bluuebunny/crossref_metadata_2025 huggingface dataset:
Femtosecond photoassociation: Coherence and implications for control in bimolecular reactions
Authors: Gross Peter , Dantus Marcos
Published: July 2002

A theoretical analysis of the recent femtosecond photoassociation spectroscopy (FPAS) experiment on mercury [U. Marvet and M. Dantus, Chem. Phys. Lett. 245, 393 (1995)] is presented. It is shown that when a thermal distribution of diatom collision pairs is excited from a free to a bound electronic state on a time scale shorter than molecular vibration, an ensemble of coherent wave packets is produced. The dynamics of these wave packets created by the photoassociation pulse can be observed by firing a second probe pulse at variable time delays, and the depletion of the first excited bound state by the probe pulse is detected via fluorescence of the remaining population. Simulations of the FPAS experiment, using both wave packet propagation techniques and perturbation theory, clearly show the vibrational dynamics of the photoassociated transients. It is also demonstrated how the FPAS technique may be used as a tool for controlling the energy, impact parameter, and orientation in bimolecular reactions.
Modeling self-contact forces in the elastic theory of DNA supercoiling
Authors: Westcott Timothy P. , Tobias Irwin , Olson Wilma K.
Published: July 2002

A DNA polymer with thousands of base pairs is modeled as an elastic rod with the capability of treating each base pair independently. Elastic theory is used to develop a model of the double helix which incorporates intrinsic curvature as well as inhomogeneities in the bending, twisting, and stretching along the length of the polymer. Inhomogeneities in the elastic constants can also be dealt with; thus, sequence-dependent structure and deformability can be taken into account. Additionally, external forces have been included in the formalism, and since these forces can contain a repulsive force, DNA self-contact can be explicitly treated. Here the repulsive term takes the form of a modified Debye–Hückel force where screening can be varied to account for the effect of added salt. The supercoiling of a naturally straight, isotropic rod in 0.1M NaCl is investigated and compared with earlier treatments of supercoiled DNA modeled by a line of point charges subject to electrostatic interactions and an elastic potential.
Correlated calculations of molecular dynamic polarizabilities
Authors: Rozyczko Piotr B. , Perera S. Ajith , Nooijen Marcel , Bartlett Rodney J.
Published: July 2002

Frequency-dependent molecular polarizabilities of several molecules N2, CO, CO2, Cl2, C2H2, COS, and CS2 are calculated by the equation-of-motion coupled cluster singles and doubles (EOM-CCSD) method. The EOM-CCSD CI-like, linear and quadratic methods for dynamic second-order properties are presented. The importance of electron correlation, the quadratic contribution, and orbital relaxation effects are assessed. London dispersion coefficients are calculated by numerical integration of the EOM-CCSD polarizabilities.
Laser-induced emission spectroscopy of matrix-isolated carbon molecules: Experimental setup and new results on C3
Authors: Čermák Ivo , Förderer Markus , Čermáková Iva , Kalhofer Stefan , Stopka-Ebeler Helmut , Monninger Gerold , Krätschmer Wolfgang
Published: July 2002

We have studied small carbon molecules using a matrix-isolation technique. Our experimental setup is described in detail. The carbon clusters were produced by evaporating graphite and trapping the carbon-vapor molecules in solid argon, where molecular growth could be induced by controlled matrix annealing. To identify the produced molecules, absorption spectroscopy in the ultraviolet (UV)-visible and infrared (IR) spectral ranges was applied. Additional characterization of the excited and ground states of the molecules was obtained from emission and excitation spectra. The molecules were excited by a pulsed dye laser system and the emission spectra were recorded with a high-sensitivity photodiode-array spectrometer. We present our measurements on linear C3. The Ã 1Πu excited state of linear C3 was populated by the electronic transition Ã 1Πu←X̃ 1Σg+, and the corresponding excitation spectra of the C3 fluorescence (Ã 1Πu→X̃ 1Σg+) and phosphorescence (ã 3Πu→X̃ 1Σg+) were studied. Comparison of excitation and absorption spectra yielded information on site effects due to the matrix environment. Emission bands in the fluorescence and phosphorescence spectra up to vibrational energies of 8500 cm−1 could be observed. The radiation lifetime of the Ã 1Πu excited state of C3 in solid argon was found to be shorter than 10 ns. The phosphorescence transition ã 3Πu→X̃ 1Σg+ decays in about 10 ms and its rise indicates fast vibrational relaxation within the triplet system. Our data support a linear ground state geometry for C3 also in solid argon.
Femtosecond dynamics of photoinduced molecular detachment from halogenated alkanes. I. Transition state dynamics and product channel coherence
Authors: Marvet Una , Zhang Qingguo , Brown Emily J. , Dantus Marcos
Published: July 2002

The direct observation of the photoinduced molecular detachment of halogens X2 from halogenated alkanes RCHX2 is presented. Three-photon excitation at 312 nm produces molecular halogens and a carbene; the halogen products are formed predominantly in the D′ state. Femtosecond pump–probe spectroscopy of the reaction reveals a fast (τ<50 fs) dissociation with no evidence of intramolecular vibrational redistribution. This is consistent with a prompt dissociation without intermediates. The experimental results demonstrate vibrational coherence in the halogen product, which requires that the reaction proceed by a concerted mechanism.
High-resolution diode laser study of H2–H2O van der Waals complexes: H2O as proton acceptor and the role of large amplitude motion
Authors: Weida Miles J. , Nesbitt David J.
Published: July 2002

Three high-resolution rovibrational bands of ortho H2 with both para and ortho H2O are observed in a slit supersonic expansion, based on direct absorption of a tunable diode laser in the ν2 bend region of H2O near 1600 cm−1. Complexes containing para H2O are responsible for a Σ←Σ type band associated with intramolecular bending excitation of H2O, while complexes containing ortho H2O exhibit two bands associated with (i) the intramolecular HOH bend (Π←Π) and (ii) an inter+intramolecular combination band (Σ←Π) corresponding to simultaneous HOH bend plus internal rotor excitation. From high-resolution line broadening studies, each upper state has a different vibrational predissociation lifetime; for bend excited para H2O complexes it is 5.1(14) nsec, while for the bend excited state and bend+internal rotor combination state of ortho H2O, it is 2.53(14) and 1.85(33) nsec, respectively. Analysis of the spectra supplemented by 2D quantum calculations indicate large amplitude, slightly hindered internal rotation of the H2O subunit in the complex. Nevertheless, the internal rotor splittings yield potential parameters that suggest ortho H2–H2O is best described with the H2 predominantly pointing towards the O atom in a H2O proton acceptor geometry.
Theoretical study of the electronic spectrum of magnesium-porphyrin
Authors: Rubio Mercedes , Roos Björn O. , Serrano-Andrés Luis , Merchán Manuela
Published: July 2002

Multiconfigurational self-consistent field (SCF) and second order perturbation methods have been used to study the electronic spectrum of magnesium-porphyrin (MgP). An extended ANO-type basis set including polarization functions on all heavy atoms has been used. Four allowed singlet states of E1u symmetry have been computed and in addition a number of forbidden transitions and a few triplet states. The results lead to a consistent interpretation of the electronic spectrum, where the Q band contains one transition, the B band two, and the N band one. The computed transition energies are consistently between 0.1 and 0.5 too low compared to the measured band maxima. The source of the discrepancy is the approximate treatment of dynamic correlation (second order perturbation theory), limitations in the basis set and the fact that all measurements have been made on substituted magnesium porphyrins.
Finite field treatment of vibrational polarizabilities and hyperpolarizabilities: On the role of the Eckart conditions, their implementation, and their use in characterizing key vibrations
Authors: Luis Josep M. , Duran Miquel , Andrés Jose L. , Champagne Benoı̂t , Kirtman Bernard
Published: July 2002

In the finite field (FF) treatment of vibrational polarizabilities and hyperpolarizabilities, the field-free Eckart conditions must be enforced in order to prevent molecular reorientation during geometry optimization. These conditions are implemented for the first time. Our procedure facilities identification of field-induced internal coordinates that make the major contribution to the vibrational properties. Using only two of these coordinates, quantitative accuracy for nuclear relaxation polarizabilities and hyperpolarizabilities is achieved in π-conjugated systems. From these two coordinates a single most efficient natural conjugation coordinate (NCC) can be extracted. The limitations of this one coordinate approach are discussed. It is shown that the Eckart conditions can lead to an isotope effect that is comparable to the isotope effect on zero-point vibrational averaging, but with a different mass-dependence.
Two-order-parameter description of liquids. I. A general model of glass transition covering its strong to fragile limit
Authors: Tanaka Hajime
Published: July 2002

Here we propose a simple physical model that may universally describe glass-transition phenomena from the strong to the fragile limit. Our model is based on the idea that there always exist two competing orderings in any liquids, (i) density ordering leading to crystallization and (ii) bond ordering favoring a local symmetry that is usually not consistent with the crystallographic symmetry. The former tries to maximize local density, while the latter tries to maximize the quality of bonds with neighboring molecules. For the phenomenological description of these competing ordering effects [(i) and (ii)] hidden in many-body interactions, we introduce density and bond order parameters, respectively. This leads to the following picture of a liquid structure: Locally favored structures with finite, but long lifetimes are randomly distributed in a sea of normal-liquid structures. Even simple liquids suffer from random disorder effects of thermodynamic origin. We argue that locally favored structures act as impurities and produce the effects of “fluctuating interactions” and “symmetry-breaking random field” against density ordering, in much the same way as magnetic impurities for magnetic ordering in spin systems. Similarly to random-spin systems, thus, we predict the existence of two key temperatures relevant to glass transition, the density ordering (crystallization) point Tm* of the corresponding pure system without frustration and the Vogel–Fulcher temperature T0. Glass transition is then characterized by these two transitions: (A) a transition from an ordinary-liquid state to a Griffiths-phase-like state at Tm*, which is characterized by the appearance of high-density metastable islands with medium-range order, and (B) another transition into a spin-glass-like nonergodic state at T0 and the resulting divergence of the lifetime of metastable islands, namely, the α relaxation time. Between Tm* and T0, a system has a complex free-energy landscape characteristic of the Griffiths-phase-like state, which leads to the non-Arrhenius behavior of α relaxation and dynamic heterogeneity below Tm*. This simple physical picture provides us with a universal view of glass transition covering the strong to fragile limit. For example, our model predicts that stronger random-disorder effects make a liquid “stronger,” or “less fragile.”
Aligning molecules with intense nonresonant laser fields
Authors: Larsen Jakob Juul , Sakai Hirofumi , Safvan C. P. , Wendt-Larsen Ida , Stapelfeldt Henrik
Published: July 2002

Molecules in a seeded supersonic beam are aligned by the interaction between an intense nonresonant linearly polarized laser field and the molecular polarizability. We demonstrate the general applicability of the scheme by aligning I2, ICl, CS2, CH3I, and C6H5I molecules. The alignment is probed by mass selective two dimensional imaging of the photofragment ions produced by femtosecond laser pulses. Calculations on the degree of alignment of I2 are in good agreement with the experiments. We discuss some future applications of laser aligned molecules.
Relativistic four-component calculations of indirect nuclear spin–spin couplings in MH4 (M=C, Si, Ge, Sn, Pb) and Pb(CH3)3H
Authors: Enevoldsen Thomas , Visscher Lucas , Saue Trond , Jensen Hans Jørgen Aagaard , Oddershede Jens
Published: July 2002

Relativistic four-component random phase approximation (RPA) calculations of indirect nuclear spin–spin coupling constants in MH4 (M=C, Si, Ge, Sn, Pb) and Pb(CH3)3H are presented. The need for tight s-functions also in relativistic four-component calculations is verified and explained, and the effect of omission of (SS–LL) and (SS–SS) two-electron integrals is investigated. Already in GeH4 we see a relativistic increase in the coupling constant by 12%, and for PbH4 the effect is a 156% increase for the one-bond coupling. Large relativistic effects are also computed for the two-bonds couplings. We find that the relativistic effects on the one-bond couplings are mainly due to scalar relativistic factors rather than spin–orbit corrections.
Enhanced saturation coverages in adsorption–desorption processes
Authors: Van Tassel Paul R. , Viot Pascal , Tarjus Gilles , Ramsden Jeremy J. , Talbot Julian
Published: July 2002

Many experimental studies of protein deposition on solid surfaces involve alternating adsorption/desorption steps. In this paper, we investigate the effect of a desorption step (separating two adsorption steps) on the kinetics, the adsorbed-layer structure, and the saturation density. Our theoretical approach involves a density expansion of the pair distribution function and an application of an interpolation formula to estimate the saturation density as a function of the density at which the desorption process commences, ρ1, and the density of the depleted configuration, ρ2. The theory predicts an enhancement of the saturation density compared with that of a simple, uninterrupted random sequential adsorption (RSA) process and a maximum in the saturation density when ρ2=(2/3)ρ1. The theoretical results are in qualitative and semiquantitative agreement with the results of numerical simulations.
Structure of the equiatomic liquid alloys K–Sb, K–Bi, and Rb–Bi over a wide temperature range
Authors: Hochgesand K. , Winter R.
Published: July 2002

We report on neutron diffraction measurements on the equiatomic liquid alloys K–Sb, K–Bi, and Rb–Bi up to temperatures of T=1800 K. The prepeaks in the resulting total structure factors S(Q) are indicative of intermediate range ordering. The shift of the prepeak positions towards smaller momentum transfer Q with increasing temperature suggests increasing distances between the structural polyanionic units. From reverse Monte Carlo (RMC) simulations the prepeak is assigned essentially to correlations between the polyvalent metal atoms (Sb or Bi), remaining even up to the highest temperatures measured. The simulations lead to models exhibiting Sb (or Bi) atoms clustered in short chains and dumbbells rather than in higher coordinated clusters or networklike structures.
Dirac’s theory of constraints in field theory and the canonical form of Hamiltonian differential operators
Authors: Olver Peter J.
Published: July 2002

A simple algorithm for constructing the canonical form of Hamiltonian systems of evolution equations with constant coefficient Hamiltonian differential operators is given. The result of the construction is equivalent to the canonical system derived using Dirac’s theory of constraints from the corresponding degenerate Lagrangian.
Hamiltonian structures for systems of hyperbolic conservation laws
Authors: Olver Peter J. , Nutku Yavuz
Published: July 2002

The bi-Hamiltonian structure for a large class of one-dimensional hyberbolic systems of conservation laws in two field variables, including the equations of gas dynamics, shallow water waves, one-dimensional elastic media, and the Born–Infeld equation from nonlinear electrodynamics, is exhibited. For polytropic gas dynamics, these results lead to a quadri-Hamiltonian structure. New higher-order entropy-flux pairs (conservation laws) and higher-order symmetries are exhibited.
Relativistic potential scattering and phase shift analysis
Authors: Arshansky R. , Horwitz L. P.
Published: July 2002

It is shown that two-body relativistic scattering cross sections can be represented in terms of phase shift analysis in essentially the same form as that of nonrelativistic scattering theory. The representation is covariant; the variable that corresponds to the orbital quantum number (and approaches it in the nonrelativistic limit) is relativistically invariant. The Levinson theorem is valid, and provides a link between the bound states, which have support in an O(2,1) invariant subspace of the full spacelike region of relative coordinates, and the scattering states that contain resonant behavior. These scattering states consequently have support in the same restricted subspace, and the same procedure may be used for the separation of variables. As an example, the resonances of an O(3,1) invariant ‘‘square well’’ are discussed.
Quantization of the classical SL(2,R) system and representations of SL(2,R) group
Authors: Plyushchay Mikhail S.
Published: July 2002

Quantization of the classical SL(2,R) system, described by (2+1)-dimensional vector Jμ, J2=const, with brackets {Jμ,Jν}=−εμνλJλ, is carried out. As a result, unitary irreducible representations of the type of discrete series Dα±, α≳0, and continuous principal series Cσϑ, σ≥1/4, ϑ∈[0,1) of the universal covering group of SL(2,R) are constructed. Different realizations of these representations, in which the operators Ĵ0, Ĵ2, or Ĵ0±Ĵ1 are diagonal are seen.
Orbital stability of solitary waves of Zakharov system
Authors: Wu Yaping
Published: July 2002

This article is concerned with the orbital stability of solitary waves of the Zakharov system. By applying the abstract results of M. Grillakis et al. [J. Funct. Anal. 74, 160–197 (1987); 94, 308–348 (1990)] and detailed spectral analysis, the stability of the solitary waves are obtained.
A chiral spin theory in the framework of an invariant evolution parameter formalism
Authors: Sarel B. , Horwitz L. P.
Published: July 2002

We present a formulation for the construction of first-order equations which describe particles with spin, in the context of a manifestly covariant relativistic theory governed by an invariant evolution parameter; one obtains a consistent quantized formalism dealing with off-shell particles with spin. Our basic requirement is that the second-order equation in the theory is of the Schrödinger–Stueckelberg type, which exhibits features of both the Klein–Gordon and Schrödinger equations. This requirement restricts the structure of the first-order equation, in particular, to a chiral form. One thus obtains, in a natural way, a theory of chiral form for massive particles, which may contain both left and right chiralities, or just one of them. We observe that by iterating the first-order system, we are able to obtain second-order forms containing the transverse and longitudinal momentum relative to a timelike vector tμtμ=−1 used to maintain covariance of the theory. This timelike vector coincides with the one used by Horwitz, Piron, and Reuse to obtain an invariant positive definite space–time scalar product, which permits the construction of an induced representation for states of a particle with spin. We discuss the currents and continuity equations. The transverse and longitudinal aspects of the particle are complementary, and can be treated in a unified manner using a tensor product Hilbert space. Introducing the electromagnetic field we find an equation which gives rise to the correct gyromagnetic ratio, and is fully Hermitian under the proposed scalar product. Finally, we show that the original structure of Dirac’s equation and its solutions is obtained in the highly constrained limit in which pμ is proportional to tμ on mass shell. The chiral nature of the theory is apparent. We define the discrete symmetries of the theory, and find that they are represented by states which are pure left or right handed.
Decomposition of time-ordered products and path-ordered exponentials
Authors: Lam C. S.
Published: July 2002

We present a decomposition formula for Un, an integral of time-ordered products of operators, in terms of sums of products of the more primitive quantities Cm, which are the integrals of time-ordered commutators of the same operators. The resulting factorization enables a summation over n to be carried out to yield an explicit expression for the time-ordered exponential, an expression which turns out to be an exponential function of Cm. The Campbell–Baker–Hausdorff formula and the non-Abelian eikonal formula obtained previously are both special cases of this result.
Discrete structures in gravity
Authors: Regge Tullio , Williams Ruth M.
Published: July 2002

Discrete approaches to gravity, both classical and quantum, are reviewed briefly, with emphasis on the method using piecewise-linear spaces. Models of three-dimensional quantum gravity involving 6j-symbols are then described, and progress in generalizing these models to four dimensions is discussed, as is the relationship of these models in both three and four dimensions to topological theories. Finally, the repercussions of the generalizations are explored for the original formulation of discrete gravity using edge-length variables.
Magneto-optical spectroscopy of d- and f-ferromagnetic materials: recent theoretical progress (Review Article)
Authors: Antonov V. N. , Yaresko A. N. , Perlov A. Ya. , Nemoshkalenko V. V. , Oppeneer P. M. , Eschrig H.
Published: July 2002

The current status of theoretical understanding of the optical and magneto-optical (MO) spectra of 3d, 4f and 5f compounds is reviewed. Energy band theory based upon the local spin-density approximation (LSDA) describes the optical and MO spectra of transition metal compounds reasonably well. Examples which we examine in detail are XPt3 compounds (with X=3d V, Cr, Mn, Fe, and Co) in the AuCu3 structure, ternary Heusler alloys NiMnSb, PdMnSb, PtMnSb, and MnBi compound. The LSDA, which is capable of describing the spectra of transition-metal alloys with high accuracy, does not suffice for lanthanide compounds having a correlated 4f shell. A satisfactory description of the optical spectra could be obtained by using a generalization of the LSDA, in which explicitly f electron Coulomb correlations are taken into account (LSDA+U approach). As examples of this group we consider CeSb and CeBi. For CeSb a record Kerr angle of 90° was very recently reported, 90° is the absolute maximum value that can be measured. It is two orders of magnitude larger than the values that are commonly measured for transition-metal compounds, and about one order of magnitude larger than values maximally achieved for other lanthanide and actinide compounds. A third group consist of uranium 5f compounds. In those compounds where the 5f electrons are rather delocalized, the LSDA describes the MO spectra reasonably well. As examples of this group we consider UAsSe and URhAl. Particular difficulties occur for the uranium compounds where the 5f electrons are neither delocalized nor localized, but more or less semilocalized. Typical examples are US, USe and UTe. The semilocalized 5f’s are, however, not inert, but their interaction with conduction electrons plays an important role. Recently achieved improvements for describing such compounds are discussed.
Approximation of turbulent conditional averages by stochastic estimation
Authors: Adrian R. J. , Jones B. G. , Chung M. K. , Hassan Yassin , Nithianandan C. K. , Tung A. T.-C.
Published: July 2002

Conditional averages of turbulent flow quantities can be approximated in terms of unconditional correlation data by means of stochastic estimation. The validity and accuracy of this procedure are investigated by comparing stochastic estimates to conditional averages measured in four turbulent flows: grid turbulence, the axisymmetric shear layer of a round jet, a plane shear layer, and pipe flow. Comparisons are made for quantities that are separated from the conditional data in time or space, and for turbulent pressures, as well as turbulent velocities. In each case, the linear estimate accurately represents large scale structure. Nonlinear quadratic estimation shows little improvement over linear estimation, because the second-order terms are small for probable values of the turbulent fluctuations.
Measurement of the drift of a droplet due to the presence of a plane
Authors: Smart Jeffrey R. , Leighton David T.
Published: July 2002

The drift of a deformable droplet of low viscosity (viscosity ratio λ=0.08) in a Couette device is examined. The drift is measured both in the plane of shear (due to the rigid outer bounding walls of the Couette device) and also normal to the plane of shear (due to the upper bounding stress-free surface). A general relationship between normal stresses induced by the deformation of a droplet in an arbitrary shear flow and the leading-order drift normal to rigid and stress-free plane surfaces is described theoretically. This relationship is consistent with previous theoretical predictions for droplet migration in shear flows, and is used to compare results from the drift measurement experiments with first-order deformation theories. The measured drift velocities are in reasonable agreement with the theory of Schowalter et al. [J. Colloid Interface Sci. 26, 152 (1968)].
On testing models for the pressure–strain correlation of turbulence using direct simulations
Authors: Speziale Charles G. , Gatski Thomas B. , Sarkar Sutanu
Published: July 2002

Direct simulations of homogeneous turbulence have, in recent years, come into widespread use for the evaluation of models for the pressure–strain correlation of turbulence. While work in this area has been beneficial, the increasingly common practice of testing the slow and rapid parts of these models separately in uniformly strained turbulent flows is shown in this paper to be unsound. For such flows, the decomposition of models for the total pressure–strain correlation into slow and rapid parts is ambiguous. Consequently, when tested in this manner, misleading conclusions can be drawn about the performance of pressure–strain models. This point is amplified by illustrative calculations of homogeneous shear flow where other pitfalls in the evaluation of models are also uncovered. More meaningful measures for testing the performance of pressure–strain models in uniformly strained turbulent flows are proposed and the implications for turbulence modeling are discussed.
A novel way of computing the Basset term in unsteady multiphase flow computations
Authors: Michaelides Efstathios E.
Published: July 2002

The inclusion of the Basset (history) term in the equation of motion of a sphere in creeping flow increases substantially the computational time and memory requirements for a numerical solution of the trajectory of the sphere. A transformation of the equation of motion of a sphere in a fluid is obtained, in which the velocity of the sphere does not appear in the history integral. Although the transformed equation is a second-order ordinary differential equation (ODE), it is explicit in the dependent variable and presents considerable advantages in reduced computational time and memory requirements.
Molecular dynamics of interface rupture
Authors: Koplik Joel , Banavar Jayanth R.
Published: July 2002

Several situations have been studied in which a fluid–vapor or fluid–fluid interface ruptures, using molecular dynamics simulations of 3000 to 20 000 Lennard-Jones molecules in three dimensions. The cases studied are the Rayleigh instability of a liquid thread, the burst of a liquid drop immersed in a second liquid undergoing shear, and the rupture of a liquid sheet in an extensional flow. The late stages of the rupture process involve the gradual withdrawal of molecules from a thinning neck, or the appearance and growth of holes in a sheet. In all cases, it is found that despite the small size of the systems studied, tens of angstroms, the dynamics is in at least qualitative accord with the behavior expected from continuum calculations, and in some cases the agreement is to within tens of percent. Remarkably, this agreement occurs even though the Eulerian velocity and stress fields are essentially unmeasurable—dominated by thermal noise. The limitations and prospects for such molecular simulation techniques are assessed.
Expansion of shear flow and vortex merger in a two-dimensional turbulent mixing layer
Authors: Chiueh Tzihong
Published: July 2002

A theoretical model is presented to describe the shear flow evolution via interplay of vortex pairing, unstable collective excitation, and turbulent diffusion. An integrable nonlinear ordinary differential equation governing the mean flow-turbulence evolution is derived. The equation does not depend on the turbulence amplitude, and thus allows for a relatively accurate determination of the flow evolution. The predicted flow expansion rate agrees well with numerical simulations [J. Fluid Mech. 100, 705 (1980)] and experiments [Phys. Fluids 22, 1011 (1979)] but differs significantly from what would be predicted using the quasilinear theory. Implications to the containment of electron plasmas are addressed.
Flow: A two-dimensional time-dependent hydrodynamical ion extraction model
Authors: Vitello P. , Cerjan C. , Braun D.
Published: July 2002

The simulation of ion extraction from a quasineutral plasma is described using a two-dimensional time-dependent numerical fluid equation code, flow. Ion motion is treated using the cold ion continuity and momentum conservation equations in the potential field of complex structures. An upwind differencing scheme in Cartesian coordinates is used to solve these fluid equations. The plasma electrons are modeled as having a Maxwell–Boltzmann distribution, which allows for ion acoustic effects inside the plasma. The growth and evolution of two-dimensional ion sheaths is treated while evaluating the ion flux collected on the extractor surface. It is found that spatial grids very much coarser than the Debye length (of order ten times larger or greater) can be successfully used to accurately model ion extraction. Although the transition region between the high density quasi-neutral plasma and the ion sheath is not resolved, the ion flux across this region is still obtained correctly. The assumption of a Maxwell–Boltzmann distribution for the electrons requires the solution of a highly nonlinear Poisson equation. Previous approaches to this problem have relied upon a Newton–Raphson root search with standard iterative techniques for the resulting linearized equations. A new methodology is described here using the recently developed strongly implicit procedure (SIP), which has been optimized for the discretized two-dimensional Laplace operator.
Steep gravity waves: Havelock’s method revisited
Authors: Vanden-Broeck Jean-Marc
Published: July 2002

Gravity waves propagating at the surface of a fluid of infinite depth are considered. The problem is formulated in terms of a series expansion attributable to Havelock [Proc. R. Soc. London Ser. A 95, 38 (1919)]. The series is truncated after a finite number of terms and the unknown coefficients are found by collocation. It is shown that this simple numerical procedure yields accurate results for waves of arbitrary steepness.
Numerical simulation of a two-dimensional turbulence experiment in magnetohydrodynamics
Authors: Verron Jacques , Sommeria Joël
Published: July 2002

Numerical simulations of a two-dimensional turbulence experiment within a confined domain are presented. The setup consists of a layer of mercury enclosed in a square box and driven by the injection of electric currents in a uniform magnetic field. The numerical finite difference model simulates the Navier–Stokes equation in two dimensions with steady forcing and linear bottom friction. Subgrid scales are modeled by an artificial constant viscosity or a higher order (biharmonic) lateral friction. The model provides an accurate representation of the (inverse) transfers of energy from the forcing scale toward the size of the domain. These transfers lead to the spontaneous appearance of a mean rotating flow, when dissipation is sufficiently weak. Thin vorticity sheets are shown to have an important role for the dynamics of the large scales. Therefore a critical spatial resolution, increasing with the bottom friction time scale, is required to get an accurate calculation. The requirement of a high resolution could not be avoided by means of the subgrid scale dissipation. Furthermore, for some low values of the dissipation, a second flow regime, without laboratory counterpart, is obtained.
Two-way coupling in shear layers with dilute bubble concentrations
Authors: Ruetsch G. R. , Meiburg E.
Published: July 2002

Direct numerical simulations are used to analyze the evolution of a temporally growing two-dimensional shear layer seeded with dilute concentrations of bubbles under gravity. The bubble concentrations are dilute enough so that bubble–bubble interactions can be neglected, but are large enough for cumulative effects of bubbles to influence the flow. The evolution of the bubble field is determined by tracking many individual bubbles, and the flow field is advanced by using the Navier–Stokes equations with a coupling term representing the effect of the bubbles on the flow. The results are interpreted in terms of the vorticity, density, and pressure fields relative to the one-way coupled or passive case. For the coupled case, a reduction in the magnitude of the vorticity and pressure gradients near the vortex center is observed. In addition to modification of the flow, it is observed that the accumulation of bubbles is smaller and the location of the equilibrium points are shifted farther from the vortex center as a result of the coupling. It is explored how these changes are modified by different Froude numbers and bubble sizes. The differences between passive and coupled cases usually increase due to larger accumulations as larger bubbles are considered. However, for certain Froude numbers an optimum coupling is observed at intermediate bubble sizes due to the absence of equilibrium points for large bubbles.
Numerical study of strongly nonlinear acoustic waves, shock waves, and streaming caused by a harmonically pulsating sphere
Authors: Yano Takeru , Inoue Yoshinori
Published: July 2002

The nonlinear propagation of spherical waves emitted from a sphere executing harmonic pulsations in an unbounded ideal gas is numerically studied without the restriction of small amplitude, namely, the weak nonlinearity. The energy dissipation due to viscosity and thermal conductivity is supposed to be negligible everywhere except for the discontinuous shock front. By the numerical analysis based on a high-resolution upwind finite difference scheme, the wave phenomena caused by the strongly nonlinear effect are revealed, which form several striking contrasts to the known results for the weakly nonlinear spherical waves. The profile of the emitted wave is rapidly distorted by the strongly nonlinear effect and shock waves are formed near the sphere. The most remarkable phenomenon is the occurrence of acoustic streaming (mean mass outflow), which gradually reduces the density of the gas near the sphere as time proceeds. In the case that the sphere pulsates with a small amplitude compared with its mean radius, beyond the shock formation distance, the wave profile is immediately transformed into an asymmetrical sawtooth-like profile. In the case that the amplitude of pulsation is comparable with the mean radius, two shocks can be formed in each wave cycle and in due course these two shocks merge into one shock.
Direct numerical simulations of bubble-laden turbulent flows using the two-fluid formulation
Authors: Druzhinin O. A. , Elghobashi S.
Published: July 2002

Direct numerical simulations (DNS) of bubble-laden isotropic decaying turbulence are performed using the two-fluid approach (TF) instead of the Eulerian–Lagrangian approach (EL). The motivation for the study is that EL requires considerable computational resources, especially for the case of two-way coupling, where the instantaneous trajectories of a large number of individual bubbles need to be computed. The TF formulation is developed by spatially averaging the instantaneous equations of the carrier flow and bubble phase over a scale of the order of the Kolmogorov length scale, which, in our case, is much larger than the bubble diameter. On that scale, the bubbles are treated as a continuum (without molecular diffusivity) characterized by the bubble phase velocity field and concentration (volume fraction). The bubble concentration, C, is assumed small enough (C⩽10−3) to neglect the bubble–bubble interactions. As a test case, direct simulation of a bubble-laden Taylor–Green vortex with one-way coupling is performed with a bubble response time of the order of the flow time scale (inverse of the mean vorticity). This simple flow allows a direct examination of the effects of the preferential accumulation of bubbles in the high-enstrophy regions of the flow on the accuracy of the two-fluid formulation. The temporal development of the maximum bubble concentration obtained from DNS agrees well with the analytical solution. DNS of the bubble-laden decaying turbulence are also performed for both cases of one-way and two-way coupling. Here, the bubble diameter and response time are much smaller than the Kolmogorov length and time scales, respectively. In this case, as expected, the effects of the preferential accumulation of the bubbles are not pronounced. The results also show that the bubble-laden flow is analogous to a stratified flow with an effective density =(1−C)ρf. Thus, due to the two-way interaction between the bubbles and carrier flow, the turbulence decay is enhanced with stable stratification, and reduced with unstable stratification.
Fast reconnection in high temperature plasmas
Authors: Kleva Robert G. , Drake J. F. , Waelbroeck F. L.
Published: July 2002

Pressure forces acting on electrons are shown to dramatically alter magnetic field line reconnection in high temperature plasmas. The electron pressure introduces a new physical scale length ρs, the ion gyroradius based on the electron temperature, into the resistive magnetohydrodynamic (MHD) equations. The single dissipation layer of resistive MHD is split into two distinct layers by this effect: a very small inner current layer and a larger flow layer. Unlike resistive MHD, the current layer is microscopic in the outflow, as well as the inflow, direction. As a consequence, the current layer is not unstable to the formation of secondary magnetic islands at low values of resistivity and patchy reconnection does not occur. The absence of a strong current sheet in the outflow region enables the magnetic nozzle controlling the outflow to open up. The magnetic reconnection rate therefore remains large as the resistivity η and ρs become small.
Very high mobility InP grown by low pressure metalorganic vapor phase epitaxy using solid trimethylindium source
Authors: Zhu L. D. , Chan K. T. , Ballantyne J. M.
Published: July 2002

Growth of very high quality unintentionally doped InP by low pressure metalorganic vapor phase epitaxy using solid trimethylindium as the indium source is described. Hall mobilities of 5370 cm2/Vs (300 K) and 131 600 cm2/Vs (77 K) with residual carrier concentration of 2.5×1014 cm−3 have been obtained. Low-temperature photoluminescence exciton spectra revealed that the neutral acceptor-bound-exciton emission was hardly observable, indicating a very low compensation ratio.
Soft x-ray emission from subpicosecond laser-produced plasmas
Authors: Kühlke D. , Herpers U. , von der Linde D.
Published: July 2002

Observation of soft x rays from a tungsten plasma produced by intense subpicosecond laser pulses is reported. X radiation of several keV photon energy was observed with just a few millijoules of laser energy. Using two consecutive pulses for the generation of the plasma the x-ray emission is maximal for a delay of about 30 to 50 ps between the laser pulses.
Defect reduction by thermal annealing of GaAs layers grown by molecular beam epitaxy on Si substrates
Authors: Lee J. W. , Shichijo H. , Tsai H. L. , Matyi R. J.
Published: July 2002

Post growth thermal annealing has been used to reduce the defect density of GaAs layers grown on Si substrates by molecular beam epitaxy. Transmission electron microscopy indicates a 100× reduction of the true defect density. Twins and stacking faults were eliminated entirely. Most misfit dislocations were confined within the first ∼150 Å GaAs layer and formed a regular and narrow network along the Si/GaAs interface. Similar results were obtained from an ion implanted and annealed specimen.
An isolated, surface-expressed I domain of the integrin αLβ2 is sufficient for strong adhesive function when locked in the open conformation with a disulfide bond
Authors: Lu Chafen , Shimaoka Motomu , Ferzly Mazen , Oxvig Claus , Takagi Junichi , Springer Timothy A.
Published: July 2002

We introduced disulfide bonds to lock the integrin αLβ2 I domain in predicted open, ligand binding or closed, nonbinding conformations. Transfectants expressing αLβ2 heterodimers containing locked-open but not locked-closed or wild-type I domains constitutively adhered to intercellular adhesion molecule-1 (ICAM-1) substrates. Locking the I domain closed abolished constitutive and activatable adhesion. The isolated locked-open I domain bound as well as the activated αLβ2 heterodimer, and binding was abolished by reduction of the disulfide. Lovastatin, which binds under the conformationally mobile C-terminal α-helix of the I domain, inhibited binding to ICAM-1 by αLβ2 with wild-type, but not locked-open I domains. These data establish the importance of conformational change in the αL I domain for adhesive function and show that this domain is sufficient for full adhesive activity.
Testosterone inhibits early atherogenesis by conversion to estradiol: Critical role of aromatase
Authors: Nathan Lauren , Shi Weibin , Dinh Hillary , Mukherjee Tapan K. , Wang Xuping , Lusis Aldons J. , Chaudhuri Gautam
Published: July 2002

The effects of testosterone on early atherogenesis and the role of
aromatase, an enzyme that converts testosterone to estrogens, were
assessed in low density lipoprotein receptor-deficient male mice fed a
Western diet. Castration of male mice increased the extent of fatty
streak lesion formation in the aortic origin compared with
testes-intact animals. Administration of anastrazole, a selective
aromatase inhibitor, to testes-intact males increased lesion formation
to the same extent as that observed with orchidectomized animals.
Testosterone supplementation of orchidectomized animals reduced lesion
formation when compared with orchidectomized animals receiving the
placebo. This attenuating effect of testosterone was not observed when
the animals were treated simultaneously with the aromatase inhibitor.
The beneficial effects of testosterone on early atherogenesis were not
explained by changes in lipid levels. Estradiol administration to
orchidectomized males attenuated lesion formation to the same extent as
testosterone administration. Aromatase was expressed in the aorta of
these animals as assessed by reverse transcription–PCR and
immunohistochemistry. These results indicate that testosterone
attenuates early atherogenesis most likely by being converted to
estrogens by the enzyme aromatase expressed in the vessel wall.
Viral capsid mobility: A dynamic conduit for inactivation
Authors: Broo Klas , Wei Jing , Marshall Dawn , Brown Fred , Smith Thomas J. , Johnson John E. , Schneemann Anette , Siuzdak Gary
Published: July 2002

Mass spectrometry and fluorescent probes have provided direct evidence that alkylating agents permeate the protein capsid of naked viruses and chemically inactivate the nucleic acid.N-acetyl-aziridine and a fluorescent alkylating agent, dansyl sulfonate aziridine, inactivated three different viruses, flock house virus, human rhinovirus-14, and foot and mouth disease virus. Mass spectral studies as well as fluorescent probes showed that alkylation of the genome was the mechanism of inactivation. Because particle integrity was not affected by selective alkylation (as shown by electron microscopy and sucrose gradient experiments), it was reasoned that the dynamic nature of the viral capsid acts as a conduit to the interior of the particle. Potential applications include fluorescent labeling for imaging viral genomes in living cells, the sterilization of blood products, vaccine development, and viral inactivationin vivo.
Intralymphatic immunization enhances DNA vaccination
Authors: Maloy Kevin J. , Erdmann Iris , Basch Veronique , Sierro Sophie , Kramps Thomas A. , Zinkernagel Rolf M. , Oehen Stefan , Kündig Thomas M.
Published: July 2002

Although DNA vaccines have been shown to elicit potent immune responses in animal models, initial clinical trials in humans have been disappointing, highlighting a need to optimize their immunogenicity. Naked DNA vaccines are usually administered either i.m. or intradermally. The current study shows that immunization with naked DNA by direct injection into a peripheral lymph node enhances immunogenicity by 100- to 1,000-fold, inducing strong and biologically relevant CD8+cytotoxic T lymphocyte responses. Because injection directly into a lymph node is a rapid and easy procedure in humans, these results have important clinical implications for DNA vaccination.
Defect in regulated secretion of P-selectin affects leukocyte recruitment in von Willebrand factor-deficient mice
Authors: Denis Cécile V. , André Patrick , Saffaripour Simin , Wagner Denisa D.
Published: July 2002

Stimulation of endothelial cells by various inflammatory mediators
leads to release of Weibel–Palade bodies and therefore to exocytosis
of both P-selectin (adhesion receptor for leukocytes) and von
Willebrand factor (vWf) (platelet ligand). The potential role of vWf in
leukocyte recruitment was investigated with the use of vWf-deficient
mice. We report a strong reduction of leukocyte rolling in venules of
vWf-deficient mice. Similarly, vWf deficiency led to a decrease in
neutrophil recruitment in a cytokine-induced meningitis model as well
as in early skin wounds. In all instances with an antibody that
preferentially recognizes plasma membrane P-selectin, we observed a
dramatic reduction in P-selectin expression at the cell surface of
vWf-deficient endothelium. With confocal microscopy, we found that the
typical rodlike shape of the Weibel–Palade body is missing in vWf
−/− endothelial cells and that part of the P-selectin content in
the vWf −/− cells colocalized with LAMP-1, a lysosomal marker.
However, intracellular P-selectin levels were similar in tumor necrosis
factor α- and lipopolysaccharide-activated cells of both genotypes.
We conclude that the absence of vWf, as found in severe von Willebrand
disease, leads to a defect in Weibel–Palade body formation. This
defect results in decreased P-selectin translocation to the cell
surface and reduced leukocyte recruitment in early phases of
inflammation.