bluuebunny/crossref_metadata_2025 huggingface dataset:
Cooperative (nonpairwise) effects in water trimers: An ab initio molecular orbital study
Authors: Mó O. , Yáñez M. , Elguero J.
Published: July 2002

High levels of ab initio molecular orbital theory were used to study the structures and binding energies of water trimers. These calculations included HF/6-311++G(2df,2p) geometry optimizations for the 17 structures considered. Harmonic vibrational energies were obtained at the HF/6–311++G(2d,2p) level. The HF potential energy surface present three minima whose geometries were refined at the MP2/6–311+G(d,p) level. The global minimum corresponds to an asymmetric cyclic structure which presents significant cooperative effects with respect to the Cs dimer. To properly describe these nonpairwise effects, ZPE (zero point energy) and correlation corrections must be taken into account. They are reflected in a stiffer intermolecular potential, shorter O–O distances, longer donor O–H bond lengths, larger energies per hydrogen bond (HB), and greater shifts of the donor O–H bond stretching frequencies than the Cs dimer. Contrarily, the other two local minima present HBs which are weaker than those of the dimer. These nonpairwise effects upon trimerization are mirrored in the topological characteristics of the electronic charge distributions of these clusters and there is a good linear relationship between the energies per HB and the charge density at the HB critical point. The binding energies for the three minima were evaluated at the MP4SDQ/6–31+G(2d,2p) level using the MP2 optimized geometries. For the remaining structures considered they were obtained at the MP2/6–311++G(2d,2p) level using the corresponding HF optimized geometries. We have also shown that nonpairwise effects can be qualitatively explained in terms of acid-base arguments.
Hypernetted and reference hypernetted chain integral equations for Lennard-Jones diatomic fluids with quadrupolar interactions
Authors: Martín C. , Lombardero M. , Lomba E.
Published: July 2002

The hypernetted chain and reference hypernetted chain integral equations are solved for quadrupolar two-center Lennard-Jones fluids and the computed fluid structure and thermodynamics are contrasted with computer simulation. A reference bridge function is determined through an empirical modification of Verlet’s approximation. The resulting reference hypernetted chain equation leads to good agreement with simulation data. On the contrary, the bare hypernetted chain approximation performs poorly, in particular as far as the equation of state is concerned, which is a well-known drawback of this closure when short ranged repulsive potentials come into play.
Microwave spectra of metal chlorides produced using laser ablation
Authors: Hensel Kristine D. , Styger Christian , Jäger Wolfgang , Merer A. J. , Gerry M. C. L.
Published: July 2002

An apparatus has been constructed to produce metal compounds using laser ablation and to investigate their rotational spectra with a microwave Fourier transform (MWFT) cavity spectrometer. The first three such compounds that have been studied are silver chloride, aluminum (I) chloride, and copper (I) chloride, produced by ablation of silver, aluminum, and copper rods in the presence of chlorine gas, using a Q-switched Nd:YAG laser (532 nm). The high resolution and sensitivity available with the MWFT cavity spectrometer have permitted the first determination of nuclear spin–rotation coupling constants for AlCl and CuCl. These constants have been used to examine the electronic structures of the molecules. Values of the rotational and nuclear quadrupole coupling constants have also been improved for the three metal chlorides.
Aluminum monochloride excited states observed by resonance-enhanced multiphoton ionization spectroscopy
Authors: Dearden David V. , Johnson Russell D. , Hudgens Jeffrey W.
Published: July 2002

AlCl was detected and characterized with 1+2, 2+1, 2+2, 3+1, and 3+2 resonance-enhanced multiphoton ionization spectroscopy using laser light between 332 and 570 nm. AlCl was generated in the gas phase with high temperature reaction of Al with InCl3, or by the reaction of Al with AgCl. New states residing between 52 000 and 66 000 cm−1 were identified. Vibrational intervals of the new Rydberg states are about 20% greater than those of AlCl (X 1Σ+), with most lying between 565–595 cm−1.
Experimental and theoretical study of oxidative addition reaction of nickel atom to O–H bond of water
Authors: Mitchell Steven A. , Blitz Mark A. , Siegbahn Per E. M. , Svensson Mats
Published: July 2002

The reaction of atomic nickel with water in the gas phase has been investigated by kinetic studies under static pressure conditions near room temperature, and by accurate quantum chemical calculations. Experimental and theoretical results are consistent with a reaction mechanism involving formation of a weakly bound nickel–water adduct, which may react further by oxidative addition of nickel to the O–H bond of water to form the insertion product HNiOH. Experimental estimates of reaction energetics have been made by using unimolecular reaction theory calculations to model rate coefficients obtained by fitting kinetic data to a simple rate equations model. These experimental estimates are in agreement with the theoretical results, and indicate that the insertion product is bound by at least 20–25 kcal/mol, relative to nickel plus water. There is also agreement that the barrier to oxidative addition is no greater than 1–2 kcal/mol, and may be smaller. This theoretical result was obtained only at the highest level treatments in the geometry optimization. The reaction mechanism involves a spin–orbit induced surface crossing from the ground state triplet surface of the reactants to a singlet surface, followed by a second crossing back to the triplet surface in the product region. The oxidative addition reaction thus proceeds on a singlet potential surface which correlates with excited (d9s1)1D nickel atoms. The electronic configuration of the 1D state is favorable for oxidative addition because repulsion between nickel and water may be reduced by sd hybridization. The presence of the low lying 1D state is responsible for the unique reactivity of nickel, among the late first row transition metal atoms, with respect to the oxidative addition reaction with water.
Computer simulation of vapor–liquid equilibria of linear quadrupolar fluids. Departures from the principle of corresponding states
Authors: Garzón Benito , Lago Santiago , Vega Carlos , Miguel Enrique de , Rull Luis F.
Published: July 2002

Vapor–liquid equilibria of different quadrupolar linear Kihara fluids have been studied, by using the Gibbs ensemble Monte Carlo technique. Coexistence curves for fluids with elongations L*=L/σ=0.3, 0.6, and 0.8 and different quadrupoles are given. We analyze the effect of quadrupole moment on critical properties. Quadrupole moment increases the critical temperature, pressure, and density. The magnitude of the increase depends on both anisotropy and quadrupole moment. A new way of reducing the quadrupole is proposed, so that the variation of critical properties due to the quadrupole follows a universal behavior. Quadrupole provokes deviations from the principle of corresponding states. A broadening of the coexistence curve is observed due to the quadrupole. The quadrupole moment increases the slope of the vapor pressure curve vs temperature inverse. Simulation data are used to describe vapor–liquid equilibria of carbon dioxide. Good agreement between simulation and experiment is achieved.
Modeling physical systems by effective harmonic oscillators: The optimized quadratic approximation
Authors: Cao Jianshu , Voth Gregory A.
Published: July 2002

A mathematical formalism is developed to map a physical system described by a general potential energy function onto one consisting of effective harmonic oscillators. The present focus is on many-body systems characterized by a temperature, so the theoretical effort is devoted to the partition function through a diagrammatic representation of its cumulant expansion in the quadratic reference system. Appropriate diagram summation and renormalization strategies lead to an ‘‘optimized quadratic approximation’’ (OQA) for both the quantum and classical partition functions of general systems. Diagrammatic methods are also used to develop accurate higher order corrections to the OQA. Applications to representative problems are presented with good success.
Surface segregation and layering in the liquid–vapor interface of a dilute bismuth:gallium alloy
Authors: Lei Ning , Huang Zhengqing , Rice Stuart A.
Published: July 2002

The density distribution normal to the liquid–vapor interface of a bismuth–gallium mixture (0.18% Bi and 99.82% Ga) has been studied by measurement of the x-ray reflectivity as a function of angle of incidence. The data obtained confirm the predicted stratification of the density distribution along the normal to the interface and the segregation of the excess surface concentration of Bi into a monolayer atop the bulk mixture.
Evolution of ion internal energy during collisional excitation in the Paul ion trap: A stochastic approach
Authors: Goeringer Douglas E. , McLuckey Scott A.
Published: July 2002

A first-order model is developed for collisional activation as effected via resonance excitation and helium buffer gas in the Paul ion trap. For an ion population at steady-state under specified experimental conditions, the kinetic theory of ion transport in gases is first used to calculate an effective temperature shown to be identical to the internal temperature for molecular ions in an atomic gas. The evolution of the ion internal energy is then followed by a random walk simulation designed to be representative of the actual collisional energy transfer process, except ion losses due to dissociation and reactive processes during collisional activation are excluded. During the simulation, inelastic ion-neutral collisions increase the average ion internal energy via small energy changes (both positive and negative) until a steady-state condition is reached in which excitation and deexcitation processes are balanced. Histogramming the simulated data reveals a Boltzmann-type internal energy distribution whose average internal energy is the same as that calculated for a true Boltzmann distribution at the same internal temperature.
Intermolecular energy transfer probabilities from trajectory calculations: A new approach
Authors: Bernshtein V. , Oref I.
Published: July 2002

A new method to calculate intermolecular energy transfer probability density function P(E′,E) from trajectory calculations is proposed. The method distinguishes between effective trajectories that contribute to P(E′,E) and those with very large impact parameter which do not. The P(E′,E) thus found obeys conservation of probability and detailed balance and is independent of the impact parameter. The method is demonstrated for benzene–Ar collisions at various temperatures and internal energies. With this method it is possible to combine ab initio inter and intramolecular potentials with trajectory calculations, obtain P(E′,E) and use that in master equation calculations to obtain rate coefficients and populations distributions without resorting to any a priori assumptions and energy transfer models. In addition, the effects of internal energy, temperature and rotations on the average energy transferred are discussed. Global potentials in center-of-mass and minimal distance coordinates which are obtained by averaging 20 000 and 50 000 trajectories are reported. It is shown that Lennard-Jones or ab initio pairwise potentials yield a Buckingham-type global potentials.
Accurate density functional calculations on frequency-dependent hyperpolarizabilities of small molecules
Authors: van Gisbergen S. J. A. , Snijders J. G. , Baerends E. J.
Published: July 2002

In this paper we present time-dependent density functional calculations on frequency-dependent first (β) and second (γ) hyperpolarizabilities for the set of small molecules, N2, CO2, CS2, C2H4, NH3, CO, HF, H2O, and CH4, and compare them to Hartree–Fock and correlated ab initio calculations, as well as to experimental results. Both the static hyperpolarizabilities and the frequency dispersion are studied. Three approximations to the exchange-correlation (xc) potential are used: the widely used Local Density Approximation (LDA), the Becke–Lee–Yang–Parr (BLYP) Generalized Gradient Approximation (GGA), as well as the asymptotically correct Van Leeuwen–Baerends (LB94) potential. For the functional derivatives of the xc potential the Adiabatic Local Density Approximation (ALDA) is used. We have attempted to estimate the intrinsic quality of these methods by using large basis sets, augmented with several diffuse functions, yielding good agreement with recent numerical static LDA results. Contrary to claims which have appeared in the literature on the basis of smaller studies involving basis sets of lesser quality, we find that the static LDA results for β and γ are severely overestimated, and do not improve upon the (underestimated) Hartree–Fock results. No improvement is provided by the BLYP potential which suffers from the same incorrect asymptotic behavior as the LDA potential. The results are however clearly improved upon by the LB94 potential, which leads to underestimated results, slightly improving the Hartree–Fock results. The LDA and BLYP potentials overestimate the frequency dependence as well, which is once again improved by the LB94 potential. Future improvements are expected to come from improved models for asymptotically correct exchange-correlation potentials. Apart from the LB94 potential used in this work, several other asymptotically correct potentials have recently been suggested in the literature and can also be expected to improve considerably upon the relatively poor LDA and GGA results, for both the static properties and their frequency dependence.
Nonadiabatic radiative lifetimes and fluorescence spectra of NO2
Authors: Santoro Fabrizio , Petrongolo Carlo
Published: July 2002

We have computed the radiative lifetimes and fluorescence spectra of the B2 nonadiabatic states of NO2 up to 18 735 cm−1, employing ab initio MRD-CI potentials refined with respect to the experimental lines up to 10 000 cm−1. The effects of the conical intersection X̃2A′/Ã2A′ between the first two adiabatic surfaces on the radiative lifetimes have been investigated by comparing the lifetimes of the nonadiabatic states with those of vibrational states of the excited electronic species. By this way we have confirmed that the anomalous long lifetimes observed for NO2 are caused by the nonadiabatic couplings, suggesting furthermore that the lifetimes of some weak absorbing states are on the contrary lowered by the conical intersection. We have compared our lifetimes with the published experimental measurements, showing that we are able to well reproduce and explain the general experimental trends and that the computed results agree with the experimental ones within a factor two or three. These differences are similar to those found among the observed lifetimes. Moreover, we have computed laser induced fluorescence spectra and compared them with the experimental ones, showing a fair agreement in the low-energy range 11 600–13 900 cm−1 and a rough agreement in the high-energy range 16 000–18 735 cm−1, and suggesting the assignments for the main 12B2 characters of some strongest bands. Finally, we have computed dispersed fluorescence spectra for some initial states, by pointing out that they can be very different because of the difference of the wave functions of the initial states.
Dynamics of Y+H2CO reactions
Authors: Stauffer Hans U. , Hinrichs Ryan Z. , Schroden Jonathan J. , Davis H. Floyd
Published: July 2002

The reactions of ground state yttrium atoms (Y) with formaldehyde (H2CO) have been studied in crossed molecular beams as a function of collision energy (Ecoll). The potential energy barrier for C–H insertion is found to lie below 12 kcal/mol. It is proposed that the reaction is initiated by C–H insertion, producing HYCHO followed by H atom migration forming H2YCO. Although Y–CO bond fission leading to YH2+CO is dominant, a secondary minor channel also leads to the production of YCO+H2. Formation of YCHO+H is not observed at 16 kcal/mol, but is clearly seen at 31 kcal/mol, indicating that D0(Y–CHO) lies between 58 and 73 kcal/mol.
Scattering theory for long-range systems at threshold
Authors: Bollé D. , Gesztesy F. , Schweiger W.
Published: July 2002

Scattering theory is discussed for long-range systems around threshold, taking into account explicitly the possibility of zero-energy eigenstates of the underlying Schrödinger Hamiltonian. Low-energy expansions are derived for the scattering amplitude in the different cases. The Coulomb-modified scattering length and effective range parameter are defined directly in terms of the scattering amplitude. Explicit formulas for these quantities are provided. No spherical symmetry of the interaction is assumed.
The separability of Maxwell’s equations in type-D backgrounds
Authors: Torres del Castillo G. F.
Published: July 2002

It is shown that in a type-D space-time that admits a two-index Killing spinor a differential operator can be constructed that maps a solution of the Maxwell equations into another solution. By considering as a background the Plebański–Demiański metric, which includes all the vacuum type-D metrics, this operator is used to obtain all the components of the electromagnetic field and the vector potential. The separated functions appearing in the solutions are shown to obey identities of the Teukolsky–Starobinsky type and the separable solutions are shown to be eigensolutions of a certain differential operator with the ‘‘Starobinsky constant’’ as the eigenvalue.
Conformal symmetry of perfect fluids in general relativity
Authors: Kramer Dietrich , Carot Jaume
Published: July 2002

Rigidly rotating perfect fluids admitting two Killing vectors ξ (stationarity), η (axial symmetry), and a proper conformal Killing vector K are studied. The three-dimensional Lie algebra [ξ,η]=0, [ξ,K]=a4K, [η,K]=a3K, a32+a42≠0 is considered. It is shown that under these assumptions there are no space-times satisfying Einstein’s field equations.
Algebraic classification of actions invariant under generalized flip moves of two-dimensional graphs
Authors: Bordemann Martin , Filk Thomas , Nowak Claus
Published: July 2002

Statistical models defined on 2-dimensional graphs are classified which are invariant under flip moves, i.e., certain local changes of the adjacency structure of the graphs. The special case of regular graphs of degree 3—which are duals of 2-dimensional triangulations—corresponds to topological models and the classification leads to metrized, associative algebras. As a novel feature flip invariant models on regular graphs of degree 4 are classified by Z2-graded metrized associative algebras. They give rise to invariants for checkered graphs. Moreover, the general case of graphs with vertices of arbitrary degree (where degree 3 does occur) is discussed. Using structure theorems for (graded,) metrized, associative algebras we prove that only the simple ideals contribute to the partition function of such models. The partition functions are computed explicitly and reveal the invariant structures of the graph under the flip moves.
Anomalous thresholds and edge singularities in electrical impedance tomography
Authors: Ciulli S. , Ispas S. , Pidcock M. K.
Published: July 2002

Studies of models of current flow behaviour in electrical impedance tomography (EIT) have shown that the current density distribution varies extremely rapidly near the edge of the electrodes used in the technique. This behaviour imposes severe restrictions on the numerical techniques used in image reconstruction algorithms. In this paper we have considered a simple two dimensional case and we have shown how the theory of end point/pinch singularities which was developed for studying the anomalous thresholds encountered in elementary particle physics can be used to give a complete description of the analytic structure of the current density near to the edge of the electrodes. As a byproduct of this study it was possible to give a complete description of the Riemann sheet manifold of the eigenfunctions of the logarithmic kernel. These methods can be readily extended to other weakly singular kernels.
Peculiar physical properties and the colossal magnetoresistance of manganites (Review)
Authors: Loktev V. M. , Pogorelov Yu. G.
Published: July 2002

An attempt is made to analyze the most important physical properties of manganites of the La-Ca-Mn-O type, which exhibit the colossal magnetoresistance effect. The primary focus is on the peculiarities of these compounds which are reflected in their crystalline, electronic, and magnetic structures and which determine the possible mechanisms by which an external magnetic field can exert a substantial influence on the transport characteristics of the current carriers in manganites. The combined effect of these factors is to create the necessary conditions for a metal-insulator phase transition that is sensitive to an external magnetic field. Another major topic in this review is a discussion of the scientific problems confronting the physics of manganites.
A theoretical study of polydisperse liquid sprays in a shear-layer flow
Authors: Katoshevski D. , Tambour Y.
Published: July 2002

A shear layer formed by two unidirectional gas streams of different velocities with a multisize (polydisperse) spray of evaporating droplets suspended in one of the gas streams is considered here. Similarity solutions are presented for the evolution in droplet size distributions across the shear layer and the effects of various initial droplet size distributions on the profiles of vapor concentrations are examined. A qualitative comparison between the present results for typical computed total mass distributions of the liquid phase and experimental data reported by Lazaro and Lasheras [Phys. Fluids A 1, 1035 (1989); Proceedings of the 22nd Symposium (International) on Combustion (The Combustion Institute, Pittsburgh, 1988), pp. 1991–1998; J. Fluid Mech. 235, 143 (1992)] shows strong similarity between the two sets of profiles. This supports the assumptions and boundary conditions employed in the present theoretical study. The general behavior of the theoretical SMD (Sauter mean diameter) distribution of the spray across the shear layer also compares well with the reported experimental results of Lazaro and Lasheras.
Heat-salt finger fluxes across a density interface
Authors: Shen Colin Y.
Published: July 2002

The heat and salt fluxes produced by salt fingering at a density interface are studied with a numerical and an analytical model. Specifically, the issue concerning the value of the heat-to-salt flux ratio is addressed. The numerical modeling based on direct numerical computation of the nonlinear governing equations obtains values around 0.5. This value is approximately the average of widely varying experimental values reported in the literature. The large difference between the theoretical flux ratio predicted based on the buoyancy maximization hypothesis and the experimentally derived flux ratio is examined with an analytical model, which includes both effects of salt stratification in the interface and salt discontinuity at the edges of the interface. Combined with the numerical model results, the analysis shows that the disagreement can be traced to the flux maximization hypothesis itself. An alternative hypothesis that maximizes convective velocity amplitude is presented which gives flux-ratio predictions consistent with the measurements. Discussion of the finger width scale and the magnitude of the salt flux is also given as well as comparison of the salt flux with laboratory measurements.
Resistive ‘‘infernal’’ modes
Authors: Charlton L. A. , Hastie R. J. , Hender T. C.
Published: July 2002

Resistive ‘‘infernal’’ modes are considered that are just below the ideal internal mode threshold. Nearest the ideal threshold, a resistive continuation of the ideal infernal mode, whose growth rate scales as η3/13 (η being the resistivity), is seen. Farther away is the conventional tearing mode with a η3/5 scaling. These scalings are found both numerically and analytically.
Temperature-gradient instability induced by conducting end walls
Authors: Berk H. L. , Ryutov D. D. , Tsidulko Yu. A.
Published: July 2002

A new rapidly growing electron-temperature-gradient instability is found for a plasma in contact with a conducting wall. The linear instability analysis is presented and speculations are given for its nonlinear consequences. This instability illustrates that conducting walls can produce effects that are detrimental to plasma confinement. This mode is of importance in open-ended systems such as mirror machines and relevant to the edge of tokamaks where field lines are open and are connected to limiters or divertors and astrophysical plasmas like the ones of the flux tubes in a solar atmosphere, with the footpoints on the photospheric level.
Tuning effect in enhanced gas dispersion under oscillatory conditions
Authors: Kurzweg Ulrich H. , Jaeger Marc J.
Published: July 2002

It is shown that the effective axial dispersion coefficient for partially mixed gases confined to cylindrical tubes and subjected to sinusoidal oscillations reaches a maximum, at a constant oscillation frequency, when the Womersley number is equal to α=3.3. Such a tuned condition in oxygen–nitrogen mixtures corresponds, for example, to tubes of 2.0 mm in diameter when the oscillation frequency is 27.7 Hz and occurs when the time for gas diffusion from the tube axis to the walls is comparable with the oscillation period.
Static shape of an acoustically levitated drop with wave–drop interaction
Authors: Lee C. P. , Anilkumar A. V. , Wang T. G.
Published: July 2002

The static shape of a drop levitated and flattened by an acoustic standing wave field in air is calculated, requiring self-consistency between the drop shape and the wave. The wave is calculated for a given shape using the boundary integral method. From the resulting radiation stress on the drop surface, the shape is determined by solving the Young–Laplace equation, completing an iteration cycle. The iteration is continued until both the shape and the wave converge. Of particular interest are the shapes of large drops that sustain equilibrium, beyond a certain degree of flattening, by becoming more flattened at a decreasing sound pressure level. The predictions for flattening versus acoustic radiation stress, for drops of different sizes, compare favorably with experimental data.
Eddy diffusivities in scalar transport
Authors: Biferale L. , Crisanti A. , Vergassola M. , Vulpiani A.
Published: July 2002

Standard and anomalous transport in incompressible flow is investigated using multiscale techniques. Eddy diffusivities emerge from the multiscale analysis through the solution of an auxiliary equation. From the latter it is derived an upper bound to eddy diffusivities, valid for both static and time-dependent flow. The auxiliary problem is solved by a perturbative expansion in powers of the Péclet number resummed by Padé approximants and a conjugate gradient method. The results are compared to numerical simulations of tracers dispersion for three flows having different properties of Lagrangian chaos. It is shown on a concrete example how the presence of anomalous diffusion in deterministic flows can be revealed from the singular behavior of the eddy diffusivity at very small molecular diffusivities.
The permeability of strongly disordered systems
Authors: Shah C. B. , Yortsos Y. C.
Published: July 2002

The critical path theory of Katz and Thompson [Phys. Rev. B 34, 8179 (1986)] is extended to estimate the permeability of strongly disordered macroscopic porous media. The theory is based on percolation concepts and for lognormal distributions predicts an asymptotic exponential dependence on the standard deviation.
Large-eddy simulation of high-Schmidt number mass transfer in a turbulent channel flow
Authors: Calmet Isabelle , Magnaudet Jacques
Published: July 2002

Mass transfer through the solid boundary of a turbulent channel flow is analyzed by means of large-eddy simulation (LES) for Schmidt numbers Sc=1, 100, and 200. For that purpose the subgrid stresses and fluxes are closed using the Dynamic Mixed Model proposed by Zang et al. [Phys. Fluids A 5, 3186 (1993)]. At each Schmidt number the mass transfer coefficient given by the LES is found to be in very good quantitative agreement with that measured in the experiments. At high Schmidt number this coefficient behaves like Sc−2/3, as predicted by standard theory and observed in most experiments. The main statistical characteristics of the fluctuating concentration field are analyzed in connection with the well-documented statistics of the turbulent motions. It is observed that concentration fluctuations have a significant intensity throughout the channel at Sc=1 while they are negligible out of the wall region at Sc=200. The maximum intensity of these fluctuations depends on both the Schmidt and Reynolds numbers and is especially influenced by the intensity of the velocity fluctuations present in the buffer layer of the concentration field. At Sc=1, strong similarities are observed between the various terms contributing to the turbulent kinetic energy budget and their counterpart in the budget of the variance of concentration fluctuations. At high Schmidt number, the latter budget is much more influenced by the small turbulent structures subsisting in the viscous sublayer. The instantaneous correlation between the spatial characteristics of the concentration field and those of the velocity field is clearly demonstrated by the presence of low- and high-concentration streaks close to the wall. The geometrical characteristics of these structures are found to be highly Sc dependent. In particular their spanwise wavelength is identical to that of the streamwise velocity streaks at Sc=1 while it is reduced by half at Sc=200. Analysis of the co-spectra between concentration and normal velocity fluctuations emphasizes the fact that the large-scale structures play an essential role in the turbulent mass transfer process at high Schmidt number. Overall the picture that emerges from this investigation fully confirms the conclusions of Campbell and Hanratty [AIChE J. 29, 221 (1983)]: high-Schmidt-number mass transfer at a solid wall is governed by the low-frequency part of the normal velocity fluctuation gradient at the wall, i.e., by the large-scale structures observed in planes parallel to the wall in the viscous sublayer.
The dynamics of vapor bubbles in acoustic pressure fields
Authors: Hao Y. , Prosperetti A.
Published: July 2002

In spite of a superficial similarity with gas bubbles, the intimate coupling between dynamical and thermal processes confers to oscillating vapor bubbles some unique characteristics. This paper examines numerically the validity of some asymptotic-theory predictions such as the existence of two resonant radii and a limit size for a given sound amplitude and frequency. It is found that a small vapor bubble in a sound field of sufficient amplitude grows quickly through resonance and continues to grow thereafter at a very slow rate, seemingly indefinitely. Resonance phenomena therefore play a role for a few cycles at most, and reaching a limit size—if one exists at all—is found to require far more than several tens of thousands of cycles. It is also found that some small bubbles may grow or collapse depending on the phase of the sound field. The model accounts in detail for the thermo-fluid-mechanic processes in the vapor. In the second part of the paper, an approximate formulation valid for bubbles small with respect to the thermal penetration length in the vapor is derived and its accuracy examined. The present findings have implications for acoustically enhanced boiling heat transfer and other special applications such as boiling in microgravity.
Mechanism of air entrainment by a disturbed liquid jet
Authors: Ohl C. D. , Og̃uz H. N. , Prosperetti A.
Published: July 2002

It was shown in recent work that the crests of surface disturbances on a falling jet are a powerful agent for air entrainment at the free surface of a liquid pool. The paper explores the opposite case in which the jet is disturbed so as to form an axisymmetric trough, rather than a crest. It is found that no air is entrained in this case. The paper concludes with some considerations on the validity of a recently proposed model for air entrainment.
A unified Monte Carlo interpretation of particle simulations and applications to non-neutral plasmas
Authors: Aydemir A. Y.
Published: July 2002

Using a ‘‘Monte Carlo interpretation’’ of particle simulations, a general description of low-noise techniques, such as the δf method, is developed in terms well-known Monte Carlo variance reduction methods. Some of these techniques then are applied to linear and nonlinear studies of pure electron plasmas in cylindrical geometry, with emphasis on the generation and nonlinear evolution of electron vortices. Long-lived l=1 and l=2 vortices, and others produced by unstable diocotron modes in hollow profiles, are studied. It is shown that low-noise techniques make it possible to follow the linear evolution and saturation of even the very weakly unstable resonant diocotron modes.
Nonlinear analysis of helix traveling wave tubes
Authors: Freund H. P. , Zaidman E. G. , Mankofsky A. , Vanderplaats N. R. , Kodis M. A.
Published: July 2002

A time-dependent nonlinear formulation of the interaction in the helix traveling wave tube is presented for a configuration in which an electron beam propagates through a sheath helix surrounded by a conducting wall. In order to describe both the variation in the wave dispersion and in the transverse inhomogeneity of the electromagnetic field with wave number, the field is represented as a superposition of waves in a vacuum sheath helix. An overall explicit sinusoidal variation of the form exp(ikz−iωt) is assumed (where ω denotes the angular frequency corresponding to the wave number k in the vacuum sheath helix), and the polarization and radial variation of each wave is determined by the boundary conditions in a vacuum sheath helix. Thus, while the field is three-dimensional in nature, it is azimuthally symmetric. The propagation of each wave in vacuo as well as the interaction of each wave with the electron beam is included by allowing the amplitudes of the waves to vary in z and t. A dynamical equation for the field amplitudes is derived analogously to Poynting’s equation, and solved in conjunction with the three-dimensional Lorentz force equations for an ensemble of electrons. Numerical examples are presented corresponding to both single- and multiwave interactions.
Shock waves in plasmas containing variable-charge impurities
Authors: Popel S. I. , Yu M. Y. , Tsytovich V. N.
Published: July 2002

Shock structures in plasmas containing variable-charge macro particles are shown to exist because of an effective dissipation associated with charging of the latter.
Multipactor saturation due to space-charge-induced debunching
Authors: Riyopoulos Spilios
Published: July 2002

As the multipactor charge builds up, the mutual repulsion overtakes the focusing effect of the rf interaction, and the thin charge ribbon formed during the initial buildup expands into a wide band. Two synergistic effects lead to saturation (a) first, spreading of the impact phases into the region where the vacuum rf retards emission (b) later, and field reversal in the front end of the bunch due to the space-charge field, regardless of the rf phase. The effective secondary yield under opposing field is zero regardless of impact energy. Steady state is achieved when the ensemble-average yield δ drops to unity. The spreads in the impact energy are less important; impacts under reversed field cause saturation even at constant (independent of impact energy) secondary yield. The bunch breakup threshold is identified from bunch stability theory. It is found that near saturation both one-surface and two-surface cascades take place simultaneously; particle orbits alternate between one- and two-surface multipactors until they are finally lost to unfavorable phase/reversed field impacts. The possibility of saturation with a tight bunch, before breakup, is also discussed.
Molecular dynamics calculation of the thermal conductivity and shear viscosity of the classical one-component plasma
Authors: Donkó Z. , Nyı́ri B.
Published: July 2002

The thermal conductivity λ and shear viscosity η of the three-dimensional classical one-component plasma (OCP) were determined by molecular dynamics experiments. In the simulations the velocity of the particles was spatially modulated, and the transport coefficients were calculated from the relaxation time of the modulation profile. The results are given for the 2⩽Γ⩽125 range of the plasma coupling parameter Γ. The reduced shear viscosity η* was found to exhibit a minimum at Γ=20 in agreement with previous calculations. In the 2⩽Γ⩽10 range our method yields η* values 20%–50% higher compared to some of the previously obtained data, while very good agreement was found at the position of the minimum of η*. The reduced thermal conductivity λ* exhibits a minimum (similarly to η*) at Γ between 15 and 20. The calculations presented here result in 30%–40% lower thermal conductivity compared to previously available data.
Acousto-optic Bragg diffraction in a LiNbO3 channel-planar composite waveguide with application to optical computing
Authors: Tsai C. S. , Zang D. Y. , Le P.
Published: July 2002

Successful experimentation on acousto-optic Bragg diffraction in a LiNbO3 composite waveguide that consists of an array of parallel but uncoupled channel waveguides directly extended to a single-mode planar waveguide and a titanium-indiffused proton-exchanged (TIPE) microlens array is reported for the first time. A channel-waveguide array, a planar waveguide, a linear TIPE microlens array, a 500-MHz surface acoustic wave transducer, and an integrating lens have all been integrated in a substrate size of 0.2×1.0×2.0 cm to form an integrated acousto-optic Bragg modulator that should find a variety of applications in optical computing, signal processing, and communications. The resulting modulator module has been utilized to perform matrix-vector multiplication.
Scanning tunneling microscopy of the a-b planes of Bi2(Ca,Sr)3Cu2O8+δ single crystal and thin film
Authors: Kirk M. D. , Eom C. B. , Oh B. , Spielman S. R. , Beasley M. R. , Kapitulnik A. , Geballe T. H. , Quate C. F.
Published: July 2002

A scanning tunneling microscope was used to image the a-b plane of a single crystal of Bi2(Ca,Sr)3Cu2O8+δ (Tc∼85 K) and a thin film grown epitaxially on a (100) SrTiO3 substrate. Images taken at room temperature in air of a freshly cleaved bulk sample clearly show the one-dimensional superstructure (27.2±0.7 Å periodicity) which is common to this phase. Several images show steps 16 Å high, suggesting that the cleavage occurs between 2122 molecular units. The registry of the superstructure from terrace to terrace is either shifted by half a superlattice parameter or is aligned. Images of evaporated thin films show features of comparable size, but not the same long-range order as the crystal. Low-temperature conductance measurements of the thin film exhibit a BCS-like energy gap with 2Δ/kTc≊12.
The orphan nuclear receptor Tlx regulates Pax2 and is essential for vision
Authors: Yu Ruth T. , Chiang Ming-Yi , Tanabe Teruyo , Kobayashi Mime , Yasuda Kunio , Evans Ronald M. , Umesono Kazuhiko
Published: July 2002

Although the development of the vertebrate eye is well described,
the number of transcription factors known to be key to this process is
still limited. The localized expression of the orphan nuclear receptor
Tlx in the optic cup and discrete parts of the central nervous system
suggested the possible role of Tlx in the formation or function of
these structures. Analyses of Tlx targeted mice revealed that, in
addition to the central nervous system cortical defects, lack of Tlx
function results in progressive retinal and optic nerve degeneration
with associated blindness. An extensive screen of Tlx-positive and
Tlx-negative P19 neural precursors identified Pax2 as a candidate
target gene. This identification is significant, because Pax2 is known
to be involved in retinal development in both the human and the mouse
eye. We find that Pax2 is a direct target and that the Tlx binding site
in its promoter is conserved between mouse and human. These studies
show that Tlx is a key component of retinal development and vision and
an upstream regulator of the Pax2 signaling cascade.
Arabidopsis cyt1
mutants are deficient in a mannose-1-phosphate guanylyltransferase and point to a requirement of N-linked glycosylation for cellulose biosynthesis
Authors: Lukowitz Wolfgang , Nickle Todd C. , Meinke David W. , Last Robert L. , Conklin Patricia L. , Somerville Christopher R.
Published: July 2002

Arabidopsis cyt1
mutants have a complex phenotype
indicative of a severe defect in cell wall biogenesis. Mutant embryos
arrest as wide, heart-shaped structures characterized by ectopic
accumulation of callose and the occurrence of incomplete cell walls.
Texture and thickness of the cell walls are irregular, and unesterified
pectins show an abnormally diffuse distribution. To determine the
molecular basis of these defects, we have cloned the
CYT1
gene by a map-based approach and found that it
encodes mannose-1-phosphate guanylyltransferase. A weak mutation in the
same gene, called
vtc1
, has previously been identified
on the basis of ozone sensitivity due to reduced levels of ascorbic
acid. Mutant
cyt1
embryos are deficient in
N-glycosylation and have an altered composition of cell wall
polysaccharides. Most notably, they show a 5-fold decrease in cellulose
content. Characteristic aspects of the
cyt1
phenotype,
including radial swelling and accumulation of callose, can be mimicked
with the inhibitor of N-glycosylation, tunicamycin. Our results suggest
that N-glycosylation is required for cellulose biosynthesis and that a
deficiency in this process can account for most phenotypic features of
cyt1
embryos.
Early changes in hepatitis C viral quasispecies during interferon therapy predict the therapeutic outcome
Authors: Farci Patrizia , Strazzera Rita , Alter Harvey J. , Farci Stefania , Degioannis Daniela , Coiana Alessandra , Peddis Giovanna , Usai Francesco , Serra Giancarlo , Chessa Luchino , Diaz Giacomo , Balestrieri Angelo , Purcell Robert H.
Published: July 2002

Despite recent treatment advances, the majority of patients with chronic hepatitis C fail to respond to antiviral therapy. Although the genetic basis for this resistance is unknown, accumulated evidence suggests that changes in the heterogeneous viral population (quasispecies) may be an important determinant of viral persistence and response to therapy. Sequences within hepatitis C virus (HCV) envelope 1 and envelope 2 genes, inclusive of the hypervariable region 1, were analyzed in parallel with the level of viral replication in serial serum samples obtained from 23 patients who exhibited different patterns of response to therapy and from untreated controls. Our study provides evidence that although the viral diversity before treatment does not predict the response to treatment, the early emergence and dominance of a single viral variant distinguishes patients who will have a sustained therapeutic response from those who subsequently will experience a breakthrough or relapse. A dramatic reduction in genetic diversity leading to an increasingly homogeneous viral population was a consistent feature associated with viral clearance in sustained responders and was independent of HCV genotype. The persistence of variants present before treatment in patients who fail to respond or who experience a breakthrough during therapy strongly suggests the preexistence of viral strains with inherent resistance to IFN. Thus, the study of the evolution of the HCV quasispecies provides prognostic information as early as the first 2 weeks after starting therapy and opens perspectives for elucidating the mechanisms of treatment failure in chronic hepatitis C.
Analysis of transforming activity of human synovial sarcoma-associated chimeric protein SYT-SSX1 bound to chromatin remodeling factor hBRM/hSNF2α
Authors: Nagai Makoto , Tanaka Shinya , Tsuda Masumi , Endo Shuichi , Kato Hiroyuki , Sonobe Hiroshi , Minami Akio , Hiraga Hiroaki , Nishihara Hiroshi , Sawa Hirofumi , Nagashima Kazuo
Published: July 2002

Human synovial sarcoma has been shown to exclusively harbor
the chromosomal translocation t(X;18) that produces the chimeric gene
SYT-SSX. However, the role of SYT-SSX in cellular transformation
remains unclear. In this study, we have established 3Y1 rat fibroblast
cell lines that constitutively express SYT, SSX1, and SYT-SSX1 and
found that SYT-SSX1 promoted growth rate in culture,
anchorage-independent growth in soft agar, and tumor formation in nude
mice. Deletion of the N-terminal 181 amino acids of SYT-SSX1 caused
loss of its transforming activity. Furthermore, association of SYT-SSX1
with the chromatin remodeling factor hBRM/hSNF2α, which regulates
transcription, was demonstrated in both SYT-SSX1-expressing 3Y1 cells
and in the human synovial sarcoma cell line HS-SY-II. The binding
region between the two molecules was shown to reside within the
N-terminal 181 amino acids stretch (aa 1–181) of SYT-SSX1 and 50 amino
acids (aa 156–205) of hBRM/hSNF2α and we found that the
overexpression of this binding region of hBRM/hSNF2α
significantly suppressed the anchorage-independent growth of
SYT-SSX1-expressing 3Y1 cells. To analyze the transcriptional
regulation by SYT-SSX1, we established conditional expression system of
SYT-SSX1 and examined the gene expression profiles. The down-regulation
of potential tumor suppressor DCC was observed among 1,176 genes
analyzed by microarray analysis, and semi-quantitative reverse
transcription–PCR confirmed this finding. These data clearly
demonstrate transforming activity of human oncogene SYT-SSX1 and also
involvement of chromatin remodeling factor hBRM/hSNF2α in human
cancer.
Holding two heads together: Stability of the myosin II rod measured by resonance energy transfer between the heads
Authors: Chakrabarty Tania , Xiao Ming , Cooke Roger , Selvin Paul R.
Published: July 2002

Myosin, similar to many molecular motors, is a two-headed dimer held together by a coiled-coiled rod. The stability of the coiled coil has implications for head–head interactions, force generation, and possibly regulation. Here we used two different resonance energy transfer techniques to measure the distances between probes placed in the regulatory light chain of each head of a skeletal heavy meromyosin, near the head–rod junction (positions 2, 73, and 94). Our results indicate that the rod largely does not uncoil when myosin is free in solution, and at least beyond the first heptad, the subfragment 2 rod remains relatively intact even under the relatively large strain of two-headed myosin (rigor) binding to actin. We infer that uncoiling of the rod likely does not play a role in myosin II motility. To keep the head–rod junction intact, a distortion must occur within the myosin heads. This distortion may lead to different orientations of the light-chain domains within the myosin dimer when both heads are attached to actin, which would explain previously puzzling observations and require reinterpretation of others. In addition, by comparing resonance energy transfer techniques sensitive to different dynamical time scales, we find that the N terminus of the regulatory light chain is highly flexible, with possible implications for regulation. An intact rod
may
be a general property of molecular motors, because a similar conclusion has been reached recently for kinesin, although whether the rod remains intact will depend on the relative stiffness of the coiled coil and the head in different motors.