bluuebunny/crossref_metadata_2025 huggingface dataset:
Many-body theory of intermolecular induction interactions
Authors: Moszyński Robert , Cybulski Sl/awomir M. , Chal/asiński Grzegorz
Published: July 2002

The second-order induction energy in the symmetry-adapted perturbation theory is expressed in terms of electron densities and polarization propagators at zero frequency of the isolated monomers. This expression is used to derive many-body perturbation series with respect to the Mo/ller–Plesset type correlation potentials of the monomers. Two expansions are introduced—one based on the standard Mo/ller–Plesset expansion of electron densities and polarization propagators, and the second accounting for the so-called response or orbital relaxation effects, i.e., for the perturbation induced modification of the monomer’s Fock operators. Explicit orbital formulas for the leading perturbation corrections that correctly account for the response effects are derived through the second order in the correlation potential. Numerical results are presented for several representative van der Waals complexes—a rare gas atom and an ion Ar–Na+, Ar–Cl−, and He–F−; a polar molecule and an ion H2O–Na+ and H2O–Cl−; two polar molecules (H2O)2; and a rare gas atom and a polar molecule Ar–HCl and He–HCl. It is shown that in the above systems, the significance of the correlation part of the induction energy varies from a very important one in the complexes of rare gas atoms and ions to a practically negligible one in the complexes of rare gases with polar molecules.
Thermochemistry of gaseous SiO(OH), SiO(OH)2, and SiO2
Authors: Hildenbrand D. L. , Lau K. H.
Published: July 2002

The gaseous species SiO(OH) and SiO(OH)2 were detected as products of the reaction of SiO2(l) and H2O(g) near 2000 K, using effusion-beam mass spectrometry. No other silicon-containing hydroxides or oxyhydroxides were observed. From third law analysis of equilibrium data, values of ΔfH°298 of SiO(OH) and SiO(OH)2 were evaluated as −118 and −213 kcal mol−1, respectively, both ±4 kcal mol−1. Several equilibrium measurements on gaseous SiO2 yielded a ΔfH°298 value of −67 kcal mol−1, which is 6–10 kcal mol−1 less stable than values given in most compilations. It appears that the new thermochemical data for SiO(OH) and SiO(OH)2, plus a literature estimate for Si2O(OH)6, can be used to model the equilibrium behavior of the silica–steam system over a wide range of temperatures and pressures.
Near ultraviolet photolysis of C2H2: A precise determination of D(HCC−H)
Authors: Mordaunt David H. , Ashfold Michael N. R.
Published: July 2002

The near ultraviolet photolysis of jet-cooled acetylene molecules has been further investigated using the technique of H atom photofragment translational spectroscopy. Analysis of the rotational structure evident in the total fragment kinetic energy spectrum yields a precise value for the C–H bond dissociation energy: D0(HCC–H)=46 074±8 cm−1 (551.2±0.1kJ mol−1).
Renormalization theory of nonuniversal thermal properties of fluids
Authors: White John A. , Zhang Sheng
Published: July 2002

A recently developed renormalization group theory for fluids can be used to calculate nonuniversal as well as universal critical point properties and also to predict the changes in pressure and other thermal properties that occur out to substantial distances away from the critical point. These features of the theory are illustrated here for a model square-well gas. The calculated pressure, temperature, and density at the critical point and transition to noncritical behavior away from the critical point for this gas are compared with measured properties of parahydrogen reported in the literature.
A thermodynamic investigation of dynamical disorder in Phase II of CBr4
Authors: Michalski Darek , White Mary Anne
Published: July 2002

CBr4 is an archetypal example of a pseudospherical molecule that can form an orientationally disordered phase (phase I, 320 K<T<365 K). At lower temperatures, phase II is the stable phase; although it must be more ordered than phase I, different techniques give conflicting results concerning order in phase II. In order to investigate dynamical disorder in phase II, the heat capacity of CBr4 has been measured in the temperature range 30–305 K. No phase transitions were found in this region. The calculated heat capacity, with contributions due to low-frequency translation and rotational-vibrations of the rigid CBr4 molecules, as well as internal motions, fits the experimentally derived Cv data to within 3%. Virtually full excitation of rigid molecule rotation-vibrations at very low temperatures (∼45 K) is consistent with the observation of a low-temperature elevation in the calculated Grüneisen parameter.
Microscopic approach to inhomogeneous polymeric liquids
Authors: Donley James P. , Rajasekaran J. J. , McCoy John D. , Curro John G.
Published: July 2002

We discuss a liquid-state theory for the equilibrium structure of inhomogeneous polymeric liquids. The theory consists of an equation for the density profile of a liquid in an external potential, which has been derived previously by density functional methods. In general, this equation must be solved by simulation techniques. However, if the chains are modeled as random walks—which is a reasonable approximation for flexible polymers at melt densities—we show that the theory reduces to a set of coupled integral equations which can be solved numerically. We present results for a single component liquid near a hard wall. Last, we show that, in the Gaussian thread limit, the theory reduces to a form that is very similar to Edwards–Helfand–Tagami ‘‘self-consistent field’’ theory. However, there are important differences between the two theories for multicomponent liquids (a blend for example) if the types of polymers are structurally dissimilar.
The generation and use of delocalized internal coordinates in geometry optimization
Authors: Baker Jon , Kessi Alain , Delley Bernard
Published: July 2002

Following on from the earlier work of Pulay and Fogarasi [J. Chem. Phys. 96, 2856 (1992)] we present an alternative definition of natural internal coordinates. This set of delocalized internal coordinates can be generated for any molecular topology, no matter how complicated, and is fully nonredundant. Using an appropriate Schmidt-orthogonalization procedure, all standard bond length, bond angle, and dihedral angle constraints can be imposed within our internal coordinate scheme. Combinatorial constraints (in which sums or differences of stretches, bends, and torsions remain constant) can also be imposed. Optimizations on some fairly large systems (50–100 atoms) show that delocalized internal coordinates are far superior to Cartesians even with reliable Hessian information available at the starting geometry.
Interionic interactions and fast-ion conduction in CaF2
Authors: Wilson Nicholas T. , Wilson Mark , Madden Paul A. , Pyper Nicholas C.
Published: July 2002

Simulations of the superionic conductor CaF2 have been undertaken, with a model for the interionic interactions derived directly from electronic structure calculations. The model includes such many-body effects as polarization and the adaptation of the size of the anion to its instantaneous environment (‘‘compression’’). Physical properties of CaF2 accurately reproduced include the phonon frequencies, the fluorite→α-PbCl2-structure phase transition and the solid-state diffusion. Analysis shows that the compression effect is much less important for fluorides than oxides but, nevertheless, significantly affects the phase transition pressure. Polarization effects, particularly of the cations, play a crucial role in allowing the anion diffusion, and the importance of a short-range contribution to the cation polarization is highlighted. These findings are contrasted with previous work using empirically parametrized shell and pair potentials, which suggest that other interactions play the dominant role.
Photochemistry and dynamics of C6H6–O2 clusters at 226 nm
Authors: DeBoer Gary , Young Mark A.
Published: July 2002

The photochemistry and dynamics of small C6H6–O2 clusters were studied in a supersonic expansion using 226 nm laser excitation and multiphoton ionization probes. We were able to detect a strong signal due to O(3P2) when mixed clusters were present in the expansion but no O atom fragments could be observed in the absence of benzene in the expansion mixture. Photofragmentation of O2 in the unique environment of the cluster is enhanced by at least three orders of magnitude compared to the isolated oxygen molecule. The kinetic energy release of the O(3P2) was determined with a time-of-flight method and found to be relatively small and characterized by a completely isotropic spatial distribution. The fine structure population of the O(3Pj) was also examined and the resultant branching fractions, P2,1,0=0.68±0.03, 0.26±0.06, 0.06±0.01, are similar to those obtained for photodissociation of isolated O2 by other workers. We also find that photochemical production of oxygen containing products, such as C6H6O, becomes feasible in larger cluster species due to solvent cage effects which trap the recoiling O atom fragments. The observed dynamics can be attributed to either excitation of the supramolecular C6H6–O2 charge-transfer state, or localized excitation of a perturbed transition in O2. The net effect of cluster absorption is to greatly enhance a chemical pathway that is only weakly observed in the separated molecules, similar to the behavior that has recently been described for the C6H6–I2 complex.
Structurally mixed molecular eigenstates of 2-fluoroethanol resulting from conformational isomerization
Authors: Hudspeth Evan , McWhorter David A. , Pate Brooks H.
Published: July 2002

Using a newly developed method of molecular-beam, infrared-microwave double-resonance spectroscopy, we are able to measure the rotational spectrum of a single molecular eigenstate of the molecular Hamiltonian near 3000 cm−1 of energy above the ground rovibrational state. This energy lies above the barrier to conformation isomerization in many molecules. In the –CH2F asymmetric C–H stretch of the Gg conformer of 2-fluoroethanol, near 2983 cm−1, we demonstrate the contribution of vibrational states localized around the Tt conformer structural minimum to the individual molecular eigenstates. The measurement demonstrates the ability of isolated molecules to use vibrational excitation to achieve geometrical rearrangement.
Electrophoresis of polyampholytes
Authors: Long Didier , Dobrynin Andrey V. , Rubinstein Michael , Ajdari Armand
Published: July 2002

We consider the motion and the deformation of Gaussian polyampholytes in free flow electrophoresis, i.e., in an applied external electric field. The electrophoretic mobility and the deformation of the chains are calculated in the linear regime, as functions of the charge distribution along the backbone and of the salt concentration. The results in salt-free solutions differ from those in solutions with a high concentration of salt even at the level of scaling laws. It is shown that in solutions with a high salt concentration, the electrophoretic mobility of a given polyampholyte strongly depends not only on its total charge but also on the details of the charge distribution along the chain. The very direction of motion can also depend on it. Indeed, even “neutral” polyampholytes, i.e., chains with equal number of positive and negative charges can move in an applied electric field. To demonstrate further these effects, we systematically compare the behavior of the linear and circular chains.
Dynamic simulation of diblock copolymer microphase separation
Authors: Groot Robert D. , Madden Timothy J.
Published: July 2002

The dissipative particle dynamics (DPD) simulation method has been used to study mesophase formation of linear (AmBn) diblock copolymer melts. The polymers are represented by relatively short strings of soft spheres, connected by harmonic springs. These melts spontaneously form a mesocopically ordered structure, depending on the length ratio of the two blocks and on the Flory–Huggins χ-parameter. The main emphasis here is on validation of the method and model by comparing the predicted equilibrium phases to existing mean-field theory and to experimental results. The real strength of the DPD method, however, lies in its capability to predict the dynamical pathway along which a block copolymer melt finds its equilibrium structure after a temperature quench. The present work has led to the following results: (1) As the polymer becomes more asymmetric, we qualitatively find the order of the equilibrium structures as lamellar, perforated lamellar, hexagonal rods, micelles. Qualitatively this is in agreement with experiments and existing mean-field theory. After taking fluctuation corrections to the mean field theory into account, a quantitative match for the locations of the phase transitions is found. (2) Where mean-field theory predicts the gyroid phase to be stable, the simulations evolve toward the hexagonally perforated lamellar phase. (3) When a melt is quenched the stable structure emerges via a nontrivial pathway, where a series of metastable phases can be formed before equilibrium is reached. The pathway to equilibrium involves a percolation of the minority phase into a network of tubes, which is destabilized by a nematic or smectic transition. (4) We conclude that either hydrodynamic interactions, or the precise form of the Onsager kinetic coefficient play an important role in the evolution of the mesophases.
Generalized adiabatic connection in density functional theory
Authors: Yang Weitao
Published: July 2002

A generalized adiabatic connection is developed for density functional theory. The method extends the well-known adiabatic connection formula and provides a general link between the Kohn–Sham and the physical system. When the complimentary error function is used as a special case, the expression for the exchange-correlation functional does not have the 1/r12 Coulomb component. The exact contributions from the physical system and the noninteracting system are established: The physical system limit has a dominant contribution, while the noninteracting system limit has no contribution.
Full configuration interaction benchmark calculations of first-order one-electron properties of BH and HF
Authors: Halkier Asger , Larsen Helena , Olsen Jeppe , Jo/rgensen Poul , Gauss Jürgen
Published: July 2002

Full configuration interaction benchmark calculations have been carried out for the electric dipole and quadrupole moments and the electric field gradient at the nuclei of BH and HF. The accuracy of perturbation theory from second to fourth order and coupled cluster theory with up to triple excitations has been investigated. For all the properties the coupled cluster models outperform the perturbation series. The convergence of the series of coupled cluster models is significantly faster and more systematic than the one of the perturbation series, and only the coupled cluster series defines a hierarchy of models with well defined levels of accuracy. The CCSD(T) model is a good approximation to the full coupled cluster singles, doubles, and triples model. It recovers 80%–90% of the full effect of triple excitations, and the small error due to the approximate description of triple excitations is comparable in size to the error due to neglect of higher-order excitations. For accurate calculations, the CCSD(T) model is the preferred model for all the properties. Our estimates of the exact dipole moments μe(HF)=1.802±0.003 D and μ0(BH)=1.356±0.004 D are in perfect agreement with the experimental values of μe(HF)=1.803±0.002 D and μ0(BH)=1.27±0.21 D.
Numerically exact diffusion coefficients for lattice systems with periodic boundary conditions. II. Numerical approach and applications
Authors: Mercier Jean-François , Slater Gary W.
Published: July 2002

In the first part of this short series [Mercier Slater and Guo, J. Chem. Phys. 110, 6050 (1999), preceding paper], we derived two algebraically exact methods to calculate the scaled diffusion coefficient D* of a particle in a lattice system with immobile obstacles and periodic boundary conditions. We showed that the method based on the Nernst–Einstein relation was much more powerful than the one based on first passage times. Indeed, the former simply reduces the problem to the solution of a system of linear equations. In this article, we now describe and test a numerical implementation for this method. We also use this implementation to treat several applications in order to demonstrate both its validity and its power.
Structure and vibrations of phenol(H2O)7,8 studied by infrared-ultraviolet and ultraviolet-ultraviolet double-resonance spectroscopy and ab initio theory
Authors: Janzen Ch. , Spangenberg D. , Roth W. , Kleinermanns K.
Published: July 2002

The vibronic spectra of jet cooled phenol(H2O)7,8 clusters were analyzed with mass selective resonance enhanced two photon ionization (R2PI) and ultraviolet-ultraviolet spectral hole burning (UV-UV SHB). A double resonance technique with an infrared (IR) laser as burn laser (IR-UV SHB) was used to measure the intramolecular OH stretching vibrations of the mass- and isomer-selected clusters. Two isomers of phenol(H2O)7 and three isomers of phenol(H2O)8 could be distinguished via SHB and their IR spectra recorded. The red- or blueshift of the electronic origin relative to the phenol monomer gives valuable hints on the hydrogen bonding between phenol and the water moiety. All IR spectra contain four characteristic groups of OH stretching vibrations which give insight into the structure of the H bonded network. The ab initio calculations show that the minimum energy structures for phenol(H2O)7,8 are very similar to the corresponding water clusters which are based on regular (H2O)8 cubes. Comparison between experiment and calculation for phenol(H2O)8 shows that phenol can attach to and insert itself in the water network.
On the growth dynamics of neutral vanadium oxide and titanium oxide clusters
Authors: Foltin M. , Stueber G. J. , Bernstein E. R.
Published: July 2002

Cluster growth dynamics of vanadium oxide and titanium oxide clusters produced by laser ablation of vanadium and titanium metal in a He gas flow seeded with up to 2% O2 are studied by covariance mapping time-of-flight mass spectrometry. Covariance mapping enables the recognition of two different distribution components in the overall homogeneous mass spectra for both vanadium oxide and titanium oxide cluster systems. The oxygen-rich component Or shows small correlated fluctuations while the oxygen-poor component Op shows large correlated fluctuations. These two cluster distribution components are observed at low ablation laser powers and low expansion gas concentrations. Fluctuations of small vanadium oxide clusters (V2O, V2O2, and V2O3) and small titanium oxide clusters (Ti2O2 and Ti2O3) are covariance determining. The less fluctuating V2O3 and Ti2O3 clusters are “nuclei” for the oxygen-rich components Or. The more fluctuating V2O and Ti2O2 are “nuclei” for the oxygen poor components Op. Correlated fluctuations or covariances within each distribution component are constant. Covariances for the different distribution components are different. Studies of mass spectra and covariances as functions of ablation laser power and expansion gas concentration imply that V2O and Ti2O2 clusters are formed in different regions of the ablation plasma plume than V2O3 and Ti2O3. We suggest that V2O3 and Ti2O3 are formed in the hot and optically dense region near the ablated metal surface and that V2O and Ti2O2 are formed in the colder plasma region farther away from the ablated metal surface. Larger vanadium oxide and titanium oxide clusters grow from these small clusters by very specific pathways which involve only uptake of VO or VO2, and TiO2, respectively.
Terahertz laser spectroscopy of the water dimer intermolecular vibrations. I. (D2O)2
Authors: Braly L. B. , Cruzan J. D. , Liu K. , Fellers R. S. , Saykally R. J.
Published: July 2002

Terahertz laser VRT spectra of the water dimer consisting of 731 transitions measured with an average precision of 2 MHz and involving four (D2O)2 intermolecular vibrations (one previously published) have been measured between 65 and 104 cm−1. The precisely determined energy level patterns differ both qualitatively and quantitatively from the predictions of several dimer potentials tested, and reveal an ordering of the intermolecular vibrations which differs dramatically from that predicted by standard normal mode analysis. Strong coupling is indicated between the low barrier tunneling motions and the intermolecular vibrations as well as among different vibrations. Particularly, the 83 cm−1 (acceptor wag) and 90 cm−1 (D2O)2 (acceptor twist) vibrations interact through a Coriolis perturbation. These spectra provide the basis for our recent determination of the water pair potential. The corresponding data set for (H2O)2 is presented in an accompanying paper.
An extension of quaternionic metrics to octonions
Authors: Marques S. , Oliveira C. G.
Published: July 2002

A treatment of a non-Riemannian geometry including internal complex, quaternionic, and octonionic space is made. Then, an interpretation of this geometry for the nonsymmetric theory of Einstein–Schrödinger, and for the unified theory of Borchsenius is showed. Finally, field equations in the extended octonionic geometry of space-time are obtained through a minimal action principle.
Unitary representations of the (4+1) de Sitter group on unitary irreducible representation spaces of the Poincaré group: Equivalence with their realizations as induced representations
Authors: Moylan P.
Published: July 2002

In a previous work we have constructed realizations of the principal continuous series of unitary irreducible representations of the simply connected covering group of the (4+1) de Sitter group on unitary irreducible representation spaces of the simply connected covering group of the Poincaré group. In this work we demonstrate the equivalence of the representations constructed in the previous work with their realizations as induced representations.
New expressions for the eigenvalues of the invariant operators of the general linear and the orthosymplectic Lie superalgebras
Authors: Nwachuku C. O. , Rashid M. A.
Published: July 2002

We obtain expansions for the eigenvalues Cp of the invariant operators (Casimir operators) of the general linear, and the orthosymplectic Lie superalgebras in terms of products of suitably defined graded power sums Pk. The resulting expressions are closed and provide unified formulas for computing the Cp’s for those superalgebras as well as their corresponding Lie algebras. The formulas are remarkably simple to suggest that the power sums used in this text could play a more basic role in the understanding of the pattern of the expansion coefficients. Explicit illustrations are given for the various series for p≤8.
Conformally symmetric radiating spheres in general relativity
Authors: Esculpi M. , Herrera L.
Published: July 2002

A method used to study the evolution of radiating anisotropic (principal stresses unequal) spheres is applied to the case in which the space-time (within the sphere) admits a one-parameter group of conformal motions. Two different kind of models are obtained, depending on the equation of state for the stresses. In one case the energy flux density at the boundary of the sphere (the luminosity) should be given as a function of the timelike coordinate in order to integrate the system of equations. In the other case the luminosity is inferred from the equation of state for the stresses. Both models are integrated numerically and their eventual applications to some astrophysical problems are discussed.
Kinematic and dynamic properties of conformal Killing vectors in anisotropic fluids
Authors: Maartens R. , Mason D. P. , Tsamparlis M.
Published: July 2002

An example from a perfect fluid FRW space-time is presented to show that a conformal Killing vector (CKV) need not map fluid flow lines into fluid flow lines. Kinematic properties of the Lie derivative along a CKV of timelike and spacelike unit vectors are derived and applied to the fluid unit four-velocity vector. Dynamic properties of special conformal Killing vectors (SCKV) in a fluid with anisotropic pressure and vanishing energy flux are obtained using Einstein’s field equations. It is shown that a SCKV maps both fluid flow lines and integral curves of na into themselves, where na is the unit spacelike vector of anisotropy. The relation between the anisotropic pressure components and the energy density is considered. By means of an example from a radiationlike viscous fluid FRW space-time it is shown that the dynamic results depend crucially on the vanishing of the energy flux vector. The extension of the dynamic results to a fluid with arbitrary stress tensor and zero energy flux vector is examined.
Spectral theory for unitary operators on a quaternionic Hilbert space
Authors: Sharma C. S. , Coulson T. J.
Published: July 2002

The spectral theorem for a unitary operator on a quaternionic Hilbert space is established and a number of related results are proved.
Schrödinger representation in Euclidean quantum field theory
Authors: Semmler U.
Published: July 2002

The Schrödinger representation of a Euclidean quantum field is constructed nonperturbatively in a new way by defining the probability amplitude ψ[u,t] as a limit of a functional integral depending on two parameters. It is shown in which sense ψ[u,t] solves the formal Schrödinger equation. Finally, the role of the potential is investigated.
Additive functionals and operators on a quaternionic Hilbert space
Authors: Sharma C. S. , Almeida D. F.
Published: July 2002

It is shown that the structure of functionals and operators on a quaternionic Hilbert space is much richer than is generally appreciated, but one has to work simultaneously with the usual definition and an unusual one of multiplication by scalar for functionals—the different definitions, of course, give rise to different vector spaces. A generalized version of the Riesz representation theorem for quaternionic Hilbert spaces is proved along with the basic theorem on the algebra of additive operators on such a space.
Relations for Clebsch–Gordan and Racah coefficients in suq(2) and Yang–Baxter equations
Authors: Nomura Masao
Published: July 2002

Relations are exploited among Clebsch–Gordan (CG) and Racah coefficients in the algebra suq(2), known as a deformation of su(2). These are used to show that the Yang–Baxter (YB) relation for the IRF (interaction round a face) model results from one of the symmetry relations for the 9-j symbol specific to suq(2), and that in an asymptotic limit this YB relation becomes the YB relation for the two-dimensional vertex model. The Racah coefficient, which has a particularly simple dependence on q, is efficiently used such that an asymptotic limit of the Racah coefficient is the CG coefficient and another limit gives the factorized S matrix of the vertex model.
Solving the two-dimensional constant quantum Yang–Baxter equation
Authors: Hietarinta Jarmo
Published: July 2002

A detailed analysis of the constant quantum Yang–Baxter equation Rk1k2j1j2 Rl1k3k1j3Rl2l3k2k3= Rk2k3j2j3 Rk1l3j1k3Rl1l2k1k2 in two dimensions is presented, leading to an exhaustive list of its solutions. The set of 64 equations for 16 unknowns was first reduced by hand to several subcases which were then solved by computer using the Gröbner-basis methods. Each solution was then transformed into a canonical form (based on the various trace matrices of R) for final elimination of duplicates and subcases. If we use homogeneous parametrization the solutions can be combined into 23 distinct cases, modulo the well-known C, P, and T reflections, and rotations and scalings R̃=κ(Q⊗Q)R(Q⊗Q)−1.
Localization and diagonalization: A review of functional integral techniques for low-dimensional gauge theories and topological field theories
Authors: Blau Matthias , Thompson George
Published: July 2002

Localization techniques for functional integrals which have recently been used to perform calculations in and gain insight into the structure of certain topological field theories and low-dimensional gauge theories are reviewed. These are the functional integral counterparts of the Mathai–Quillen formalism, the Duistermaat–Heckman theorem, and the Weyl integral formula, respectively. In each case, the necessary mathematical background (Euler classes of vector bundles, equivariant cohomology, topology of Lie groups) is introduced, and the finite dimensional integration formulae described. Then some applications to path integrals are discussed and an overview of the relevant literature is given. The applications include supersymmetric quantum mechanics, cohomological field theories, phase space path integrals, and two-dimensional Yang–Mills theory.
Radiation-dominated quantum Friedmann models
Authors: Lemos Nivaldo A.
Published: July 2002

Radiation-filled Friedmann–Robertson–Walker universes are quantized according to the Arnowitt–Deser–Misner formalism in the conformal-time gauge. Unlike previous treatments of this problem, here both closed and open models are studied, only square-integrable wave functions are allowed, and the boundary conditions to ensure self-adjointness of the Hamiltonian operator are consistent with the space of admissible wave functions. It turns out that the tunneling boundary condition on the universal wave function is in conflict with self-adjointness of the Hamiltonian. The evolution of wave packets obeying different boundary conditions is studied, and it is generally proven that all models are nonsingular. Given an initial condition on the probability density under which the classical regime prevails, it is found that a closed universe is certain to have an infinite radius, a density parameter Ω=1 becoming a prediction of the theory. Quantum stationary geometries are shown to exist for the closed universe model, but oscillating coherent states are forbidden by the boundary conditions that enforce self-adjointness of the Hamiltonian operator.
R-matrix theory, formal Casimirs and the periodic Toda lattice
Authors: Morosi Carlo , Pizzocchero Livio
Published: July 2002

The nonunitary r-matrix theory and the associated bi- and triHamiltonian schemes are considered. The language of Poisson pencils and of their formal Casimirs is applied in this framework to characterize the biHamiltonian chains of integrals of motion, pointing out the role of the Schur polynomials in these constructions. This formalism is subsequently applied to the periodic Toda lattice. Some different algebraic settings and Lax formulations proposed in the literature for this system are analyzed in detail, and their full equivalence is exploited. In particular, the equivalence between the loop algebra approach and the method of differential-difference operators is illustrated; moreover, two alternative Lax formulations are considered, and appropriate reduction algorithms are found in both cases, allowing us to derive the multiHamiltonian formalism from r-matrix theory. The systems of integrals for the periodic Toda lattice known after Flaschka and Hénon, and their functional relations, are recovered through systematic application of the previously outlined schemes.
Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry: Supplement VIII, Halogen Species Evaluation for Atmospheric Chemistry
Authors: Atkinson R. , Baulch D. L. , Cox R. A. , Hampson R. F. , Kerr J. A. , Rossi M. J. , Troe J.
Published: July 2002

This paper updates and extends part of the previous data base of critical evaluations of the kinetics and photochemistry of gas-phase chemical reactions of neutral species involved in atmospheric chemistry [J. Phys. Chem. Ref. Data 9, 295 (1980); 11, 327 (1982); 13, 1259 (1984); 18, 881 (1989); 21, 1125 (1992); 26, 521 (1997); 26, 1329 (1997); 28, 191 (1999)]. The present evaluation is limited to the inorganic halogen family of atmospherically important reactions. The work has been carried out by the authors under the auspices of the IUPAC Subcommittee on Gas Phase Kinetic Data Evaluation for Atmospheric Chemistry. Data sheets have been prepared for 102 thermal and photochemical reactions, containing summaries of the available experimental data with notes giving details of the experimental procedures. For each thermal reaction, a preferred value of the rate coefficient at 298 K is given together with a temperature dependence where possible. The selection of the preferred value is discussed and estimates of the accuracies of the rate coefficients and temperature coefficients have been made for each reaction. For each photochemical reaction the data sheets list the preferred values of the photoabsorption cross sections and the quantum yields of the photochemical reactions together with comments on how they were selected. The data sheets are intended to provide the basic physical chemical data needed as input for calculations that model atmospheric chemistry. A table summarizing the preferred rate data is provided, together with an appendix listing the available values of enthalpies of formation of the reactant and product species.
Vortex dipole rebound from a wall
Authors: Orlandi Paolo
Published: July 2002

Accurate numerical simulations of vortex dipoles impinging on flat boundaries have revealed interesting new features. In the case of free-slip boundaries the dipole does not rebound from the wall. In the case of nonslip walls rebounding occurs and complex interactions of secondary and tertiary vortices appear. The numerical simulation of the first dipole rebound from the wall agrees with experimental visualizations. Numerical experiments extending in time beyond the real experiments show multiple rebounding. Each rebound is associated with the detachment of a secondary vorticity layer from the wall, these layers merge, and at a value of Reynolds number Re=1600, form a new dipole. This dipole has sufficient circulation to induce on itself a motion in the opposite direction to the motion of the initial dipole.
On the relaxation of single hard-sphere gases
Authors: Schürrer Ferdinand , Kügerl Georg
Published: July 2002

The nonlinear Boltzmann equation is solved to examine the Maxwellization of a spatially uniform hard-sphere gas using the Laguerre moment method. The computations are carried out for two different classes of initial conditions. Emphasis is layed on the characteristic times for the relaxation of the distribution function toward the equilibrium. As a result, in the thermal energy range the relaxation takes place within few mean collision times, regardless of the initial state. Depending on whether the high-energy tail is initially overpopulated or underpopulated the relaxation time in this part of the spectrum is a function of the molecular velocity via 1/v or log v, respectively. The solutions are compared with those of the linearized Boltzmann equation.
The pressure–dilatation correlation in compressible flows
Authors: Sarkar S.
Published: July 2002

Simulations of simple compressible flows have been performed to enable the direct estimation of the pressure–dilatation correlation. The generally accepted belief that this correlation may be important in high-speed flows has been verified by the simulations. The pressure–dilatation correlation is theoretically investigated by considering the equation for fluctuating pressure in an arbitrary compressible flow. This leads to the isolation of a component of the pressure–dilatation that exhibits temporal oscillations on a fast time scale. Direct numerical simulations of homogeneous shear turbulence and isotropic turbulence show that this fast component has a negligible contribution to the evolution of turbulent kinetic energy. Then, an analysis for the case of homogeneous turbulence is performed to obtain a formal solution for the nonoscillatory pressure–dilatation. Simplifications lead to a model that algebraically relates the pressure–dilatation to quantities traditionally obtained in incompressible turbulence closures. The model is validated by direct comparison with the simulations.
Stabilization of the Rayleigh–Taylor instability by convection in smooth density gradient: Wentzel–Kramers–Brillouin analysis
Authors: Bud’ko A. B. , Liberman M. A.
Published: July 2002

A self-consistent analytical model based on the Wentzel–Kramers–Brillouin (WKB) approximation is developed to investigate the suppression of the Rayleigh–Taylor (RT) instability by the combined effect of the convective mass flow and structured profiles both, in subsonic and supersonic flow regimes. The eigenvalue problem for the instability growth rates σ is reduced to the problem of solving the system of algebraic equations. In static stratified plasma the eigenvalues spectrum σn(k) (n=0,1,2,...) is found for any density profile. In the presence of steady-state mass flow the growth rate is obtained as an implicit function of the transverse wave number and as a functional of the unperturbed profiles. The cutoff wave number is expressed explicitly as the function of the unperturbed variables. Applicability of the WKB approach implies Fr=v2/gL≪1 (Fr is the local Froude number, L is the stratification length scale), still it yields satisfactory agreement with numerical solutions of the boundary value problem for the RT growth rates in the ablatively accelerated plasma of larger targets with sharp density gradient (Fr∼1).
Representation of volume-preserving maps induced by solenoidal vector fields
Authors: Thyagaraja A. , Haas F. A.
Published: July 2002

A general representation of finite-volume-preserving maps induced by solenoidal vector fields in periodic cylinders is derived. An important special case is the area preserving Hamiltonian maps which include the standard mapping. Applications to computational problems in plasma physics are briefly indicated.
Hamiltonian description of axisymmetric vortex flows and the system of vortex rings
Authors: Novikov E. A.
Published: July 2002

The Hamiltonian equations for the trajectories of fluid particles in axisymmetric vortex flows are obtained. On the basis of these equations, the system of thin vortex rings is considered. The problem of the interaction of two vortex rings has a general analytic solution. The problems of stochastization of the system of three and more vortex rings as well as stochastization of trajectories of fluid particles in the system of two and more vortex rings (with appropriate initial conditions) are discussed.
The electron-acoustic mode
Authors: Gary S. Peter , Tokar Robert L.
Published: July 2002

This paper examines electrostatic modes in an unmagnetized, homogeneous, Vlasov plasma with three Maxwellian components: ions, hot electrons, and cool electrons. In such a plasma, the electron-acoustic mode with frequencies between the ion and electron plasma frequencies may propagate with light damping. The conditions that allow propagation of this mode, which is distinct from the well-known ion-acoustic and Langmuir waves, are given in detail; approximate necessary conditions are 10≲Th/Tc and 0<nc<0.8ne, where the subscripts c, h, and e refer to the cool and hot electron components and the total electron population, respectively.
Hydrodynamic modification of the horseshoe vortex at a vertical pier junction with ground
Authors: Gupta A. K.
Published: July 2002

By mounting a delta-wing-like passive device on the front part of the junction between a vertical bluff body cylinder and the flat ground, the horseshoe vortex flow structure has been modified significantly. Model test results of a flow visualization study are described to show these changes. The results will have an important and favorable influence on the local scour prevention around river bridge piers
The effect of variable viscosity on the interfacial stability of two-layer Poiseuille flow
Authors: Pinarbasi A. , Liakopoulos A.
Published: July 2002

In this paper the linear stability analysis of the interface between two Newtonian liquids with temperature-dependent viscosity in plane Poiseuille flow is presented. A piecewise linear temperature profile is considered. The linearized equations describing the evolution of small, two-dimensional disturbances are derived and the stability problem is formulated as an eigenvalue problem for a set of ordinary differential equations. The continuous eigenvalue problem is solved numerically by a pseudospectral method based on Chebyshev polynomial expansions. The method leads to a generalized matrix eigenvalue problem, which is solved by the QZ algorithm. Results on the onset of instability are presented in the form of stability maps for a range of thickness ratios, disturbance wave numbers, imposed temperature differences, constant-temperature viscosity ratios, thermal conductivity ratios, and Reynolds numbers. Increasing the imposed temperature difference, constant-temperature viscosity ratio, or Reynolds number can have a stabilizing or destabilizing effect, depending on the flow configuration (thickness ratio) and disturbance wavelength. Increasing the thermal conductivity ratio has a destabilizing effect on the interface for all configurations and disturbance wavelengths.
Oscillations of a deformed liquid drop in an acoustic field
Authors: Shi Tao , Apfel Robert E.
Published: July 2002

The oscillations of an axially symmetric liquid drop in an acoustic standing wave field in air have been studied using the boundary integral method. The interaction between the drop oscillation and sound field has been included in this analysis. Our computations focus on the frequency shift of small-amplitude oscillations of an acoustically deformed drop typical of a drop levitated in air. In the presence or absence of gravity, the trend and the magnitude of the frequency shift have been given in terms of drop size, drop deformation, and the strength of the sound field. Our calculations are compared with experiments performed on the United States Microgravity Laboratory (USML-1) and with ground-based measurements, and are found to be in good agreement within the accuracy of the experimental data.
Particle dispersion and deposition in direct numerical and large eddy simulations of vertical pipe flows
Authors: Uijttewaal W. S. J. , Oliemans R. V. A.
Published: July 2002

The motion of dense particles in a turbulent gas flow has been studied by means of numerical simulations. The single-phase turbulent pipe flow was modelled using Direct Numerical Simulation and Large Eddy Simulation. At tube Reynolds numbers of 5300, 18300 and 42000 particles with dimensionless relaxation times ranging from 5 to 104 were released. Assuming the system to be dilute, the characteristics of particle dispersion, deposition and concentration distribution were studied under various conditions of gravity and lift. This study shows that for small particles the deposition process is governed by the properties of the near-wall layer where the wall-normal turbulence intensity is low, while for large inertial particles turbulent dispersion determines the chances for particles to hit the tube wall. The motion of the latter particles appears to scale properly with the Lagrangian integral time scale of the turbulence. Furthermore we demonstrated the segregation of particles towards the wall, as a result of particle-turbulence interaction.
On the stability of the Hartmann layer
Authors: Lingwood R. J. , Alboussière T.
Published: July 2002

In this paper we are concerned with the theoretical stability of the laminar Hartmann layer, which forms at the boundary of any electrically conducting fluid flow under a steady magnetic field at high Hartmann number. We perform both linear and energetic stability analyses to investigate the stability of the Hartmann layer to both infinitesimal and finite perturbations. We find that there is more than three orders of magnitude between the critical Reynolds numbers from these two analyses. Our interest is motivated by experimental results on the laminar–turbulent transition of ducted magnetohydrodynamics flows. Importantly, all existing experiments have considered the laminarization of a turbulent flow, rather than transition to turbulence. The fact that experiments have considered laminarization, rather than transition, implies that the threshold value of the Reynolds number for stability of the Hartmann layer to finite-amplitude, rather than infinitesimal, disturbances is in better agreement with the experimental threshold values. In fact, the critical Reynolds number for linear instability of the Hartmann layer is more than two orders of magnitude larger than experimentally observed threshold values. It seems that this large discrepancy has led to the belief that stability or instability of the Hartmann layer has no bearing on whether the flow is laminar or turbulent. In this paper, we give support to Lock’s hypothesis [Proc. R. Soc. London, Ser. A 233, 105 (1955)] that “transition” is due to the stability characteristics of the Hartmann layer with respect to large-amplitude disturbances.
Relationship between wall pressure and velocity-field sources
Authors: Chang Peter A. , Piomelli Ugo , Blake William K.
Published: July 2002

The objective of this investigation is to study the velocity-field sources for the fluctuating wall pressure, determine their locations in the boundary layer, and investigate their physics. The velocity-field sources and partial wall pressures were computed from a database generated by a direct numerical simulation of a low Reynolds number, fully developed, turbulent channel flow. Results show that the mean-shear (MS) and turbulence-turbulence (TT) partial pressures (πMS and πTT, respectively) are the same order of magnitude. The buffer region dominates most of the wave number range; the viscous shear layer is significant at the highest wavenumbers; the buffer and logarithmic regions are important at low wavenumbers. Over most of the wavenumber range, the contribution from the buffer region is the dominant TT component; in the low-wavenumber range, the viscous shear layer, buffer region, and logarithmic region are significant; in the medium and high wavenumbers the viscous shear layer and buffer region dominate. The most important TT partial pressures are π23TT, π13TT and π12TT from the buffer region. It is conjectured that π23TT and π13TT may be generated by quasi-streamwise vortices. π12TT may be due to near-wall shear layers and spanwise vortices. π23TT, π22TT and π33TT from the viscous shear layer are the dominant high-wavenumber partial pressures.
Theory of the collisional presheath in an oblique magnetic field
Authors: Riemann K.-U.
Published: July 2002

In the limit of a small Debye length (λD→0), the plasma boundary layer in front of a negative absorbing wall is split up into a collision-free planar space charge sheath and a quasineutral presheath, where the ions are accelerated to ion sound speed (Bohm criterion). Usually the presheath mechanism depends decisively on collisional friction of the ions, on ionization, or on geometric ion current concentration. If the ion dynamics in the presheath is dominated by a magnetic field (nearly) parallel to the wall, an additional effect must be considered to provide an ion transport to the wall. The special cases (a) of an ion transport by field lines intersecting the wall at a finite angle and (b) of an ion transport by collisions result in somewhat contradictory conclusions. To get a coherent picture, a hydrodynamic model of the presheath is investigated accounting for an oblique magnetic field and for collisions. The limiting cases (a) and (b) are discussed, and it is shown that (in plane geometry) the presheath ion acceleration depends always on elementary processes. The main effect of a strong magnetic field is to ‘‘compress’’ the collisional presheath into a thin layer with a characteristic extension of the ion gyroradius ρi.