bluuebunny/crossref_metadata_2025 huggingface dataset:
Polarizable point-charge model for water: Results under normal and extreme conditions
Authors: Svishchev Igor M. , Kusalik Peter G. , Wang Jian , Boyd Russel J.
Published: July 2002

Molecular dynamics simulations of liquid water under normal and extreme conditions are performed using the polarizable point-charge (PPC) model. This efficient three-site model explicitly incorporates results from ab initio studies of the water molecule in applied electric fields. The structural, thermodynamic, and dielectric properties, and the self-diffusion coefficient are examined at a number of temperatures ranging from 263 to 573 K. These simulation results are compared with available experimental data along the liquid–vapor coexistence line; the agreement is very good for all properties studied. The temperature of maximum density for the PPC model is found to coincide with the experimentally observed value of 277 K. The spatial coordination of water molecules in the liquid and the anisotropy of the self-diffusion tensor are analyzed at various state points. Increased directional anisotropy in the local translational diffusion, suggestive of prenucleation phenomena, can be observed at T=263 K. Above T=473 K the local translational anisotropy becomes rather insensitive to temperature variation indicating a weakening of the correlations between water molecules. Rototranslational dynamics and nonlinear polarization effects arising in polarizable models for water are discussed along with their phenomenological implications. The dimer properties for the PPC potential are also reported.
The Jacobian factor in free energy simulations
Authors: Boresch Stefan , Karplus Martin
Published: July 2002

The role of Jacobian factors in free energy simulations is described. They provide a simple interpretation of ‘‘moment of inertia correction’’ and ‘‘dynamic stretch free energy’’ terms in such simulations. Since the relevant Jacobian factors can often be evaluated analytically by use of the configurational partition function of a polyatomic molecule, it is possible to subtract them from the simulation results when they make unphysical contributions. An important case arises in alchemical simulations that use a single topology method and introduce dummy particles to have the same number of atoms in the initial and final state. The more general utility of the Jacobian factors for simulations of complex systems is briefly discussed.
Numerical simulation of the isomerization of HCN by two perpendicular intense IR laser pulses
Authors: Dion C. M. , Chelkowski S. , Bandrauk A. D. , Umeda H. , Fujimura Y.
Published: July 2002

Isomerization of HCN is studied numerically for a laser excitation configuration of two perpendicular intense IR pulses. This scheme confines the molecule to a plane and promotes proton transfer along the curved reaction path. It is shown that internal rotation of the CN group enhances isomerization when compared to a fixed C≡N orientation model. Isomerization rates with rotation exceed those without rotation of the CN by about a factor of 3. Internal rotation also enhances dissociation and destroys phase control of the isomerization. It is found that at intensities I∼1013 W/cm2, maximum isomerization occurs with negligible dissociation for a 2 ps pulse excitation. Maximum isomerization is also found for one field frequency resonant with the CH bend frequency ωbend and the other perpendicular frequency at 2ωbend.
Statistical mechanics of a fluid in contact with a curved wall
Authors: Poniewierski A. , Stecki J.
Published: July 2002

Formal expressions are derived for the surface tension of a fluid in contact with a hard spherical wall or a cavity. A method is provided for expansions in powers of curvature and applied to the first correction. Four contributions are found, one of them of the form due to Tolman.
Activated diffusion of benzene in NaY zeolite: Rate constants from transition state theory with dynamical corrections
Authors: Jousse Fabien , Auerbach Scott M.
Published: July 2002

We calculated transition state theory and exact rate coefficients for benzene jumps in Na-Y zeolite between 150 and 500 K. This is the first exact flux correlation function rate calculation for a non-spherical molecule inside a zeolite. We calculated rates for jumps between SII and W sites, located near Na ions in 6-rings and in 12-rings windows, respectively. Partition function ratios were calculated using Voter’s displacement vector method. A general Arrhenius behavior is observed over the whole temperature range for all processes. The activation energies are close to the difference between the minimum energies in the sites, and between the sites and the transition states. The calculated prefactors present reasonable values around 1012–1013 s−1, in good agreement with nuclear magnetic resonance relaxation experiments. We were not able to decompose the prefactors into simple vibrational and entropic components, and therefore a complete calculation of the rate constant seems necessary to obtain reliable values. In three of the four types of motions investigated, the transition state theory rate constant is approximately equal to the more exact correlation function rate constant. However, in the case of the W→W jump, transition state theory is qualitatively wrong. This is due to the fact that the minimum energy path from one W site to another is very unstable and intersects the SII→SII minimum energy pathway, so a slight perturbation sends the molecule to a SII site instead of the W site. As a consequence, the prefactor for the W→W jump is found to be almost one order of magnitude smaller than the prefactor for the W→ SII jump, although the activation energies are similar.
Finite chain length effects on the coil–globule transition of stiff-chain macromolecules: A Monte Carlo simulation
Authors: Ivanov V. A. , Paul W. , Binder K.
Published: July 2002

We study the effect of finite chain length on the collapse transition of stiff-chain macromolecules by means of a Monte Carlo simulation within the framework of the bond fluctuation lattice model. Variable stiffness of the chains was modeled by introducing a potential depending on the angle between successive bonds and we introduced an additional quasi-Lennard-Jones potential between monomer units which are not nearest neighbors along the chain to model the quality of the solvent. Chains of length up to 200 monomer units were simulated. For the flexible case these chains are long enough to determine the θ-temperature, but for higher stiffnesses we show systematic effects in the dependence of the apparent transition temperature on the stiffness. For fixed chain lengths we determine apparent phase diagrams and give the apparent transition points and points of ideal chain size as a function of stiffness. We report on the occurrence of a toroidal structure in our model and characterize this structure by local and global packing and orientational ordering.
Experimental determination of the ground-state inversion splitting in D3O+ by microwave spectroscopy
Authors: Araki Mitsunori , Ozeki Hiroyuki , Saito Shuji
Published: July 2002

Inversion–rotation transitions of the fully deuterated hydronium ion, D3O+, were observed for the first time by microwave spectroscopy. The ion was generated in a hollow cathode cell by dc-glow discharge of a mixture of D2O and D2. Twenty six P- and Q-branch transitions were measured precisely for the lowest pair levels of inversion motion in the frequency region of 220–565 GHz. The ground-state inversion splitting and effective molecular constants for the upper and lower levels were determined by a least-squares fit of the measured line frequencies. The inversion splitting was determined to be 15.355 504 4(45) cm−1, where the number in parentheses denotes three standard deviations of the fit.
Rotational dynamics of neutral red: Do ionic and neutral solutes experience the same friction?
Authors: Dutt G. B. , Singh M. K. , Sapre A. V.
Published: July 2002

The rotational dynamics of neutral and cationic forms of the phenazine dye neutral red has been studied in n-alcohols, amides, and aprotic solvents using picosecond time-resolved fluorescence depolarization spectroscopy. While both the neutral and cationic forms of neutral red experienced more or less the same friction in alcohols, the cationic form experienced 16%–26% more friction in amides and aprotic solvents exceptions being formamide and propylene carbonate (PC). The results were analyzed in terms of the Stokes–Einstein–Debye (SED) hydrodynamic theory and dielectric friction theories of Nee–Zwanzig and van der Zwan–Hynes. Both the Nee–Zwanzig and van der Zwan–Hynes dielectric friction theories overestimate the dielectric friction contribution for the neutral form of neutral red in alcohols. The rotational dynamics of neutral form of neutral red in N, N-dimethyl formamide (DMF), N, N-dimethyl acetamide (DMA), N, N-dimethyl propionamide (DMP), and dimethyl sulphoxide (DMSO) is adequately described by the hydrodynamic model with the stick boundary condition. However, it overestimates the friction experienced in formamide, and to a certain extent in PC wherein for both forms similar reorientation times were observed. As the cations are strongly solvated by amides only 60%–70% of the friction experienced in DMF, DMA, and DMP can be accounted for by the SED theory.
An information-entropic study of correlated densities of the water molecule
Authors: Hồ Minhhuy , Weaver Donald F. , Smith Vedene H. , Sagar Robin P. , Esquivel Rodolfo O. , Yamamoto Shigeyoshi
Published: July 2002

The Shannon entropy of the water molecule was calculated at different correlation levels including full configuration interaction (CI) for the D95 basis set. The results show that an analysis of both the position and momentum space entropy yields insights into the characteristics of different correlated methods from the density perspective and provides an alternative way of interpreting the wave function. Various changes in the electronic densities intrinsic to these correlation methods are also related to concepts within the information entropy framework.
Thermodynamic expansion of nucleation free-energy barrier and size of critical nucleus near the vapor-liquid coexistence
Authors: Koga Kenichiro , Zeng X. C.
Published: July 2002

Nucleation free-energy barrier height and size of the critical nucleus are expanded in powers of the chemical potential difference between the supersaturated vapor (or expanded liquid) in the metastable state and the saturated vapor-liquid system in the stable equilibrium state at the same temperature. The coefficients in the expansions are expressed in terms of the thermodynamic properties at the stable equilibrium state. Comparisons with the results obtained from the density-functional calculation for nucleation of the Lennard–Jones fluid show that systematic improvement in predicting properties of the critical nucleus, either liquid droplet or vapor cavity, is achieved by adding the higher order terms in the expansions. The scaling relations proposed by McGraw and Laaksonen are found to be good approximations to the general expansion; in particular, the barrier height displacement appearing in these scaling relations is naturally given as the second order coefficient in the expansion of the barrier height.
The solvation of chloride by methanol—surface versus interior cluster ion states
Authors: Cabarcos Orlando M. , Weinheimer Corey J. , Martı́nez Todd J. , Lisy James M.
Published: July 2002

A combined experimental and theoretical structural study of methanolated chloride anions has been conducted, utilizing infrared vibrational spectroscopy and ab initio electronic structure calculations. These results represent the first such study for an anion with a nonaqueous solvent. The principal question addressed is whether the cluster ions assume structures reflecting surface or interior solvated states. The vibrational spectra in the O–H stretching regions, for Cl−(CH3OH)1–8,10,12, and calculated O–H vibrational bands for Cl−(CH3OH)1–4, consistently indicate that the chloride anion undergoes surface solvation. The behavior is remarkably similar to that of hydrated anions (chloride, bromide, and iodide) with large polarizabilities. This suggests that the asymmetric hydration of these anions lies not necessarily in the nature of the solvent, but in the nature of the anion.
Solid–liquid phase equilibrium for binary Lennard-Jones mixtures
Authors: Hitchcock Monica R. , Hall Carol K.
Published: July 2002

Solid–liquid phase diagrams are calculated for binary mixtures of Lennard-Jones spheres using Monte Carlo simulation and the Gibbs–Duhem integration technique of Kofke. We calculate solid–liquid phase diagrams for the model Lennard-Jones mixtures: argon–methane, krypton–methane, and argon–krypton, and compare our simulation results with experimental data and with Cottin and Monson’s recent cell theory predictions. The Lennard-Jones model simulation results and the cell theory predictions show qualitative agreement with the experimental phase diagrams. One of the mixtures, argon–krypton, has a different phase diagram than its hard-sphere counterpart, suggesting that attractive interactions are an important consideration in determining solid–liquid phase behavior. We then systematically explore Lennard-Jones parameter space to investigate how solid–liquid phase diagrams change as a function of the Lennard-Jones diameter ratio, σ11/σ22, and well-depth ratio, ε11/ε22. This culminates in an estimate of the boundaries separating the regions of solid solution, azeotrope, and eutectic solid–liquid phase behavior in the space spanned by σ11/σ22 and ε11/ε22 for the case σ11/σ22<0.85.
Vibronic spectroscopy of jet-cooled indazole: S1↔S spectra and mode assignments
Authors: Jalviste Erko , Temps F.
Published: July 2002

The S0↔S1 electronic transition of 1H-indazole has been investigated by laser-induced fluorescence (LIF) excitation and dispersed fluorescence (DF) spectroscopies in a supersonic free jet expansion. Ground and excited state vibrational modes were assigned based on the observed band frequencies and intensities, ab-initio frequency calculations, and available infrared absorption data. Unexpected “symmetry forbidden” transitions were observed in the DF, which, according to our assignments, would originate from the 271 excited state in-plane fundamental to several ground state out-of-plane fundamentals. Accepting the assignments, this observation would indicate a vibronic coupling between the S1 (or S0) state (A′) and an excited nπ*-type electronic state of A″ symmetry.
First-order correction to classical nucleation theory: A density functional approach
Authors: Barrett Jonathan
Published: July 2002

It is shown that the classical expression for the change in grand potential of a system on formation of a cluster of radius R is modified by a factor [1−(2w+6δT)/R], to first order in 1/R, where w is a correction due to the nonzero compressibilities of liquid and vapor (near the triple point, w is approximately equal to the product of liquid compressibility and surface tension), and δT is the coefficient in the expression relating the surface tension of the droplet, γ(R), to the planar surface tension, γ∞, i.e., γ(R)=γ∞(1−2δT/R). An expression for δT is derived involving the pair and triplet correlation functions and the density profile of the planar surface. This complements the expression for δT involving the pair distribution function derived by Blokhuis and Bedeaux; the equivalence of the two expressions in the low density limit is demonstrated. Calculations of δT and w are performed using mean-field density functional theory for the Yukawa potential and an r−6 potential, as well as using the square-gradient approximation. δT is found to be negative for all conditions investigated; its magnitude depends on the potential used, and tends to increase with increasing temperature. However, the ratio δT/w is found to be relatively insensitive to potential and to temperature, being between about −1.2 and −1.5 for the conditions investigated. The effect of using a weighted density approximation in place of the local density approximation for the hard-sphere part of the potential is estimated in a square-gradient approximation and found to be small.
Influences of concerted cluster diffusion on single-file diffusion of CF4 in AlPO4-5 and Xe in AlPO4-31
Authors: Sholl David S. , Lee Cha Kun
Published: July 2002

Traditional models of single-file diffusion in zeolites assume that adsorbate diffusion mechanisms at high and dilute pore loadings are identical. Atomistic models of single file systems, in contrast, have shown that clusters of adsorbed molecules can diffuse via concerted motions that are not available to individual adsorbates. We report molecular dynamics simulations of CF4 clusters in AlPO4-5 and Xe clusters in AlPO4-31. We have used these simulations to characterize the diffusion and dissociation mechanisms and rates of these clusters as a function of temperature and cluster size. We have also used our MD results to parametrize coarse-grained models of adsorbed cluster dynamics. Using kinetic Monte Carlo simulations of our coarse-grained models, we have simulated pores containing hundreds of interacting clusters on microsecond time scales. These simulations offer the first opportunity to examine single-file diffusion in models that accurately account for the existence of concerted cluster diffusion. Our simulations show that concerted cluster motions make large contributions to single-file mobilities. The deviations of the computed single-file mobilities from the well-known result for hard sphere diffusion are discussed, as are the implications of using the latter model to interpret experimental measurements of single-file diffusion.
Crossed beam reaction of phenyl radicals with unsaturated hydrocarbon molecules. I. Chemical dynamics of phenylmethylacetylene (C6H5CCCH3;X 1A′) formation from reaction of C6H5(X 2A1) with methylacetylene, CH3CCH(X 1A1)
Authors: Kaiser R. I. , Asvany O. , Lee Y. T. , Bettinger H. F. , Schleyer P. v. R. , Schaefer H. F.
Published: July 2002

The chemical reaction dynamics to form phenylmethylacetylene, C6H5CCCH3(X 1A′), via reactive collisions of the phenyl radical C6H5(X 2A1) with methylacetylene, CH3CCH(X 1A1), are unraveled under single collision conditions in a crossed molecular beam experiment at a collision energy of 140 kJ mol−1. The laboratory angular distribution and time-of-flight spectra of C9H8+ at m/e=116 indicate the existence of a phenyl radical versus hydrogen replacement pathway. Partially deuterated methylacetylene, CH3CCD(X 1A1), was used to identify the site of the carbon–hydrogen bond cleavage. Only the loss of the acetylenic hydrogen atom was observed; the methyl group is conserved in the reaction. Electronic structure calculations reveal that the reaction has an entrance barrier of about 17 kJ mol−1. Forward-convolution fitting of our data shows that the chemical reaction dynamics are on the boundary between an osculating complex and a direct reaction and are governed by an initial attack of the C6H5 radical to the π electron density of the C1 carbon atom of the methylacetylene molecule to form a short lived, highly rovibrationally excited (C6H5)HCCCH3 intermediate. The latter loses a hydrogen atom to form the phenylmethylacetylene molecule on the A′2 surface. The phenylallene isomer channel was not observed experimentally. The dynamics of the title reaction and the identification of the phenyl versus hydrogen exchange have a profound impact on combustion chemistry and chemical processes in outflows of carbon stars. For the first time, the reaction of phenyl radicals with acetylene and/or substituted acetylene is inferred experimentally as a feasible, possibly elementary reaction in the stepwise growth of polycyclic aromatic hydrocarbon precursor molecules and alkyl substituted species in high temperature environments such as photospheres of carbon stars and oxygen poor combustion systems.
Global geometry optimization of silicon clusters using the space-fixed genetic algorithm
Authors: Iwamatsu Masao
Published: July 2002

The space-fixed genetic algorithm originally proposed by Niesse and Mayne [J. Chem. Phys. 105, 4700 (1996)] is modified and used to study the lowest energy structure of small silicon clusters by employing empirical interatomic potentials. In this new space-fixed genetic algorithm, a gradient-free simplex method, rather than the conventional gradient-driven conjugate gradient minimization employed by Niesse and Mayne, is selected by virtue of its flexibility and applicability to any form of interatomic potentials for which the calculation of derivatives is difficult. Using two empirical three-body potentials, we calculated the ground state structure up to Si15 successfully using this new genetic algorithm based on the simplex method. The effect of angular dependent three-body potentials on the cluster structures is examined and compared with the experimental results.
Exact solvability of the Mullins nonlinear diffusion model of groove development
Authors: Broadbridge P.
Published: July 2002

The Mullins equation for the development of a surface groove by evaporation–condensation is yt=yxx/1+y2x. It is pointed out that this is the equation of the potential for the field variable Θ satisfying the nonlinear diffusion equation Θt=∂x[Θx/1+Θ2]. The latter has already been solved exactly, with boundary conditions corresponding exactly to those specified by Mullins. The depth of a groove at a grain boundary is predicted exactly without first making the linear (small-slope) approximation. For some types of initial data, the Cauchy problem may be solved for some related equations.
A family of nonlinear Klein–Gordon equations and their solutions
Authors: Grundland A. M. , Infeld E.
Published: July 2002

Various forms of the nonlinear Klein–Gordon equation are seen to have exact, solitonlike solutions when separation of variables is postulated. The family for which these exact solutions are found includes the sine-Gordon equation as a special case. An interesting conclusion obtained is that both the soliton–antisoliton and breather solutions, hitherto known for the sine-Gordon equations result from a broader class of Klein–Gordon equations. The method can be extended to other equations.
Supersymmetric quantum mechanics and the Korteweg–de Vries hierarchy
Authors: Grant Aaron K. , Rosner Jonathan L.
Published: July 2002

The connection between supersymmetric quantum mechanics and the Korteweg–de Vries (KdV) equation is discussed, with particular emphasis on the KdV conservation laws. It is shown that supersymmetric quantum mechanics aids in the derivation of the conservation laws, and gives some insight into the Miura transformation that converts the KdV equation into the modified KdV equation. The construction of the τ function by means of supersymmetric quantum mechanics is discussed.
Second harmonic generation in nonlinear optical films
Authors: Bao Gang , Dobson David C.
Published: July 2002

Second harmonic generation, an important phenomenon in nonlinear optics, is modeled in this work. The model is derived from a nonlinear system of Maxwell’s equations, which overcomes the known shortcomings of some commonly used models in the literature. Existence and uniqueness of solutions are established by a combination of a variational approach and the contraction mapping principle. Some numerical results are also presented.
Quantization of Lie group and algebra of G2 type in the Faddeev–Reshetikhin–Takhtajan approach
Authors: Sasaki Norihito
Published: July 2002

Based on the quantized universal enveloping (QUE) algebras, a quantization of the automorphism group of some nonassociative algebras is given in the formulation employing noncommuting matrix entries. A quantum group of G2 type included in this scheme is studied in detail in the Faddeev–Reshetikhin–Takhtajan (FRT) approach. Also in the formulation employing noncommuting matrix entries, the QUE-algebra of G2 type is reconstructed through the pairing induced by the R-matrix between the quantum group and the QUE-algebra.
Relativistic spin networks and quantum gravity
Authors: Barrett John W. , Crane Louis
Published: July 2002

Relativistic spin networks are defined by considering the spin covering of the group SO(4), SU(2)×SU(2). Relativistic quantum spins are related to the geometry of the two-dimensional faces of a 4-simplex. This extends the idea of Ponzano and Regge that SU(2) spins are related to the geometry of the edges of a 3-simplex. This leads us to suggest that there may be a four-dimensional state sum model for quantum gravity based on relativistic spin networks that parallels the construction of three-dimensional quantum gravity from ordinary spin networks.
Liquid oscillations in a basin with varying surface area
Authors: Green Theodore
Published: July 2002

Changes of the Helmholtz mode of oscillation in a basin associated with the variation of basin water-surface area with that of water level are calculated. The oscillation period decreases slightly and higher harmonics appear in the spectrum.
Development of enstrophy and spectra in numerical turbulence
Authors: Herring Jackson R. , Kerr Robert M.
Published: July 2002

Decaying isotropic turbulence with initial Taylor microscale Reynolds number (Rλ≤258) is studied via direct numerical simulations (DNS), with spectral resolution ≤2563. DNS results are compared with two-point closure, in the form of the direct interaction approximation (DIA) and the test field model (TFM). The goals of this study are to understand the time-dependence of enstrophy and spectra as they evolve from random initial conditions, and to assess and interpret differences between DNS and closure. Two time scales are identified in the DNS. The first is that for the development of normalized enstrophy production (velocity derivative skewness) and is independent of Rλ. The second is that for the saturation of the enstrophy which follows after a longer period of near exponential growth and is strongly Rλ dependent. For ν≠0, the DIA represents the time development of both integral quantities, such as enstrophy and spectra, with surprising accuracy in spite of its lack of invariance to random large-scale sweeping. The TFM has significant energy range errors, which we attribute to its Markovianization assumption.
Bootstrap current induced by fusion born alpha particles
Authors: Hsu C. T. , Shaing K. C. , Gormley R. P. , Sigmar D. J.
Published: July 2002

The bootstrap current produced by fusion born alpha particles is obtained, retaining effects of slowing down drag, pitch angle scattering, and arbitrary aspect ratio. The result is presented both as a summation of a rapidly converging series and a simple Padé approximation good for arbitrary aspect ratio. Quantitative results are derived using the International Thermonuclear Experimental Reactor (ITER) [Plasma Phys. Controlled Nucl. Fusion Res. (International Atomic Energy Agency, Vienna, 1989), Vol. 3, p. 214] parameters.
Convective cells in nonuniform dusty plasmas
Authors: Shukla P. K. , Varma R. K.
Published: July 2002

It is shown that purely damped convective cell modes acquire a real frequency in the presence of static charged dust grains in a nonuniform magnetized dusty plasma. The frequency of the two-dimensional mode is induced by the plasma density gradient and corresponds to charge density waves arising out of the plasma non-neutrality. The dynamics of weakly interacting finite-frequency convective cell modes is governed by the generalized Navier–Stokes equation. The presence of the new term arising from the dust inhomogeneity in the latter provides the possibility of a two-dimensional dipolar vortex solution in dusty plasmas.
Cnoidal standing waves and the transition to the traveling hydraulic jump
Authors: Bridges Thomas J.
Published: July 2002

Standing waves of finite amplitude in an enclosed basin are considered. It is shown that the existing theory of Tadjbakhsh and Keller [J. Fluid Mech. 3, 442 (1960)] breaks down at a critical value of the amplitude dependent upon the depth. By formal perturbation expansion it is found that the solutions change to cnoidal standing waves at this critical value of the amplitude. An experiment is constructed which verifies this theory and also elucidates the wavefield when the amplitude is much larger than the critical amplitude. Based on the experimental observations, a cascade theory is proposed to explain the transition to a traveling hydraulic jump.
Existence and calculation of sharp boundary magnetohydrodynamic equilibrium in three-dimensional toroidal geometry
Authors: Berk H. L. , Freidberg J. P. , Llobet X. , Morrison P. J. , Tataronis J. A.
Published: July 2002

The problem of sharp boundary, ideal magnetohydrodynamic equilibria in three-dimensional toroidal geometry is addressed. The sharp boundary, which separates a uniform pressure, current-free plasma from a vacuum, is determined by a magnetic surface of a given vacuum magnetic field. The pressure balance equation has the form of a Hamilton–Jacobi equation with a Hamiltonian that is quadratic in the momentum variables, which are the two covariant components of the magnetic field on the outer surface of the plasma. The condition of finding a unique solution on the outer surface is identical with finding phase-space tori in nonlinear dynamics problems, and the Kolmogorov–Arnold–Moser (KAM) theorem guarantees that such solutions exist for a wide band of parameters. Perturbation theory is used to calculate the properties of the magnetic field just outside the plasma. Special perturbation theory is needed to treat resonances and it is explicitly shown that there are bands of pressure where there are no solutions.
An experimental study of rivulet instabilities in centrifugal spin coating of viscous Newtonian and non-Newtonian fluids
Authors: Fraysse Nathalie , Homsy George M.
Published: July 2002

An experimental study on the onset and evolution of centrifugally driven rivulets is presented, which aims to investigate the influence on the instability of various experimental conditions (drop volume and rotational frequency), the wetting properties of the liquid (surface tension and contact angle), and fluid viscoelasticity. The apparatus allows continuous observation of the drop shapes following an impulsive spin-up of the substrate, and these are analyzed by digital image analysis. The flows exhibit an onset time, or, equivalently, a critical radius, before which the drop spreads axisymmetrically. Data on drop spreading are compared with simple predictions of lubrication theory. The measured azimuthal wave number and growth rate of the instability are in good agreement with the linear stability analysis of Troian et al. [Europhys. Lett. 10, 25 (1989)], as long as the critical radius is taken from the experiment itself. The most unstable wavelength is found to be independent of both drop size and rotation speed in the range of parameters investigated, as observed previously by Melo et al. [Phys. Rev. Lett. 63, 1958 (1989)]. On the other hand, a change in the wetting properties of the liquid significantly modifies the critical radius, which, in turn, affects the number of fingers, with the nonwetting fluid exhibiting a smaller critical radius. This trend is in agreement with the mechanism of instability that is linked to the presence of a capillary ridge near the edge of the drop. No qualitative nor quantitative difference in behavior has been observed between a Boger fluid having a relaxation time of about 1 s, and its Newtonian solvent, in the experimental conditions considered.
Resistive destabilization of cycloidal electron flow and universality of (near-) Brillouin flow in a crossed-field gap
Authors: Christenson Peggy J. , Chernin David P. , Garner Allen L. , Lau Y. Y.
Published: July 2002

It is shown that a small amount of dissipation, caused by current flow in a lossy external circuit, can produce a disruption of steady-state cycloidal electron flow in a crossed-field gap, leading to the establishment of a turbulent steady state that is close to, but not exactly, Brillouin flow. This disruption, which has nothing to do with a diocotron or cyclotron instability, is fundamentally caused by the failure of a subset of the emitted electrons to return to the cathode surface as a result of resistive dissipation. This mechanism was revealed in particle simulations, and was confirmed by an analytic theory. These near-Brillouin states differ in several interesting respects from classic Brillouin flow, the most important of which is the presence of a microsheath and a time-varying potential minimum very close to the cathode surface. They are essentially identical to that produced when (i) injected current exceeds a certain critical value [P. J. Christenson and Y. Y. Lau, Phys. Plasmas 1, 3725 (1994)] or (ii) a small rf electric field is applied to the gap [P. J. Christenson and Y. Y. Lau, Phys. Rev. Lett. 76, 3324 (1996)]. It is speculated that such near-Brillouin states are generic in vacuum crossed-field devices, due to the ease with which the cycloidal equilibrium can be disrupted. Another novel aspect of this paper is the introduction of transformations by which the nonlinear, coupled partial differential equations in the Eulerian description (equation of motion, continuity equation, Poisson equation, and the circuit equation) are reduced to an equivalent system of very simple linear ordinary differential equations.
Collapse of cycloidal electron flows induced by misalignments in a magnetically insulated diode
Authors: Garner A. L. , Lau Y. Y. , Chernin D.
Published: July 2002

The effect of a slight misalignment in the magnetic field on a magnetically insulated diode is investigated. It is found that a slight tilt in the magnetic field, with a minute component along the dc electric field, completely destabilizes the cycloidal electron flow in the crossed-field gap. The final state consists of the classical Brillouin flow superimposed by a turbulent background, together with a slow electron drift across the gap. This disruption of the cycloidal flow is quite insensitive to the emission current density, and is due to the accumulation of space charge in the gap caused by the magnetic misalignment. This result was obtained from a one-dimensional simulation code. It reinforces the notion that the turbulent, near Brillouin-like states are generic in ALL vacuum crossed-field devices.
Density profile consistency and its relation to the transport of trapped versus passing electrons in tokamaks
Authors: Baker D. R. , Rosenbluth M. N.
Published: July 2002

A formal expression for the canonical steady-state density profile in a tokamak can be obtained from the Fokker–Planck-type diffusion equation derived from the Vlasov equation in the limit of anomalous diffusion due to strong turbulence. Here we derive an explicit expression for this canonical profile for a tokamak with arbitrary cross section and aspect ratio. The resulting profile is independent of the spatial dependence of the diffusion coefficient, but does depend on the relative diffusion of trapped versus passing particles. Under conditions where only the trapped particles transport due to interactions with the turbulence the profiles are considerably flatter than if both the trapped and passing transport the same. The steepness of the calculated profile depends on the ratio of the diffusion coefficients for passing and trapped particles. The calculated profiles are compared with measured profiles from the tokamak known as DIII-D [J. L. Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159]. Density profiles for a typical International Thermonuclear Experimental Reactor (ITER) [R. Aymar, Fusion Eng. Design 24, 977 (1984)] plasma are also derived.
Boundary effects on dust-ion-acoustic and dust-acoustic waves in collisional dusty plasmas
Authors: Shukla P. K. , Rosenberg M.
Published: July 2002

The dispersion relations for dust-ion-acoustic and dust-acoustic waves are obtained, taking into account boundary and collisional effects in unmagnetized dusty plasma waveguides. The relevance of this investigation to low-temperature laboratory dusty plasmas is discussed.
Single-mode waveguide microlenses and microlens arrays fabrication in LiNbO3 using titanium indiffused proton exchange technique
Authors: Zang D. Y. , Tsai C. S.
Published: July 2002

Single-mode planar waveguide microlenses and microlens arrays in LiNbO3 have been fabricated using a combination of titanium diffusion and proton exchange processes. The lenses have demonstrated properties such as very short focal length, micron focal spot size, and large angular field of view that are desirable for applications in integrated and fiber-optic signal processing and computing as well as communication systems.
Compensation in epitaxial cubic SiC films
Authors: Segall B. , Alterovitz S. A. , Haugland E. J. , Matus L. G.
Published: July 2002

Hall measurements on four n-type cubic SiC films epitaxially grown by chemical vapor deposition on (100) Si substrates are reported. Detailed analyses of the temperature-dependent carrier concentrations indicate that the samples are highly compensated (>90%), contrary to the assumption of no compensation made in previous studies of similarly prepared SiC films. Donor ionization energies ED are found to be less than one-half the values previously reported. The values for ED and the donor concentration ND, combined with results for small bulk platelets with nitrogen donors, suggest the relation ED(ND)=ED(0)−αN1/3D for cubic SiC. A curve fit gives α≂2.6×10−5 meV cm and ED(0)≂48 meV, which is the generally accepted value of ED(0) for nitrogen donors in cubic SiC.
Substrate hole current and oxide breakdown
Authors: Chen I. C. , Holland S. , Young K. K. , Chang C. , Hu C.
Published: July 2002

It is known that when an n-channel metal-oxide-semiconductor field-effect transistor is biased with a high positive gate voltage, a hole current appears in the substrate cathode. Recent experiments indicate that the holes are generated within the oxide. We show that this hole generation mechanism is linked to oxide time-dependent breakdown. When the hole fluence reaches a certain critical value, breakdown occurs. This is in agreement with a hole-trapping-induced breakdown model. For very thin oxides the hole generation rate can become so low that the substrate hole current is dominated by the tunneling of valence-band electrons which is not expected to contribute to oxide breakdown. A different mechanism of hole generation such as hot-hole tunneling from the anode may be responsible for oxide breakdown in the important case of low gate voltage (<6 V).
Dislocation microstructures on flat and stepped Si surfaces: Guidance for growing high-quality GaAs on (100) Si substrates
Authors: Lo Y. H. , Wu M. C. , Lee H. , Wang S. , Liliental-Weber Z.
Published: July 2002

Type-I dislocations at the GaAs/Si interface are beneficial because they effectively relax the mismatched stress, but do not propagate into the GaAs film. Accordingly, the best way to grow a low defect density GaAs film on a Si substrate is to form as many as possible type-I dislocations or, equivalently, to suppress other kinds of defects. The high-resolution transmission electron microscopy study shows that most of the type-I dislocations are formed at the double step on a Si surface. It is further determined that the silicon surface steps are mainly due to the substrate tilting instead of the heating before growth. Based on our study, the (100) Si substrate with double steps along both [110] and [11̄0] axes provides the best condition for growing low defect density GaAs on Si substrates.
Self-phase modulation and optical limiting of a low-power CO2 laser with a nematic liquid-crystal film
Authors: Khoo I. C. , Michael R. R. , Finn G. M.
Published: July 2002

The extraordinarily large thermal refractive index gradient of a nematic liquid crystal is employed for transverse self-phase modulation of a cw CO2 laser. The observed optical intensity limiting and switching are in good agreement with theoretical expectations and provide useful guidelines for infrared-beam-limiting applications.
TbOF complex centers in ZnS thin-film electroluminescent devices
Authors: Okamoto Kenji , Yoshimi Takuya , Miura Shoshin
Published: July 2002

The effect of oxygen doping in ZnS:TbF thin-film electroluminescent (EL) devices has been studied. We concluded that the TbOF complex center is more efficient than the TbF complex center in ZnS thin-film EL devices. The improvement is attributed to the decreased nonradiative emission process of Tb and/or enlarged lattice distortions which generate electron-hole pairs efficiently at the Tb excitation site.
Loss of brain-derived neurotrophic factor-dependent neural crest-derived sensory neurons in neurotrophin-4 mutant mice
Authors: Liebl Daniel J. , Klesse Laura J. , Tessarollo Lino , Wohlman Todd , Parada Luis F.
Published: July 2002

Peripheral ganglion neurons confer sensory information including touch, pain, temperature, and proprioception. Sensory modality is linked to specific neurotrophin (NTF) requirements. NT-3 supports survival of neurons that differentiate primarily into proprioceptors whereas nerve growth factor and brain-derived neurotrophic factor (BDNF) support subpopulations that transmit nociception and mechanoreception, respectively. We examined sensory neurons of gene-targeted mouse mutants at the NT-4, BDNF, NT-3, and TrkA loci. We show that NT-4 functions early in gangliogenesis, upstream of BDNF. In the absence of NT-4 function, BDNF-dependent, TrkB-expressing neurons fail to appear. The results are consistent with the model that precursor cells intended to become BDNF-dependent mechanoreceptors instead differentiate into NT-3-dependent proprioceptive neurons.
WASp verprolin homology, cofilin homology, and acidic region domain-mediated actin polymerization is required for T cell development
Authors: Zhang Jinyi , Shi Fabio , Badour Karen , Deng Yupu , McGavin Mary K. H. , Siminovitch Katherine A.
Published: July 2002

All members of the Wiskott–Aldrich syndrome protein (WASp) family contain a carboxyl-terminal verprolin homology, cofilin homology, and acidic region (VCA) domain that binds and activates the Arp2/3 complex, thereby linking these proteins to the induction of actin polymerization. Although the VCA domain imbues WASp and other WASp family members with the capacity to modulate cytoskeletal organization, little is known about the impact of this domain activity on lymphoid cell function. Here we demonstrate that T cell-restricted expression of VCA domain-deleted WASp (WASpΔVCA) in WAS−/−mice engenders a severe early block in T lymphopoiesis associated with impaired T cell antigen receptor αβ expression and a consequent failure to generate single-positive CD4+and CD8+T cells. These latter defects, which are not observed in WAS−/−mice, are associated with impaired induction of cellular actin polymerization and a failure in the terminal differentiation of double-negative thymocytes. These findings indicate that WASp family proteins play an essential role in modulating the signaling events required for early thymocyte development and reveal their capacity to subserve this role to depend on VCA domain-mediated actin polymerization.
Fold prediction of helical proteins using torsion angle dynamics and predicted restraints
Authors: Zhang Chao , Hou Jingtong , Kim Sung-Hou
Published: July 2002

We describe a procedure for predicting the tertiary folds of α-helical proteins from their primary sequences. The central component of the procedure is a method for predicting interhelical contacts that is based on a helix-packing model. Instead of predicting the individual contacts, our method attempts to identify the entire patch of contacts that involve residues regularly spaced in the sequences. We use this component to glue together two powerful existing methods: a secondary structure prediction program, whose output serves as the input to the contact prediction algorithm, and the tortion angle dynamics program, which uses the predicted tertiary contacts and secondary structural states to assemble three-dimensional structures. In the final step, the procedure uses the initial set of simulated structures to refine the predicted contacts for a new round of structure calculation. When tested against 24 small to medium-sized proteins representing a wide range of helical folds, the completely automated procedure is able to generate native-like models within a limited number of trials consistently.
Antiviral activities of the soluble extracellular domains of type I interferon receptors
Authors: Han Chun-Sheng , Chen Yizhen , Ezashi Toshihiko , Roberts R. Michael
Published: July 2002

Alternative splicing leads to the expression of multiple isoforms of the subunits (IFNAR1 and IFNAR2) of the type I IFN receptor. Here we describe two transcripts representing extracellular forms of ovine IFNAR1 and show that soluble extracellular forms of both IFNAR2 and IFNAR1, prepared in recombinant form inEscherichia coli, have antiviral (AV) activity in the absence of IFN. Exposure of Madin-Darby bovine kidney cells to the extracellular domain (R2E) of IFNAR2 at concentrations as low as 10 nM afforded complete protection against vesicular stomatitis virus and led to the rapid activation of the transcription factors ISGF3 and GAF. Although R2E can bind IFN (Kd≈70 nM), activity was observed irrespective of whether or not ligand was present. R2E was inactive on mouse L929 cells but active on L929 cells expressing a membraneanchored, ovine/human chimeric IFNAR2 with an ovine extracellular domain. The data suggest that AV activity is conferred by the ability of soluble R2E to associate with the transfected IFNAR2 subunit rather than resident murine IFNAR1. Soluble extracellular forms of IFNAR1 have lower AV activity than R2E on Madin-Darby bovine kidney cells but are less species-specific and protect wild-type L929 cells as efficiently as the transfected cell line, presumably by interacting with one of the murine receptor subunits.
Imaging transcriptional regulation of p53-dependent genes with positron emission tomography
in vivo
Authors: Doubrovin Michael , Ponomarev Vladimir , Beresten Tatiana , Balatoni Julius , Bornmann William , Finn Ronald , Humm John , Larson Steven , Sadelain Michel , Blasberg Ronald , Gelovani Tjuvajev Juri
Published: July 2002

A noninvasive method for molecular imaging of the activity of different signal transduction pathways and the expression of different genes
in vivo
would be of considerable value. It would aid in understanding the role specific genes and signal transduction pathways have in various diseases, and could elucidate temporal dynamics and regulation at different stages of disease and during various therapeutic interventions. We developed and assessed a method for monitoring the transcriptional activation of endogenous genes by positron-emission tomography (PET) imaging. The
HSV1-tk
/
GFP
(
TKGFP
) dual reporter gene was used to monitor transcriptional activation of p53-dependent genes. A retrovirus bearing the
Cis
-p53/TKGFP reporter system was constructed in which the
TKGFP
reporter gene was placed under control of an artificial cis-acting p53-specific enhancer. U87 glioma and SaOS-2 osteosarcoma cells were transduced with this retrovirus and used to establish xenografts in rats. We demonstrated that DNA damage-induced up-regulation of p53 transcriptional activity correlated with the expression of p53-dependent downstream genes, such as p21, in U87 (wild-type p53), but not in SaOS-2 osteosarcoma (p53 −/−) cells. We showed that PET, with [
124
I]FIAU (2′-fluoro-2′-deoxy-1-β-
d
-arabinofuranosyl-5-[
124
I]iodouracil) and the
Cis
-p53TKGFP reporter system, is sufficiently sensitive to image the transcriptional regulation of genes in the p53 signal transduction pathway. These imaging results were confirmed by independent measurements of p53 activity and the expression levels of downstream genes (e.g., p21) by using conventional molecular-biological assays. PET imaging of p53 transcriptional activity in tumor xenografts by using the
Cis
-p53TKGFP reporter system may be useful in assessing novel therapeutic approaches.