bluuebunny/crossref_metadata_2025 huggingface dataset:
Photodissociation dynamics of acetylacetone: The OH product state distribution
Authors: Yoon Min-Chul , Choi Young S. , Kim Sang Kyu
Published: July 2002

Acetylacetone in the supersonic jet, which exists predominantly as an enolic form, is found to give rise to the OH fragment after the π–π* transition induced by the UV absorption. The nascent OH product state distributions are determined using a laser-induced fluorescence technique at the excitation wavelengths of 291 and 266 nm. The OH fragment is vibrationally cold, and its rotational state distribution is peaked at N=3 or 4 at the pump wavelength of 291 or 266 nm, respectively. No fluorescence from the excited acetylacetone has been observed even in the energy region near the origin, suggesting the ultrafast nonradiative processes of the excited state. From the measured OH product state distributions, the upper bound for the dissociation energy of the acetylacetone is estimated to be 90.3 kcal/mol. The ratios of Λ-doublets and spin–orbit states of the OH fragment are also measured. A slight preference of the OH fragment in the Π3/22 state over the Π1/22 state is observed. The Π−/Π+ ratios, determined by the relative intensity ratios of Q and P (or R) lines, are found to be less than unity, suggesting the preferential cleavage of the C–OH bond on the molecular plane probably due to a relatively strong intramolecular hydrogen bonding of the enolic acetylacetone. The prior calculation reproduces the experiment quite well for the OH rotational state distribution at 291 nm, while it does not for that at 266 nm. This suggests that the transition state in the acetylacetone dissociation, at the low energy near threshold, may be completely loosely defined on the potential energy surface which does not have a barrier to recombination, and it becomes tightened as the energy increases above the reaction threshold.
Electronic structure of tris(8-hydroxyquinoline) aluminum thin films in the pristine and reduced states
Authors: Johansson N. , Osada T. , Stafström S. , Salaneck W. R. , Parente V. , dos Santos D. A. , Crispin X. , Brédas J. L.
Published: July 2002

The electronic structure of tris(8-hydroxyquinoline) aluminum (Alq3) has been studied in the pristine molecular solid state as well as upon interaction (doping) with potassium and lithium. We discuss the results of a joint theoretical and experimental investigation, based on a combination of x-ray and ultraviolet photoelectron spectroscopies with quantum-chemical calculations at the density functional theory level. Upon doping, each electron transferred from an alkali metal atom is stored on one of the three ligands of the Alq3 molecule, resulting in a new spectral feature (peak) in the valence band that evolves uniformly when going from a doping level of one to three metal atoms per Alq3 molecule.
Phase separation of polymer blend films near patterned surfaces
Authors: Kielhorn L. , Muthukumar M.
Published: July 2002

The effect of a patterned surface on the phase separation kinetics of a thin polymer film has been investigated using the Cahn–Hilliard–Cook model in three dimensions with the addition of a short range surface potential. We have observed pattern-induced spinodal waves perpendicular to the surface creating “checkerboard”-like composition fluctuations for a wide range of patterns used in this study. The number of such layers strongly depends on the magnitude of the thermal noise. For sufficiently thin films, where the film thickness is smaller than the spinodal wavelength, spinodal decomposition can be arrested if the surface potential and the characteristic size of the pattern are chosen accordingly, enabling the transfer of surface patterns to the film material. The kinetic pathways through which the equilibrium states are reached delicately depend on the particular pattern, its size, and the film thickness.
Fractional power dependence of mean lifetime of electron transfer reaction on viscosity of solvent
Authors: Okada Akira
Published: July 2002

Dynamical effects of a solvent (environment) on an electron transfer (ET) reaction are investigated by using the Sumi–Marcus reaction–diffusion equation; this equation describes the time evolution of population distribution function of a reactant in a slow nuclear coordinate system. Assuming that viscosity of the solvent (environment) is proportional to a relaxation time scale of the slow nuclear mode, power dependence of a mean lifetime of ET on the relaxation time scale becomes the same as the one on the viscosity. Therefore, the former power dependence is investigated instead of the latter, and it is found that the power in the limit of the (infinitely) large relaxation time scale is 1−r when r<1, and 0 when 1⩽r, where r is the ratio of the reorganization energy of fast nuclear modes to the slow nuclear mode. However, this limit cannot always be reached in a realistic situation. Therefore, the present theory is extended to a large but finite relaxation time scale. The values of the power obtained by the present theory are in reasonable agreement with the ones calculated numerically by W. Nadler and R. A. Marcus [J. Chem. Phys. 86, 3906 (1987)]. Finally, a difficulty in numerical calculations is shown. An expansion of the population distribution function in some basis set of functions is common in numerical calculations. However, the use of that finite basis set of functions which is independent of the relaxation time scale leads to a value of the power that is either zero or unity in the limit of the large relaxation time scale, and as such cannot reproduce the correct asymptotic behavior of the mean lifetime.
Comment on “Improving protein circular dichroism calculations in the far-ultraviolet through reparametrizing the amide chromophore” [J. Chem. Phys. 109, 782 (1998)]
Authors: Woody Robert W. , Sreerama Narasimha
Published: July 2002

Circular dichroism spectra for 23 proteins have been calculated using transition parameters from experiments on model amides and semiempirical molecular orbital (MO) calculations. The results are substantially better than those reported by Hirst. The improvements result primarily from using distributed dipoles (monopoles), rather than the point dipoles used by Hirst.
Evidence of three-body correlation functions in Rb+ and Sr2+ acetonitrile solutions
Authors: D’Angelo P. , Pavel N. V.
Published: July 2002

The local structure of Sr2+ and Rb+ ions in acetonitrile has been investigated by x-ray absorption spectroscopy (XAS) and molecular dynamics simulations. The extended x-ray absorption fine structure above the Sr and Rb K edges has been interpreted in the framework of multiple scattering (MS) formalism and, for the first time, clear evidence of MS contributions has been found in noncomplexing ion solutions. Molecular dynamics has been used to generate the partial pair and triangular distribution functions from which model χ(k) signals have been constructed. The Sr2+ and Rb+ acetonitrile pair distribution functions show very sharp and well-defined first peaks indicating the presence of a well organized first solvation shell. Most of the linear acetonitrile molecules have been found to be distributed like hedgehog spines around the Sr2+ and Rb+ ions. The presence of three-body correlations has been singled out by the existence of well-defined peaks in the triangular configurations. Excellent agreement has been found between the theoretical and experimental data enforcing the reliability of the interatomic potentials used in the simulations. These results demonstrate the ability of the XAS technique in probing the higher-order correlation functions in solution.
Evidence for the stress-thermal rule in an elastomer subjected to simple elongation
Authors: Broerman A. W. , Venerus D. C. , Schieber J. D.
Published: July 2002

Anisotropic thermal diffusivity is examined in a cross-linked silicone elastomer deformed in simple elongation. Thermal diffusivity both parallel and perpendicular to the stretch direction is measured using an optical technique called forced Rayleigh scattering. The thermal diffusivity is found to increase linearly with stretch ratio to a value 10% larger than the equilibrium value for a stretch ratio of two. Measurements of the birefringence and tensile stress were used to evaluate the stress-optic rule, which was found to be valid. The difference between measured thermal diffusivities parallel and perpendicular to the stretch direction and tensile stress data were used to show the thermal conductivity and stress tensors are linearly related. These data appear to be the first direct evaluation of the stress-thermal rule in a deformed polymeric material.
Rotationally resolved pulsed field ionization photoelectron bands for O2+(a 4Πu, v+=0–18) in the energy range of 16.0–18.0 eV
Authors: Song Y. , Evans M. , Ng C. Y. , Hsu C.-W. , Jarvis G. K.
Published: July 2002

We have obtained rotationally resolved pulsed field ionization-photoelectron (PFI-PE) spectra of O2 in the energy range of 16.0–18.0 eV, covering ionization transitions O2+(a 4Πu, v+=0–18,J+)←O2(X 3Σg., v″=0,N″). Although these vibrational PFI-PE bands for O2+(a 4Πu, v+) have significant overlaps with those for O2+(X 2Πg) and O2+(A 2Πu), we have identified all O2+(a 4Πu, v+=0–18) bands by simulation of spectra obtained using supersonically cooled O2 samples with rotational temperatures ≈20 and 220 K. While the v+=4–18PFI-PE bands represent the first rotationally resolved photoelectron data for O2+(a 4Πu), the PFI-PE bands for O2+(a 4Πu, v+=10–18) are the first rotationally resolved spectroscopic data for these levels. The simulation also allows the determination of accurate ionization energies, vibrational constants, and rotational constants for O2+(a 2Πu, v+=0–18). The observed intensities of spin–orbit components for the majority of O2+(a 2Πu, v+) vibrational bands are in accordance with the forced spin–orbit/rotational autoionization mechanism.
Dynamic response of an irreversible catalytic reaction to periodic variation of the reactant’s pressure
Authors: López A. C. , Albano E. V.
Published: July 2002

The dynamic response of the Ziff, Gulari, and Barshad model for the catalytic oxidation of CO, to a periodic variation of the external pressure is studied close to the CO-poisoning transition. Due to the applied pressure, a considerable enhancement in the catalytic activity is observed when the system is briefly driven into the CO-poisoned state. However, saturation of catalysts’ surface can be induced by applying signals of either long period or high amplitude.
The polarized infrared and Raman spectra of α-T6 single crystal: An experimental and theoretical study
Authors: Degli Esposti Alessandra , Fanti Marianna , Muccini Michele , Taliani Carlo , Ruani Giampiero
Published: July 2002

The polarized infrared absorption and Raman spectra on the bc plane of a single crystal of α-sexithienyl (α-T6) have been recorded in the range from 50 up to 3500 cm−1. The intermolecular interactions give rise to the Davydov splitting of the ungerade phonon energy levels originated by the molecular pyramidalization along with the CCH bending modes. A μ-Raman investigation on the crystal surface allows identification of the perturbed local packing of the molecules, which are spectroscopically crystal defects. The selective resonant enhancement of the scattering intensity of some Raman modes, obtained by exciting at λex=632.8 nm, indicates that these defects are associated with the lowering of the electronic excited states due to the increased π-orbital overlapping. Moreover, the resonant enhancement allows detection of some of the gerade intramolecular phonons not observed in the bulk. The comparison between the far-infrared absorption spectra recorded at room temperature and at 80 K evidences three lattice phonons. The spectral analysis, based on a simulation, which uses a scaled ab initio molecular force field, well interprets the experimental data assigning all the internal phonons and elucidates some of the effects of the intermolecular interactions. The calculations are performed at the HF/6-31 G* level of the theory to obtain the harmonic frequencies and the dipole moment and polarizability tensor derivatives of the α-T6 molecule. Both polarized infrared and Raman spectra simulations are carried out calculating the optical response of a unit cell of the α-T6 single crystal belonging to the C2h factor group in the monoclinic P21/n space group.
Influence of hydrogen on the stability of positively charged silicon dioxide clusters
Authors: Schenkel T. , Schlathölter T. , Newman M. W. , Machicoane G. A. , McDonald J. W. , Hamza A. V.
Published: July 2002

Spectra of positively charged secondary ions from thermally grown SiO2 films were recorded in a time-of-flight secondary ion mass spectrometry scheme. Ablation of cluster ions was induced by the impact of slow (4 keV/u) Au69+ projectiles. The intensities of SixOyHz+, (x=1–22, y=1–44, z=0–7) clusters are found to depend sensitively on the oxygen to silicon ratio and also on the hydrogen content. We find that oxygen rich clusters, y=2x+1, and, in one case, y=2x+2, can be stabilized by the incorporation of two additional hydrogen atoms in the cluster.
Weakly convergent expansions of a plane wave and their use in Fourier integrals
Authors: Weniger E. Joachim
Published: July 2002

The Fourier transform of an irreducible spherical tensor is normally computed with the help of the Rayleigh expansion of a plane wave in terms of spherical Bessel functions and spherical harmonics. The angular integrations are then trivial. However, the remaining radial integral containing a spherical Bessel function may be so complicated that the applicability of Fourier transformation is severely restricted. As an alternative, the use of weakly convergent expansions of a plane wave in terms of complete orthonormal sets of functions is suggested. The weakly convergent expansions of a plane wave are constructed in such a way that their application in Fourier integrals leads to expansions of the Fourier or inverse Fourier transform that converge with respect to the norm of either the Hilbert space L2(R3) or the Sobolev space W(1)2(R3). Accordingly, these weakly convergent expansions may be viewed as distributions that are defined on either L2(R3) or W(1)2(R3). The properties of some complete orthonormal sets of functions, in particular their Fourier transforms, are also studied. Shibuya and Wulfman [Proc. R. Soc. London Ser. A 286, 376 (1965)] derived an expansion of a plane wave involving the four-dimensional spherical harmonics. It is shown that this Shibuya–Wulfman expansion is also a distribution which is defined on the Sobolev space W(1)2(R3). Finally, as an application it is shown how weakly convergent expansions can be used profitably for the construction of addition theorems.
A class of perfect fluid metrics with flat three-dimensional hypersurfaces
Authors: Wolf Thomas
Published: July 2002

The class of perfect fluid and vacuum space-times with a family of flat three-slices and a tensor of exterior curvature covariantly constant within these slices is examined and the corresponding solutions are found. It is shown that this class contains the class of metrics with three commuting Killing vectors. Therefore, e.g., all known stationary metrics with cylindrical or plane symmetry are generalized. An instruction is given for constructing perfect fluid metrics with this symmetry and a connection to a vacuum across surfaces p=0. Thereby the equation of state of the interior rotating perfect fluid can be arbitrarily chosen and the positivity of density and pressure can be forced. A geometric criterion of the interior metric with rotating matter is found that decides whether the exterior solution is stationary or static. Besides solutions with three symmetries, inhomogeneous metrics also are found. Among them is a solution with one symmetry and rotating, expanding, shearing, and accelerating perfect fluid. All resulting vacuum solutions are already known.
About vacuum solutions of Einstein’s field equations with flat three-dimensional hypersurfaces
Authors: Wolf Thomas
Published: July 2002

The class of vacuum space-times with a family of flat three-slices and a traceless tensor of exterior curvature Kab is examined. Metrics without symmetry and solutions describing gravitational radiation are obtained. It turns out that there is a correlation between rank (Kab) and the Petrov type. Although the resulting solutions are already known, the richness of the class of space-times with flat slices becomes obvious. An example is given of a metric with one-parameter manifold of families of flat slices.
Group theoretical aspects of the extended interacting boson model
Authors: Kota V. K. B. , Van der Jeugt J. , De Meyer H. , Vanden Berghe G.
Published: July 2002

Group theoretical aspects of the extended interacting boson model (gIBM) with s, d, and g bosons are discussed. The basis states and branching rules are given for the seven dynamical symmetries. Casimir operators and their eigenvalues are obtained and enable one to write down energy formulas.
Initial value problem for colliding gravitational plane waves. I
Authors: Hauser Isidore , Ernst Frederick J.
Published: July 2002

A new form of the general solution of the initial value problem for colliding gravitational plane waves with collinear polarization is obtained. The solution of the linear hyperbolic field equation for ψ(u,v) is expressed as a linear superposition ∫da g(σ)ω(u,v,σ) of a one-parameter family of basic solutions of the form ω(u,v,σ)=ω1(u,σ)ω2(v,σ), where u and v are arbitrary null coordinates and σ is the parameter. The coefficients g(σ) in this superposition are expressed in terms of the initial data by using a generalization of an integral transform obtained by Abel in his solution of a tautochrone problem of classical particle mechanics.
Noncommutative differential geometry of matrix algebras
Authors: Dubois-Violette Michel , Kerner Richard , Madore John
Published: July 2002

The noncommutative differential geometry of the algebra Mn (C) of complex n×n matrices is investigated. The role of the algebra of differential forms is played by the graded differential algebra C(sl(n,C),Mn (C))=Mn (C)⊗Λsl(n,C)*,sl(n,C) acting by inner derivations on Mn (C). A canonical symplectic structure is exhibited for Mn (C) for which the Poisson bracket is, to within a factor i, the commutator. Also, a canonical Riemannian structure is described for Mn (C). Finally, the analog of the Maxwell potential is constructed and it is pointed out that there is a potential with a vanishing curvature that is not a pure gauge.
Inverse spectral transform for the Ishimori equation: I. Initial value problem
Authors: Konopelchenko B. G. , Matkarimov B. T.
Published: July 2002

The initial value problem for the (2+1)-dimensional integrable generalization of the (1+1)-dimensional continuous Heisenberg ferromagnet model (Ishimori-I,II equations) is solved by the ∂̄ and nonlocal Riemann–Hilbert problems (NRHP) method.
Initial and boundary conditions for diffusive linear transport problems
Authors: Malvagi F. , Pomraning G. C.
Published: July 2002

The initial and boundary layer analyses of an asymptotic expansion that yields a diffusive description of linear particle transport are carried out in some generality. This yields initial and boundary conditions that apply to the diffusion equation previously reported in the literature. Effects treated include boundary curvature, the variation of the transport boundary condition along the bounding surface, and spatial and temporal variations of the interaction coefficients (cross sections) in the initial and boundary layers.
Nonrelativistic limit of supersymmetric theories
Authors: de Azcárraga José A. , Ginestar Damián
Published: July 2002

The superfield formulation of the nonrelativistic limit of supersymmetric theories is given using a precise group-theoretical definition. The procedure is applied to the Wess–Zumino superfield Lagrangian and to supersymmetric QED.
Dirac Hamiltonian with Coulomb potential and spherically symmetric shell contact interactiona)
Authors: Dittrich J. , Exner P. , Šeba P.
Published: July 2002

Spherically symmetric Hamiltonians describing a Dirac particle in Coulomb potential combined with contact interaction on a sphere (typically, a δ-shell interaction) are constructed. The point spectrum is studied numerically for the case of scalar and vector δ shells. A comparison of two possible definitions of δ-shell coupling constants is also given.
On the symmetry groups of the intrinsic generalized wave and sine–Gordon equations
Authors: Tenenblat K. , Winternitz P.
Published: July 2002

The intrinsic generalized sine–Gordon and wave equations are integrable n-dimensional generalizations of the sine–Gordon and wave equations, respectively. It is shown that their Lie-point symmetry groups are finite-dimensional. They consist only of translations in the first case and translations and dilations in the second. The symmetries are used to obtain classes of exact invariant solutions.
On a gravitational Aharonov–Bohm effect
Authors: Jensen Bjo/rn , Kučera Jaromír
Published: July 2002

In this article we study the properties of certain quantum systems outside a rotating cosmic gauge string. It is shown that particle energies couple to the angular momentum density in the string, even though the particles are constrained only to move outside the string core where the Riemannian curvature vanishes identically. This effect may be looked upon as a gravitational analog of the Aharanov–Bohm effect. Due to the coupling of the particle energy to the angular momentum density in the string, it is shown that the angular momentum spectrum of the quantum particle is shifted by a constant amount. This, it is argued, will give rise to an nonvanishing angular momentum density in the quantum vacuum outside the rotating string. It is also shown that a quantum particle might experience an infinite blueshift in the presence of closed timelike curves outside such a string.
On exact solutions of the regularized long-wave equation: A direct approach to partially integrable equations. II. Periodic solutions
Authors: Parker A.
Published: July 2002

In this second of two articles (designated I and II), the bilinear transformation method is used to obtain stationary periodic solutions of the partially integrable regularized long-wave (RLW) equation. These solutions are expressed in terms of Riemann theta functions, and this approach leads to a new and compact expression for the important dispersion relation. The periodic solution (or cnoidal wave) can be represented as an infinite sum of sech2 ‘‘solitary waves’’: this remarkable property may be interpreted in the context of a nonlinear superposition principle. The RLW cnoidal wave approximates to a sinusoidal wave and a solitary wave in the limits of small and large amplitudes, respectively. Analytic approximations and error estimates are given which shed light on the character of the cnoidal wave in the different parameter regimes. Similar results are presented in brief for the related RLW Boussinesq (RLWB) equation.
Harmonic analysis on spannors
Authors: Moylan P.
Published: July 2002

The spannor representation of the universal covering group of the conformal group of Minkowski space–time is studied herein. It is an example of an indecomposable representation. Various parallelizations for spannors are described. Normalized K̃-finite basis fields for the spannors are introduced, and a computation of the actions of the scale generator and other generators of the conformal group on these basis fields is given. The irreducibility of all irreducible composition factors is established, and it is decided which ones are infinitesimally unitary. Two sets of generators for maximal Abelian subalgebras of the enveloping algebra of the conformal group are introduced. The action of one of the sets on K̃-finite basis fields in the spannor representation is studied; this is important for understanding of the problem of degeneracy in the spannor representation, since K̃ types occur with multiplicity 2. Many calculations make use of the lowest (highest) weight module structures of the unitarizable and positive (negative) energy, irreducible composition factors; and, thus, many of our results and techniques can be used to initiate a study of the quantum deformations of the associated Lie algebra representations. Results on the Poincaré content of the unitarizable, irreducible composition factors are stated, and unitary equivalences between certain of these factors are established.
Spherically symmetric manifolds which admit five isometries
Authors: Azad H. , Ziad M.
Published: July 2002

Spherically symmetric space–times which admit a five-dimensional group of motions are studied and it is shown that it can never be locally a maximal group of motions but globally it could be a maximal group.
Coisotropic regularization of singular Lagrangians
Authors: Ibort A. , Marín-Solano J.
Published: July 2002

We present an alternative approach to the usual treatments of singular Lagrangians. It is based on a Hamiltonian regularization scheme inspired on the coisotropic embedding of presymplectic systems. A Lagrangian regularization of a singular Lagrangian is a regular Lagrangian defined on an extended velocity phase space that reproduces the original theory when restricted to the initial configuration space. A Lagrangian regularization does not always exists, but a family of singular Lagrangians is studied for which such a regularization can be described explicitly. These regularizations turn out to be essentially unique and provide an alternative setting to quantize the corresponding physical systems. These ideas can be applied both in classical mechanics and field theories. Several examples are discussed in detail.
Sklyanin determinant, Laplace operators, and characteristic identities for classical Lie algebras
Authors: Molev Alexander
Published: July 2002

The structure of quantized enveloping algebras called twisted Yangians, which are naturally associated with the B, C, and D series of the classical Lie algebras is studied. An explicit formula for the formal series (the Sklyanin determinant), whose coefficients are free generators of the center of the twisted Yangian is obtained. As a corollary a new system of algebraically independent generators of the center of the universal enveloping algebra for the orthogonal and symplectic Lie algebras is obtained and the characteristic polynomial for the matrix formed by the generators of these Lie algebras is found.
Directly interacting massless particles—A twistor approach
Authors: Bette Andreas
Published: July 2002

Twistor phase spaces are used to provide a general description of the dynamics of a finite number of directly interacting massless spinning particles, forming a closed relativistic massive and spinning system with an internal structure. A Poincaré invariant canonical quantization of the so obtained twistor phase space dynamics is performed.
The q-phase-difference operator and two-mode q-coherent states
Authors: Yang Yaping , Feng Weiguo , Wu Xiang
Published: July 2002

In this paper, we introduce unitary and Hermitian phase-difference operators for the two modes of the electromagnetic field in the q-deformed case. The q-creation and annihilation operators of phase-difference quanta are given, and the algebraic properties of some operators in phase space are discussed. The phase-difference properties of two-mode q-coherent states are investigated.
Large amplitude gravitational waves
Authors: Alı̀ G. , Hunter John K.
Published: July 2002

We derive an asymptotic solution of the Einstein field equations which describes the propagation of a thin, large amplitude gravitational wave into a curved space–time. The resulting equations have the same form as the colliding plane wave equations without one of the usual constraint equations.
Refutation of stability proofs for dipole vortices
Authors: Nycander J.
Published: July 2002

Five stability proofs for dipole vortices (modons) that have been presented by various authors are examined. It is shown that they are all incorrect, and that westward-propagating dipoles are in fact unstable, in contradiction to some of the proofs.
Exterior flow of the Kida ellipse
Authors: Dahleh Marie D.
Published: July 2002

The exterior flow of a Kida ellipse that has a large chaotic zone is studied. The spatial variability that is characterized by the separation rate of nearby particles in the chaotic region is determined. In order to quantify this variability, a method for calculating the finite time spreading rate is developed. By examining the individual particle trajectories, it is seen that particles that have a high spreading rate are particles that pass near a stagnation point of the flow. The effect of the addition of vorticity to particles in the exterior flow is examined.
Three-dimensional instability of strained vortices in a stably stratified fluid
Authors: Miyazaki Takeshi , Fukumoto Yasuhide
Published: July 2002

The linear stability of unbounded strained vortices in a stably stratified fluid is investigated theoretically. The problem is reduced to a Floquet problem which is solved numerically. The three-dimensional elliptical instability of Pierrehumbert type [Phys. Rev. Lett. 57, 2157 (1986)] is shown to be suppressed by the stable stratification and it disappears when the Brunt–Väisälä frequency exceeds unity. On the other hand, two classes of new instability mode are found to occur. One appears only when the Brunt–Väisälä frequency is less than 2, whereas the other persists for all values of the Brunt–Väisälä frequency. The former mode is related to a parametric resonance of internal gravity waves, and the latter modes are related to superharmonic parametric instability.
Gas-dynamic study of pulsed desorption flows into a vacuum
Authors: Sibold D. , Urbassek H. M.
Published: July 2002

The one-dimensional adiabatic expansion of gas desorbing into a vacuum is studied. The gas is assumed to desorb thermally during a finite period of time from a plane surface. Both the boundary conditions of particle reflection at the surface and sticking at the surface (recondensation) are treated. The general solution of the space- and time-dependent flow field is presented for arbitrary adiabatic coefficient γ. Explicit results are derived and discussed for the case of a monoatomic gas (γ=5/3) and a diatomic gas (γ=7/5 and γ=9/7). The cases of a gas with a large number of degrees of freedom (γ=1), and of γ=3 as a particularly transparent example of the analysis, are treated separately.
Second-type self-similar solutions to the strong explosion problem
Authors: Waxman Eli , Shvarts Dov
Published: July 2002

The flow resulting from a strong explosion at the center of an ideal gas sphere, whose density drops with the distance r from the origin as r−ω, is assumed to approach asymptotically the self-similar solutions by Sedov and Taylor. It is shown that the Sedov–Taylor (ST) solutions that exist only for ω≤5 and are probably the most familiar example for self-similar solutions of the first type fail to describe the asymptotic flow obtained for 3≤ω≤5. New second-type self-similar solutions that describe the asymptotic flow for 3≤ω≤5, as well as for ω≥5, are presented and analyzed. The shock waves described by these solutions are accelerating while the shock waves described by the ST solutions for ω≤3 are decelerating. The new solutions are related to a new singular point in Guderley’s map. They exist only for ω values smaller than some ωc that depends upon the adiabatic index of the gas. The asymptotic flow obtained for ω≥ωc is discussed in a subsequent paper.
The unsteady Stokes resistance of arbitrarily oriented, finite-length cylinders
Authors: Loewenberg M.
Published: July 2002

The unsteady (oscillatory) Stokes resistance is numerically computed for finite-length, circular cylinders using a first-kind, boundary integral formulation for low and moderate dimensionless frequencies. Translational oscillations parallel and perpendicular to the cylinder axis (axial and transverse oscillations) are considered. For arbitrarily oriented cylinders with length-to-width ratios in the range: 1/10≤a/b≤10, a simple, previously proposed, expression accurately (±5%) describes the oscillatory Stokes resistance at all frequencies in terms of steady Stokes and inviscid flow resistance parameters. Numerical calculations for spheroids agree with previous results; the results for cylinders and transverse oscillations of oblate spheroids are new.
The fast kinematic magnetic dynamo and the dissipationless limit
Authors: Finn John M. , Ott Edward
Published: July 2002

This paper investigates the possibility of utilizing the equation for the magnetic field in a perfectly conducting fluid to obtain information on the kinematic magnetic dynamo in the limit where the magnetic Reynolds number Rm (equivalently, the conductivity) approaches infinity. Since the limit Rm→∞ is highly singular, it is not immediately clear that such an approach is possible. Recently, however, it has been proposed that the growth rate for the fastest growing mode at finite Rm, γmax(Rm), has a limit, γ∞=limRm→∞γmax (Rm), which can be obtained directly from the flux through a macroscopic area, using the equation for the magnetic field in a perfectly conducting fluid. The utility of this is that it reduces the problem to one of investigating the chaotic dynamics of the trajectories of fluid elements convected by the flow. Numerical experiments with finite Rm on a simple class of models will be presented. These numerical experiments support the idea that γ∞ can be obtained from the perfect conductivity equation and elucidate the nature of the singular limiting process yielding this result.
Similarity transformations of radiation hydrodynamic equations and investigation on laws of radiative conduction
Authors: Jun Zhang , Wenbing Pei
Published: July 2002

Self-similar solutions for the radiation hydrodynamic equations are discussed under the boundary conditions of constant density, temperature, radiative flux, and pressure. The scaling laws of radiative energy and penetration depth, which are independent of any boundary condition, are given as functions of boundary temperature and density. These scaling laws have the same power exponents of temperature and density as those obtained from only radiation conduction equations and are in good agreement with the numerical simulations and experimental results.
Fast dynamos with finite resistivity in steady flows with stagnation points
Authors: Lau Yun-Tung , Finn John M.
Published: July 2002

Results of the kinematic fast dynamo problem for two classes of steady incompressible flows are presented. These flows are the ABC flow and the spatially aperiodic flow of Lau and Finn [Physica D 57, 238 (1992)]. In a range of parameters, these three-dimensional flows have stagnation points (A and B type) and there are chaotic streamlines. The chaos is associated with the intermingled stable and unstable manifolds of the stagnation points. In the aperiodic flow the chaos takes the form of chaotic scattering. The growth rate of the dynamos for the aperiodic flow is found to obey a certain scaling law with resistivity η (as η→0), from which the results are extrapolated to the limit η→0 (infinite magnetic Reynolds number). Numerical results are presented indicating that fast kinematic dynamos can exist in these flows and that chaotic flow is a necessary condition. The structure of the dynamo magnetic fields is also shown, in particular, the relationship between the regions of maximal field strength and the invariant dynamical structures of the aperiodic flow. For the aperiodic flow, the unstable mode has a real frequency and these regions consist of two fingers of oppositely directed field. These regions rotate about a streamline (the one-dimensional unstable manifold) coming out of the type A stagnation point. For the ABC flow with A=B=C, it is found that there are two dynamo modes: an oscillating mode and a purely growing mode. The mode crossing occurs at magnetic Reynolds number between 300 and 350, with the purely growing mode dominating for larger magnetic Reynolds numbers. For the oscillating mode, the region of large ‖B‖ is similar to that for the aperiodic flow. For the purely growing mode, the region of large ‖B‖ is localized in single fingers about the one-dimensional unstable manifolds. The distribution function of ln‖B‖ is observed to be approximately Gaussian for both modes of the ABC flow. The distribution function for the mode found for the aperiodic flow has a much more complex structure, apparently associated with the escape of streamlines in chaotic scattering.
Gyrokinetic simulations of E×B velocity-shear effects on ion-temperature-gradient modes
Authors: Cohen Bruce I. , Williams Timothy J. , Dimits Andris M. , Byers Jack A.
Published: July 2002

Data from several current tokamak experiments indicate that the equilibrium perpendicular velocity field can become strongly sheared accompanying the transition from the L mode to the H mode, i.e. improved, confinement, and that fluctuation levels are reduced. Linear theory suggests that velocity shear can stabilize ion-temperature-gradient (ITG) modes when the frequency shift experienced by the mode due to the radial dependence of the Doppler shift is comparable to the growth rate. To confirm the predictions of linear theory and to explore nonlinear issues, e.g., self-generated shear flows, saturation amplitudes, and the concomitant energy transport levels, two- and three-dimensional gyrokinetic simulations of ITG modes have been performed. The simulations were done with and without magnetic shear in a slab configuration using the partially linearized (δf) algorithm to reduce statistical noise. The simulations confirm theoretical analyses of the stabilizing and destabilizing effects of imposed perpendicular velocity fields. The ion energy transport levels at saturation follow the trends of the linear growth rates and the mixing-length estimates. The gyrokinetic simulations are in qualitative agreement with the results of gyrofluid simulations, and exhibit saturation amplitudes and energy transport similar to those in gyrofluid simulations. These transport levels are generally lower than those typically reported in the laboratory experiments; including toroidal driving terms significantly increases the transport levels in the simulations.
Dispersion driven instability in miscible displacement in porous media
Authors: Yortsos Y. C. , Zeybek M.
Published: July 2002

The effect of dispersion on the stability of miscible displacement in rectilinear porous media is examined. Following a convection–dispersion (CDE) formalism, the base state of Tan and Homsy [Phys. Fluids 29, 3549 (1986)] at conditions of unfavorable mobility contrast is analyzed. Emphasis is placed on the dependence of the dispersion coefficient on flow rate (e.g., mechanical dispersion). It is found that such a dependence induces a destabilizing contribution at short wavelengths. This effect, which is in contrast to the stabilization commonly associated with dispersion, is highly pronounced near the onset of the displacement. It is also near this onset that, for a certain condition, the cutoff wavenumber becomes infinitely large. An analytical expression is derived for this condition and the origin and implications of the instability are discussed. It is also suggested that the present CDE formulation may be inadequate in providing stability criteria for a range of unstable flows.
The formation of spilling breaking water waves
Authors: Duncan J. H. , Philomin V. , Behres M. , Kimmel J.
Published: July 2002

Photographs from high-speed movies of the profiles of a mechanically generated, gentle spilling breaking water wave are presented. It is found that as the wave steepens a bulge forms on the forward face of the wave near the crest and capillary waves form on the water surface ahead of the ‘toe’ of the bulge (see Fig. 1). The toe of the bulge then moves rapidly down the forward face of the wave and a train of large-amplitude waves with short wavelength grows rapidly on the surface of the bulge. These waves quickly break down into a random pattern indicating that the flow has become turbulent.
Instability of a plane centered rarefaction wave
Authors: Velikovich Alexander , Phillips Lee
Published: July 2002

An analytic small-amplitude theory of the instability of a plane centered rarefaction wave (which has recently been discovered numerically by Yang et al.) is presented. A finite-difference (FCT) calculation is performed and compares well with the theory. The instability manifests itself as perturbation growth on the wave’s trailing edge. The asymptotic value approached by the perturbed velocity of the trailing edge is expressed as kδx0a0u∞(M,γ), where k is the perturbation wave number, δx0 is the constant perturbation amplitude of the leading edge, a0 is the sound speed in the unperturbed gas, and u∞(M,γ) is a dimensionless function that depends on the adiabatic exponent, γ, and the strength of the rarefaction wave, M, taken as the ratio of sound speeds behind and ahead of it. This function is essentially determined by the way the perturbed rarefaction wave is formed, e.g., by moving a corrugated piston from a gas-filled space or by interaction of a plane shock wave with a rippled contact interface between two different gases. The function u∞(M,γ) is shown to vanish in the limiting cases of strong and weak rarefaction waves, its asymptotic form in the latter case (M→1) being u∞(M,γ)=[(γ+1)(1−M)/(γ−1)]1/2. This instability, which, for example, might develop in high-gain nuclear fusion pellets accelerated by laser beams, appears to be relatively harmless for these types of applications, since neither the perturbations of density and pressure at any given value of x/t, nor the integral ∫(δρ)dx taken over the width of the rarefaction wave increase in time.
Lévy stable distributions for velocity and velocity difference in systems of vortex elements
Authors: Min I. A. , Mezić I. , Leonard A.
Published: July 2002

The probability density functions (PDFs) of the velocity and the velocity difference field induced by a distribution of a large number of discrete vortex elements are investigated numerically and analytically. Tails of PDFs of the velocity and velocity difference induced by a single vortex element are found. Treating velocities induced by different vortex elements as independent random variables, PDFs of the velocity and velocity difference induced by all vortex elements are found using limit distribution theorems for stable distributions. Our results generalize and extend the analysis by Takayasu [Prog. Theor. Phys. 72, 471 (1984)]. In particular, we are able to treat general distributions of vorticity, and obtain results for velocity differences and velocity derivatives of arbitrary order. The PDF for velocity differences of a system of singular vortex elements is shown to be Cauchy in the case of small separation r, both in 2 and 3 dimensions. A similar type of analysis is also applied to non-singular vortex blobs. We perform numerical simulations of the system of vortex elements in two dimensions, and find that the results compare favorably with the theory based on the independence assumption. These results are related to the experimental and numerical measurements of velocity and velocity difference statistics in the literature. In particular, the appearance of the Cauchy distribution for the velocity difference can be used to explain the experimental observations of Tong and Goldburg [Phys. Lett. A 127, 147 (1988); Phys. Rev. A 37, 2125, (1988); Phys. Fluids 31, 2841 (1988)] for turbulent flows. In addition, for intermediate values of the separation distance, near exponential tails are found.
Weakly nonlocal gravity–capillary solitary waves
Authors: Yang T.-S. , Akylas T.R.
Published: July 2002

There is evidence from previous analytical work based on a model equation and from numerical computations that gravity–capillary solitary waves in a liquid layer are nonlocal—they feature oscillatory tails of constant amplitude—when the Bond number τ is less than 1/3. Here, the full gravity–capillary wave problem is examined and these tails are calculated asymptotically in the weakly nonlinear regime. For given values of τ (0<τ<1/3) and Froude number F slightly greater than 1, there exists a one-parameter family of weakly nonlocal solitary waves characterized by the phase shift of the tails relative to the main peak. The tail amplitude depends on the phase shift and is exponentially small with respect to the wave peak amplitude. Predictions of the asymptotic theory are confirmed by numerical computations using a spectral method.
An illustration of the link between ribs and hyperbolic instability
Authors: Leblanc Stéphane , Godeferd Fabien S.
Published: July 2002

The hyperbolic instability in planar Taylor–Green cells is investigated by direct numerical simulations at Reynolds number 400. The mechanism of this three-dimensional instability is a stretching of the vorticity perturbation along the principal axis of strain in accordance with well-known results of rapid distortion theory and stability analysis, leading to the formation of spanwise counter-rotating vortices (or ribs) in the irrotational hyperbolic stagnation point regions, as described by Kerr and Dold [J. Fluid Mech. 276, 307 (1994)].
Excitation of electron Bernstein and ion Bernstein waves by extraordinary electromagnetic pump: Kinetic theory
Authors: Sharma R. P. , Kumar Atul , Kumar Raj , Tripathi Y. K.
Published: July 2002

In the present paper, parametric decay instability of an extraordinary electromagnetic (EM) wave into a high-frequency electron Bernstein wave and a low-frequency ion Bernstein wave is studied. The guiding center formalism is followed to obtain the coupling coefficients including kinetic effects. The momentum space integrals in the coupling coefficients are solved in the dipole approximation of the pump wave and in the limits, k⊥ρe<1 for electron Bernstein wave and k⊥ρi<1 for ion Bernstein wave. Analytical expressions for homogeneous growth rate and threshold are given explicitly. Applications of the present investigation are pointed out to ionospheric modification experiment and fusion plasmas.
Laser-induced fluorescence measurement of the dynamics of a pulsed planar sheath
Authors: Goeckner M. J. , Malik Shamim M. , Conrad J. R. , Breun R. A.
Published: July 2002

Using laser-induced fluorescence (LIF) the ion density near the edge of an expanding plasma sheath has been measured. These measurements utilized a transition of N+2 [the P12 component of the X 2Σ+g(ν=0)→B 2Σ+u(ν=0) band] in a N2 plasma. The strength of the laser-induced fluorescence was used as a measure of the temporally and spatially varying ion density. The expanding sheath was produced by applying a −5 kV pulse to a polished planar electrode in the plasma source ion implantation device [J. R. Conrad et al., J. Vac. Sci. Technol. A 8, 3146 (1990)]. The laser beam was aligned normal to the surface and was reflected off the center of the electrode. The LIF diagnostic used here is nonperturbing whereas previous researchers have used Langmuir probes, which perturb the plasma, to make their measurements. As such, the data reported here represent a benchmark measurement of pulsed sheaths and allow a better comparison between experimental measurements and theoretical predictions. It has been found that the sheath edge moves approximately 16 times faster than the ion-acoustic velocity during the early part of the pulse, t<1 μs, and then slows to approximately the ion-acoustic velocity after 6 μs. In addition to the LIF measurements, a biased probe was used far from the cathode to determine the sheath edge location. Good agreement is found when the LIF and probe data are compared. The LIF data also are compared to the predictions of a simulation that is based on a time-varying two-fluid model of the sheath [G. A. Emmert and M. A. Henry, J. Appl. Phys. 71, 113 (1992)]. While the predictions of the model show moderate agreement with the data, substantial discrepancies are observed. These discrepancies are attributed to a number of physical phenomena that are not included in the present model.