bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling and analysis of moving temperature patterns on catalytic surfaces
Authors: Colin Pierre , Balakotaiah Vemuri
Published: July 2002

A mathematical model that predicts moving temperature and concentration patterns on nonisothermal catalytic surfaces is developed and analyzed. The model accounts for a slow change of the surface activity of the catalyst, diffusion of the species, conduction of heat, convection from the fluid phase, and a Langmuir–Hinshelwood-type kinetic expression. It is shown that this model predicts ignition, extinction, and homogeneous oscillations for a wide range of parameter values. It is found that the model does not predict stationary temperature patterns for typical values of the transport coefficients. However, the model predicts moving (oscillating) temperature and concentration patterns for typical parameter values. The calculations show that these spatiotemporal patterns exist in regions near the homogeneous Hopf bifurcation point indicating that homogeneous oscillations are unlikely to occur. It is also found that the typical size of these moving patterns is of the order of 1 cm2 and the period of oscillation is smaller but of the same order of magnitude as the period of homogeneous oscillation.
Resonance enhanced multiphoton ionization of the P2 radical
Authors: Howe J. D. , Ashfold M. N. R. , Hudgens J. W.
Published: July 2002

P2 radicals have been detected using resonance-enhanced multiphoton ionization (REMPI) spectroscopy via the C 1∑+u–X 1∑+g transition. The simple structure of the origin band of this transition, which is free from significant perturbations, is amenable to straightforward simulation and thus provides an excellent route both for the detection and the characterization of ground state P2 radicals.
Solid-state dynamic processes in complex systems analyzed by two-dimensional isotropic–anisotropic correlation nuclear magnetic resonance
Authors: Frydman L. , Vallabhaneni S. , Lee Y. K. , Emsley L.
Published: July 2002

We describe the application of a recently developed two-dimensional nuclear magnetic resonance (2D NMR) technique, variable-angle correlation spectroscopy, to the analysis of molecular motions in complex unlabeled solids. This technique separates the broad anisotropic chemical shift line shapes of nuclei in a sample according to the isotropic shift of each site. It can therefore be used to characterize molecular reorientations by monitoring the changes that these processes introduce in the resolved powder patterns as a function of temperature. Using the 13C NMR anisotropies of dimethylsulfone as a test case, we explored the potential applications of following such an approach. It was found that in contrast to what happens in nonexchanging systems, the anisotropic line shapes resolved by the variable-angle technique on an exchanging solid are different from line shapes that at similar temperatures can be recorded from a nonrotating sample. An explanation for these differences is presented, and the complete theory required to extract kinetic and geometric information from the experimental 2D line shapes is introduced and illustrated with computer simulations. The capability of this approach to analyze motions in complex systems is further demonstrated with a natural-abundance 13C variable-temperature NMR analysis of L-tyrosine ethyl ester; a reorienting compound possessing up to 11 inequivalent carbon sites in the solid phase.
Formation of ArCl(B,C), Ar(3P2), and Cl* by the three-body ionic-recombination reaction of Ar+(2P3/2)+Cl−+Ar
Authors: Tsuji Masaharu , Ide Masahiro , Muraoka Toshihiko , Nishimura Yukio
Published: July 2002

The three-body ionic-recombination reaction of Ar+(2P3/2)/Cl−/Ar leading to ArCl(B,C), Ar(3P2), and Cl(4s,4s′,4p,4p′,4d,5p,6s) has been studied spectroscopically in an Ar flowing afterglow. The results obtained are compared with our previous data for the Ar+(2P3/2)/Cl−/He reaction in order to examine the effects of the third-body atom. The Ar+(2P3/2)/Cl−/Ar reaction populates lower energy states of [Ar+(2P3/2)Cl−] intermediates than the Ar+(2P3/2)/Cl−/He one, so that the branching ratio of excimer becomes large in comparison with those of predissociation channels, the lower ArCl(B) state is preferentially excited than the ArCl(C) state, the ArCl(B) state is populated in lower vibrational levels, and the electronic state distribution of Cl* shifts to lower energy. On the basis of these findings, it is concluded that the heavy Ar atom is a more efficient acceptor of the excess energy than the light He atom in the Ar+(2P3/2)/Cl−/Rg (Rg=rare gas) system.
Solvation: Effects of molecular size and shape
Authors: Chan Hue Sun , Dill Ken A.
Published: July 2002

Sharp et al. have raised the question: If a solute and its solvent have different molecular sizes, how should solvation interaction energies be extracted from transfer experiments? Is the chemical potential more appropriately given by ‘‘classical’’ solution theory or by Flory–Huggins theory? We study a general statistical mechanical treatment of solvation, a generalized Flory–Huggins theory, and related treatments by Hildebrand and by Sharp et al. to determine the physical basis for size-dependent terms in the chemical potential, and their limitations and applicabilities. We find that the extra entropy in the Flory–Huggins theory does not arise from the disparity of sizes of solute and solvent, from free volume, or from artifacts of approximations. Rather, when solutes and solvents have sufficient complexity that they can ‘‘interfere’’ with each other in solution, there is an ‘‘entropy of coupling’’ of translational freedom to excluded volume or internal or rotational degrees of freedom. The Flory–Huggins theory approximates this coupling entropy for polymeric solutes or solvents, but not for other systems. Proper extraction of contact free energies requires proper subtraction of the coupling term. This study rationalizes several experimental and simulation studies, and indicates that coupling entropies that depend on molecular size and shape are often needed to treat complex solvation processes.
Ab initio studies of the complexes of benzene with carbon monoxide and formaldehyde
Authors: Nagy Peter I. , Ulmer Charles W. , Smith Douglas A.
Published: July 2002

Benzene...carbon monoxide and benzene...formaldehyde complexes are studied using ab initio methods with the highest calculations at the MP4SDTQ/6–31+G**//MP2/6–31+G** level. The benzene...carbon monoxide dimer forms a π complex of Cs symmetry where the CO top is nearly parallel with the benzene plane. In the benzene...formaldehyde π complex the dimer is without any symmetry. In this arrangement a weak hydrogen bond is expected between the elements where benzene acts as the acceptor, while in a linear benzene...formaldehyde alignment benzene has been identified as a weak hydrogen bond donor to the carbonyl oxygen. Changes in the intramolecular geometric parameters upon dimerization are small. Interaction energy of the benzene...CO dimer seems to be underestimated compared to the experimental value. No experimental value has been found for the benzene...formaldehyde binding energy. The calculated value is more negative by 0.7 kcal/mol with reference to the benzene...CO dimer. Calculated intermolecular vibrational frequencies are in partial agreement with the experiment. The stretching frequency of the benzene...CO dimer is well reproduced, intermolecular bending and torsional frequencies are overestimated. The intramolecular vibrational frequencies for the monomers show over and underestimation in the high and low frequency ranges, respectively. Experimental results in the literature suggest an almost free internal rotation of the CO top above benzene. The calculated barrier to internal rotation is 0.01 kcal/mol in good agreement with the experimental value. Based on this theoretical value the model with the almost free internal rotation was supported. Analysis for the benzene...formaldehyde dimer suggests more hindered rotation, if at all, with a H2CO top.
Accurate Fermi contact couplings of radicals using multireference perturbation theory
Authors: Chawla Sanjay , Messmer Richard P.
Published: July 2002

Reliable calculations of hyperfine coupling constants of π radicals, in particular, the Fermi contact term, have been found to pose a major theoretical problem. We present accurate Fermi contact couplings (FCCs) at the H and heavy atom nuclei in the CH, OH and CH3 radicals using generalized perturbation theories based on a multiconfigurational self-consistent-field (MCSCF) wave function. In addition, we analyze the spin polarization and correlation contributions to the FCC, using the CH radical as a test case. The success of our method is explained on the basis of the use of MCSCF optimized orbitals, as well as the renormalization of the coefficients of the MCSCF configurations. The scheme is more economical and of equal accuracy compared to the best results in the literature, as well as in very good agreement with experiment.
An ab initio benchmark study of the H+CO→HCO reaction
Authors: Woon David E.
Published: July 2002

The H+CO→HCO reaction has been characterized with correlation consistent basis sets at five levels of theory in order to benchmark the sensitivities of the barrier height and reaction ergicity to the one-electron and n-electron expansions of the electronic wave function. Single and multireference methods are compared and contrasted. The coupled cluster method RCCSD(T) was found to be in very good agreement with Davidson-corrected internally-contracted multireference configuration interaction (MRCI+Q). Second-order Mo/ller–Plesset perturbation theory (MP2) was also employed. The estimated complete basis set (CBS) limits for the the barrier height (in kcal/mol) for the five methods, including harmonic zero-point energy corrections, are MP2, 4.66; RCCSD, 4.78; RCCSD(T), 4.15; MRCI, 5.10; and MRCI+Q, 4.07. Similarly, the estimated CBS limits for the ergicity of the reaction are: MP2, −17.99; RCCSD, −13.34; RCCSD(T), −13.79; MRCI, −11.46; and MRCI+Q, −13.70. Additional basis set explorations for the RCCSD(T) method demonstrate that aug-cc-pVTZ sets, even with some functions removed, are sufficient to reproduce the CBS limits to within 0.1–0.3 kcal/mol.
Annihilation kinetics of liquid crystal defects
Authors: Liu Chester , Muthukumar M.
Published: July 2002

The time evolution of liquid crystalline configurations quenched suddenly from an isotropic state to a nematic state is simulated using the Monte Carlo method. In the Metropolis sampling procedure, we have employed the Frank free energy involving the splay, bend, and twist elastic constants, surface anchoring energy, and external aligning fields. In the nondiffusive regime, we have derived a scaling law for the defect density ρ(t) in terms of time t after quench in space dimension d as ρ(t)∼t−ν with ν=d(d+1)/(d2+2d−1), which is a generalization of a previous scaling argument in two dimensions. Our simulation results are in agreement with this value of ν in both two and three dimensions. While surface anchoring tends to slow down the kinetics of defect annihilation, elastic anisotropy is found to exert no effect on the value of ν in two dimensions. In the presence of external aligning fields, ρ(t) is found to decay exponentially with t.
Chlorine atom addition reaction to isoprene: A theoretical study
Authors: Lei Wenfang , Zhang Renyi
Published: July 2002

Ab initio molecular orbital calculations have been performed to investigate the structures and energies of the adduct isomeric radicals arising from the addition reaction of chlorine atom to isoprene. Geometry optimizations of the four isomers of the Cl–isoprene adduct were carried out with density function theory (DFT)-B3LYP and the energies were computed using the single-point calculation of several methods, including MP2, MP4, and CCSD(T). The most energetically favorable isomer was found to be that with Cl addition to the terminal C1-position (isomer I). At the CCSD(T)/6-311G** level of theory, the energies of the other three isomeric radicals relative to I were 1.4, 14.2, and 14.7 kcal mol−1 for Cl additions to C4- (isomer IV), C3- (isomer III), and C2-positions (isomer II), respectively. The activation energy for Cl migration from the more energetic isomers to the lower energy ones (i.e., from isomers II to I or III to IV) was found to be relatively low (2.8–4.7 kcal mol−1), indicating that isomerization of the Cl-isoprene adduct is likely to dominate the chemistry. The present theoretical results are compared to available experimental data based on product analysis for the Cl-isoprene reaction system to infer the atmospheric degradation mechanism of isoprene initiated by chlorine atoms.
R-matrix approach to lattice integrable systems
Authors: Bl/aszak Maciej , Marciniak Krzysztof
Published: July 2002

An r-matrix formalism is applied to the construction of the integrable lattice systems and their bi-Hamiltonian structure. Miura-like gauge transformations between the hierarchies are also investigated. In the end the ladder of linear maps between generated hierarchies is established and described.
Integrable three-particle systems, hidden symmetries and deformations of the Calogero–Moser system
Authors: Rañada Manuel F.
Published: July 2002

The properties of some three-particle systems are studied using as an approach the theory of generalized or hidden symmetries. It is proven that the existence of a family that possesses, in addition to the Energy function, two nonlinear constants of motion in involution. This family of integrable systems is obtained by considering deformations of the Lagrangian of the Calogero–Moser system. Finally, the generalization of the results to the general case of n particles is discussed.
Why the Thirring model cannot fulfill canonical anticommutation relations
Authors: Baumann Klaus
Published: July 2002

Perturbative and constructive arguments show that the Gross–Neveu model with N≥2 flavor degrees is asymptotically free. Assuming canonical anticommutation relations (CAR), which are quite plausible, and some technicalities, we show for the Thirring model, i.e., the case of N=1 flavor, that the fields are necessarily free. This explains from an axiomatic point of view the different behavior depending on the number of flavors.
Discrete decay and continuous measurement
Authors: Kanter Marek
Published: July 2002

In this paper measurement is defined via an observation operator with simple, purely discrete spectrum. When the quantum Zeno effect holds, a continuously measured quantum process with fixed initial state ‘‘freezes’’ in that state. Such states are called ‘‘regular’’ states. If the final state is also fixed, then movement from the initial state to the final state is forced to occur, whether or not the initial state is regular. This forced movement is studied here in the context of a discrete state model which contains a distinguished nonregular state which mediates all transitions. In this model the forced movement between any two states satisfies a ‘‘least action’’ principle, wherein action is identified with change of state transitions. For completeness, the model is also studied when only the initial state is fixed under continuous measurement. In that case the distinguished state exhibits exact exponential decay at all times. The model is of interest independently of issues related to continuous measurement inasmuch as it is a ‘‘discrete’’ approximation to a ‘‘continuous’’ decay model. More precisely, the distinguished state exhibits exact exponential decay on a finite time interval which expands without bound as the discrete decay products densely approach the continuum. This unexpected result provides a striking confirmation of Fermi’s ‘‘golden rule.’’
Universality classes for asymptotic behavior of relaxation processes in systems with dynamical disorder: Dynamical generalizations of stretched exponential
Authors: Vlad Marcel Ovidiu , Metzler Ralf , Nonnenmacher Theo F. , Mackey Michael C.
Published: July 2002

The asymptotic behavior of multichannel parallel relaxation processes for systems with dynamical disorder is investigated in the limit of a very large number of channels. An individual channel is characterized by a state vector x which, due to dynamical disorder, is a random function of time. A limit of the thermodynamic type in the x-space is introduced for which both the volume available and the average number of channels tend to infinity, but the average volume density of channels remains constant. Scaling arguments combined with a stochastic renormalization group approach lead to the identification of two different types of universal behavior of the relaxation function corresponding to nonintermittent and intermittent fluctuations, respectively. For nonintermittent fluctuations a dynamical generalization of the static Huber’s relaxation equation is derived which depends only on the average functional density of channels, ρ[W(t′)]D[W(t′)], the channels being classified according to their different relaxation rates W=W(t′), which are random functions of time. For intermittent fluctuations a more complicated relaxation equation is derived which, in addition to the average density of channels, ρ[W(t′)]D[W(t′)], depends also on a positive fractal exponent H which characterizes the fluctuations of the density of channels. The general theory is applied for constructing dynamical analogs of the stretched exponential relaxation function. For nonintermittent fluctuations the type of relaxation is determined by the regression dynamics of the fluctuations of the relaxation rate. If the regression process is fast and described by an exponential attenuation function, then after an initial stretched exponential behavior the relaxation process slows down and it is not fully completed even in the limit of very large times. For self-similar regression obeying a negative power law, the relaxation process is less sensitive to the influence of dynamical disorder. Both for small and large times the relaxation process is described by stretched exponentials with the same fractal exponent as for systems with static disorder. For large times the efficiency of the relaxation process is also slowed down by fluctuations. Similar patterns are found for intermittent fluctuations with the difference that for very large times and a slow regression process a crossover from a stretched exponential to a self-similar algebraic relaxation function occurs. Some implications of the results for the study of relaxation processes in
Eichler integrals and string theory
Authors: Sandoval Leonidas
Published: July 2002

In this article, it is shown how to obtain objects called Eichler integrals in the mathematical literature that can be used for calculating scattering amplitudes in string theory. These Eichler integrals are also new examples of Eichler integrals with poles.
The Kirkwood–Buckingham variational method and the boundary value problems for the molecular Schrödinger equation
Authors: Pupyshev V. I. , Scherbinin A. V. , Stepanov N. F.
Published: July 2002

The approach based on the multiplicative form of a trial wave function within the framework of the variational method, initially proposed by Kirkwood and Buckingham, is shown to be an effective analytical tool in the quantum mechanical study of atoms and molecules. As an example, the elementary proof is given to the fact that the ground state energy of a molecular system placed into the box with walls of finite height goes to the corresponding eigenvalue of the Dirichlet boundary value problem when the height of the walls is growing up to infinity.
Reconstruction of secular polynomials for Hubbard model from energy perturbation series for weak and strong coupling cases
Authors: Bracken Paul , Čı́žek Jiri
Published: July 2002

An interpolation technique which is based on the strongly and weakly correlated cases of the one-dimensional Hubbard model is proposed. The input information consists of the perturbation expansions which are obtained from the Lieb–Wu equations in both limits. The Hubbard model is used to describe cyclic polyene rings, and for the case of N=6 sites, this would correspond to benzene. The technique has been applied to several symmetries of the model. It has been shown that the exact secular problem can be reconstructed for these symmetries, and the results for N=6 agree exactly with the results which have been obtained in a different way, that is, by using a full configuration interaction calculation.
Two families of nonstandard Poisson structures for Newton equations
Authors: Marciniak Krzysztof , Rauch-Wojciechowski Stefan
Published: July 2002

Two families of nonstandard two-dimensional Poisson structures for systems of Newton equations are studied. They are closely related either with separable systems or with the so-called quasi-Lagrangian systems. A theorem characterizing the general form of bi-Hamiltonian formulation for separable systems in two and in n dimensions is formulated and proved.
Cauchy horizon end points and differentiability
Authors: Beem John K. , Królak Andrzej
Published: July 2002

Cauchy horizons are shown to be differentiable at end points where only a single null generator leaves the horizon. A Cauchy horizon fails to have any null generator end points on a given open subset iff it is differentiable on the open subset and also iff the horizon is (at least) of class C1 on the open subset. Given the null convergence condition, a compact horizon which is of class C2 almost everywhere has no end points and is (at least) of class C1 at all points.
Black holes, bandwidths and Beethoven
Authors: Kempf Achim
Published: July 2002

It is usually believed that a function φ(t) whose Fourier spectrum is bounded can vary at most as fast as its highest frequency component ωmax. This is, in fact, not the case, as Aharonov, Berry, and others drastically demonstrated with explicit counterexamples, so-called superoscillations. It has been claimed that even the recording of an entire Beethoven symphony can occur as part of a signal with a 1 Hz bandwidth. Bandlimited functions also occur as ultraviolet regularized fields. Their superoscillations have been suggested, for example, to resolve the trans-Planckian frequencies problem of black hole radiation. Here, we give an exact proof for generic superoscillations. Namely, we show that for every fixed bandwidth there exist functions that pass through any finite number of arbitrarily prespecified points. Further, we show that, in spite of the presence of superoscillations, the behavior of bandlimited functions can be characterized reliably, namely through an uncertainty relation: The standard deviation ΔT of samples φ(tn) taken at the Nyquist rate obeys ΔT⩾1/4ωmax. This uncertainty relation generalizes to variable bandwidths. For ultraviolet regularized fields we identify the bandwidth as the in general spatially variable finite local density of degrees of freedom.
Temperature jump and thermal creep slip: Rigid sphere gas
Authors: Loyalka S. K.
Published: July 2002

The half-space problems of temprature jump and thermal creep slip are solved for a rigid sphere gas based on the linearized Boltzmann equation. The SN method is used, and it is shown that accurate results for the jump/slip coefficients and the temperature, density, and velocity can be obtained in relatively short computational times. The previously reported variational results for the jump/slip coefficients are found to be quite good (1%–3% error). It is noted, however, that for rigid sphere molecules the Knudsen layer is somewhat thinner than for the BGK model. The creep slip coefficient is in good agreement with the experimental data of Annis [J. Chem. Phys. 57, 2898 (1972)], but for other quantities experimental data are needed.
Compressible boundary-layer stability theory
Authors: Reed Helen L. , Balakumar Ponnampalam
Published: July 2002

In comparing their results with those of Mack, Wazzan et al. [Phys. Fluids 27, 331 (1984)] found major differences regarding the effect of free-stream Mach number on viscous instability. The present work shows that all past observations concerning stability behavior in the supersonic range based on Mack’s work are accurate to the level of approximation involved. More importantly, as a consequence of discussions with Mack (private communication) prompted by this dispute, this paper clarifies the fluid-property assumptions made by him in his earlier calculations and demonstrates the sensitivity of stability calculations to the accuracy of the basic state.
Interaction of a streamwise vortex with a turbulent mixing layer
Authors: Bell James H. , Mehta Rabindra D.
Published: July 2002

An experimental study has been conducted on the interaction of a single streamwise vortex with a plane turbulent mixing layer. The vortex was generated by a half-delta wing mounted in the settling chamber of a blower-driven wind tunnel. The single-stream mixing layer originated from the top lip of the extended contraction exit, with a tripped initial boundary layer. Initially, the vortex rode below the mixing layer and its effect was to locally distort the mean velocity and Reynolds stress distributions in the lower (high-speed) side of the mixing layer. Once the vortex became embedded within the mixing layer, the distortion, which took the form of a wrinkle, spread through the whole width of the layer. The induced extra strain rates in the mixing layer resulted in the generation of additional, relatively large, Reynolds stresses in the regions around the vortex. In particular, the secondary shear stress (∼(u′w′)) reached a maximum absolute value equivalent to about 40% of the maximum primary shear stress (∼(u′v′)), thus indicating significantly increased three-dimensionality in the mixing layer. The position and behavior of ∼(u′w′) was found to be strongly correlated with that of the streamwise vorticity. The vortex appeared to decay rapidly once it was embedded in the mixing layer; the maximum mean streamwise vorticity was observed to decay as approximately 1/X2 in this region. By the last measurement station, almost all the effects in the mixing layer resulting from the vortex, and the identity of the vortex itself, had subsided.
Mixed convection on a vertical needle with heated tip
Authors: Wang C. Y.
Published: July 2002

The mixed convection on a vertical slender adiabatic paraboloid with a tip heat source is studied. The boundary layer equations admit similarity solutions that are governed by a nondimensional free-stream parameter γ and a heat source parameter α. Numerical results show for aiding flow (α>0) the solutions are unique and for opposing flow (α<0) the solutions may be unique, dual, or nonexistent. Velocity and temperature profiles are obtained.
Conductivity of a relativistic plasma
Authors: Braams Bastiaan J. , Karney Charles F. F.
Published: July 2002

The collision operator for a relativistic plasma is reformulated in terms of an expansion in spherical harmonics. In this formulation the collision operator is expressed in terms of five scalar potentials that are given by one-dimensional integrals over the distribution function. This formulation is used to calculate the electrical conductivity of a uniform electron–ion plasma with infinitely massive ions.
Modification of sawteeth by second harmonic electron-cyclotron heating in a tokamak
Authors: Snider R. T. , Content D. , James R. , Lohr John , Mahdavi M. A. , Prater R. , Stallard B.
Published: July 2002

Electron-cyclotron heating, using second harmonic waves launched from the low field side, has a strong effect on sawteeth in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. I, p. 159]. There are two distinct methods and mechanisms for sawtooth period extension. In the first case a factor of 6 increase in sawtooth period, compared to that during Ohmic heating, occurs with electron-cyclotron heating (ECH) localized near the q=1 surface. The sawtooth period increases faster than linearly with increasing ECH power. Qualitatively, there is agreement with sawtooth suppression models, although details of the accompanying m/n=1/1 oscillation appear inconsistent. The second case is a technique for complete sawtooth suppression in which impurities and off-axis heating broaden the current profile and drive q0>1. It appears that the temperature and the current and q profiles inside q=1 are determined by the magnetohydrodynamic activity, which is modified by ECH, and not by the ECH power deposition directly.
Effects of long-wavelength dissipation on beam-driven Langmuir turbulence
Authors: Robinson P. A. , Newman D. L. , Rubenchik A. M.
Published: July 2002

The effects of long-wavelength dissipation on beam-driven Langmuir turbulence are investigated using numerical simulations that include both weak and strong turbulence effects. Strong-turbulence wave collapses occur concurrently with weak-turbulence energy cascades if the long-wavelength damping is sufficiently small relative to the growth rate of the beam-unstable waves. Above a threshold damping level, only the weak-turbulence backscatter cascade is observed, and it becomes increasingly truncated as the damping increases, eventually consisting of only a single backscatter. A simple Lotka–Volterra model gives an excellent description of the periodic evolution observed in the weak-turbulence regime. Suppression of the usual backscatter cascade by long-wavelength damping enables intense beam-aligned density troughs to form, which trap and duct Langmuir waves.
Laser implosion of gas-filled microballoons
Authors: Piriz A. R. , Giudice G. E. , Wouchuk J. G.
Published: July 2002

A relatively simple model for the implosion of a strong shock driven by a spherical shell which is ablatively imploded by a laser pulse is presented. The dynamics of the shock and the shell is completely described by three dimensionless parameters containing all the information regarding the laser and the target. The model is applied to recent experiments which produce the highest neutron yield, and a good agreement with simulations and experimental results is found.
Electric and magnetic structure of an edge plasma in a tokamak with a helical magnetic limiter
Authors: Takamura S. , Ohnishi N. , Yamada H. , Okuda T.
Published: July 2002

A strong modification of plasma potential was observed at the plasma surface with the application of a helical field produced by a set of local helical coils. The structure of magnetic field lines was computed by a field line tracing code for a tokamak plasma. Such a field structure was examined in the experiment using electron beam probing. The potential formation and the magnetic structure are discussed referring to the scheme of an ergodic magnetic limiter.
Nonlinear evolution equations for thin liquid films with insoluble surfactants
Authors: De Wit A. , Gallez D. , Christov C. I.
Published: July 2002

The dynamics of a free-liquid film with insoluble surfactants is followed until film rupture with a simple model based on three nonlinear evolution equations for the film thickness, the surfactants concentration and the tangential velocity of the fluid in the film. This model is derived asymptotically from the full Navier–Stokes equations for free films and incorporates the effect of van der Waals attraction, capillary forces and Marangoni forces due to gradients of surface tension. Different stability regimes are observed numerically for periodic and fixed boundary conditions and several initial conditions. Furthermore, the role of the relevant parameters (Hamaker constant, tension, Marangoni number) on the rupture time is assessed and comparison is made with the flow dynamics for a liquid film with insoluble surfactants on a solid substrate.
Chaotic advection in point vortex models and two-dimensional turbulence
Authors: Babiano A. , Boffetta G. , Provenzale A. , Vulpiani A.
Published: July 2002

The dynamics of passively advected particles in either integrable or chaotic point vortex systems and in two-dimensional (2-D) turbulence is studied. For point vortices, it is shown that the regular or chaotic nature of the particle trajectories is not determined by the Eulerian chaoticity of the vortex motion, but rather by pure Lagrangian quantities, such as the distance of an advected particle from the vortex centers. In fact, each point vortex turns out to be surrounded by a regular island, where the advected particles are trapped and their Lagrangian Lyapunov exponent is zero, even though the vortex itself may perform a chaotic trajectory. In the field between the vortices, passive particles undergo chaotic advection with an associated positive Lyapunov exponent. For well-separated vortices, even at large times, the advected particles do not cross the boundary between the chaotic sea and the regular islands surrounding the vortices. A similar situation holds in the case of forced-dissipative 2-D turbulence, where particles trapped in the interior of the coherent structures have a null Lagrangian Lyapunov exponent, while those in the background turbulent sea move chaotically. This gives clear evidence of the important role played by chaotic advection, even in complex Eulerian flows.
Nonlinear transformation of waves in finite water depth
Authors: Kaihatu James M. , Kirby James T.
Published: July 2002

The formulation of a nonlinear frequency domain parabolic mild-slope model is detailed. The resulting model describes two-dimensional wave transformation and nonlinear coupling between frequency components. Linear dispersion and transformation characteristics are dictated by fully-dispersive linear theory, an improvement over weakly-dispersive Boussinesq theory. Both the present model and a weakly-dispersive nonlinear frequency domain model are compared to laboratory data for both two-dimensional wave transformation and pure shoaling. It is found that, in general, data-model comparisons are enhanced by the present model, particularly in instances where the wave condition is outside the shallow water range.
Direct numerical simulation of near-interface turbulence in coupled gas-liquid flow
Authors: Lombardi Paolo , De Angelis Valerio , Banerjee Sanjoy
Published: July 2002

Turbulence structures near the interface between two flowing fluids have been resolved by direct numerical simulation. As a first step the interface has been kept flat, corresponding closely to the recent gas-liquid flow experiments of Rashidi and Banerjee [Phys. Fluids A 2, 1827 (1990)], with the fluids coupled through continuity of velocity and shear stress boundary conditions. For density ratios between the fluids typical of air and water, the turbulence characteristics on the gas side are quite similar to that in wall regions. The liquid side shows larger velocity fluctuations close to the interface and ejections originate closer to the interface. The mean velocity distribution, turbulence intensities, Reynolds stress and various other statistical measures are significantly altered compared to those in the wall region of channel flows. Quasi-streamwise vortices form in the areas between high and low shear stress on both sides of the interface. At any given instant, about a fifth of these appear to be coupled across the interface. Whether the others are, but the coupling is too weak for the detection technique used, or were coupled previously remains an open question. In any case, sweeps usually occur on the high shear stress side of these vortices and ejections on the low shear stress side. Significant coupling exists across the interface with over 60% of the Reynolds stress in the region close to the interface being associated with coupled events –the main coupling coming through gas ejection-liquid ejection events over low shear stress regions, with a lesser but significant number of gas sweep-liquid sweep events over high shear stress regions.
Spatial simulation of the instability of channel flow with local suction/blowing
Authors: Chung M. Yongmann , Sung Hyung Jin , Boiko Andrey V.
Published: July 2002

A direct numerical simulation was made of instability in a spatially evolving channel flow. A local surface suction/blowing was imposed at the upper wall (x/h=20). A Tollmien–Schlichting (TS) wave was superimposed on the laminar channel flow at the inflow. At the outflow, the buffer domain technique was applied to suppress the reflection of outgoing waves. The influence of the local suction/blowing on the linear and nonlinear instabilities of the flow was examined. It was found that the local suction/blowing increases the disturbance energy significantly in the interaction zone for subcritical (Re=5000) and supercritical (Re=10 000) cases. The effects of the blowing strength (0⩽AS⩽0.1) and the initial TS wave amplitude (0⩽ATS⩽2.0) on the subcritical channel flow were scrutinized. Two regimes of the wave/flow interaction were found by varing AS, i.e., “monotonic” and “vortex splitting” regimes.
Growth of ideal magnetohydrodynamic modes driven slowly through their instability threshold: Application to disruption precursors
Authors: Callen J. D. , Hegna C. C. , Rice B. W. , Strait E. J. , Turnbull A. D.
Published: July 2002

The growth of an ideal magnetohydrodynamic (MHD) instability in a high-temperature plasma is calculated in the case where the plasma β is driven slowly through its instability threshold. The MHD perturbation grows faster than exponentially, approximately as exp[(t/τ)3/2]. Its characteristic growth time τ∼(32)2/3γ̂MHD−2/3γh−1/3 is a hybrid of the ideal MHD incremental growth rate γ̂MHD and the heating rate γh. This simple model agrees well with the observed growth of disruption precursors in high β DIII-D [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] discharges having strongly peaked pressure profiles, where the observed growth times of ⩾10−4 s are significantly slower than the typical ideal MHD time scale of ⩽10−5 s.
Effect of impurity trapping on the capacitance-voltage characteristics of n-GaAs/N-AlGaAs heterojunctions
Authors: Tan K. L. , Lundstrom M. S. , Melloch M. R.
Published: July 2002

We have studied the capacitance-voltage (C-V) characteristics of Schottky barriers on inverted n-GaAs/N-AlGaAs and normal N-AlGaAs/n-GaAs heterojunctions. Impurities introduced during film growth produced a negative sheet charge of 6.0×1011 cm−2 at the interface of the inverted n-GaAs/N-AlGaAs heterojunction. The effectiveness of GaAs quantum wells in trapping these impurities was investigated. GaAs quantum wells 20 Å wide were placed in intervals of 2500 Å for the first 0.75 μm of the AlGaAs layer; in the last 0.25 μm, the periodicity of the quantum wells was progressively decreased by half with the last quantum well placed at about 160 Å from the GaAs/AlGaAs interface. The resulting measured interface charge concentration of 4.4×1010 cm−2 is more than a magnitude lower than measured before the use of the quantum wells and is essentially at the limit of the accuracy of the C-V technique for this structure.
Epitaxial growth of Ge films on GaAs (285–415 °C) by laser photochemical vapor deposition
Authors: Tavitian V. , Kiely C. J. , Geohegan D. B. , Eden J. G.
Published: July 2002

Epitaxial Ge films have been grown on [001]GaAs by laser photochemical vapor deposition (LPVD) for substrate temperatures (Ts) in the 285≤Ts≤415 °C interval. Films 400–700 Å thick were grown at rates up to ∼1 Å s−1 (0.6–5 nm/min) by photodissociating GeH4 with an ArF (193 nm) laser in parallel geometry (i.e., laser beam passes above and parallel to substrate). The quality of the Ge/GaAs heterointerfaces has been examined by plan-view and cross-sectional transmission electron microscopy. Convergent beam electron diffraction, selected area diffraction patterns, and lattice images show the LPVD films to be epitaxial, whereas films grown in the absence of ultraviolet laser radiation (i.e., by conventional chemical vapor deposition) are amorphous (Ts=305 °C) or heavily defected polycrystalline (Ts=415 °C). Epitaxy apparently hinges on the laser production of a species which migrates more than 10–60 mean free paths to reach the substrate (or is collisionally transformed en route into a more stable species) and dramatically alters the surface chemistry. The large number of collisions involved points to the transformation of photochemically generated GeH2 or GeH3 into Ge2H6 as the key gas phase reaction.
Recruitment of intron-encoded and co-opted proteins in splicing of the bI3 group I intron RNA
Authors: Bassi Gurminder S. , de Oliveira Daniela M. , White Malcolm F. , Weeks Kevin M.
Published: July 2002

Detectable splicing by the
Saccharomyces
cerevisiae
mitochondrial bI3 group I intron RNA
in
vitro
is shown to require both an intron-encoded protein, the
bI3 maturase, and the nuclear-encoded protein, Mrs1. Both proteins
bind independently to the bI3 RNA. The bI3 maturase binds as a monomer,
whereas Mrs1 is a dimer in solution that assembles as two dimers,
cooperatively, on the RNA. The active six-subunit complex has a
molecular mass of 420 kDa, splices with a
k
cat
of 0.3 min
−1
, and binds
the guanosine nucleophile with an affinity comparable to other group I
introns. The functional bI3 maturase domain is translated from within
the RNA that encodes the intron, has evolved a high-affinity
RNA-binding activity, and is a member of the LAGLIDADG family of DNA
endonucleases, but appears to have lost DNA cleavage activity. Mrs1 is
a divergent member of the RNase H fold superfamily of dimeric DNA
junction-resolving enzymes that also appears to have lost its nuclease
activity and now functions as a tetramer in RNA binding. Thus, the bI3
ribonucleoprotein is the product of a process in which a
once-catalytically active RNA now obligatorily requires two
facilitating protein cofactors, both of which are compromised in their
original functions.
Evaluation of the role of heterogeneous nuclear ribonucleoprotein A1 as a host factor in murine coronavirus discontinuous transcription and genome replication
Authors: Shen Xiaolan , Masters Paul S.
Published: July 2002

Viruses with RNA genomes often capture and redirect host cell
components to assist in mechanisms particular to RNA-dependent RNA
synthesis. The nidoviruses are an order of positive-stranded RNA
viruses, comprising coronaviruses and arteriviruses, that employ a
unique strategy of discontinuous transcription, producing a series of
subgenomic mRNAs linking a 5′ leader to distal portions of the genome.
For the prototype coronavirus mouse hepatitis virus (MHV),
heterogeneous nuclear ribonucleoprotein (hnRNP) A1 has been shown to be
able to bind
in vitro
to the negative strand of the
intergenic sequence, a cis-acting element found in the leader RNA and
preceding each downstream ORF in the genome. hnRNP A1 thus has been
proposed as a host factor in MHV transcription. To test this hypothesis
genetically, we initially constructed MHV mutants with a very
high-affinity hnRNP A1 binding site inserted in place of, or adjacent
to, an intergenic sequence in the MHV genome. This inserted hnRNP A1
binding site was not able to functionally replace, or enhance
transcription from, the intergenic sequence. This finding led us to
test more directly the role of hnRNP A1 by analysis of MHV replication
and RNA synthesis in a murine cell line that does not express this
protein. The cellular absence of hnRNP A1 had no detectable effect on
the production of infectious virus, the synthesis of genomic RNA, or
the quantity or quality of subgenomic mRNAs. These results strongly
suggest that hnRNP A1 is not a required host factor for MHV
discontinuous transcription or genome replication.
tRNA-like recognition of group I introns by a tyrosyl-tRNA synthetase
Authors: Myers Christopher A. , Kuhla Birte , Cusack Stephen , Lambowitz Alan M.
Published: July 2002

The
Neurospora crassa
mitochondrial tyrosyl-tRNA synthetase (CYT-18 protein) functions in splicing group I introns by promoting the formation of the catalytically active RNA structure. Previous work suggested that CYT-18 recognizes a conserved tRNA-like structure of the group I intron catalytic core. Here, directed hydroxyl-radical cleavage assays show that the nucleotide-binding fold and C-terminal domains of CYT-18 interact with the expected group I intron cognates of the aminoacyl-acceptor stem and D-anticodon arms, respectively. Further, three-dimensional graphic modeling, supported by biochemical data, shows that conserved regions of group I introns can be superimposed over interacting regions of the tRNA in a
Thermus thermophilus
TyrRS/tRNA
Tyr
cocrystal structure. Our results support the hypothesis that CYT-18 and other aminoacyl-tRNA synthetases interact with group I introns by recognizing conserved tRNA-like structural features of the intron RNAs.
Structure and function in rhodopsin: Mass spectrometric identification of the abnormal intradiscal disulfide bond in misfolded retinitis pigmentosa mutants
Authors: Hwa John , Klein-Seetharaman Judith , Khorana H. Gobind
Published: July 2002

Retinitis
pigmentosa (RP) point mutations in both
the intradiscal (ID) and transmembrane domains of rhodopsin cause
partial or complete misfolding of rhodopsin, resulting in loss of
11-
cis
-retinal binding. Previous work has shown that
misfolding is caused by the formation of a disulfide bond in the ID
domain different from the native Cys-110–Cys-187 disulfide bond in
native rhodopsin. Here we report on direct identification of the
abnormal disulfide bond in misfolded RP mutants in the transmembrane
domain by mass spectrometric analysis. This disulfide bond is between
Cys-185 and Cys-187, the same as previously identified in misfolded RP
mutations in the ID domain. The strategy described here should be
generally applicable to identification of disulfide bonds in other
integral membrane proteins.
Ultrasensitive detection and identification of fluorescent molecules by FCS: Impact for immunobiology
Authors: Földes-Papp Zeno , Demel Ulrike , Tilz Gernot P.
Published: July 2002

An experimental application of fluorescence correlation
spectroscopy is presented for the detection and identification of
fluorophores and auto-Abs in solution. The recording time is between 2
and 60 sec. Because the actual number of molecules in the unit volume
(confocal detection volume of about 1 fl) is integer or zero, the
fluorescence generated by the molecules is discontinuous when
single-molecule sensitivity is achieved. We first show that the
observable probability,
N
, to find a single fluorescent
molecule in the very tiny space element of the unit volume is
Poisson-distributed below a critical bulk concentration c*. The
measured probability means we have traced, for example, 5 ×
10
10
fluorophore molecules per ml of bulk solution. The
probability is related to the average frequency,
C
, that
the volume of detection contains a single fluorescent molecule and to
the concentration,
c
, of the bulk solution. The
analytical sensitivity of an assay is calculated from the average
frequency
C
. In the Goodpasture experiment, we
determined as analytical sensitivity a probability of 99.1% of
identifying one single immune complex. Under these conditions, a single
molecule event is proven. There exist no instrumental assumptions of
our approach on which the experiment itself, the theoretical
background, or the conclusion are based. Our results open up a broad
field for analytics and diagnostics in solution, especially in
immunology.