bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of the H atom abstraction of D adsorbed on Si(100)
Authors: Buntin Steven A.
Published: July 2002

Product HD kinetic energy distributions are reported for the incident gas phase H atom abstraction of D adsorbed on a monodeuteride-terminated Si(100) surface. The H atoms are generated by laser photolysis of HI and have well-defined kinetic energies in the range of 1–3 eV. For an incident H atom average kinetic energy of 〈EH〉=1.1 eV, the HD product kinetic energy distribution has a mean value of 〈EHD〉=1.2–1.3 eV and extends up to the nominal available-energy limit, providing dynamical evidence for a direct Eley–Rideal mechanism for this abstraction reaction. For 〈EH〉=1.5 and 3.2 eV, the HD product kinetic energy distribution broadens relative to that for 〈EH〉=1.1 eV while 〈EHD〉 remains unchanged, suggesting that energy loss to the substrate becomes more significant and the reaction becomes less Eley–Rideal-like for these higher energies. The results are compared with recent classical trajectory calculations.
High pressure studies of the rhodium–hydrogen system in diamond anvil cell
Authors: Tkacz Marek
Published: July 2002

High pressure x-ray studies on the rhodium–hydrogen system have been performed up to 8 GPa at ambient temperature. Formation of the hydride phase has been observed at 3.8 GPa of hydrogen pressure, accompanied by a rapid increase of the lattice volume of rhodium metal. Decomposition pressure has been determined as 2.5 GPa. This value has been used for the calculation of the standard free energy of formation of the rhodium hydride, giving a value of 11 500 cal/mol H2. The thermodynamic properties of the RhH have been discussed. The pressure-concentration isotherm has been calculated, using the x-ray data and the “universal” volume increase during the hydride phase formation.
Kramers-type picture for crystal nucleation
Authors: Reguera D. , Rubı́ J. M. , Pérez-Madrid A.
Published: July 2002

We introduce a new scheme to analyze the kinetics of homogeneous nucleation in terms of a global order parameter. Our approach is based on the application of the internal degrees of freedom formalism to derive a kinetic equation of the Kramers type formulated for a global reaction coordinate. We provide explicit expressions for the quantities and coefficients involved in the process, suitable for simulation. In addition, our picture recovers in the quasistationary case the transition rate obtained from the method of reactive flux. The equation we present may provide a link between theoretical approaches to homogeneous nucleation (generally formulated in terms of a kinetic equation of the Fokker–Planck type) and simulations (which mostly employ linear response theory). In this context, our scheme provides a theoretical framework to interpret and extend the results obtained in recent simulations.
Structural and electronic properties of small Cun clusters using generalized-gradient approximations within density functional theory
Authors: Massobrio Carlo , Pasquarello Alfredo , Dal Corso Andrea
Published: July 2002

Neutral and anionic Cun clusters (Cu2, Cu3, Cu6 and Cu7−) are studied within density functional theory via the local density approximation (LDA) and the generalized-gradient approximation (GGA) of Perdew and Wang for exchange and correlation. Three different levels of improvement upon the LDA are considered. In the first level, the GGA correction to the exchange-correlation energy is evaluated using the electronic density and the atomic coordinates obtained in the LDA calculation. In the second level, the electronic density is obtained self-consistently within the GGA while keeping the LDA structural configurations. In the third level, both electronic density and ionic positions are obtained fully self-consistently within the GGA. We found that the first level of approximation is already sufficient to correct the overbinding found in the LDA. With respect to the LDA, the self-consistent GGA enhances the electron charge accumulation around the nuclei by depleting the interatomic bonding regions.
Resonance-enhanced multiphoton ionization photoelectron spectroscopy of Rydberg states of N2O below the X 2Π ionization limit
Authors: Scheper C. R. , Kuijt J. , Buma W. J. , de Lange C. A.
Published: July 2002

A three-photon resonance-enhanced multiphoton ionization spectroscopic study on N2O is carried out in the spectral range from 80 000 cm−1 up to the lowest ionization limit at 103 963 cm−1. High-resolution photoelectron spectroscopy is used to identify and characterize the observed excited states. Eighteen origins are reported which have either not been assigned before or are reassigned now. Moreover, the photoelectron spectra taken at higher-lying resonances often show extensive vibronic coupling with the near-resonant vibronic manifolds built on lower-lying origins.
Photoelectron spectroscopy, gas phase acidity, and thermochemistry of tert-butyl hydroperoxide: Mechanisms for the rearrangement of peroxyl radicals
Authors: Clifford Eileen P. , Wenthold Paul G. , Gareyev Roustam , Lineberger W. Carl , DePuy Charles H. , Bierbaum Veronica M. , Ellison G. Barney
Published: July 2002

The 3.531 eV negative ion photoelectron spectra of the hydroperoxide ion and the tert-butylperoxide ion have been studied. We find HO2−+ℏω351.1 nm→HO2+e− EA[HO2,X̃ 2A″]=1.089±0.006 eV, (CH3)3COO−+ℏω351.1 nm→(CH3)3COO+e− EA[(CH3)3COO,X̃ 2A″]=1.196±0.011 eV. The photoelectron spectra show detachment to the ground state of the peroxyl radicals and to a low lying electronic state. The intercombination gaps are measured to be ΔE(X̃ 2A″–Ã 2A′)[HO2]=0.871±0.007 eV and ΔE(X̃ 2A″–2A′)[(CH3)3COO]=0.967±0.011 eV. The gas phase acidity of (CH3)3COOH was measured in a tandem flowing afterglow-selected ion flow tube (FA-SIFT) to be ΔacidG298=363.2±2.0 kcal mol−1 and we find ΔacidH298[(CH3)3COO–H]=370.9±2.0 kcal mol−1. Use of ΔacidH298[(CH3)3COO–H] and EA[(CH3)3COO] leads to the bond energies DH298[(CH3)3COO–H]=85±2 kcal mol−1 and D0[(CH3)3COO–H]=83±2 kcal mol−1. The thermochemistry of the alkylperoxyl radicals, RO2, is reviewed. A mechanism for the rearrangement of chemically activated peroxyl radicals is proposed [RO2]X̃ 2A″→[RO2]*Ã 2A′→aldehydes/ketones+HO(2Π), [RO2]X̃ 2A″→[RO2]*Ã 2A′ →alkenes+HO2(X̃ 2A″).
The ab initio model potential method: Third-series transition metal elements
Authors: Casarrubios Marcos , Seijo Luis
Published: July 2002

In this paper we present nonrelativistic and relativistic core ab initio model potentials (AIMPs) and valence basis sets for La and the third-series transition metal elements. The relativistic AIMPs are derived from atomic Cowan–Griffin calculations; they are made of a spin-free part and a one-electron spin-orbit operator according to Wood and Boring. The core potentials correspond to the 62-electron core [Cd,4f]. The valence basis sets are optimized and spin-orbit corrected. We present monitoring spin-free calculations on the atoms, singly ionized ions and monohydrides of the ten elements, which show a good performance overall. A spin-free-state-shifted spin-orbit-configuration interaction calculation on Pt, which uses empirical spin-free data and which is expected to be essentially free from spin-free deficiencies, points out that the quality of the spin-orbit operators is very good.
Viscous fingering in reaction-diffusion systems
Authors: De Wit A. , Homsy G. M.
Published: July 2002

The problem of viscous fingering is studied in the presence of simultaneous chemical reactions. The flow is governed by the usual Darcy equations, with a concentration-dependent viscosity. The concentration field in turn obeys a reaction–convection–diffusion equation in which the rate of chemical reaction is taken to be a function of the concentration of a single chemical species and admits two stable equilibria separated by an unstable one. The solution depends on four dimensionless parameters: R, the log mobility ratio, Pe, the Peclet number, α, the Damköhler number or dimensionless rate constant, and d, the dimensionless concentration of the unstable equilibrium. The resulting nonlinear partial differential equations are solved by direct numerical simulation over a reasonably wide range of Pe, α, and d. We find new mechanisms of finger propagation that involve the formation of isolated regions of either less or more viscous fluid in connected domains of the other. Both the mechanism of formation of these regions and their effects on finger propagation are studied in some detail.
Zero electron kinetic energy spectroscopy of the ArCl− anion
Authors: Lenzer Thomas , Yourshaw Ivan , Furlanetto Michael R. , Reiser Georg , Neumark Daniel M.
Published: July 2002

Zero electron kinetic energy (ZEKE) spectroscopy has been utilized to study the Ar3540Cl− anion and the X1/2, I3/2 and II1/2 electronic states of neutral ArCl. Well-resolved progressions in the low-frequency vibrations of the anion and the neutral complexes are observed in the ZEKE spectra. From our spectroscopic data we construct model potential functions for the anion and three neutral states. This yields refined values for the neutral state splittings and the first accurate experimental ArCl− anion potential. Absolute uncertainties for Rm and ε in all potentials are estimated to be ±0.08 Å and ±0.6 meV, respectively.
The nematic–isotropic phase transition in linear fused hard-sphere chain fluids
Authors: Jaffer K. M. , Opps S. B. , Sullivan D. E.
Published: July 2002

We present a modification of the generalized Flory dimer theory to investigate the nematic (N) to isotropic (I) phase transition in chain fluids. We focus on rigid linear fused hard-sphere (LFHS) chain molecules in this study. A generalized density functional theory is developed, which involves an angular weighting of the dimer reference fluid as suggested by decoupling theory, to accommodate nematic ordering in the system. A key ingredient of this theory is the calculation of the exact excluded volume for a pair of molecules in an arbitrary relative orientation, which extends the recent work by Williamson and Jackson for linear tangent hard-sphere chain molecules to the case of linear fused hard-sphere chains with arbitrary intramolecular bondlength. The present results for the N–I transition are compared with previous theories and with computer simulations. In comparison with previous studies, the results show much better agreement with simulations for both the coexistence densities and the nematic order parameter at the transition.
Laser spectroscopy of CCH in the 36 600–39 700 cm−1 region
Authors: Chiang Whe-Yi , Hsu Yen-Chu
Published: July 2002

This work reports on nine new bands of the CCH B̃ 2A′←X̃ 2Σ+ system recorded by laser-induced fluorescence. In addition, the 37 946 and 38 107 cm−1 bands were revisited in a supersonic molecular beam. Of these 11 bands, ten were in the 36 600–39 700 cm−1 range; they were rotationally analyzed and assigned as transitions from low vibronic levels, X̃(0,v2,v3) (v2=2–4 and v3=0–1) of CCH to two vibrational levels (T,T+1221) of the B̃ state. Although previously observed as too weak, these bands were enhanced in this work by relaxing vibrationally excited CCH with SF6, instead of He or Ar. The rotationally resolved 35 939 cm−1 band, the band outside of the range of 36 600–39 700 cm−1, is assigned as B̃(T+775)←X̃(0,91,0). These new bands, together with transitions previously reported, are simultaneously analyzed in a nonlinear fit. This work obtained the first spectroscopic parameters of X̃(0,20,0), X̃(0,22,0), X̃(0,22,1), B̃(T+775), and B̃(T+1221), and further improved the spectroscopic parameters of X̃(0,31,0), X̃(0,33,0), and B̃(T). Though the rotational constants of the B̃ state have a very small vibrational dependence, their spin-rotation constants increase by a factor of 2 within 1221 cm−1, suggesting that the B̃ state is perturbed by the nearby unobserved C̃ 2A″ state via the Renner–Teller effect.
Pulse sequence symmetries in the nuclear magnetic resonance of spinning solids: Application to heteronuclear decoupling
Authors: Edén Mattias , Levitt Malcolm H.
Published: July 2002

We develop the average Hamiltonian theory of a class of symmetrical radio-frequency pulse sequences in the NMR of rotating solids. Theorems are presented which allow one to predict the elimination of many average Hamiltonian terms, without detailed calculation. These results are applied to the problem of heteronuclear decoupling in the presence of rapid magic angle spinning. We present sequences which minimize the number of heteronuclear terms at the same time as recoupling the homonuclear interactions of the irradiated spins. The performance of the new sequences is tested on C13 labeled calcium formate. Experimental measurements of double-quantum H1 excitation indicate a relationship between good heteronuclear decoupling of the observed spin species and efficient recoupling of the irradiated spin species. The heteronuclear decoupling performance of the new sequences is significantly better than that obtained with an unmodulated radio-frequency field. The decoupling performance is improved further by breaking the pulse sequence symmetry in a controlled fashion.
A lattice model for fluids with directional interactions
Authors: Aranovich Grigoriy , Donohue Paul , Donohue Marc
Published: July 2002

Here we analyze a lattice model for fluids with directional interactions in the framework of the Ono–Kondo theory. The free energy of the system is represented as an explicit function of the temperature and bulk density. It is shown that the model predicts both order–disorder and vapor–liquid phase transitions. This theory predicts a tricritical point where the vapor–liquid and order–disorder phase transitions both disappear. Also, it predicts retrograde condensation where the boundary of phase stability becomes a multivalued function of concentration. In addition, predictions of the theory are compared with Monte Carlo simulation data. It is shown that the partition function cannot be factored to predict separately the contributions of, for example, dispersion and hydrogen bonding interactions.
XPS studies of Ru-polypyridine complexes for solar cell applications
Authors: Rensmo H. , Westermark K. , Södergren S. , Kohle O. , Persson P. , Lunell S. , Siegbahn H.
Published: July 2002

A series of Ru-polypyridine dyes has been studied with electron spectroscopy using AlKα and synchrotron radiation. Both pure complexes and complexes adsorbed on nanostructured TiO2 (anatase) surfaces have been examined and special emphasis was given to the dye complex cis-bis(4,4′-dicarboxy-2,2′-bipyridine)-bis-(isothiocyanato)-ruthenium(II) [Ru(dcbpy)2(NCS)2]. The measurements provide information concerning the energy level matching between the dyes and the TiO2, which is of importance in photoinduced charge transfer reactions and in applications such as dye-sensitized solar cells. The measurements also support the general picture of bonding of carboxylated complexes to the surfaces via the carboxyl groups of a single bi-isonicotinic acid ligand, and that, for Ru(dcbpy)2(NCS)2, the NCS-ligand–TiO2 interaction is small. Corroborative support is provided via quantum chemical calculations on the ligand (bi-isonicotinic acid) adsorbed on a TiO2 anatase (101) surface.
Simulations of collision-induced absorption of hydrogen on Ni(111)
Authors: Kindt James T. , Tully John C.
Published: July 2002

Experiments by Ceyer and co-workers [Faraday Discuss. Chem. Soc. 91, 437 (1991)] have demonstrated that hydrogen atoms adsorbed on the Ni(111) surface can be driven below the surface under the impact of a hyperthermal (>2 eV) rare gas atom beam. We have modeled these experiments using classical molecular dynamics (MD) simulations, with the goal of elucidating the mechanism of this collision-induced absorption (CIA) process. The simulations favor a mechanism involving direct impact of the rare gas atom with an adsorbed hydrogen atom. The MD results are consistent with experiment in showing that the CIA efficiency increases with rare gas atomic mass for Ar, Kr, and Xe; interestingly, they predict a reversal of this trend when the rare gas is changed from Ne to He. These results are interpreted in terms of a crossover from a light collider regime of very efficient direct impulsive collisions to a massive collider regime of direct collisions strongly coupled to substrate dynamics and relaxation. The simulated CIA cross sections scaled approximately with normal incident collision energy, consistent with experiment. A hydrogen isotope effect, in which CIA was enhanced for deuterium with respect to hydrogen, was found in the simulations where none was observed experimentally. We show that this discrepancy may come from quantum effects, due to zero-point energies and to energy dissipation by electron-hole pair excitations, which tend to counteract and approximately cancel the isotopic difference observed in a purely classical simulation.
Structural and thermodynamic properties of freely-jointed hard-sphere rings and chains
Authors: Lin C.-T , Stell G. , Kalyuzhnyi Yu. V.
Published: July 2002

In this paper, we employ the product-reactant Ornstein–Zernike approach (PROZA) developed by the authors to investigate the structural and thermodynamic properties of freely-jointed hard-sphere ring fluids. Using an M×m component sticky two-point (S2P) model and specifying an appropriate association rule between various species, the associating monomers will form M rings with each ring composed of m beads in the complete-association limit. Applying the PROZA to such a Hamiltonian and considering the limit of complete association, we are able to derive analytical expressions for the average monomer–monomer radial distribution function (RDF) as well as its intermolecular and intramolecular contributions and a closed form of the compressibility pressure. To test the theory, we also perform Monte Carlo simulations for the freely-jointed hard-sphere ring model over a wide range of densities and ring sizes. Compared to the simulation results, we find that the predictions of the PROZA for the compressibility factor of flexible ring melts are quantitatively accurate and the average monomer–monomer RDF g(r) is in excellent agreement with the simulation data over a wide range of densities that includes the polymer-melt regime. Based upon such a comparison as well as theoretical considerations, we conclude that ring-size independence of g(r) is a quantitatively accurate approximation and also that the g(r) of rings will be a good approximation for melts of long chains. Finally, we find that we must go beyond our PROZA framework in order to accurately obtain the separate intramolecular and intermolecular parts of g(r), for which we give a quantitatively satisfactory recipe.
Hard-sphere fluids in very narrow cylindrical pores
Authors: Mon K. K. , Percus J. K.
Published: July 2002

Properties of fluids in narrow pores have been studied extensively in recent years. In this paper, we study the pore radius dependence of hard-sphere fluids in very narrow cylindrical pores via Monte Carlo simulations to explore the crossover from strictly one-dimensional to quasi-one-dimensional fluids. Our study shows that this crossover is not simple. We observe, for constant pressure, an unexpected but interesting nonmonotonic variation of density and compressibility as the pore radius is varied near the value of one hard-sphere diameter (σhs). These results are related to the onset of configurations with two or more nearest-neighbor hard spheres across the diameter of the cylindrical pore.
A “modified Lennard-Jones oscillator” model for diatom potential functions
Authors: Hajigeorgiou Photos G. , Le Roy Robert J.
Published: July 2002

A flexible new analytical representation for the internuclear potential energy of a diatomic molecule is proposed and tested. The new model may be thought of as a generalization of the prototypical Lennard-Jones (2n,n) function, with the form V(R)=De[1−(Re/R)ne−β(z)z]2, where z=(R−Re)/(R+Re) is a dimensionless radial distance variable which approaches 1 as R→∞. This form explicitly incorporates the theoretically predicted attractive inverse-power asymptotic behavior V(R)=D−Cn/Rn associated with most potential energy curves. This “modified Lennard-Jones” (MLJ) function is tested against other flexible forms for the potential energy by performing nonlinear least-squares fits both to known numerical potential curves and to spectroscopic line positions.
Molecular dynamics simulations of gel (LβI) phase lipid bilayers in constant pressure and constant surface area ensembles
Authors: Venable Richard M. , Brooks Bernard R. , Pastor Richard W.
Published: July 2002

The results of a series of molecular dynamics simulations of the gel state of a dipalmitoylphosphatidylcholine bilayer at 293 K are described. The simulations, ranging from 40 ps to 2.5 ns, show clearly that: a flexible cell geometry is essential during equilibration; Ewald summation of electrostatics is superior to spherical cutoff methods; water exchange with the carbonyl group of chain 2 takes place on the ns time scale, while there is almost no hydration of chain 1. There is overall good agreement (D-spacing, chain tilt, fraction gauche, and area compressibility modulus) with experiment, though the surface area per lipid is slightly underestimated. The randomization of torsion 1 of chain 2 from exclusively gauche minus (as specified in the initial condition modeled from the crystal structure of a related lipid) to a mixture of g+/g− over the course of approximately 2 ns is a critical feature of the study. The torsional equilibration proceeded steadily when simulating at constant surface tension, but was effectively quenched by simulation at constant area. The associated presence of conformational degeneracy of this torsion, and conformational disorder in the upper region of chain 2, is most likely associated with the seemingly anomalous infrared (IR) results for gauche bonds in the upper region of the chains. It may also be a characteristic of the gel phase, and be related to the long time required for the gel to subgel transition.
Density functional calculations of hypothetical neutral hollow octahedral molecules with a 48-atom framework: Hydrides and oxides of boron, carbon, nitrogen, aluminum, and silicon
Authors: LaViolette Randall A. , Benson Michael T.
Published: July 2002

We computed via first-principles density functional theory calculations (employing both the local density and generalized gradient approximations) the dimensions, bond lengths and angles, binding energy, and HOMO–LUMO gap of the following hypothetical neutral hollow octahedral molecules: B48H24, C48H48, C96H80 (formed by bonding two C48H48 molecules), N48H24, Al48H24, and Si48H48; B24O24, C24O24, N24O24, Al24O24, and Si24O24. Each molecule consists of a large hollow framework of six puckered eight-membered rings whose planes are either mutually perpendicular or parallel, so that each molecule possesses only eight- and nine-membered rings. The hydrides have their hydrogen atoms attached only to the two-atom bridging sites on the framework. The oxides have their oxygen atoms occupying exclusively the two-atom bridging sites of the framework alternating with the (B, C, N, Al, Si) atoms exclusively occupying the three-atom bridging sites. We also calculated the infrared spectra of the C48H48 and the C24O24 molecules. For the sake of comparison, we also examined the hypothetical octahedral C48 fullerene cuboctohedron (possessing four-, six-, and eight-membered rings) studied by Dunlap and Taylor. The molecules based on carbon would be the most stable; those based on nitrogen would be the least stable, if at all.
Quantum stochastic calculus, operation valued stochastic processes, and continual measurements in quantum mechanics
Authors: Barchielli A. , Lupieri G.
Published: July 2002

The physical idea of a continual observation on a quantum system has been recently formalized by means of the concept of operation valued stochastic process (OVSP). In this article, it is shown how the formalism of quantum stochastic calculus of Hudson and Parthasarathy allows, in a simple way, for constructing a large class of OVSP’s that in particular contains the quantum counting processes of Davies and Srinivas and continual ‘‘Gaussian’’ measurements. This result is obtained by means of a stochastic dilation of the OVSP’s: at the level of the enlarged system probabilities turn out to be expressed in terms of projection valued measures associated with certain time-dependent, commuting, self-adjoint operators.
Nonlinear equations with superposition formulas and exceptional group G2. III. The superposition formulas
Authors: Gagnon L. , Hussin V. , Winternitz P.
Published: July 2002

Superposition formulas are derived expressing the general solution of several different systems of nonlinear ordinary differential equations in terms of a fundamental set of particular solutions. The equations, as well as the superposition formulas, are induced by the action of the exceptional Lie group G2 (complex or real) on a homogeneous space G2/G, where G⊆G2 is a maximal subgroup of G2. When G is either parabolic, or simple, three particular solutions are needed. When G is SL(2,C)×SL(2,C) (or one of its real forms), then two particular solutions suffice.
Effects of the shear viscosity on the character of cosmological evolution
Authors: Huang Wung-Hong
Published: July 2002

Bianchi type I cosmological models are studied that contain a stiff fluid with a shear viscosity that is a power function of the energy density, such as η=αεn. These models are analyzed by describing the cosmological evolutions as the trajectories in the phase plane of Hubble functions. The simple and exact equations that determine these flows are obtained when 2n is an integer. In particular, it is proved that there is no Einstein initial singularity in the models of 0≤n<1. Cosmologies are found to begin with zero energy density and in the course of evolution the gravitational field will create matter. At the final stage, cosmologies are driven to the isotropic Friedmann universe. It is also pointed out that although the anisotropy will always be smoothed out asymptotically, there are solutions that simultaneously possess nonpositive and non-negative Hubble functions for all time. This means that the cosmological dimensional reduction can work even on matter fluid having shear viscosity. These characteristics can also be found in any-dimensional models.
Linear dynamical systems, coherent state manifolds, flows, and matrix Riccati equation
Authors: Berceanu S. , Boutet de Monvel L.
Published: July 2002

The classical motion and the quantum evolution determined by linear Hamiltonians on coherent state manifolds of compact and noncompact Hermitian symmetric space structure are considered. A matrix Riccati equation, with opposite signs of the quadratic terms in the compact and noncompact cases, determines the classical motion and the quantum evolution. The geometric meaning of equations of motion as flow on a complex Grassmann manifold and his noncompact dual is emphasized. Possibilities of generalizing the results to flag manifolds are pointed out.
Casimir operators of the exceptional group G2
Authors: Bincer A. M. , Riesselmann K.
Published: July 2002

Degree 2 and 6 Casimir operators of G2 are calculated in explicit form, with the generators of G2 written in terms of the subalgebra A2.
Nonlocal symmetries of the KdV equation
Authors: Guthrie Graeme A. , Hickman Mark S.
Published: July 2002

A loop algebra of nonlocal isovectors of the Korteweg–de Vries (KdV) equation is introduced which is derived from the bi-Hamiltonian structure of that equation by inverting the usual recursion operator. These symmetries are lifted to the modified KdV (mKdV) equation and two related evolution equations via coordinate transformations. The effect on the algebraic structure of the isovector algebra is considered. Possible applications of these symmetries include recovering Miura-type transformations.
Space–time topology change and stringy geometrya)
Authors: Aspinwall Paul S. , Greene Brian R. , Morrison David R.
Published: July 2002

Recent work which has significantly honed the geometric understanding and interpretation of the moduli space of certain N=2 superconformal field theories is reviewed. This has resolved some important issues in mirror symmetry and has also established that string theory admits physically smooth processes which can result in a change in topology of the spatial universe. Recent work which illuminates some properties of physically related theories associated with singular spaces such as orbifolds is described.
The geometrical approach to multidimensional inverse scattering
Authors: Enss Volker , Weder Ricardo
Published: July 2002

We prove that in multidimensional short-range potential scattering the high velocity limit of the scattering operator of an N-body system determines uniquely the potential. For a given long-range potential the short-range potential of the N-body system is uniquely determined by the high velocity limit of the modified Dollard scattering operator. Moreover, we prove that any one of the Dollard scattering operators determines uniquely the total potential. We obtain as well a reconstruction formula with an error term. Our simple proof uses a geometrical time-dependent method.
Continuous wavelet transforms on the sphere
Authors: Holschneider Matthias
Published: July 2002

In this very short paper we shall construct a continuous wavelet analysis based on dilations translations and rotations on the sphere. It is the analog of the construction proposed by Murenzi [in his thesis, 1990], on R2. At small scale we shall recover the Euclidian structure of the sphere. At large scale we obtain that the wavelet transform decays rapidly because the sphere is compact.
Algebraic structure of discrete zero curvature equations and master symmetries of discrete evolution equations
Authors: Ma Wen-Xiu , Fuchssteiner Benno
Published: July 2002

An algebraic structure related to discrete zero curvature equations is established. It is used to give an approach for generating master symmetries of the first degree for systems of discrete evolution equations and an answer to why there exist such master symmetries. The key of the theory is to generate nonisospectral flows (λt=λl, l⩾0) from the discrete spectral problem associated with a given system of discrete evolution equations. Three examples are given.
An estimation theoretical characterization of coherent states
Authors: Fujiwara Akio , Nagaoka Hiroshi
Published: July 2002

We introduce a class of quantum pure state models called the coherent models. A coherent model is an even-dimensional manifold of pure states whose tangent space is characterized by a symplectic structure. In a rigorous framework of noncommutative statistics, it is shown that a coherent model inherits and expands the original spirit of the minimum uncertainty property of coherent states.
Nonmonotonic temperature dependence of the mass transfer rate during isotopic phase separation of He3–He4 solid mixtures
Authors: Gan’shin A. N. , Grigor’ev V. N. , Maidanov V. A. , Omelaenko N. F. , Penzev A. A. , Rudavskii E. Ya. , Rybalko A. S.
Published: July 2002

It is discovered that the characteristic phase separation time constant of solid He3–He4 mixtures at low temperatures exhibits a nonmonotonic temperature dependence with a minimum. It means that the rate of mass transport slows down at very low temperatures, and the corresponding values of the effective diffusion coefficient also depend nonmonotonically on temperature and concentration and differ significantly from the spin diffusion coefficient measured earlier in NMR experiments on quantum diffusion. The discovered nonmonotonicity may be associated with the influence of the nonuniform field of elastic stresses in the crystal because of the difference in the molar volumes of the phases.
Self-similar solution of the second kind for a convergent viscous gravity current
Authors: Diez Javier Alberto , Gratton Roberto , Gratton Julio
Published: July 2002

The axisymmetric flow of a very viscous fluid toward a central orifice is studied. In a recent paper, a self-similar solution for this problem has been found. The self-similarity is of the second kind and hence the flow remembers its initial condition only through a nondimensional constant which characterizes it. In this work this convergent flow is studied experimentally (using silicone oils) by measuring the front position and the height profile as a function of time. It is verified that the self-similar solution properly describes the flow within a certain interval of the cavity radius, where values are obtained for the similarity exponent δ in agreement (accounting for experimental errors) with the theoretical value 0.762... . The transition to the self-similar flow is also simulated numerically and numerical values are obtained for the time closure for different initial conditions. These simulations also show the theoretical self-similar flow after the cavity closure, which is very difficult to observe experimentally.
Kinetic models for diffusion in shear flow
Authors: Garzó V. , López de Haro M.
Published: July 2002

The kinetic models proposed by Garzó et al. [Phys. Fluids A 1, 380 (1989)] and Gross and Krook [Phys. Rev. 102, 593 (1956)] are used to study diffusion in uniform shear flow. The kinetic equations are solved by means of a generalized Chapman–Enskog expansion for Maxwell molecules. The main peculiarity of the method is that the successive approximations to the velocity distribution functions retain all orders in the shear rate. Two cases are considered, namely the self-diffusion and the tracer diffusion in a system under shear flow. The relevant transport properties and the distribution functions are explicitly obtained. A comparison with previous results derived from the Boltzmann equation and between both models is carried out.
The dissipation range spectrum
Authors: Manley Oscar P.
Published: July 2002

It is shown that the available data for the skewness of velocity derivatives in turbulent flows are consistent with a simple exponential decay in the dissipation range, i.e., ∼exp(−ak), rather than ∼exp(−akm), where m≳1.
Statistical inelastic cross-section model for the Monte Carlo simulation of molecules with continuous internal energy
Authors: Koura Katsuhisa
Published: July 2002

The statistical inelastic cross-section [SICS(C)] model for the Monte Carlo simulation of molecules with continuous internal energy is derived by modifying the SICS(D) model for discrete internal energy [Phys. Fluids A 4, 1782 (1992)]. The SICS(C) model specified for rotationally inelastic collisions with the use of Parker’s rotational energy gain function is applied to the Monte Carlo simulation of a nitrogen shock wave and compared with the SICS(D) model.
Resonance of longshore currents under topographic forcing
Authors: Falqués A. , Iranzo V. , Montoto A.
Published: July 2002

The steady perturbation caused in a longshore flow by a bottom undulation is considered. The bedforms are assumed to be alongshore periodic, with crests in the cross-shore direction and with a small amplitude in order for linear theory to be applicable. The inviscid shallow-water equations are considered in order to investigate topographic resonance, that is, the condition under which the perturbation in the flow reaches a maximum. Since upstream edge waves held stationary by the mean flow are solutions to the homogeneous resonance equations, the existence of such flows gives rise to the existence of resonances of infinite amplitude (linear, inviscid theory). For a maximum local Froude number of the basic flow F of less than 1, the flow is found to behave subcritically according to classic channel flow theory. In addition, neither steady edge waves nor infinite amplitude resonances exist in this case. However, by numerical simulation, a finite maximum in the flow perturbation as a function of bedform wavelength is found. This topographic resonance is rather weak and wide banded. For a bedform height of 1% the local water depth, the perturbation on the flow may typically be 4% of the mean current. The resonant wavelength is between two and three times the distance of the peak longshore current to the shoreline, lV, when the current profile has a maximum at some distance offshore, or nearly four times the cross-shore length scale of the sandbars, l, for a flow profile monotonically increasing to a constant current far offshore. For F≳1 resonances of infinite amplitude are found. For every F, lV, and l, there is an infinite set of resonant modes with an increasing cross-shore complexity when the mode number increases, similarly to edge waves. The resonant wavelength increases with F and with lV. Some implications on the growth of transverse sandbar families and cuspidal coast are discussed.
Waveguide mode deformation in free-electron lasers
Authors: Fajans J. , Wurtele J. S.
Published: July 2002

The free-electron laser (FEL)-induced modifications to the vacuum waveguide modes are calculated for low-frequency FEL’s. Typically, the mode modifications are large and exhibit complicated axial behaviors. In addition to the wave component with a near vacuum wavenumber, components at two upshifted wavenumbers must be analyzed. Electron beam surface charges and currents are also important. At low gain, effects from all three roots of the FEL dispersion relation must be included. The dominant modification is due to the electron beam space-charge wave.
Wave propagation in one-dimensional inhomogeneous random media with an application to lower hybrid waves in fusion plasmas
Authors: Spigler Renato , Grossmann William
Published: July 2002

Wave propagation in a one-dimensional stratified medium, whose physical parameters are subject to random fluctuations, is considered. The complex-valued reflection coefficient obeys, as a function of the slab width L, a stochastic Riccati differential equation, associated with an initial-value problem. Using this fact, solving first the boundary-value problem satisfied by the field on [0,L] is avoided. The reflection coefficient can be computed numerically by a Monte-Carlo-type procedure by the generation of suitable sequences of random numbers aimed at constructing realizations of the stochastic processes that enter the refractive index. An application is made to the propagation of ‘‘lower hybrid waves’’ in thermonuclear (fusion) plasmas for realistic models of the deterministic density profile using available experimental data for the statistics of the random fluctuations. Several functional forms of the density profile are considered, and the relevant physical approximations are discussed. This work generalizes a previous analysis that did not include the inhomogeneous background density profile. Results show that the reflected power is a sensitive function of the form of the density profile.
Stability of a Z pinch with rising current
Authors: Cochran F. L. , Robson A. E.
Published: July 2002

A two-dimensional resistive magnetohydrodynamic (MHD) code is used to examine the stability of a Z pinch to m=0 perturbations under conditions of rising current. The current waveform is chosen to keep the unperturbed radius of the pinch constant in time. At low current the perturbations oscillate without growing, or grow only weakly, contrary to the predictions of ideal MHD theory. At high current, the perturbations grow at a rate consistent with ideal MHD. The point of transition can be described in terms of a critical Lundquist number; this increases with the wavelength of the perturbation, and is about 160 for ka=0.3, where k is the wavenumber and a is the pinch radius. These results may explain recent experimental observations of anomalously stable dense Z pinches.
Resistive Z-pinch stability theory
Authors: Culverwell I. D. , Coppins M.
Published: July 2002

The linear time evolution of axisymmetric perturbations in a time-dependent Z-pinch equilibrium is studied using a one-dimensional linearized initial value code. A critical Lundquist number of the order of 50–100 is found, below which the pinch is stable. This result may account for the enhanced stability of recent deuterium fiber pinch experiments.
A k-space integral equation for describing propagation through a strongly inhomogeneous plasma density profile
Authors: Myer R. C. , Fried B. D.
Published: July 2002

A k-space integral equation is derived that describes the propagation of electromagnetic waves induced by an external source of charge or current in a magnetized plasma (B=B0ẑ) having an arbitrary density variation in the x̂ direction. The nonlocal k-space dielectric tensor kernel is derived keeping finite ion Larmor radius ρi corrections to all orders without the use of an expansion in the inverse density gradient scale length LN so that the effect of a strongly inhomogeneous plasma density profile (LN≊ρi) on wave propagation in the ion cyclotron range of frequencies can be studied. The integral equation is solved numerically in the electrostatic limit to study the capacitive excitation of ion Bernstein waves for frequencies near the second harmonic of the ion cyclotron frequency (ω≊2Ωi). The spectrum of weakly damped eigenmodes for a plasma having a large region of uniform density and a highly nonuniform edge is found to consist of numerous ‘‘uniform plasma’’ modes and an electrostatic drift mode that propagates only in the edge region. Asymmetries in the radial structure of these modes, which arise from the diamagnetic drift of particles in the plasma edge, result in an asymmetric distribution of wave energy launched in the directions parallel and antiparallel to the diamagnetic current. The surface electrostatic drift mode is found to be the dominant mode of oscillation as the wave frequency approaches the second harmonic of the ion cyclotron frequency.
Impurity rotation in hydrogen and helium plasmas
Authors: Rowan William L. , Meigs A. G. , Hickok R. L. , Schoch P. M. , Yang X. Z. , Zhang B. Z.
Published: July 2002

Rotation velocities were measured in two Ohmically heated discharges and compared to neoclassical predictions evaluated with measurements of Ti, ni, and Er. Hydrogen fueled one of the discharges, and helium was the principal fuel for the other. For both discharges, poloidal rotation is consistent with the momentum balance equation. In contrast, current neoclassical theory predicts the toroidal rotation velocity only in the case of negligible ion–neutral charge exchange and neutral ionization.
Relativistic fluid model of the resistive hose instability
Authors: Siambis John G.
Published: July 2002

A systematic analysis of the hose instability using the relativistic fluid formulation is reported. In its basic nature, the hose instability is a macroscopic, low-frequency instability, hence a fluid model should, in principle, give an accurate account of the hose instability. It has been found that for zeroth-order beam displacements, giving rise to rigid beam displacements, the fluid wave equation and resulting dispersion relation are identical to the spread-mass model and the energy-group model results. When first-order fluid displacements are included as well, giving rise to compressible, nonfrozen displacements in the axial direction and beam cross-section distortion in the radial direction, then there is obtained a wave equation similar, but not identical to the multicomponent model. The dispersion relation is solved for numerically. The hose instability growth rate is found to be similar to the multicomponent model result, over part of the beam frame, real hose frequency range.
Ion temperature profile deconvolution and corrections to confinement parameters in spheromaks
Authors: Mayo R. M. , Hurlburt D. J. , Fernández J. C.
Published: July 2002

The purpose of this work is to determine ion temperature profiles in spheromaks [Nucl. Fusion 19, 489 (1979)] for the first time. Knowledge of the ion temperature profile is necessary in the correct calculation of plasma confinement parameters. The work herein details the calculation of ion temperature profiles for the Compact Torus Experiment (CTX) [Nucl. Fusion 28, 1555 (1988)] and S-1 [Phys. Rev. Lett. 46, 188 (1981)] spheromaks. Data from single chord Doppler ion temperature measurements in these devices have been analyzed with the aid of a one-dimensional equilibrium charge state transport code. Using electron temperature and density profiles from Thomson scattering, and estimates for transport rates, a most probable position for the emission of line radiation can be determined and correlated with the measured Doppler ion temperature, thus generating an ion temperature profile. From this ion temperature profile determination, plasma confinement parameters for the small solid flux conserver CTX [Phys. Fluids B 2, 1342 (1990)] spheromak can be determined, and confinement parameters for S-1 can be reevaluated, eliminating the previous confinement calculations assumption Ti(r)=Te(r). The CTX device has a calculated volume averaged beta 〈β〉 ranging from 3% to 8% and a volume averaged energy confinement time 〈τE〉 between 14 and 35 μsec; while the S-1 spheromak has 〈β〉 between 15% and 40% and 〈τE〉 ranging from 30 to 70 μsec.
Effects of diamagnetic drift and compressibility on linear resistive magnetohydrodynamic stability of stellarators
Authors: Charlton L. A. , Leboeuf J.-N.
Published: July 2002

To study diamagnetic and compressibility effects for stellarators, toroidally averaged equations describing resistive pressure-driven modes are developed. Linear stability results are given for cylindrical geometry and toroidal geometry. When toroidal coupling is included, both the analytical theory and numerical calculations show that diamagnetic effects can be destabilizing when they are small. However, for parameters appropriate to the Advanced Toroidal Facility (ATF) experimental device [J. F. Lyon et al., Fusion Technol. 10, 179 (1986)] the diamagnetic effects are large and stabilizing. The linear results also show that for the specific cases reported here compressibility effects tend to cancel toroidal effects. Cylindrical calculations which include diamagnetic effects may then be adequate for the study of the experimental data.
Wilton ripples phenomenon with a background current
Authors: Ma Yan-Chow
Published: July 2002

The Wilton ripples phenomenon with a background current is examined in detail. The case of most interest occurs when the current is near the threshold value such that direct resonance occurs for the Wilton ripples. Interaction between the fundamental mode and its second harmonic occurs on a much faster time scale. Bifurcation diagrams and stability analysis are presented.
Thermal excitation of waves on quantized vortices
Authors: Barenghi C. F. , Donnelly Russell J. , Vinen W. F.
Published: July 2002

We consider the problem of the thermal excitation of vortex waves on quantized vortex lines and rings. We discover that this problem, which has been largely neglected in the past, apparently presents a ‘‘free energy catastrophe’’ above about 1.85 K; that is, if one naively proceeds to compute the thermally induced vortex waves on rings or lines, the entropy of the waves becomes so large that the free energy of vortices is driven negative, and superfluidity could not exist. Various mechanisms for eliminating this problem are considered; it appears to be necessary to consider the relevant system to be the vortex plus the bath in order to avoid double counting of states. We comment on the relevance of this calculation to the problem of thermally induced nucleation of vortices.
The effect of hydrodynamic turbulence on Langmuir waves in a slightly ionized plasma
Authors: Parish J. L. , Raitt W. J.
Published: July 2002

Two-dimensional, hydrodynamic turbulence, and the transport of enstrophy by this turbulence from large spatial scales to small spatial scales, is considered. This fluid is taken as slightly ionized, with a small number of ions in equilibrium with it. These ions produce a fluctuating magnetic field that induces a fluctuating electric field, either or both of which alter the distribution function of the electrons present. Considering a cold-electron plasma, it is found that turbulence can heat the electrons, increase their Langmuir frequency and decrease the Landau damping.