bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental and simulated vibrational spectra of H2 absorbed in amorphous ice: Surface structures, energetics, and relaxations
Authors: Hixson Holly G. , Wojcik Marek J. , Devlin Matthew S. , Devlin J. Paul , Buch V.
Published: July 2002

Infrared spectra are reported for thin films of deuterated microporous amorphous ice formed at 12 K and saturated with absorbed molecular hydrogen. This paper focuses on both the influence of the surface-bound H2 on the absorption bands of the OD groups that dangle from the micropore surfaces and the behavior of the induced infrared bands of the stretching mode of H2 itself. Both structural changes and the relaxation of ortho-H2 to para-H2 are apparent from variations in the observed spectra with time and temperature. A reasonably detailed interpretation of the complex spectral behavior has been possible through simulation of spectra for H2 interacting with the surface of amorphous ice clusters generated previously in a classical trajectory study of the cluster growth through the accumulation and relaxation of individual water molecules. Potential minima were calculated with respect to H2 coordinates on the cluster surface and a qualitative interpretation of adsorbate–surface bonding provided via the partitioning of the H2⋅⋅D2O interactions into relatively weak van der Waals interactions, and stronger electrostatic interactions between the H2 quadrupole and the H2O dipole and quadrupole. The net binding of H2 to the surface is dominated by several van der Waals interactions with neighboring D2O molecules, but in most of the calculated minima, H2 also forms a single electrostatic bond to a surface molecule. While such electrostatic bonds provide only a small fraction of the binding energy, they appear to influence strongly the observed spectra. Observed H2-induced shifts in the dangling OD bands appear to be caused by electrostatic bonding of H2 to a D atom of a dangling OD and a significant fraction of the H2 intensity is proposed to originate from H2 molecules which are electrostatically bonded to dangling oxygen atoms on the surface. Calculations (which do not contain adjustable parameters) reproduce quite well several features of the measured spectra, including the splitting of the dangling OD band, the shift of this band due to binding of H2, and the frequency and the width of the H2 band.
Primary product channels in the photodissociation of methane at 121.6 nm
Authors: Mordaunt D. H. , Lambert I. R. , Morley G. P. , Ashfold M. N. R. , Dixon R. N. , Western C. M. , Schnieder L. , Welge K. H.
Published: July 2002

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of CH4(CD4) at 121.6 nm. Contrary to the previous consensus view, we find simple C–H bond fission to be the dominant primary process following excitation at this wavelength. The resulting CH3 fragments are formed with very high levels of internal excitation: Some (∼25%) possess so much internal energy that they must undergo subsequent unimolecular decay. The present experiments do not provide a unique determination of the products of this secondary decay process, but statistical arguments presented herein suggest that they will be predominantly CH and H2 fragments. Similar considerations point to a significant role for the direct three body process yielding the same products H+H2+CH. This overall pattern of energy disposal can be rationalized by assuming that most of the initially prepared CH4(Ã 1T2) molecules undergo rapid internal conversion (promoted by the Jahn–Teller distortion of this excited state) to high vibrational levels of the ground state prior to fragmentation. The realization that CH4 photodissociation at 121.6 nm yields CH3 (and CH) fragments, rather than methylene radicals, will necessitate some revision of current models of the hydrocarbon photochemistry prevailing in the atmospheres of the outer planets and some of their moons, notably Titan.
Enhanced coalescence upon laser desorption of fullerene oxides
Authors: Beck Rainer D. , Stoermer Carsten , Schulz Christof , Michel Rudi , Weis Patrick , Bräuchle Götz , Kappes Manfred M.
Published: July 2002

C60Ox, x=1–3 were prepared by exposing C60 solutions to ozone. After high performance liquid chromatography purification, these materials were studied by laser desorption time of flight mass spectroscopy. These mass spectra suggest that fullerene oxides undergo laser desorption induced coalescence more efficiently than pure C60. A correlation between the fragmentation of the desorbed parent species and the observed coalescence products both in yield and distribution suggests that efficient gas phase coalescence involves at least one reactive species, such as C58, produced by fragmentation of the desorbed fullerenes or fullerene derivatives.
Rotational spectra for off-center endohedral atoms at C60 fullerene
Authors: Hernández-Rojas J. , Bretón J. , Gomez Llorente J. M.
Published: July 2002

Rotational spectra for endohedral Li+@C60 and Na+@C60 are calculated at different temperatures. Most of the features in these spectra are related with the degree of anisotropy in the atom–cage interaction. While the low anisotropy for Na+@C60 results in rather simple spectra with the 2B oscillation typical of a diatomic molecule, the more eccentric and anisotropic Li+@C60 produces complex spectra with rotational and librational bands. Some interesting effects are induced by the cage rotation, which has been incorporated through a semiclassical formalism.
Stimulated emission pumping spectroscopy of HCP near the isomerization barrier: EVIB≤25 315 cm−1
Authors: Ishikawa Haruki , Chen Yit-Tsong , Ohshima Yasuhiro , Wang Jianghong , Field Robert W.
Published: July 2002

Rotation-vibration levels in the energy region near the HCP↔HPC X̃ 1Σ+ isomerization barrier (i.e., the HPC geometry) were observed by HCP Ã–X̃ and C̃–X̃ stimulated emission pumping (SEP) spectroscopy. Unlike HCN↔HNC, where the unstable isomer corresponds to a local minimum on the potential energy surface, the HPC isomer corresponds to a saddle point. In the Ã–X̃ SEP spectra, the l=0 and 2 components of pure bend overtone levels (0,26≤v2≤42,0) and the bend–CP stretch combination levels, (0,24,1) and (0,26,1), were sampled. On the other hand, in the C̃–X̃ SEP spectra, which sample l=0 components exclusively, 2ω2:1ω3 polyads were identified. These polyads appeared in the SEP spectra as a regular, easily recognizable pattern. Since the C̃–X̃ SEP spectra appeared to be almost totally Franck–Condon nonselective (a large fraction of the predicted total density of l=0 levels was observed), the polyad pattern was the key to vibrational assignments of highly excited vibrational levels. It was found that the bending vibration exhibits very regular (Morse-type) behavior up to at least v2=42. However, an abrupt change was found in the v2-dependence of all of the vibrational fine structure constants above v2=36, E(0,36,0)=22 048 cm−1. In addition, a sudden turning on of perturbations in the pure bend overtone levels was also observed to occur at v2=32. These abrupt changes in the level structure could be related to an abrupt change in curvature of the potential energy surface along directions perpendicular to the bending coordinate when the bending coordinate is far from equilibrium.
On the consequences of nonremovable derivative couplings. I. The geometric phase and quasidiabatic states: A numerical study
Authors: Yarkony David R.
Published: July 2002

Conical intersections complicate the computational treatment of nuclear dynamics in the adiabatic state basis through the geometric phase effect and singularities in the derivative couplings. The diabatic representation seeks to eliminate these difficulties. However, the adiabatic to diabatic state transformation is necessarily approximate in a polyatomic molecule since the derivative couplings cannot be rigorously removed. This point is rarely considered when constructing approximate diabatic states. The nonremovable part of the derivative couplings is investigated by considering the integral of the derivative coupling along closed loops in the vicinity of the 1 2A′–2 2A′ seam of conical intersections in H3.
Theoretical study of vibronic spectra and photodissociation pathways of methane
Authors: Mebel Alexander M. , Lin Sheng-Hsien , Chang Chung-Hung
Published: July 2002

Potential energy surfaces for the first singlet and triplet excited states of methane have been studied using multireference configuration interaction (MRCI) and equation-of-motion coupled cluster (EOM-CCSD) ab initio molecular orbital calculations. The vertical excitation energies for the 1T2 and 3T2 states are computed to be 10.64–10.66 and 10.25–10.30 eV, respectively. Two minima are found on the first excited singlet surface, 1 (∼C3v) and 2 (C2v), with adiabatic excitation energies of 9.16–9.25 and 8.39–8.52 eV, respectively. No minima is located on the triplet surface. Vibronic spectra, calculated based on the geometries, vibrational frequencies, and normal modes of the ground and excited states, reproduce well the experimental results. The spectra due to the 3s(C2v)←1t2 transition start at ∼8.63 eV and form a broad underlying continuum. The 3s(C3v)←1t2 transition is shown to be responsible for the minor fine structure observed in the experimental absorption spectra between 9.5 and 10.6 eV. Dissociation pathways leading to various photofragmentation products are discussed on the basis of the calculated minimal energy pathways of H and H2 elimination. Production of CH3(2A2″) and fast hydrogen atoms, the major channel observed experimentally, is speculated to occur either via the S0←S1 internal conversion or, more likely, via the S1(1A″)→T1(3A) intersystem crossing followed by fast dissociation in the triplet state. Spin–orbit coupling between S1 and T1 has been calculated to be about 45 cm−1.
Investigation of a grid-free density functional theory (DFT) approach
Authors: Glaesemann Kurt R. , Gordon Mark S.
Published: July 2002

Density functional theory (DFT) has gained popularity, because it can frequently give accurate energies and geometries. Because evaluating DFT integrals fully analytically is usually impossible, most implementations use numerical quadrature over grid points, which can lead to numerical instabilities. To avoid these instabilities, the Almlöf-Zheng (AZ) grid-free approach was developed. This approach involves application of the resolution of the identity (RI) to evaluate the integrals. The focus of the current work is on the implementation of the AZ approach into the electronic structure code GAMESS, and on the convergence of the resolution of the identity with respect to basis set in the grid-free approach. Both single point energies and gradients are calculated for a variety of functionals and molecules. Conventional atomic basis sets are found to be inadequate for fitting the RI, particularly for gradient corrected functionals. Further work on developing auxiliary basis set approaches is warranted.
Flexing analysis of steric exchange repulsion accompanying ethane internal rotation
Authors: Goodman Lionel , Gu Hongbing
Published: July 2002

Relaxation calculations using natural bond orbitals are carried out for the Puali exchange steric repulsion changes associated with ethane internal rotation using sufficiently extended Dunning correlation-consistent and Gaussian orbitals, so that interpretations are not basis set dependent. These calculations show that the total exchange repulsion is strongly dependent on the C–C bond lengthening that accompanies rotation. The effect of this relaxation is that the total exchange repulsion favors the eclipsed conformer by ∼6 kcal/mol, far greater than previous estimates obtained without skeletal relaxation. The source of the skeletal flexing dependence stems from the strong dependence of the orthogonalization-induced-energy-shift of the σCH NBO and carbon core orbitals on RC–C. Strong basis set dependence is found for the pairwise repulsions. Extended basis set calculations attribute an antibarrier energy change to anti/syn repulsions and a barrier-forming one to gauche repulsions. These senses are opposite to the barrier-forming anti/syn CH pairwise interactions found in previous ethane steric considerations.
Shape of dielectric relaxation curves of ethylene glycol oligomer–water mixtures
Authors: Shinyashiki Naoki , Sudo Seiichi , Abe Wataru , Yagihara Shin
Published: July 2002

Dielectric measurements of water mixtures of ethylene glycol oligomer (EGO) with 1–6 repeat units were carried out in the frequency range of 100 MHz–30 GHz at 25 °C. One relaxation process due to water and EGO was observed for each mixture. If the number of repeat units of EGO is larger than three, the water mixtures show a broad and symmetric relaxation curve. On the other hand, if the number of repeat units of EGO is two or less, the mixtures show a broad and asymmetric relaxation curve. The two types of relaxation curves observed in the EGO–water mixtures reflect the size of the EGO molecule. The asymmetric relaxation curve is due to the cooperative motion of water and EGO molecules in the EGO–water cluster for smaller EGO–water mixtures. In contrast, the symmetric dielectric relaxation curve is a result of the variation of local structure in larger EGO–water mixtures. The larger EGO molecules cannot move cooperatively and behave as a geometrical constraint to the motion of water clusters.
Parametrizing a polarizable force field from ab initio data. I. The fluctuating point charge model
Authors: Banks Jay L. , Kaminski George A. , Zhou Ruhong , Mainz Daniel T. , Berne B. J. , Friesner Richard A.
Published: July 2002

We have developed a polarizable force field for peptides, using all-atom OPLS (OPLS-AA) nonelectrostatic terms and electrostatics based on a fluctuating charge model and fit to ab initio calculations of polarization responses. We discuss the fitting procedure, and specific techniques we have developed that are necessary in order to obtain an accurate, stable model. Our model is comparable to the best existing molecular mechanics force fields in reproducing quantum-chemical peptide energetics. It also accurately reproduces many-body effects in many cases. We believe that straightforward extensions of our linear-response electrostatic model will significantly improve the accuracy for those cases that the present model does not adequately address.
On the adiabatic to diabatic states transformation near intersections of conical intersections
Authors: Yarkony David R.
Published: July 2002

A Hamiltonian, Hd,(2), “rigorously” diabatic in the vicinity of Rx, a point of conical intersection, is constructed using second-order degenerate perturbation theory. Near an Rx on a C2v seam of conical intersection of two states of different symmetry, Hd,(2) may exhibit a confluence with a Cs seam of conical intersection of two states of the same symmetry. Thus by construction, there exists a “rigorous” diabatic representation of the vicinity of this confluence. A procedure for defining a unique linear combination of the degenerate states at a conical intersection is found to be useful for determining the parameters for Hd,(2) and for identifying approximate symmetries in situations where point group symmetry is rigorously absent.
Simulation of preferential Cu2+ solvation in aqueous ammonia solution by means of Monte Carlo method including three-body correction terms
Authors: Pranowo Harno D. , Rode Bernd M.
Published: July 2002

A canonical ensemble Monte Carlo simulation was performed for Cu2+ in 18.57% aqueous ammonia solution at a temperature of 293.16 K, using ab initio pair potentials and three-body potentials for Cu–H2O–H2O, Cu–H2O–NH3 and Cu–NH3–NH3 interactions. The first solvation shell consists of three water and three ammonia molecules, and the second shell of 11.6 water and 10.2 ammonia molecules. The structure of the solvated ion is discussed in terms of radial distribution functions, coordination number, and angular distributions.
Informational–statistical thermodynamics of a complex system
Authors: Fonseca Alexandre F. , Mesquita Marcus V. , Vasconcellos Áurea R. , Luzzi Roberto
Published: July 2002

We apply a statistical–thermodynamic approach to the study of a particular physical system (two sets of nonlinearly coupled oscillators), driven far away from equilibrium. Such a system displays a kind of complex behavior consisting in the so-called Fröhlich effect leading in steady-state conditions to a nonequilibrium phase condensation resembling the Bose–Einstein condensation of systems in equilibrium. A kind of “two-fluid model” arises: the “normal nonequilibrium phase” and Fröhlich condensate or “nonequilibrium superphase,” which is shown to be an attractor of the system. We work out some aspects of the irreversible thermodynamics of this dissipative complex system. Particular nonlinear properties are discussed and Lyapunov exponents determined. This kind of system gives a good modeling of polar vibration modes in polymers and biopolymers.
On a new class of completely integrable nonlinear wave equations. I. Infinitely many conservation laws
Authors: Nutku Y.
Published: July 2002

We point out a class of nonlinear wave equations which admit infinitely many conserved quantities. These equations are characterized by a pair of exact one-forms. The implication that they are closed gives rise to equations, the characteristics and Riemann invariants of which are readily obtained. The construction of the conservation laws requires the solution of a linear second-order equation which can be reduced to canonical form using the Riemann invariants. The hodograph transformation results in a similar linear equation. We discuss also the symplectic structure and Bäcklund transformations associated with these equations.
Scaling properties of total energy of heavy positive ions in d-dimensions
Authors: March N. H.
Published: July 2002

Using density functional theory in the asymptotic limit of very heavy positive ions, it is demonstrated that the well-known scaling property of the total energy E3(Z, N) of an ion with N electrons and atomic number Z is a special case of the d-dimensional result Ed(Z, N)=Z(4+4d−d2)/d(4−d)fd (N/Z):d≠4.
Quantum-mechanical path integrals with Wiener measure for all polynomial Hamiltonians. II
Authors: Daubechies Ingrid , Klauder John R.
Published: July 2002

The coherent-state representation of quantum-mechanical propagators as well-defined phase-space path integrals involving Wiener measure on continuous phase-space paths in the limit that the diffusion constant diverges is formulated and proved. This construction covers a wide class of self-adjoint Hamiltonians, including all those which are polynomials in the Heisenberg operators; in fact, this method also applies to maximal symmetric Hamiltonians that do not possess a self-adjoint extension. This construction also leads to a natural covariance of the path integral under canonical transformations. An entirely parallel discussion for spin variables leads to the representation of the propagator for an arbitrary spin-operator Hamiltonian as well-defined path integrals involving Wiener measure on the unit sphere, again in the limit that the diffusion constant diverges.
A time-harmonic Green’s function technique and wave propagation in a stratified nonreciprocal chiral slab with multiple discontinuities
Authors: He Sailing
Published: July 2002

A transversely polarized time-harmonic electromagnetic plane wave normally incident on a stratified nonreciprocal chiral (bi-isotropic) slab with multiple discontinuities in the parameters is considered. Up- and down-going modes are identified and used to rewrite Maxwell’s equations. Two Green’s function matrices are introduced to express the internal modes in terms of the down-going incident modes. Ordinary differential equations for the Green’s functions are given together with the boundary conditions. Several operators are introduced to treat the multiple discontinuities in the permittivity and permeability. The method provides an efficient way to calculate the internal fields as well as the scattered fields.
Stochastic dynamical interpolation formula to time-reversible diffusion processes
Authors: Misawa Tetsuya
Published: July 2002

A diffusion process of which the diffusion property is preserved under time reversal is called a time-reversible diffusion process. A typical example of such a process is found in Nelson’s stochastic mechanics. Within the extended framework of stochastic integrals proposed by Itô, a sort of the interpolation formula, called a stochastic dynamical interpolation formula, to the time-reversible diffusion process, is derived from a pinned process for the diffusion process. It is shown that the stochastic interpolation formula is regarded as an extention of Lévy’s interpolation formula, which is obtained on the basis of a pinned Brownian motion.
Existence of primary fields as a generalization of the Lüscher–Mack theorem
Authors: Hadjiivanov Ludmil K.
Published: July 2002

The existence of Virasoro primary fields in two-dimensional scale invariant theories is proved in the standard axiomatic setting.
Bosonization in background of metric and gauge
Authors: Charpentier Eric , Gawȩdzki Krzysztof
Published: July 2002

The paper extends the two-dimensional bosonization formulas on a Riemann surface to the case with Abelian gauge field. The result is used to bosonize the massless Thirring model and as an alternative route to the chiral bosonization.
Condition on the symmetry-breaking solution of the Schwinger–Dyson equation
Authors: Cheng G. , Kuo T. K.
Published: July 2002

We derive a condition for a nontrivial solution of the Schwinger–Dyson equation to be accompanied by a Goldstone bound state. It implies that, for quenched planar QCD, although chiral symmetry breaking occurs when there is a cutoff, the continuum limit fails to exist.
On quantum cohomology rings for hypersurfaces in CPN−1
Authors: Jinzenji Masao
Published: July 2002

Using the torus action method, we construct a one-variable polynomial representation of quantum cohomology ring for degree k hypersurface in CPN−1. The results interpolate the well-known result of CPN−2 model and the one of Calabi–Yau hypersuface in CPN−1. We find in the k⩽N−2 case, the principal relation of this ring has a very simple form compatible with toric compactification of moduli space of holomorphic maps from CP1 to CPN−1.
Nonsingular Hankel functions as a new basis for electronic structure calculations
Authors: Bott E. , Methfessel M. , Krabs W. , Schmidt P. C.
Published: July 2002

As a basis for electronic structure calculations, Gaussians are inconvenient because they show unsuitable behavior at larger distances, while Hankel functions are singular at the origin. This paper discusses a new set of special functions which combine many of the advantageous features of both families. At large distances from the origin, these “smoothed Hankel functions” resemble the standard Hankel functions and therefore show behavior similar to that of an electronic wave function. Near the origin, the functions are smooth and analytical. Analytical expressions are derived for two-center integrals for the overlap, the kinetic energy, and the electrostatic energy between two such functions. We also show how to expand such a function around some point in space and discuss how to evaluate the potential matrix elements efficiently by numerical integration. This supplies the elements needed for a practical application in an electronic structure calculation.
Spin structure of antiferromagnetic disclination
Authors: Ivanov B. A. , Kireev V. E. , Voronov V. P.
Published: July 2002

The spin structure of magnetic disclination, viz., spin inhomogeneity manifested in antiferromagnets in the presence of a dislocation, is analyzed. The analysis is carried out on the basis of the discrete model as well as in the specially constructed generalized continual theory based on the local introduction of the antiferromagnetism vector l and taking into account the possibility of a change in the length of this vector. In the solution without singularities constructed for disclination, the modulus of the antiferromagnetism vector vanishes at the center of the disclination. In the discrete model, the disclination energy depends significantly on the arrangement of spins near the core and on the type of their interaction. With the proposed model of spin arrangement, the results of numerical analysis based on the discrete and continual models are in good agreement. It is shown that planar disclinations are stable to the emergence of spins from the easy plane.
Amplitude–frequency dependence of doppleron–phonon resonance
Authors: Tsymbal L. T. , Cherkasov A. N.
Published: July 2002

The amplitude-frequency dependence of the doppleron–phonon resonance (DPR) in tungsten is investigated experimentally and theoretically. The analysis is based on the general theoretical consideration of propagation and interaction of ultrasonic and electromagnetic modes in metals. It has been established that the weak doppleron–phonon interaction takes place in tungsten in the frequency range used (10–500 MHz). The complicated experimental amplitude–frequency dependence of DPR can be explained under the assumption of predominance of the induction mechanism of interaction between the electronic and ionic subsystems of the metal. It is found that under such an assumption, this dependence is of a general character and is not determined by the shape of the Fermi surface of the metal.
Nanostructure of superconducting Nb–Ti alloys
Authors: Lazarev B. G. , Ksenofontov V. A. , Mikhailovskii I. M. , Velikodnaya O. A.
Published: July 2002

The paper reports on the results of high-resolution field emission microscopy technique and its application for a qualitative analysis of the local chemical composition of superconducting Nb–Ti alloys. Previous investigations based on field ion microscopy revealed that a superconducting wire made of Nb–60 at. %Ti alloy contains thin and extended regions with a composition close to Nb, which form current-carrying paths with a density 4×1010 cm−2. The formation of such zones is associated with rising diffusion occurring during thermal treatment and correlates with high critical currents. The development of the high-resolution field emission microscopy has made it possible to refine considerably in the present work the distribution of precipitated components and the configuration of the current-carrying system. The obtained results confirm that high current-carrying properties of Nb–Ti alloys are due to the presence of phases with a composition close to pure Nb precipitated as a result of thermomechanical treatment and forming a three-dimensional lattice of inclusions.
On the influence of drag effect on acoustic modes in two-condensate relativistic superfluid systems
Authors: Vil’chinskiı̆ S. I.
Published: July 2002

Expressions for the velocities of acoustic excitations in relativistic two-condensate superfluid systems are derived taking into account the reciprocal drag of superfluid motions (drag effect). The influence of drag effect on acoustic modes in this system is considered. It is shown that the inclusion of drag effect does not change the nature of acoustic excitation vibrations, but changes the velocities of second, third and fourth sounds.
On transfer of motion in a system of two-dimensional superfluid Bose-gases separated by a thin layer
Authors: Terentjev S. V. , Shevchenko S. I.
Published: July 2002

The effect of transfer of motion between superfluid 2D Bose gases separated by a thin layer is predicted on the basis of microscopic calculations. It is shown that the effect exists at nonzero temperatures as well as at T=0 only for a closed secondary circuits. The dependence of the drag current on temperature and thickness of the layer is determined for charged Bose gases. An experiment is proposed for measuring the predicted effect.
Electromagnetic and acoustic waves in layered organic conductors (a review)
Authors: Kirichenko O. V. , Peschansky V. G.
Published: July 2002

The review is devoted to theoretical investigations of propagation of electromagnetic and acoustic waves in layered conductors of organic origin. Attention is focussed on spectroscopic possibilities for studying the electron structure of organic quasi-two-dimensional conductors, which is of great importance for understanding physical processes in these materials. High-frequency and magnetoacoustic effects considered in this review are typical of quasi-two-dimensional conductors and quite informative. The analysis of these effects makes it possible to study in detail the electron energy spectrum and relaxation properties of charge carriers in layered conductors.
Lower critical fields of textured high-temperature superconductors. III. Experimental study of the anisotropy of the fields Hc1 of the HTSC YBa2Cu3O7−δ
Authors: Finkel’ V. A. , Derevyanko V. V.
Published: July 2002

The dependence of the critical currents Ic of textured samples of the HTSC YBa2Cu3O7−δ (T=77.3 K) on the value of an external magnetic field Hext applied beforehand is investigated for various angles between the field direction and the sample. By a technique developed in the present study the measured angular dependence of Ic(H) is used to determine the values of the lower critical fields Hc1 of the HTSC YBa2Cu3O7−δ in two directions: the direction of the principal axis of the orthorhombic lattice, Hc1ab=89 Oe, and in the perpendicular direction, Hc1c=383 Oe; the ratio of the effective masses of the electron in the two directions is mc/mab=18.5.
New bounds on the permeability of a random array of spheres
Authors: Beasley J. D. , Torquato S.
Published: July 2002

The recently derived variational principle of Rubinstein and Torquato (submitted to J. Fluid Mech.) is applied to obtain new rigorous two- and three-point upper bounds on the fluid permeability k for slow viscous flow around a random array of identical spheres which may penetrate one another in varying degrees. The n-point bounds involve up to n-point correlation function information. Both bounds are simplified and computed for the special case of mutually impenetrable spheres for a wide range of sphere volume fractions. The three-point bound is sharp and provides significant improvement over the two-point bound, especially at high sphere volume fractions (low porosities). It is the sharpest upper bound on k for a random array of impenetrable spheres developed to date and begins to approach the Kozeny–Carman empirical relation at low porosities.
A sensitive test for accuracy in evaluation of molecular collision number in the direct-simulation Monte Carlo method
Authors: Koura Katsuhisa
Published: July 2002

A sensitive test is proposed for the accuracy in the evaluation of the molecular collision number in the direct-simulation Monte Carlo method: It is shown that the translational relaxation in a spatially homogeneous box, which is divided into a number of cells where the simulation of molecular collisions is carried out, is a simple but sensitive test problem for the evaluation of the lower limit of mean molecular number per cell, 〈ΔN〉min, and the computation time, as well as for the verification of the accuracy in the evaluation of the collision number. It is confirmed from the test that 〈ΔN〉min is about 1 for the null-collision (NC) technique but about 30 for the time-counter (TC) technique, and the computation time for the NC technique is about 10%–40% larger than that for the TC technique in the simulation with the same molecular number.
Note on the bound of growth rate for unstable parallel shear flow
Authors: Gu Daifang
Published: July 2002

The classical result for the bound of growth rate for unstable inviscid parallel shear flows has been improved to reflect the dependence of the bound on wave numbers: β≤(k/(4k2+π2)1/2) ×max‖U′‖.
Flow patterns and organization within a turbulent spot
Authors: Sankaran R. , Antonia R. A. , Bisset D. K. , Sokolov M.
Published: July 2002

Recent investigations of turbulent spots within laminar flat plate boundary layers have identified coherent structures which make up the spots. The present paper focuses on the topology of the flow associated with these structures. An array of X probes aligned in the direction normal to the wall has been used to obtain u (streamwise) and v (normal to the wall) velocity fluctuations at three streamwise locations both in and outside the plane of symmetry. The ‘‘most probable spot’’ (MPS), which contains the statistically most probable number of structures, is presented and discussed in the context of instantaneous realizations, overall ensemble averages with respect to the spot leading edge, and averages conditioned on individual structures within the spot. Sectional streamlines for single realizations and for a composite of averages conditioned on the structures show a succession of saddles and foci similar to the topology of a turbulent boundary layer. Percentage contributions from individual structures to the Reynolds stresses are similar to those previously reported for the large-scale motion in a turbulent boundary layer, except near the trailing edge of the spot. The height and spacing of structures, and the distributions of spanwise vorticity and strain rate, all appear to be more regular away from the plane of symmetry.
Effect of splitter plate on unsteady flows around a body of revolution at incidence
Authors: Degani David
Published: July 2002

Numerical solutions of the thin-layer approximation of the three-dimensional Navier–Stokes equations have been obtained for flows around an ogive-cylinder body with and without a splitter plate. The numerical results were compared qualitatively with experimental surface pressure, hot-wire anemometer measurements, and smoke visualization of the leeside vortex flow field at high angles of attack. Both experimental and numerical results show that the presence of a splitter plate in the leeward plane of symmetry suppresses the low-frequency, large-scale von Kármán vortex shedding from the body and leaves the two primary vortices symmetric and almost parallel to the upper surface of the body. It is suggested that the presence of the splitter plate prevents the interaction between flows on either side of the symmetry plane. As a result of the enforced symmetry (a) the antisymmetric mode of the convective instability near the apex of the body cannot be excited and therefore the vortices remain symmetric, staying low and parallel to the upper body surface; and (b) the antisymmetric mode of the absolute instability mechanism cannot be initiated, which suppresses the alternate shedding of vortices from the cylindrical portion of the body. On the other hand, high-frequency fluctuations of the shear layer (which could be the result of a local instability not subject to symmetry conditions) remain virtually unaffected by the presence of the splitter plate.
Turbulent structure and entrainment in heated jets: The effect of initial conditions
Authors: Russ S. , Strykowski P. J.
Published: July 2002

The turbulent structure in the near field of heated jets was investigated at a Reynolds number of 10 000 for density ratios between 1.0 and 0.5; the corresponding mixing and entrainment of these low density jets was examined. The exit conditions of the jet were carefully controlled using extension tubes to alter the nozzle-exit boundary layer thickness, as well as using screens to generate turbulent conditions while keeping the boundary layer thickness approximately constant. Heated jets with initially laminar exit conditions were dominated by the formation and pairing of vortex structures. Increasing the boundary layer thickness produced longer wavelength structures which saturated and paired at farther downstream distances; under these conditions jet momentum mixing was reduced, giving rise to an increase in the jet potential core length. The shear layer structures also became more organized as the jet density was reduced relative to the density of the ambient fluid, resulting in increased momentum mixing and a dramatic visual spreading of the jet. Turbulent exit conditions disrupted the formation of these large-scale vortex structures for all of the density ratios investigated, producing smaller spreading rates and significantly lower mixing and entrainment.
Dispersion, permeability heterogeneity, and viscous fingering: Acoustic experimental observations and particle-tracking simulations
Authors: Tchelepi H. A. , Orr F. M. , Rakotomalala N. , Salin D. , Wouméni R.
Published: July 2002

Stable and unstable displacement experiments were performed in millstone and limestone cores. Concentration histories at ten locations along the core samples were obtained by acoustic measurements. Particle-tracking simulations of the displacements were also made utilizing permeability distributions measured with a permeameter. The combination of experimental observations and simulations indicate that superstable (M<1) displacements suppress the influence of heterogeneity; this suppression was reflected in smaller apparent dispersivities as the mobility ratio decreased below unity. In the millstone, which exhibited random heterogeneity, two-dimensional particle-tracking simulations reproduce with reasonable accuracy the growth of the fingered region in unstable displacements. In homogeneous porous media, concentration histories obtained in three-dimensional simulations did not differ significantly from their two-dimensional counterparts. In the more heterogeneous limestone, unstable displacements accentuated the influence of heterogeneity leading to longer transition zones. Two distinct flow regimes were observed in unstable displacements: (1) an initial period of rapid transition zone growth and (2) a subsequent period in which leading and trailing edges of the transition zone travel at nearly constant velocities.
The effect of externally applied oscillating electric fields on the l=1 and l=2 diocotron modes in non-neutral plasmas
Authors: Spencer Ross L.
Published: July 2002

A high-frequency oscillating electric field can change the properties of diocotron modes in non-neutral plasmas. The effect depends crucially on the azimuthal mode number, m, of the applied field. For m=0,±1 there is no effect, and for applied standing waves there is also no effect. But if the applied field has the form of a traveling wave with ‖m‖≥2, the frequency of stable diocotron modes can be modified and for ‖m‖≥3, the l=2 instability of hollow density profiles can be stabilized. The analytic results are verified with a nonlinear fluid simulation of an infinitely long non-neutral plasma.
Radio frequency effects on confinement and equilibrium in a magnetic mirror
Authors: Goodman D. L. , Petty C. C.
Published: July 2002

The effect of radio frequency (rf) power in the ion cyclotron frequency range on radial ion transport and plasma equilibrium for the Constance B magnetic mirror [Phys. Rev. Lett. 58, 1853 (1987)] is presented. Low-frequency rf power increases the ion perpendicular loss rate, which then becomes the dominant ion loss mechanism. The observed radial transport is much in excess of that calculated by classical, neoclassical, or stochastic wave–particle models, with implications for limits on heating in magnetic mirrors and charge state selection in electron cyclotron resonance ion sources. With sufficient rf power, the ion loss rate rivals the ionization source (plasma production) rate, with a resultant loss of plasma equilibrium. The effect of rf power on the ambipolar potential is also studied, and is shown to be an electron effect whose scaling with applied field is consistent with theory.
A fast numerical method for the Alfvén spectrum of ideal magnetohydrodynamics
Authors: Novo Sylvia , Rojo Jesús , Núñez Manuel
Published: July 2002

A numerical method is developed to find the Alfvén spectrum of axisymmetric toroidal equilibria. It involves the numerical integration of ordinary differential equations only and is therefore faster than finite element algorithms; moreover, the Alfvén frequencies of every flux surface may be calculated independently from the rest. The method is applied to the Shafranov–Solov’ev equilibrium for the first Fourier modes, and the results are compared to those obtained with the geometrical optics approximation. This comparison shows which of the parameters of the equilibrium are relevant on each branch of spectral points. Other features observed in the graphics obtained by this algorithm are also analyzed.
Density dependence of electron mean-free paths from low to high energies
Authors: Gordon F. I. , Djaoui A.
Published: July 2002

The mean-free path of an electron scattering inelastically in a degenerate electron gas at zero temperature has been calculated, using the dielectric function/self-energy formalism of Ritchie [Phys. Rev. 114, 644 (1959)]. The objective of this work is to model accurately the mean-free path as a function of photoelectron energy and density for use in the calculation of extended x-ray absorption fine structure (EXAFS) [Rev. Mod. Phys. 53, 1 (1981)] of fluids and plasmas whose structure can be described by a radial distribution function. Results are compared with proposed universal mean-free path formulas and it is found that a more generalized version of a power law provides an adequate fit. A table of the best-fit parameters of this law at a range of densities of interest for the EXAFS colliding shock experiments on foil targets is presented.
Plasma wake-field effects on high-current relativistic electron beam transport in the ion-focused regime
Authors: Miller J. D. , Schneider R. F. , Weidman D. J. , Nguyen K. T.
Published: July 2002

Modulation of the beam current has recently been observed during ion-focused regime (IFR) transport of a high-power relativistic electron beam propagating through a low-density background plasma. Injecting a high-current, high-energy electron beam into an IFR channel immersed in a background plasma induces plasma oscillations. These background plasma oscillations, induced by the rise-time portion of the beam ejecting plasma electrons from the vicinity of the beam into the background plasma, give rise to a modulated axial electric field. This field travels with the beam leading to beam energy and current oscillations. In the experiment, a 1.7 MeV, 1 kA, rise-time-sharpened electron beam is propagated on a KrF excimer laser-produced IFR channel in trimethylamine (TMA) gas, which is immersed in a low-density plasma-filled transport tube. Experimental measurements, analytical theory, and detailed computer simulations are presented demonstrating modulation of this high-current relativistic electron beam near the low-density background plasma frequency.
A predictive Monte Carlo simulation of two-fluid flow through porous media at finite mobility ratio
Authors: DeGregoria A. J.
Published: July 2002

Diffusion-limited aggregation simulations of two-fluid flow through porous media at infinite mobility ratio are extended to finite mobility ratio of driving to driven fluid. Curves for sweep efficiency at breakthrough are generated that come close to data obtained on two-dimensional sand packs.
Electron confinement in a bumpy torus without the influence of hot-electron rings—‘‘Ring killer’’ experiment
Authors: Hillis D. L. , Wilgen J. B. , Cobble J. A. , Davis W. A. , Hiroe S. , Rasmussen D. A. , Richards R. K. , Uckan T. , Jaeger E. F. , Hankins O. E. , Goyer J. R. , Solensten L.
Published: July 2002

The ELMO Bumpy Torus (EBT) [Plasma Physics and Controlled Nuclear Fusion (IAEA, Vienna, 1975), Vol. II, p. 141] normally has an energetic electron ring in each of its 24 mirror sectors. The original intention of using this hot-electron population was to provide an average local minimum in the magnetic field (through its diamagnetism) to stabilize the simple interchange and flute modes, which otherwise are theoretically inherent in a closed-field-line bumpy torus. To study the confinement properties of a bumpy torus without the influence of hot-electron rings, a water-cooled stainless steel limiter in each mirror sector was extended into the plasma to the ring location; this eliminated the hot-electron ring population. These limiters were aptly named ‘‘ring killers.’’ Electron temperature, density, space potential, and plasma fluctuations have been measured during the ring killer experiment and are compared to standard EBT operation. The results of these experiments indicate that the hot-electron rings in EBT do enhance the core plasma properties of EBT and do, in fact, reduce plasma fluctuations; however, these improvements are not large in magnitude. These measurements and recent theoretical models suggest that simple interchange/flute modes are stabilized, or fluctuation levels reduced, well before that condition is obtained for average minimum-B stabilization. Several possible mechanisms for this stabilization are discussed.
Vortex-in-cell simulation of bubble competition in a Rayleigh–Taylor instability
Authors: Zufiria Juan A.
Published: July 2002

The penetration of a front of light fluid into a heavy fluid in a Rayleigh–Taylor unstable flow is studied by using a vortex-in-cell numerical algorithm. The algorithm is used to simulate the competition among the bubbles that are formed in the interface when the density ratio of the two fluids is very large. Several multifrequency initial conditions and the effect of surface tension have been considered. It is found that the front moves with constant acceleration. The values obtained for the acceleration of the front are in very good agreement with experimental results obtained by Read [Phys. D 12, 45 (1984)] and with the results obtained by using Zufiria’s [Phys. Fluids 31, 440 (1988)] model.
Term-by-term tests of stress-transport turbulence models in a three-dimensional boundary layer
Authors: Schwarz Walter R. , Bradshaw Peter
Published: July 2002

Current Reynolds-averaged turbulence models for use in three-dimensional flows have mostly been derived by straightforward generalization of two-dimensional models; the results are rather unsatisfactory. Reynolds-stress transport methods perform better than methods based on an (isotropic) eddy viscosity. One approach to the problem is a term-by-term check of Reynolds-stress transport models against experiments or simulation data in representative three-dimensional flows. In this paper a comparison of four models for the turbulent-transport ‘‘diffusion’’ terms, and four models for the pressure–strain ‘‘redistribution’’ terms, with measurements (including all six Reynolds stresses and all ten triple products) in the thin boundary layer on the flat floor of a duct with a 30° bend in the horizontal plane is presented. The conclusion is that the pressure–strain models perform remarkably well, and that although the triple-product models often perform badly, they represent rather small terms and therefore do not cause large errors in Reynolds-stress predictions. The implication is that inadequate modeling of the dissipation must be the main cause of errors in predictions of three-dimensional boundary layers.