface of the liquid bridge so that the time-dependent temperature and velocity fields can be properly described by the dynamic model of an azimuthally traveling wave.