bluuebunny/crossref_metadata_2025 huggingface dataset:
Gauge-invariant calculation of nuclear magnetic shielding constants at the coupled–cluster singles and doubles level
Authors: Gauss Jürgen , Stanton John F.
Published: July 2002

The gauge-including atomic orbital (GIAO) method for the gauge-invariant calculation of nuclear magnetic shielding constants has been implemented at the coupled-cluster singles and doubles (CCSD) level. A brief description of the theory and its computational requirements is provided. Finally, the GIAO–CCSD method is applied to calculate nuclear shielding constants of N2O with two different basis sets, the larger of which contains 153 contracted Gaussian functions.
Coherent control over the photodissociation of CH3I
Authors: Kleiman Valeria D. , Zhu Langchi , Allen Jeanette , Gordon Robert J.
Published: July 2002

Coherent phase control of the photodissociation of CH3I has been achieved by quantum mechanical interference between competing paths. The control was accomplished by exciting the parent molecules with three UV photons of frequency ω1 and one VUV photon of frequency ω3=3ω1. Varying the phase difference between the two laser beams resulted in a modulation of the I+ and CH+3 signals, without affecting the parent ion signal. We propose a mechanism in which control occurs over the photodissociation step to produce CH3+I*, followed by ionization of the neutral fragments by additional UV photons.
Impulsive stimulated light scattering from glass-forming liquids. I. Generalized hydrodynamics approach
Authors: Yang Yongwu , Nelson Keith A.
Published: July 2002

The generalized hydrodynamics equations, including nonlocal behavior in time, are used to calculate density response functions for impulsive stimulated light scattering (ISS) from glass-forming liquids. Single-exponential relaxation dynamics are considered first, and then the results are generalized to account for complex relaxation. Thermal diffusion, acoustic, and relaxation modes are described. The time dependence of both acoustic and relaxation modes yields the structural relaxation dynamics. In addition, the relative amplitudes of the modes in ISS data yield the Debye–Waller factor fq, called the nonergodicity parameter in mode-coupling theory (MCT) of glass-forming liquids, in the limit of low wave vector q. This permits testing of MCT predictions of a square-root cusp in fq→0(T) at a crossover temperature Tc. The information which can be obtained through ISS is compared in theoretical and practical terms to that obtainable through low-frequency light scattering spectroscopy.
Impulsive stimulated light scattered from glass-forming liquids. II. Salol relaxation dynamics, nonergodicity parameter, and testing of mode coupling theory
Authors: Yang Yongwu , Nelson Keith A.
Published: July 2002

Impulsive stimulated light scattering was used to investigate structural relaxation dynamics of the organic glass-former salol. Relaxation dynamics on ns–ms time scales are monitored in the 238.7–280.5 K temperature range and found to be described well by the Kohlrausch–Williams–Watts relaxation function. The nonergodicity parameter fq of mode-coupling theory (MCT) in the limit of the small wave vector q is evaluated. Its temperature dependence shows a square-root cusp at a crossover temperature of Tc=266 K, consistent with neutron scattering measurements at high q and with light scattering results. This supports the MCT prediction of a cusp in fq(T) at a q-independent temperature.
On the effectiveness of monomer-, dimer-, and bond-centered basis functions in calculations of intermolecular interaction energies
Authors: Williams Hayes L. , Mas Eric M. , Szalewicz Krzysztof , Jeziorski Bogumil/
Published: July 2002

A range of basis sets differing in the location of basis functions has been explored from the point of view of the effectiveness of calculating the electrostatic, induction, dispersion, and exchange components of intermolecular interaction energies. Possible location strategies range from monomer-centered basis sets, through the dimer-centered ones, to sets with functions centered at the intermolecular bond. It is shown that the most effective approach is to use the so-called ‘‘monomer plus’’ basis sets containing, in addition to monomer-centered functions and bond functions, a small number of functions centered on the interacting partner. Using such basis sets for He2 and (H2O)2 the best values to date have been obtained for several interaction energy components. The conclusions from this work are relevant also for supermolecular calculations of interaction energies.
Direct numerical solution of the Ornstein–Zernike integral equation and spatial distribution of water around hydrophobic molecules
Authors: Ikeguchi Mitsunori , Doi Junta
Published: July 2002

The Ornstein–Zernike integral equation (OZ equation) has been used to evaluate the distribution function of solvents around solutes, but its numerical solution is difficult for molecules with a complicated shape. This paper proposes a numerical method to directly solve the OZ equation by introducing the 3D lattice. The method employs no approximation the reference interaction site model (RISM) equation employed. The method enables one to obtain the spatial distribution of spherical solvents around solutes with an arbitrary shape. Numerical accuracy is sufficient when the grid-spacing is less than 0.5 Å for solvent water. The spatial water distribution around a propane molecule is demonstrated as an example of a nonspherical hydrophobic molecule using iso-value surfaces. The water model proposed by Pratt and Chandler is used. The distribution agrees with the molecular dynamics simulation. The distribution increases offshore molecular concavities. The spatial distribution of water around 5α-cholest-2-ene (C27H46) is visualized using computer graphics techniques and a similar trend is observed.
Ab initio pair potential parameter set for the interaction of a rigid and a flexible water model and the complete series of the halides and alkali cations
Authors: Tóth Gergely
Published: July 2002

The ab initio calculations were performed at the quantum chemical levels of the second and fourth order Moller–Plesset perturbation theory. The binding energies were corrected for the basis set superposition error. Exactly 420 different ion–water configurations were chosen for each ion to scan the potential surface entirely. The parametrized analytical potential functions, the radial distribution functions, and the solvation internal energies obtained by molecular dynamics simulations were compared to previous simulations and experimental results.
Free energy calculations of small molecules in dense amorphous polymers. Effect of the initial guess configuration in molecular dynamics studies
Authors: van der Vegt Nico F. A. , Briels Wim J. , Wessling Matthias , Strathmann Heiner
Published: July 2002

The excess free energy of small molecules in the amorphous polymers poly(ethylene) and poly(dimethylsiloxane) was calculated, using the test-particle-insertion method. The method was applied to polymer configurations obtained from molecular dynamics simulations with differently prepared initial guess configurations. It was found that the calculated solubility coefficients strongly depend on the quality of the initial guess configuration. Slow compression of dilute systems, during which process only the repulsive parts of the nonbonded Lennard-Jones potentials are taken into account, yields polymer melts which are better relaxed, and which offer lower solubilities for guest molecules compared with polymer melts generated at the experimental density or prepared by compressing boxes with soft-core nonbonded potentials. For the last two methods initial stresses relax by straining the internal modes (bond angles, torsion angles) of the chains.
Unit cells for the simulation of hexagonal ice
Authors: Hayward J. A. , Reimers J. R.
Published: July 2002

A number of periodic lattices have historically been used to represent ice-1h in computer simulations. These vary in size, shape, and method of generation, and while they have served their intended purposes, their properties have rarely been documented in detail and their intercompatibility is unknown. We develop a method for generating sets of internally consistent lattices and apply it to determine eight unit cells containing from 96 to 768 water molecules in both near-cubic and slab arrangements. It can easily be applied to generate additional (larger) cells or representations of specific crystal faces. Each unit cell in this set has zero net dipole moment and minimal net quadrupole moment and is optimized using four different criteria to measure the randomness of the hydrogen bonding; if required, these criteria can easily be modified to suit the intended application and alternate sets thus generated. We find that Cota and Hoover’s much used constraint for selecting unit cells with zero dipole moment is too restrictive, not permitting a fully random hydrogen-bonding network; also, unit-cell generation methods based on potential-energy minimization are found to prefer unrepresentative, highly ordered structures.
A new method for excited states: Similarity transformed equation-of-motion coupled-cluster theory
Authors: Nooijen Marcel , Bartlett Rodney J.
Published: July 2002

We present the first application of the similarity transformed equation-of-motion coupled-cluster method (STEOM-CC) to calculate excited states. STEOM-CC theory arises from a similarity transform of the second quantized Hamiltonian which strongly reduces the coupling between singly excited determinants and more highly excited configurations. Consequently, excitation energies can be obtained to a good approximation by diagonalizing the transformed Hamiltonian in the space of single excitations only. The STEOM method is applied to obtain the valence excitation spectrum of the pyridine molecule. The accuracy of STEOM is shown to be comparable to current state of the art methods like equation-of-motion coupled-cluster theory and CASPT2, whereas the computational requirements of STEOM are very modest compared to the above methods.
Natural chemical shielding analysis of nuclear magnetic resonance shielding tensors from gauge-including atomic orbital calculations
Authors: Bohmann Jonathan A. , Weinhold Frank , Farrar Thomas C.
Published: July 2002

Nuclear magnetic shielding tensors computed by the gauge including atomic orbital (GIAO) method in the Hartree–Fock self-consistent-field (HF-SCF) framework are partitioned into magnetic contributions from chemical bonds and lone pairs by means of natural chemical shielding (NCS) analysis, an extension of natural bond orbital (NBO) analysis. NCS analysis complements the description provided by alternative localized orbital methods by directly calculating chemical shieldings due to delocalized features in the electronic structure, such as bond conjugation and hyperconjugation. Examples of NCS tensor decomposition are reported for CH4, CO, and H2CO, for which a graphical mnemonic due to Cornwell is used to illustrate the effect of hyperconjugative delocalization on the carbon shielding.
Use of negative complex potential as absorbing potential
Authors: Ge Jiu-Yuan , Zhang John Z. H.
Published: July 2002

Exact numerical calculation shows that the use of negative complex potential (NCP) significantly improves the efficiency of wavefunction absorption over that of negative imaginary potential (NIP) in scattering applications. The improvement in absorption is especially significant in the case of low energy scattering with de Broglie wavelength larger than the length of absorbing potential. The addition of a negative real potential to the pure imaginary potential speeds up the absorption of wavefunction by effectively shortening its de Broglie wavelength. Explicit TD numerical calculation for a one-dimensional model demonstrates the effectiveness of the NCP and shows that the reflection from the absorbing potential can effectively be eliminated by using optimized absorbing parameters for the energy in question. In addition, comparison of the exact numerical calculation with semiclassical WKB analysis casts a serious doubt on the quantitative value of using WKB analysis at low energies with de Broglie wavelengths much larger than the absorbing length and/or for polynomial absorbing potentials higher than linear.
Effect of geometrical conformation on the short-time photodissociation dynamics of 1-iodopropane in the A-band absorption
Authors: Zheng Xuming , Phillips David Lee
Published: July 2002

We have taken resonance Raman spectra and made absolute Raman cross section measurements at six excitation wavelengths for 1-iodopropane. The resonance Raman spectra have most of their Raman intensity in features that may be assigned as fundamentals, overtones, and combination bands of three Franck–Condon active vibrational modes (the nominal C–I stretch, the nominal CCC bend, and the nominal CCI bend) for the trans and gauche conformations of 1-iodopropane. The resonance Raman and absorption cross sections of the trans and gauche conformations of 1-iodopropane were simulated using a simple model and time-dependent wave packet calculations. The results of the simulations were used in conjunction with the vibrational normal-mode coefficients to find the short-time photodissociation dynamics of trans and gauche conformers of 1-iodopropane in terms of internal coordinate changes. The trans and gauche conformers display significantly different Franck–Condon region photodissociation dynamics, which indicates that the C–I bond breaking is conformational dependent. In particular, there are large differences in the trans and gauche short-time photodissociation dynamics for the torsional motion (xGBx) about the GB carbon–carbon bond and the GBC angle (where C=α-carbon atom attached to the iodine atom, B=β-carbon atom attached to the α-carbon atom, G=methyl group carbon atom attached to the β-carbon atom). The major differences in the trans and gauche A-band short-time photodissociation dynamics can be mostly explained by the position of the C–I bond in the trans and gauche conformers relative to the plane of the three carbon atoms of the n-propyl group of 1-iodopropane.
Hydrodynamic effects on domain growth in off-critical polymer blends
Authors: Chen Hao , Chakrabarti Amitabha
Published: July 2002

Starting from a coarse-grained model, which includes hydrodynamic interactions, we numerically study kinetics of phase separation in a two-dimensional model of a polymer blend. In the absence of hydrodynamics, domains grow according to the Lifshitz–Slyozov power law without any evidence of pinning, regardless of the relative concentration of the blend components. In the presence of hydrodynamics, our results suggest that critical and off-critical mixtures are characterized by different growth laws. For the critical composition of the blend, hydrodynamic effects are found to be important during the entire growth process and this leads to a larger growth exponent. On the other hand, for off-critical blends, hydrodynamic effects are important only for blends with volume fraction not far away from the critical value. Even in these cases, domains show a faster growth only at intermediate times under the influence of hydrodynamics; subsequently the growth exponent crosses over to a Lifshitz–Slyozov value at late times. This gives strong support to the argument that the apparent pinning of domain growth observed in experiments on off-critical polymer blends may be due to this crossover phenomenon.
The thermal conductivity of an equimolar methane–ethane mixture in the critical region
Authors: Sakonidou E. P. , van den Berg H. R. , ten Seldam C. A. , Sengers J. V.
Published: July 2002

We have measured the thermal conductivity of an equimolar mixture of methane and ethane in the vicinity of the critical point. The new experimental data confirm that the thermal conductivity of a mixture does not diverge at the critical point but crosses over to a finite limiting behavior at the critical point. A quantitative representation of the thermal-conductivity data has been obtained in terms of a recently developed extension of the mode-coupling theory that incorporates the crossover between the behavior of the thermal conductivity close to and far away from the critical point. The same theory enables us also to predict other transport properties of the mixture in the vicinity of the critical point.
A unified model for ion conduction in crystals of β- and β″-alumina structure
Authors: Meyer Martin , Maass Philipp , Bunde Armin
Published: July 2002

We discuss a recently developed microscopic model for ion conduction in crystals of the β- and β″-alumina type, where the charge transport is associated with the motion of certain mobile defects. Monte Carlo simulations of the model provide a good quantitative description of the mixed alkali effect based on the fact that different types of mobile ions have a different preference to become part of a mobile defect. By taking into account the Coulomb interaction between the effectively charged defects and the disorder present in the conduction plane, also the conductivity spectra can be understood. We further show that our model reproduces the non-Arrhenius behavior of the dc-conductivity found in Na-β″-alumina.
Excited-state reversible geminate reaction. II. Contact geminate quenching
Authors: Agmon Noam
Published: July 2002

Results of Part I are extended to include the effect of contact quenching by the geminate partner. The long-time and asymptotic solutions derived herein are compared with numerical and experimental data for excited-state proton transfer to solvent, involving both quenching and two different lifetimes.
Effective Hamiltonians for degenerate and quasidegenerate direct perturbation theory of relativistic effects
Authors: Kutzelnigg Werner
Published: July 2002

After a review of a compact formulation of degenerate and quasidegenerate perturbation theory and the related effective Hamiltonians, a new analysis of the Foldy–Wouthuysen (FW) transformation as a sequence of two transformations is given, where the first transformation decouples electronic from positronic states, while the second transformation reestablishes the normalization. It is then shown that other transformations are possible which share the first part with the FW transformation, but which differ in the second part, by choosing different model spaces. The notorious singularities of the FW transformation in the presence of a Coulomb potential are entirely due to the second step, where the model space is that of all electronic states. Direct perturbation theory as well as its quasidegenerate generalization result as special cases with model spaces related to eigenfunctions of the nonrelativistic Hamiltonian. Additional flexibility results from the option between a unitary transformation and a transformation in intermediate normalization. Nonperturbative alternatives to the perturbation expansion are discussed. The new formalism is particularly useful for the application of direct perturbation theory to relativistic multiconfiguration SCF theory.
Photodetachment of free hexahalogenometallate doubly charged anions in the gas phase: [ML6]2−, (M=Re, Os, Ir, Pt; L=Cl and Br)
Authors: Wang Xue-Bin , Wang Lai-Sheng
Published: July 2002

We report the first observation and photodetachment photoelectron spectroscopic study of a series of hexahalogenometallates dianions MCl62− (M=Re, Os, Ir, and Pt) and MBr62− (M=Re, Ir, and Pt) in the gas phase. All of these species were found to be stable as free gaseous doubly charged anions. Photoelectron spectra of all the dianions were obtained at several detachment photon energies. The photon-energy-dependent spectra clearly revealed the dianion nature of these species and allowed the repulsive Coulomb barriers to be estimated. The binding energies of the second excess electron in MCl62− (M=Re, Os, Ir, Pt) were determined to be 0.46 (5), 0.46 (5), 0.82 (5), and 1.58 (5) eV, respectively, and those in MBr62− (M=Re, Ir, Pt) to be 0.76 (6), 0.96 (6), and 1.52 (6) eV, respectively. A wealth of electronic structure information about these metal complexes were obtained and low-lying and highly-excited electronic states of the corresponding singly charged anions were observed. Detachment from metal d orbitals or ligand orbitals were observed and could be clearly distinguished; detachments from the metal d-orbitals all occur at low binding energies whereas those from the ligand-dominated orbitals all take place at rather high binding energies. We also found a remarkable correlation between electron affinities measured in vacuo and the redox potentials obtained in the solution phase of these species.
A computational study of homogeneous liquid–vapor nucleation in the Lennard-Jones fluid
Authors: Shen Vincent K. , Debenedetti Pablo G.
Published: July 2002

Umbrella sampling Monte Carlo simulations are used to calculate free energy barriers to homogeneous liquid–vapor nucleation in the superheated Lennard-Jones fluid. The calculated free energy barriers decrease with increased superheating and vanish at the spinodal curve. A statistical geometric analysis reveals the existence of two types of voids: Small interstitial cavities, which are present even in the equilibrium liquid, and much larger cavities that develop as the system climbs the nucleation free energy barrier. The geometric analysis also shows that the average cavity size within the superheated liquid is a function of density but not of temperature. The critical nucleus for the liquid–vapor transition is found to be a large system-spanning cavity that grows as the free energy barrier is traversed. The weblike cavity is nonspherical at all superheatings studied here, suggesting a phenomenological picture quite different from that of classical nucleation theory.
A colloidal crystal modeled by bead–spring cubes
Authors: Denneman A. I. M. , Jongschaap R. J. J. , Mellema J.
Published: July 2002

In experiments colloidal crystals are usually polycrystalline. This polycrystallinity depends on the shear history, while a stable orientation of the crystallites is obtained by preshearing the sample for a sufficiently long time. To predict the linear viscoelastic properties of a colloidal crystal, the crystallites immersed in a Newtonian fluid are modeled by bead–spring cubes. Explicit constitutive equations are obtained, which describe the stress response on the applied small-amplitude oscillatory shear flow with frequency ω. These constitutive equations are used to predict the dynamic moduli G′(ω) and G″(ω) belonging to the specific configurations of the polycrystalline sample before and after preshearing. The influence of the preshear process on the dynamic moduli is discussed and for a sample that is presheared for a sufficiently long time the following results are obtained: (i) for high frequencies G′(ω) is constant and G″(ω)=η∞ω (where η∞ is the viscosity contribution of the fluid surrounding a crystallite) and (ii) for lower frequencies G′(ω) is nearly constant and G″(ω) is proportional to ω−1/2. The theoretical results obtained in this paper are consistent with experimental results found in literature. It is finally noted that the bead–spring formalism in this paper shows explicitly that the static modulus belonging to the crystallites is identical to the high frequency limit of the storage modulus G′(ω).
Raman frequency noncoincidence effect of confined liquid dimethyl sulfoxide
Authors: Czeslik Claus , Kim Yoo Joong , Jonas Jiri
Published: July 2002

Raman spectroscopy was used to study the short-range structure of liquid dimethyl sulfoxide (DMSO) confined to porous silica glasses in the pore radius range from 15 to 56 Å. By analyzing isotropic and anisotropic Raman spectra of DMSO in the SO stretching vibration region, we found the noncoincidence of the in-phase and out-of-phase SO vibrational wavenumbers increasing with decreasing pore radius. This behavior is in contrast to that found earlier for other confined polar liquids. Using a theory derived by Logan and results of ab initio calculations, the liquid structures of DMSO, acetone, N,N-dimethyl formamide in the proximity of silica surfaces are proposed.
Ammonia synthesis at low temperatures
Authors: Rod T. H. , Logadottir A. , Nørskov J. K.
Published: July 2002

Density functional theory (DFT) calculations of reaction paths and energies for the industrial and the biological catalytic ammonia synthesis processes are compared. The industrial catalyst is modeled by a ruthenium surface, while the active part of the enzyme is modeled by a MoFe6S9 complex. In contrast to the biological process, the industrial process requires high temperatures and pressures to proceed, and an explanation of this important difference is discussed. The possibility of a metal surface catalyzed process running at low temperatures and pressures is addressed, and DFT calculations have been carried out to evaluate its feasibility. The calculations suggest that it might be possible to catalytically produce ammonia from molecular nitrogen at low temperatures and pressures, in particular if energy is fed into the process electrochemically.
The non-separability of “dielectric” and “mechanical” friction in molecular systems: A simulation study
Authors: Kumar P. V. , Maroncelli M.
Published: July 2002

Simulations of the time-dependent friction controlling rotational, translational, and vibrational motions of dipolar diatomic solutes in acetonitrile and methanol have been used to examine the nature of “dielectric” friction. The way in which electrical interactions increase the friction beyond that present in nonpolar systems is found to be rather different than what is anticipated by most theories of dielectric friction. Long-range electrostatic forces do not simply add an independent contribution to the friction due to short-ranged or “mechanical” sources (modeled here in terms of Lennard-Jones forces). Rather, the electrical and Lennard-Jones contributions are found to be strongly anticorrelated and not separable in any useful way. For some purposes, the mechanism by which electrical interactions increase friction is better viewed as a static electrostriction effect: electrical forces cause a subtle increase in atomic density in the solute’s first solvation shell, which increases the amplitude of the force fluctuations derived from the Lennard-Jones interactions, i.e., the mechanical friction. However, electrical interactions also modify the dynamics of the friction, typically adding a long-time tail, which significantly increases the integral friction. Both of these effects must be included in a correct description of friction in the presence of polar interactions.
Experimental evidence of structural evolution in ultrafine cobalt particles stabilized in different polymers—From a polytetrahedral arrangement to the hexagonal structure
Authors: Dassenoy F. , Casanove M.-J. , Lecante P. , Verelst M. , Snoeck E. , Mosset A. , Ely T. Ould , Amiens C. , Chaudret B.
Published: July 2002

Ultrafine cobalt particles have been reproducibly synthesized by decomposition of an organometallic precursor in the presence of a stabilizing polymer. The size of the stable monodisperse colloids thus obtained is seen to strongly depend on the nature of the polymer: around 4.2 nm diameter in polyphenylenoxide (PPO) and around 1.4 nm diameter in polyvinylpyrrolidone (PVP). Investigations by wide angle x-ray scattering (WAXS) and high-resolution transmission electron microscopy (HRTEM) give evidence for a size dependence of the structural organization, and hence for a close relationship between structure and synthesis conditions. Co/PPO particles exhibit a hexagonal compact structure with the metal–metal bond length of the bulk material while Co/PVP ones display an original structure. We show that the unusual features of the experimental data in Co/PVP clearly point to a nonperiodic polytetrahedral structure. Successful simulations of the HRTEM and WAXS results have been obtained using models built on the basis of a polytetrahedral arrangement.
Ion association and electrical conductance minimum in Debye–Hückel-based theories of the hard sphere ionic fluid
Authors: Weingärtner Hermann , Weiss Volker C. , Schröer Wolffram
Published: July 2002

We investigate the ion–ion pair association equilibrium in the hard sphere ionic fluid as predicted by several Debye–Hückel-based theories. The degree of dissociation is examined in a pairing theory based on Ebeling’s definition of the association constant, and in extensions of this theory for ion pair–free ion interactions by Fisher and Levin, and ion pair–ion pair interactions by Weiss and Schröer. The effect of an increase of the dielectric constant on the degree of dissociation is studied. In all cases, minima of the density-dependent degree of dissociation along isotherms are found at low ionic density, at least over part of the temperature range. The loci of these minima in the temperature-density plane are calculated. The results are compared with conductivity data. We discuss a possible resemblance of the system’s behavior in a region in which the degree of association changes rapidly with a tricritical scenario, in which a line of second-order insulator–conductor phase transitions intersects the coexistence curve.
Propagators for massive vector fields in anti-de Sitter space-time using Stueckelberg’s Lagrangian
Authors: Janssen H. , Dullemond C.
Published: July 2002

Expressions are found for homogeneous and inhomogeneous propagators for vector fields of arbitrary mass in anti-de Sitter space-time using a generalization of Stueckelberg’s Lagrangian for a massive vector field. The massless case (quantum electrodynamics) is also considered by taking the appropriate zero-mass limit.
The evolution partial differential equation u
t=u
x
x
x+3(u
x
x
u2 +3u2x
u)+3u
x
u4
Authors: Calogero F.
Published: July 2002

The evolution equation ut=uxxx+3(uxxu2 +3u2xu)+3uxu4, u=u(x,t), is integrable; it can be (exactly) linearized by an appropriate change of (dependent) variable. Hence several explicit solutions of the partial differential equation (PDE) can be exhibited; some of them display a remarkable solitronic phenomenology.
A Bäcklund transformation and nonlinear superposition formula of a modified Korteweg–de Vries-type bilinear equation
Authors: Hu Xing-Biao
Published: July 2002

A modified Korteweg–de Vries (mKdV)-type bilinear equation that has been shown to pass the three-soliton solution and the Painlevé tests [see, J. Math. Phys. 2094, 28 (1987); 2572, 31 (1990)] is considered herein. In this article, a Bäcklund transformation and a nonlinear superposition formula are proven rigorously. As an application of the obtained results, N-soliton solutions are obtained. Therefore, the integrability of this mKdV-type bilinear equation is confirmed.
Zeros of the Jimbo, Miwa, Ueno tau function
Authors: Palmer John
Published: July 2002

We introduce a family of local deformations for meromorphic connections on P1 in the neighborhood of a higher rank (simple) singularity. Following the scheme in Malgrange [Mathematique et Physique, Progress in Mathematics (Birkhäuser, Boston, 1983), Vol. 37, pp. 381–400, ibid., pp. 401–426; ibid., pp. 427–438] we use these local models to prove that the zeros of the tau function, introduced by Jimbo, Miwa, and Ueno in their pioneering work on “Birkhoff” deformations at irregular singular points [Physica D 2, 306–352; 2, 407–448 (1981); 4, 26–46 (1983); Publ. RIMS Kyoto Univ. 17-2, 703–721 (1981)], occur at precisely those points in the deformation space at which a certain Birkhoff–Riemann–Hilbert problem fails to have a solution.
The shallow-water nonlinear Schrödinger equation in Lagrangian coordinates
Authors: Osborne A. R. , Boffetta G.
Published: July 2002

The nonlinear Schrödinger equation in Lagrangian coordinates (LNLS) is derived from the Lagrangian form of the Korteweg–de Vries equation (LKdV) using a multiscale averaging approach. The resultant LNLS equation describes fluid particle motions for a small-amplitude, narrow-banded nonlinear wave field propagating in shallow water. Particle motion is discussed for LNLS in the context of the inverse scattering transform (IST) for both infinite-line and periodic boundary conditions and explicit expressions for the trajectories of fluid parcels are derived. It is demonstrated that, even in the presence of nonlinear effects, a particle trajectory can be orthogonally decomposed into nonlinear horizontal and vertical motions which, given the initial conditions, evolve independently of each other. It is shown how the presence of a long, slowly varying wave component, which is interpreted as radiation stress, influences the motion of fluid particles. Finally it is also shown how these results may be used in an experimental context for the study of data described approximately by the LNLS equation is discussed.
Interfacial electrohydrodynamic instability: The Kath and Hoburg model revisited
Authors: Castellanos A. , González A.
Published: July 2002

A revision of the Kath and Hoburg model for the interfacial electrohydrodynamic instability in the case of two miscible fluids having identical properties but differing conductivities is attempted. Experiments performed by the same authors to validate their model show that the measured wave numbers do not correspond to the modes of maximum instability. In fact, the model is unable to give a preferred wave number. An analysis of the possible explanations for this discrepancy is made. In this Brief Communication the existence of a transient surface tension as the wave number selection mechanism is proposed.
The sudden expansion of a gas cloud into vacuum revisited
Authors: Tzuk Y. , Barmashenko B. D. , Bar I. , Rosenwaks S.
Published: July 2002

The sudden expansion into vacuum of a gas cloud, with initial centrally symmetric density and temperature profiles, is studied theoretically for different values of the specific heat ratio γ. Models treating the expansion are discussed, in particular, a model for isentropic expansion and a model for spatially isothermal expansion. For γ→1, the density of the gas obtained from the former model for late stages of the expansion, approaches a Gaussian spatial profile which is the exact solution to the latter model. A description by a Gaussian profile can be, for some important cases, approximately correct even for large deviations of γ from one. For a spherically symmetric flow, the maximum difference (for any given time and distance from the center of symmetry) between the densities obtained from the above two models is 11% for γ=7/5. For γ=1.28, which corresponds to the expansion of lead azide detonation products previously studied in the author’s laboratory, the difference is 9%. It is also shown that in practice it is more convenient to use the model for isothermal expansion to describe the density profile since it does not depend on γ, which is very often not exactly known. Finally, for γ→1, a relation between the density and the temperature is obtained which is not dependent on their initial distributions.
The transient electrodynamic forces at laser–plasma interaction
Authors: Hora Heinrich
Published: July 2002

In the discussion of the concerning of the transient terms of the nonlinear (ponderomotive) forces of laser–plasma interaction, the recent result by Zeidler et al. [Phys. Fluids 28, 372 (1985)] can be used to confirm the strength of Schlüter’s term and to provide an analogy to conclude a more general transient term.
Self-consistent study of a perpendicular collisionless and nonresistive shock
Authors: Lembege B. , Dawson J. M.
Published: July 2002

Particle dynamics and field behavior associated with a perpendicular collisionless supercritical and viscous shock are investigated by use of numerical simulation. A one-dimensional, relativistic, fully electromagnetic and nonperiodic particle simulation code (for both electrons and ions) is used where self-consistent space-charge effects and induced effects are totally included. The principal field patterns of the shock (trailing wave train, ramp, and foot region) are studied in detail and are shown to have scale lengths mainly dictated by ion dynamics; the behavior of the corresponding plasma currents associated with the different field components is also presented. Ions are shown to suffer successive ‘‘acceleration–trapping–detrapping’’ at the shock front, and locally in the trailing wave train of the downstream region through combined effects of the electrostatic and magnetic fields. While detrapped, the reflected ions describe very large Larmor orbits and cause a ring distribution; a large rapid nonstochastic ion heating results from this ion gyration. This heating (resistivity-free) is the main source of dissipation and is responsible for large field damping. Competitive effects such as particle stochasticity, particle trapping, wave damping, wave overtaking, and dispersion effects are shown to interact with each other and to affect the overall dissipation mechanism. Comparison with previous works is also discussed. Various Mach number situations are considered, leading to the definition of a transitory regime between subcritical and supercritical regimes and of a corresponding critical threshold of the electrostatic field. In contrast with the supercritical regime, the subcritical regime is characterized by a low density of trapped-reflected ions, a broad ion distribution function with a weak tail, and a weak adiabatic bulk ion heating.
An experimental study of dynamics of drop formation
Authors: Zhang Xiaoguang , Basaran Osman A.
Published: July 2002

A liquid being ejected from a nozzle emanates from it as discrete, uniformly sized drops when the flow rate is sufficiently low. In this paper, an experimental study is presented of the dynamics of a viscous liquid drop that is being formed directly at the tip of a vertical tube into ambient air. The evolution in time of the drop shape and volume is monitored with a time resolution of 1/12 to 1 ms. Following the detachment of the previous drop, the profile of the new growing drop at first changes from spherical to pear-shaped. As time advances, the throat of the pear-shaped drop takes on the appearance of a liquid thread that connects the bottom portion of the drop that is about to detach to the rest of the liquid that is pendant from the tube. The focus here is on probing the effects of physical and geometric parameters on the universal features of drop formation, paying special attention to the development, extension, and breakup of the liquid thread and the satellite drops that are formed subsequent to its breakup. The role of surfactants in modifying the dynamics of drop formation is also studied. The effects of finite inertial, capillary, viscous, and gravitational forces are all accounted for to classify drastically different formation dynamics and to elucidate the fate of satellite drops following thread rupture.
On the universality of the Kolmogorov constant
Authors: Sreenivasan Katepalli R.
Published: July 2002

All known data are collected on the Kolmogorov constant in one-dimensional spectral formula for the inertial range. For large enough microscale Reynolds numbers, the data (despite much scatter) support the notion of a ‘‘universal’’ constant that is independent of the flow as well as the Reynolds number, with a numerical value of about 0.5. In particular, it is difficult to discern support for a recent claim that the constant is Reynolds number dependent even at high Reynolds numbers.
Symmetry breaking bifurcation in finite disk flow
Authors: Radel Victoria S. , Szeri Andras Z.
Published: July 2002

In the Reynolds number range 8 771⩽Re⩽13 300, we find three distinctly different solutions for flow between finite, corotating disks. The basic flow, present for all Reynolds numbers, displays symmetry with respect to midplane, while the other two solutions are asymmetric, exist only within the specified Reynolds number range, and are mirror images of one another. These solutions were obtained in mixed formulation of the steady state problem. To circumvent the Babushka–Brezzi stability criteria yet solve for steady state, we follow Zienkiewicz and Woo and adjoin the time asymptotic form of the equation of mass conservation, in artificial compressibility form, to the steady state Navier–Stokes equations. The system so obtained is nonsingular and yields to easy solution by Galerkin’s method. To follow particular solution branches, we employ parametric continuation.
Length scales of turbulence in stably stratified mixing layers
Authors: Smyth William D. , Moum James N.
Published: July 2002

Turbulence resulting from Kelvin–Helmholtz instability in layers of localized stratification and shear is studied by means of direct numerical simulation. Our objective is to present a comprehensive description of the turbulence evolution in terms of simple, conceptual pictures of shear–buoyancy interaction that have been developed previously based on assumptions of spatially uniform stratification and shear. To this end, we examine the evolution of various length scales that are commonly used to characterize the physical state of a turbulent flow. Evolving layer thicknesses and overturning scales are described, as are the Ozmidov, Corrsin, and Kolmogorov scales. These considerations enable us to provide an enhanced understanding of the relationships between uniform-gradient and localized-gradient models for sheared, stratified turbulence. We show that the ratio of the Ozmidov scale to the Thorpe scale provides a useful indicator of the age of a turbulent event resulting from Kelvin–Helmholtz instability.
The critical temperature of dry impact for tiny droplet impinging on a heated surface
Authors: Wang An-Bang , Lin Chih-Hung , Chen Chi-Chang
Published: July 2002

Leidenfrost temperature has long been taken as the criterion to classify characteristic regimes for not only heat transfer but also hydrodynamic impact patterns for droplet impinging on a heated surface. This work classifies the characteristic impact regimes and reveals that the hydrodynamic impact parameter influences the classification of characteristic impact regimes. For each individual impact, the required surface temperature for the dry impact, which has no direct liquid–solid-contact during the impact period is much lower than the Leidenfrost temperature. The squeeze film effect has been successfully used to explain the corresponding mechanism for it.
Effect of antenna orientation and plasma anisotropy on the directivity of fast wave antenna radiation
Authors: Pavlov I. P. , Heikkinen J. A.
Published: July 2002

Asymmetry in the transverse wave number spectrum of the radiated power of a screenless fast wave antenna at an ion cyclotron range of frequencies is calculated with a model that takes into account the nonsymmetry of the plasma surface impedance matrix for an inhomogeneous tokamak plasma in front of the antenna. The directivity of the wave number spectrum transverse to the ambient magnetic field caused by the asymmetry in the surface impedance is found to be strongly asymmetric with respect to the parallel wave number by the effect of the nonperpendicular angle between the antenna current strap and the magnetic field. The latter is shown to be responsible also for the asymmetry in the parallel wave number spectrum of an undirected antenna, and can lead to deviations of order ≤30% in the corresponding spectrum of a phased antenna array with directivity. The consequences of the observed effects to the antenna performance in the current drive applications as well as in excitation of poloidally asymmetric spectra are discussed.
Experimental study of shock formation in a dusty plasma
Authors: Luo Q.-Z. , D’Angelo N. , Merlino R. L.
Published: July 2002

An experimental investigation of the effect of negatively charged dust on ion acoustic shock formation in a Q machine is described. Ion acoustic compressional pulses were observed to steepen as they traveled through a dusty plasma if the percentage of the negative charge in the plasma on the dust grains was ≳75%.
Turbulence driven by two-stream instability in a magnetized plasma
Authors: Goldman Martin V. , Crary Frank , Newman David L. , Oppenheim Meers
Published: July 2002

Although the nonlinear evolution of the two-stream instability has been a subject of numerical and theoretical studies for many years, recent spacecraft measurements of nonlinear electrostatic bipolar wave structures in the auroral ionosphere have prompted new studies. Using parallel two-dimensional (2D) particle-in-cell electrostatic simulations with initially counterstreaming cold electrons in a magnetized plasma, the evolution of wave turbulence and particle distributions has been followed for over 10 000 inverse plasma frequencies. Ions are introduced in the frame of one of the electron beams (motivated by measurements). Wave turbulence evolves in at least five separate stages. After tens of ωe−1, trapping produces bipolar wave structures with long range order across B. After hundreds of ωe−1, the structures break up and whistlers appear. After thousands of ωe−1, ion cyclotron waves driven by a bump-on-tail in the evolved electron distribution become prominent. After 6000 ωe−1, the ion cyclotron waves appear to undergo parametric decay into oblique waves.
Effective mass filtering: Giant quantum amplification of the photocurrent in a semiconductor superlattice
Authors: Capasso Federico , Mohammed Khalid , Cho Alfred Y. , Hull Robert , Hutchinson Albert L.
Published: July 2002

We report the first observation of an extremely large photocurrent amplification phenomenon at very low voltages in a superlattice of Al0.48In0.52As (35 Å)/Ga0.47In0.53As (35 Å), grown by molecular beam epitaxy. Responsivities at λ=1.3 μm are as high as 4×103 A/W at 1.4 V corresponding to a current gain of 2×104. Significant gains (≂50) were obtained at voltages as low as 20 mV. The detectivity D* at λ=1.3 μm and 1-kHz modulation frequency is 1011 (cm Hz1/2/W) at 0.2-V bias. This effect, which represents a new quantum type photoconductivity, is caused by the large difference in the tunneling rates of electrons and holes through the superlattice barriers, associated with their large effective mass difference (effective mass filtering). Superlattice effective mass filters represent a new class of low voltage, high gain photodetectors. A unique feature, which makes them potentially extremely versatile, is the ability to tune the gain and gain-bandwidth product over a wide range by varying the superlattice period and/or duty factor.
New native oxide of InP with improved electrical interface properties
Authors: Robach Y. , Joseph J. , Bergignat E. , Commere B. , Hollinger G. , Viktorovitch P.
Published: July 2002

The electrical properties of InP insulator interface were improved by using a new native oxide between gate insulator and the semiconductor. This phosphorus-rich oxide identified as In(PO3)y polyphosphate was grown anodically. Capacitance-voltage measurements on this metal-insulator-semiconductor structure yielded an interface state density as low as 4×1010 cm−2 eV−1 and were nearly free of hysteresis in the depletion and accumulation region.