bluuebunny/crossref_metadata_2025 huggingface dataset:
Propagation in a plasma of a laser beam smoothed by longitudinal spectral dispersion
Authors: Walraet François , Riazuelo Gilles , Bonnaud Guy
Published: February 2003

The paraxial propagation of a large-bandwidth laser beam focused by a grating onto a plasma is examined. Plasma nonlinearity inducing filamentation is modeled by ion-acoustic waves via three-dimensional simulations. The resulting longitudinal spectral dispersion (LD) smoothing is compared to the standard transverse dispersion (TD) smoothing. The coherence time and size, the peak intensity and the propagation angle of the speckles are addressed. The influence of a transverse plasma flow is discussed. No special advantage of one of the smoothing techniques is observed for controlling filamentation. Only beam bending appears to be reduced when LD is used instead of TD.
Laboratory and computer simulations of super-Alfvénic shocks in a weakly ionized medium
Authors: Mourenas D. , Vierne J. , Simonet F. , Krauz V. I. , Nikulin S. , Mialton V. V. , Karakin M. A.
Published: February 2003

High Mach number shocks have been studied experimentally at the Kurchatov Institute in 2000 with the PF3 Plasma Focus [N. V. Filippov et al., Phys. Lett. A 211, 168 (1996)]. The main goal of these especially designed laboratory experiments was to provide data against which to test current numerical models, in order to better understand the interaction of a high-velocity plasma jet with a partially ionized gas in a strong transversal magnetic field. Observed magnetic field compression, plasma density profile evolution, and shock slowing down, are well reproduced by a two-dimensional hybrid code HAWAI2D [B. Lembege and F. Simonet, Phys. Plasmas 8, 3967 (2001)] with Monte Carlo collisions. Some of the code initial conditions are directly obtained from the numerous diagnostics installed in the experimental chamber. Others are derived from jet velocity and background density measurements, making use of a simplified model of jet propagation from the pinch. Ion-neutral collisions play a central role in the shock dynamics, as demonstrated in the simulations.
On low-frequency dust-modes in a collisional and streaming dusty plasma with dust charge fluctuation
Authors: Islam M. K. , Nakashima Y. , Yatsu K. , Salimullah M.
Published: February 2003

Lighter particles streaming, dust charge fluctuation, and collisional effects on the dust-modes, viz., the dust-lower-hybrid (DLH) and dust-acoustic (DA) waves, have been examined analytically using fluid model. In the absence of dust charge fluctuation and collisional effects, two-stream instability of these modes occurs when the ion streaming velocity exceeds the parallel phase velocity of the respective mode. Moreover, if either ion or electron streaming velocity exceeds the wave parallel phase velocity, then the collisions of ions or electrons with neutral gas atoms/molecules give destabilizing effect on the dust-modes provided the dust charge fluctuation is negligible. Dust charge fluctuation gives the damping of either of these modes. It is found that charge fluctuation and collisional effects on the DA-mode are greater than those of the DLH-mode. Hence, the DLH-mode is more stable compared to the DA-mode.
Equilibrium of field reversed configurations with rotation. IV. Two space dimensions and many ion species
Authors: Qerushi Artan , Rostoker Norman
Published: February 2003

In a previous paper [N. Rostoker and A. Qerushi, Phys. Plasmas 9, 3057 (2002)] a generalized Grad–Shafranov equation for the plasma flux function was derived which provides a complete description of equilibria of field reversed configurations with rotation. In this paper this fundamental equation is solved for two space dimensions and many ion species. The following fusion fuels are considered: D–T, D–He3, and p–B11. Using periodic boundary conditions the original differential equation is converted to an equivalent integral equation which involves a Green’s function. The integral equation is solved by iteration. Approximate solutions are found for all the fusion fuels considered using a two-dimensional equilibrium model for one type of ion [A. Qerushi and N. Rostoker, Phys. Plasmas 9, 5001 (2002)]. They are used as starting trial functions of the iterations. They turn out to be so close to the real solutions that only a few iterations are needed.
Fully nonlinear phenomenology of the Berk–Breizman augmentation of the Vlasov–Maxwell system
Authors: Vann R. G. L. , Dendy R. O. , Rowlands G. , Arber T. D. , d’Ambrumenil N.
Published: February 2003

The Berk–Breizman augmentation of the Vlasov–Maxwell system is widely used to model self-consistent resonant excitation and damping of wave fields by evolving energetic particle populations in magnetic fusion plasmas. The key model parameters are the particle annihilation rate νa, which drives bump-on-tail structure, and the linear wave damping rate γd. A code, based on the piecewise parabolic method, is used to integrate the fully nonlinear Berk–Breizman system of equations across the whole (νa,γd) parameter space. The results of this code show that the system’s behavior can be classified into one of four types, each of which occurs in a well-defined region of parameter space: chaotic, periodic, steady state, and damped. The corresponding evolution in (x,v) phase space is also examined.
Self-fields in a free-electron laser with helical wiggler and ion-channel guiding
Authors: Esmaeilzadeh Mahdi , Mehdian Hassan , Willett Joseph E. , Aktas Yildirim M.
Published: February 2003

The self-electric and self-magnetic fields of a relativistic electron beam passing through a helical wiggler and ion channel are analyzed. A numerical study shows that for a low energy, high density beam the self-fields can produce very large effects.
Cold phase fluid model of the longitudinal dynamics of space-charge-dominated beams
Authors: de Hoon Michiel J. L. , Lee Edward P. , Barnard John J. , Friedman Alex
Published: February 2003

The dynamics of a longitudinally cold, charged-particle beam can be simulated by dividing the beam into slices and calculating the motion of the slice boundaries due to the longitudinal electric field generated by the beam. On each time step, the beam charge is deposited onto an (r, z) grid, and an existing (r, z) electrostatic field solver is used to find the longitudinal electric field. Transversely, the beam envelope equation is used for each slice boundary separately. In contrast to the g-factor model, it can be shown analytically that the repulsive electric field of a slice compressed to zero length is bounded. Consequently, this model allows slices to overtake their neighbors, effectively incorporating mixing. The model then effectively describes a cold fluid in longitudinal z, vz phase space. Longitudinal beam compression calculations based on this cold phase fluid model showed that slice overtaking reflects local mixing, while the global phase space structure is preserved.
Elastic and transport cross sections for argon in hydrogen plasmas
Authors: Krstić P. S. , Schultz D. R. , Chung Taekyun
Published: February 2003

Accurate elastic differential scattering cross sections have been computed through a fully quantum mechanical approach for hydrogen ions (H+, D+, or T+) colliding with argon in the center of mass energy range of 0.1 to 200 eV. With the appropriate weight functions, these differential cross sections have been integrated to yield the momentum transfer, viscosity, and integral elastic cross sections. The momentum transfer and viscosity cross sections have been extended to higher collision energies (up to 10 keV) using a classical, three-body scattering method. Results have been compared with those of a simple model and previous recommended values. Knowledge of these cross sections, tabulated and made available through the world wide web, is of use to modeling and diagnostics of gaseous electronic, fusion, and other plasma environments.
Experimental observation of plasma formation and current transfer in fine wire expansion experiments
Authors: Duselis Peter U. , Kusse Bruce R.
Published: February 2003

When several kA pulses are passed through single, fine 25-μm-diam wires the wire material heats, melts, vaporizes, and expands. Initially the voltage across and current through a wire increases until an abrupt voltage collapse occurs. After this collapse the voltage remains at a relatively small value while the current continues to increase. The voltage collapse has been attributed to the formation of plasma around the wire and a transfer of current to this highly conducting coronal plasma. Interferometry has confirmed the plasma formation, but the current transfer has only been postulated. This Letter reports additional confirmation of the plasma formation as well as the first direct observation of the current transfer.
Localized measurement of turbulent fluctuations in tokamaks with coherent scattering of electromagnetic waves
Authors: Mazzucato E.
Published: February 2003

Localized measurements of short-scale turbulent fluctuations in tokamaks are still an outstanding problem. In this paper, the method of coherent scattering of electromagnetic waves for the detection of density fluctuations is revisited. Results indicate that the proper choice of frequency, size and launching of the probing wave can transform this method into an excellent technique for high-resolution measurements of those fluctuations that plasma theory indicates as the potential cause of anomalous transport in tokamaks. The best spatial resolution can be achieved when the range of scattering angles corresponding to the spectrum of fluctuations under investigation is small. This favors the use of high frequency probing waves, such as those of far infrared lasers. The application to existing large tokamaks is discussed.
Characteristics of runaway electrons in a tokamak
Authors: Schroder K. A. , Gentle K. W.
Published: February 2003

The characteristics of the high-energy tail (∼1 MeV) on the electron distribution in a tokamak have been determined with a new technique from measurements of the heat flux to a movable target on the outboard midplane. The measurements give the absolute value of f(v) and a direct estimate of the diffusion coefficient for energetic electrons at the edge. From this information, average confinement time, diffusion coefficient, and source position can be inferred. The diffusion coefficients are in the range 1–4 m2/s, comparable with those of the bulk particles, which are much larger than the values expected for energetic electrons subject only to electrostatic turbulence that would account for the bulk diffusivity.
Blobs and front propagation in the scrape-off layer of magnetic confinement devices
Authors: Bian N. , Benkadda S. , Paulsen J.-V. , Garcia O. E.
Published: February 2003

In this paper we show the self-consistent evolution of an isolated density perturbation in models of tokamak scrape-off layer turbulence. Our purpose is to explain the possible mechanisms responsible for radial propagation of density perturbations observed in the scrape-off layer. Results of both two-dimensional numerical simulations and one-dimensional quasilinear modeling of the propagative events are presented, and shown to be consistent with many experimental observations. The role of sheath dissipation for front propagation and turbulent mixing is also addressed.
Radiative Alfvén condensation instability
Authors: Goswami Rajiv , Singh Raghvendra , Kaw Predhiman
Published: February 2003

The coupling of magnetic perturbations to the radiative condensation instability has been investigated. It is shown that drift-like perturbations can couple the shear Alfvén waves to the radiative thermal mode and give rise to a radiative Alfvén condensation instability.
Effects of flow shear on the ion temperature gradient modes in a high β plasma slab
Authors: Gao Zhe , Dong J. Q. , Liu G. J. , Ying C. T.
Published: February 2003

The integral eigenmode equations derived previously for the study of drift instabilities in a sheared slab plasma with arbitrary β (plasma pressure/magnetic pressure) are extended. These equations are used to investigate the effects of flow shear on the ion temperature gradient driven (ITG) modes. It is found that the destabilizing effect of a parallel velocity shear, V0′, is weakened by the finite β effect, especially in case of weak magnetic shear. However, the perpendicular velocity shear, VE′, still effectively stabilizes these modes even in high β regions. A large enough VE′ can completely stabilize the ITG mode at arbitrary β. In addition, the effect of VE′ is highly enhanced in weak magnetic shear regions. When the parallel flow coexists with the perpendicular flow, the comprehensive flow effect depends on the relative sign of these velocity shears. The modes with higher growth rates may be stabilized by a smaller VE′ for V0′VE′>0.
Injection of electrons into the three-dimensional helical wiggler field in a free electron laser
Authors: Mirzanejhad S.
Published: February 2003

Injection of electrons into a three-dimensional magnetostatic helical wiggler in the resonance region is studied numerically. Similarly to the one-dimensional situation, a considerable reduction in the amplitude of oscillations about the steady-state orbits is found when an axial magnetic field is changed adiabatically. On the other hand, interesting quasihelical trajectories are found for an annular beam in the resonance region. Therefore it seems the gain reduction in the magnetoresonance region can be overcome by this new injection method.
Convective cell formation in a Z pinch
Authors: Kesner J.
Published: February 2003

Closed field line confinement systems can develop convective cells when the magnetohydrodynamic interchange stability criterion is violated. Using a previously derived set of reduced equations [V. P. Pastukhov and N. V. Chudin, Plasma Phys. Rep. 27, 907 (2001)] it is shown that a true steady state solution can exist. For an assumed large-scale vortex pattern, the plasma pressure profile that is implied by these convective flows as well as the nonlocal heat flux resulting from the convective flows is calculated.
Evidence of Lévy stable process in tokamak edge turbulence
Authors: Jha R. , Kaw P. K. , Kulkarni D. R. , Parikh J. C. ,   ADITYA Team
Published: February 2003

The time series of floating potential and poloidal electric field fluctuations in the edge plasma of ohmically heated ADITYA tokamak [Phys. Plasmas 4, 4292 (1997)] are analyzed for self-similarity. It is observed that the distribution function of a sum of n data points converges to a self-similar distribution of Lévy scale index, α=1.1–1.3 for n⩽40 and α=1.8–2.0 for larger n. This shows that the scaling properties of small scale fluctuations are non-Gaussian and those of large scale fluctuations are Gaussian. Implication of this observation to our understanding of plasma transport is discussed.
Four-mode calculation of resonance states of intermediate complexes in the SN2 reaction Cl−+CH3Cl′→ClCH3+Cl′−
Authors: Schmatz Stefan , Hauschildt Jan
Published: February 2003

A four-dimensional coupled-cluster [CCSD(T)] potential energy surface has been constructed for the collinear symmetric SN2 reaction Cl−+CH3Cl′→ClCH3+Cl′−. Making use of the filter diagonalization method and optical potentials, bound states as well as resonance states up to energies far above the dissociation threshold have been calculated. Most of the bound and resonance states up to the first overtone of the symmetric C–H stretching vibration could be assigned four quantum numbers (symmetric C–H stretching mode, umbrella bending mode, intra- and intermolecular C–Cl stretching modes). The effect of different average lifetimes of resonance states with gerade and ungerade symmetry that was found in previous two-mode calculations [J. Chem. Phys. 114, 5233 (2001)] is preserved in the four-dimensional calculations. The range of the resonance widths has become appreciably more widespread because many resonance states with excitation in the high-frequency C–H stretching mode and the umbrella bending mode are particularly long-living. The linewidths of the resonances and, therefore, the coupling to the continuum become larger with excitation in any mode. However, this increase as a function of a quantum number is not monotonic due to the strong mixing of the many progressions in the intermolecular stretching mode of the intermediate complex.
Correlation between rate of folding, energy landscape, and topology in the folding of a model protein HP-36
Authors: Mukherjee Arnab , Bagchi Biman
Published: February 2003

We explore the correlation between the energy landscape and topology in the folding of a model protein (chicken villin headpiece HP-36) by using a force-field which incorporates the effects of water through a hydropathy scale and the role of helical propensity of amino acids through a nonlocal harmonic potential. Each amino acid is represented by one side chain atom which is attached to the backbone Cα atom. Sizes and interactions of all the side chain residues are different and depend on the hydrophobicity of a particular amino acid, whereas helical propensities are incorporated in the interaction of Cα atoms. Simulations have been carried out by quenching from a fixed high temperature to two different low temperatures for many initial random configurations. The simulated structures resemble the real native state rather closely, with the root mean square deviation of the best structure being 4.5 Å. Moreover, the structure shows both the helices and bends at the appropriate positions of the model protein. The simplified model allows the study of energy landscape and also of the correlation between energy landscape with the dynamics of folding and topology. The initial part of folding is very fast, followed by two distinct slow stages, with the last stage being certainly the rate determining of the folding process. The initial fast dynamics is primarily due to hydrophobic collapse. The very slow last stage of folding is accompanied by a significant and sharp increase in the relative contact order parameter but relatively small decrease in energy. Analysis of the time dependence of the formation of the individual contact pairs show rich and complicated dynamics, where some contacts wait for a long time to form. This seems to suggest that the slow late stage folding is due to long range contact formation and also that the free energy barrier is entropic in origin. Results have been correlated with the theories of protein folding.
Particle acceleration in three-dimensional tearing configurations
Authors: Nodes Christoph , Birk Guido T. , Lesch Harald , Schopper Rüdiger
Published: February 2003

In three-dimensional electromagnetic configurations that result from unstable resistive tearing modes, particles can be efficiently accelerated to relativistic energies. To prove this, resistive magnetohydrodynamic simulations are used as input configurations for successive test particle simulations. The simulations show the capability of three-dimensional nonlinearly evolved tearing modes to accelerate particles perpendicular to the plane of the reconnecting magnetic field components. The simulations differ considerably from analytical approaches by involving a realistic three-dimensional electric field with a nonhomogeneous component parallel to the current direction. The resulting particle spectra exhibit strong pitch-angle anisotropies. Typically, about 5%–8% of an initially Maxwellian distribution is accelerated to the maximum energy levels given by the macroscopic generalized electric potential structure. Results are shown for both, nonrelativistic particle acceleration that is of interest, e.g., in the context of auroral arcs and solar flares, and relativistic particle energization that is relevant, e.g., in the context of active galactic nuclei.
Self-consistent discharge characteristics of collisional helicon plasmas
Authors: Cho Suwon , Lieberman M. A.
Published: February 2003

The steady-state fluid equations and Maxwell’s equations are solved self-consistently in helicon plasmas. When the Trivelpiece-Gould mode dominates the power transfer, the density profile is relatively flat in the central region compared to the case where there is significant bulk heating by the helicon wave. It is shown that the density changes from a relatively flat to steep profile at a certain value of the magnetic field, and this is related to the mode coupling and decoupling of the helicon and Trivelpiece-Gould modes.
Ab initio three-body interactions for water. I. Potential and structure of water trimer
Authors: Mas Eric M. , Bukowski Robert , Szalewicz Krzysztof
Published: February 2003

A new ab initio three-body potential for water has been generated from the Hartree–Fock method and symmetry-adapted perturbation theory calculations performed at 7533 trimer geometries. The calculated nonadditive energies were then fitted to a physically motivated analytic formula containing representations of short-range exchange contributions and damped induction terms. To our knowledge, this is the first time the short-range nonadditive interactions have been explicitly included in a potential for water. The fitted nonadditive potential was then applied, together with an accurate ab initio pair potential, SAPT-5s, to evaluate the effects of nonadditivity on the structure and energetics of water trimer.
Ab initio three-body interactions for water. II. Effects on structure and energetics of liquid
Authors: Mas Eric M. , Bukowski Robert , Szalewicz Krzysztof
Published: February 2003

A new ab initio three-body potential [E. M. Mas et al., J. Chem. Phys. 118, 4386 (2003), preceding paper] has been employed, together with an accurate ab initio pair potential SAPT-5s, in Monte Carlo simulations of liquid water in the canonical ensemble at ambient conditions. Analysis of radial distribution functions from these simulations illustrates the profound effect nonadditive forces have on the hydrogen-bonded structure of the liquid. Simulations using only the two-body potential give one hydrogen bond per molecule less than observed experimentally, radial distribution functions far from measured ones, and the internal energy underestimated relative to the experimental value. When three-body effects are introduced, all these quantities become significantly closer to experimental ones. In particular, we show that three-body effects result in a reorientation of water molecules leading to significantly increased number of hydrogen bonds. Our simulations indicate that three-body effects contribute 14.5% to the internal energy of water, whereas four- and higher-body effects contribute 1.4%. Since the ab initio three-body terms which are relevant for simulations are well reproduced by the nonadditive portion of classical induction interaction, our work confirms to some extent the validity of polarization models of water.
Potential energy surfaces for the Al+O2 reaction
Authors: Pak Michael V. , Gordon Mark S.
Published: February 2003

We present a systematic multiconfigurational study of the lowest two doublet potential energy surfaces of atomic aluminum with molecular oxygen. The most likely products, AlO and AlO2, are expected to figure prominently in subsequent reactions to form Al2O3. The main reaction pathways on both surfaces invariably lead to the formation of cyclic AlO2, possibly followed by isomerization to the lower-energy linear AlO2 isomer. A reaction path leading from Al+O2 directly to AlO+O was not located. However, both AlO2 isomers can dissociate to AlO+O with no barrier beyond endothermicity. There is also no barrier for the reaction of AlO2 with AlO to form Al2O3, and this reaction is highly exothermic.
Negative Fukui functions: New insights based on electronegativity equalization
Authors: Bultinck Patrick , Carbó-Dorca Ramon , Langenaeker Wilfried
Published: February 2003

Fukui functions have been calculated for large numbers of organic molecules, and were found to always be positive. Numeric and algebraic considerations allowed the identification of several boundary conditions for negative values for Fukui functions. Negative Fukui functions are found to be very unlikely, except when very short interatomic distances are present. Recent hypotheses concerning the occurrence of negative Fukui functions are strongly supported by the present approach.
The near-threshold absorption spectrum of N2
Authors: Jungen Ch. , Huber K. P. , Jungen M. , Stark G.
Published: February 2003

A new comprehensive multichannel quantum defect study of the near-threshold absorption of N214 has been carried out over the energy range 118 720–125 425 cm−1. A nearly complete understanding of the rotationally cold spectra reported earlier [K. P. Huber and Ch. Jungen, J. Chem. Phys. 92, 850 (1990); K. P. Huber et al., ibid. 100, 7957 (1994)] has been achieved in the region where core-excited s and d Rydberg levels built on the A 2Πu state of the ion interact with the series of p and f complexes converging to the lowest vibrational levels of X 2Σg+. The interactions reduce to a purely electronic quantum defect matrix which, after suitable transformations, accounts for the observed perturbed structures and intensities arising from vibronic coupling, rotational l uncoupling, and the different geometries of the X and A ion cores. The final calculations converged with 42 nonzero quantum defect parameters reproducing the 597 upper-state rovibronic levels with a standard deviation of 1.12 cm−1. The results have been used to calculate the R(0) line oscillator strengths in terms of eight nonvanishing electronic dipole transition moments, the latter treated as parameters that were fitted to photoelectrically measured band absorption f values. The calculations satisfactorily reproduce the observed oscillator strength distribution. Using ab initio calculated core properties for ground state N2+, the long-range model for a nonpenetrating Rydberg electron interacting with a quadrupolar and polarizable ion core predicts the diagonal f quantum defects in reasonable agreement with the results of the least-squares fits. Similar to NO, deviations from predictions by the same model for the diagonal d quantum defects arise primarily from the strong sσ∼dσ interchannel coupling and from the intrachannel interaction of the dπg Rydberg with the 1πg valence orbital, which, in contrast to 2π of NO, is occupied not in the ground state of N2, but in the electronically excited precursor states a′ 1Σu−, w 1Δu, and b′ 1Σu+.
Dissociation dynamics of the Ã 2A″ state of vinyl radical
Authors: Mann Aaron M. , Chen Xiangling , Lozovsky Vladimir A. , Moore C. Bradley
Published: February 2003

H or D atom velocities are reported for dissociation from several vibrational levels in the lowest 2500 cm−1 of the Ã 2A″ state of two vinyl radical isotopomers, C2H3 and CD2CH. Energies are inferred for the acetylene product and compared with both theoretical and experimental work. An upper bound is given for the rate of H-atom scrambling between the α and β carbon atoms.
Cascading third-order Raman process studied by six-wave mixing broadband multiplex coherent anti-Stokes Raman scattering spectroscopy
Authors: Kano Hideaki , Hamaguchi Hiro-o
Published: February 2003

The broadband multiplex coherent anti-Stokes Raman scattering (CARS) spectroscopy in the frequency domain was used to study six-wave mixing processes of simple aromatic molecules such as benzene and toluene. The six-wave mixing CARS signals were observed to have similar spectral profiles to those of the four-wave mixing CARS signals, but the peak positions were located in the overtone and the combination regions. By examining the concentration dependence, the six-wave mixing CARS signals were attributed to the so-called parallel-cascading third-order process. It is shown that the parallel-cascading process is useful for differentiating the contribution of the inhomogeneous broadening to the spectral profile.
Semiclassical implementation of the mapping Hamiltonian approach for nonadiabatic dynamics using focused initial distribution sampling
Authors: Bonella S. , Coker D. F.
Published: February 2003

A new semiclassical approach to implementing the mapping Hamiltonian formulation of nonadiabatic dynamics is presented. The approach involves using initial distributions of mapping oscillator variables that focus the sampling in such a way as to recover individual trajectory motion over the occupied state potential surface. The usual semiclassical implementation of the mapping Hamiltonian approach only recovers this feature after ensemble averaging. We test the approach on several model problems and show that it converges with very few trajectories compared to the usual approach.
Dielectric relaxation spectroscopy of 2-propanol–water mixtures
Authors: Sato Takaaki , Buchner Richard
Published: February 2003

The complex dielectric spectra of 2-propanol–water mixtures were determined at seven molar fractions of 2-propanol, X=0.03, 0.065, 0.14, 0.3, 0.5, 0.7, and 0.9 at 25 °C in the frequency range 0.1⩽ν/GHz⩽89 with the help of time domain reflectometry in 0.1⩽ν/GHz⩽25 and waveguide interferometry in 13⩽ν/GHz⩽89. In the alcohol-rich region of 0.3⩽X⩽1.0, a description of the ε*(ν) spectra requires the superposition of the three relaxation processes. The dominating low-frequency dispersion (j=1) follows a Cole–Cole equation. Additionally, two Debye equations (j=2 and 3) with the relaxation times of τ2∼10–20 ps and τ3∼1–2 ps are required to fit the high-frequency part of the spectrum. The three processes are assigned to the cooperative dynamics of the H-bond system (j=1), a rotation of singly H-bonded alcohol monomers at the ends of chainlike structure (j=2), possibly connected to the formation of bifurcate hydrogen bonds, and a flipping motion of free OH group (j=3). In the region of X<0.3, the intermediate alcohol monomer process becomes inseparable. Here, a two process model with a Cole–Cole equation for the main dispersion and a high-frequency Debye process for the fast switching mode gives the best fit. Based on the dielectric relaxation mechanism of the pure constituents proposed in the literatures [J. Barthel et al., Chem. Phys. Lett. 165, 369 (1990), and R. Buchner et al., Chem. Phys. Lett. 306, 57 (1999)], a composition-dependent relaxation behavior of the mixtures is discussed.
Association in electrolyte solutions: Rodlike polyelectrolytes in multivalent salts
Authors: González-Mozuelos P. , Olvera de la Cruz M.
Published: February 2003

We describe a new approach to determine the degree of association between ionic components in complex electrolyte solutions. We use the electrostatic contribution to the free energy that arises from the exact separation into long and short ranged parts of the correlation functions in a dilute electrolyte solution to determine the effective charge of the various ionic components. We describe the short-ranged direct correlations between different ionic components with delta functions whose strength give the direct association between them. The association is determined self-consistently by minimizing the resulting free energy, which contains long and short range correlations contributions. Association between like charges is mediated by direct association between opposite charges. We analyze rodlike polyelectrolyes in monovalent and/or multivalent salts. We find a broad minimum in the absolute value of effective rod charge at long distances as a function of monovalent salt concentration. This minimum is due to the association of the salt ions among themselves. We also determine the number of multivalent and monovalent ions associated to the rods. The degree of association is a function of the smallest length scale, which determines the electrostatic potential between ionic species at contact.
Quantum dynamics for dissipative systems: A numerical study of the Wigner–Fokker–Planck equation
Authors: Zhang Shesheng , Pollak Eli
Published: February 2003

A study is provided of dynamics and dissipative tunneling in a symmetric quartic double well potential. The numerical solution for the position autocorrelation function obtained through the Wigner–Fokker–Planck equation is compared with numerically exact results of Stockburger and Mak [J. Chem. Phys. 110, 4983 (1999)]. We find that the Wigner–Fokker–Planck dynamics agree well with the numerically exact computations, they account for both quantum coherences as well as quantum tunneling phenomena. This, in contrast to the mixed quantum classical approximation, which does not perform as well.
Ion concentrations and velocity profiles in nanochannel electroosmotic flows
Authors: Qiao R. , Aluru N. R.
Published: February 2003

Ion distributions and velocity profiles for electroosmotic flow in nanochannels of different widths are studied in this paper using molecular dynamics and continuum theory. For the various channel widths studied in this paper, the ion distribution near the channel wall is strongly influenced by the finite size of the ions and the discreteness of the solvent molecules. The classical Poisson–Boltzmann equation fails to predict the ion distribution near the channel wall as it does not account for the molecular aspects of the ion–wall and ion–solvent interactions. A modified Poisson–Boltzmann equation based on electrochemical potential correction is introduced to account for ion–wall and ion–solvent interactions. The electrochemical potential correction term is extracted from the ion distribution in a smaller channel using molecular dynamics. Using the electrochemical potential correction term extracted from molecular dynamics (MD) simulation of electroosmotic flow in a 2.22 nm channel, the modified Poisson–Boltzmann equation predicts the ion distribution in larger channel widths (e.g., 3.49 and 10.00 nm) with good accuracy. Detailed studies on the velocity profile in electro-osmotic flow indicate that the continuum flow theory can be used to predict bulk fluid flow in channels as small as 2.22 nm provided that the viscosity variation near the channel wall is taken into account. We propose a technique to embed the velocity near the channel wall obtained from MD simulation of electroosmotic flow in a narrow channel (e.g., 2.22 nm wide channel) into simulation of electroosmotic flow in larger channels. Simulation results indicate that such an approach can predict the velocity profile in larger channels (e.g., 3.49 and 10.00 nm) very well. Finally, simulation of electroosmotic flow in a 0.95 nm channel indicates that viscosity cannot be described by a local, linear constitutive relationship that the continuum flow theory is built upon and thus the continuum flow theory is not applicable for electroosmotic flow in such small channels.
Density functional theory for nonuniform polymers: Accurate treatment of the effect of attractive interactions
Authors: Patra Chandra N. , Yethiraj Arun
Published: February 2003

A density functional theory is presented for the effect of fluid–fluid and fluid–surface attractive interactions on the structure of polymers at surfaces. The theory treats the ideal gas free-energy functional exactly and uses a weighted density approximation for the hard chain contribution to the excess free-energy functional. The attractive interactions are calculated using the bulk fluid direct correlation function obtained from the polymer reference interaction site model theory. The predictions of the theory are in good agreement with computer simulation results for the density profiles of freely rotating fused-sphere chains at surfaces for a wide range of densities and temperatures. The results emphasize the importance of using different approximations for the hard sphere and attractive interactions in density functional theories for polymers.
Hydrogen bonding in liquid methanol, methylamine, and methanethiol studied by molecular-dynamics simulations
Authors: Kosztolányi T. , Bakó I. , Pálinkás G.
Published: February 2003

Molecular-dynamics computer simulations have been carried out on liquid methanol, methylamine, and methanethiol. The local structure of the liquids was studied based on radial distribution functions and the density projections of the neighboring molecules obtained on the basis of simulated molecular configurations. The extent of hydrogen bonding was investigated by direct analysis of the connectivity of molecules forming hydrogen-bonded clusters in these liquids. By this analysis, the methanol molecules were found to form linear chainlike structures. The local structure of hydrogen-bonded molecules of methylamine proved to be rather space filling due to the great extent of chain branching. Methanethiol molecules also proved to form hydrogen bonds forming small compact clusters. No evidence was found, however, for the clustering of hydrophobic methyl groups in any of the liquids. The quality of simulations was checked by derivation of neutron total and composite radial distribution functions and by comparison of those with available experimental data.
Comment on “Atomization energies and enthalpies of formation of the SnBin (n=1–3) gaseous molecules by Knudsen cell mass spectrometry” [J. Chem. Phys. 116, 6957 (2002)]
Authors: Figgen Detlev , Müller Wolfgang , Schweizer Marcus , Stoll Hermann , Peterson Kirk A.
Published: February 2003

Preliminary theoretical results, cited by Meloni and Gingerich in the title paper, are supplemented by new information (a) on the structure of the SnBi2 molecule, and (b) on computed atomization energies for SnBin (n=1–3).
An efficient orbital transformation method for electronic structure calculations
Authors: VandeVondele Joost , Hutter Jürg
Published: February 2003

An efficient method for optimizing single-determinant wave functions of medium and large systems is presented. It is based on a minimization of the energy functional using a new set of variables to perform orbital transformations. With this method convergence of the wave function is guaranteed. Preconditioners with different computational cost and efficiency have been constructed. Depending on the preconditioner, the method needs a number of iterations that is very similar to the established diagonalization–DIIS approach, in cases where the latter converges well. Diagonalization of the Kohn–Sham matrix can be avoided and the sparsity of the overlap and Kohn–Sham matrix can be exploited. If sparsity is taken into account, the method scales as O(MN2), where M is the total number of basis functions and N is the number of occupied orbitals. The relative performance of the method is optimal for large systems that are described with high quality basis sets, and for which the density matrices are not yet sparse. We present a benchmark calculation on a DNA crystal containing 2×12 base pairs, solvent and counter ions (2388 atoms), using a TZV(2d,2p) basis (38 688 basis functions) and conclude that the electronic structure of systems of this size can now be studied routinely.
Ab initio study of the vibronic and spin–orbit structure in the X 2Π electronic state of CCCH
Authors: Perić M. , Mladenović M. , Tomić K. , Marian C. M.
Published: February 2003

Potential energy surfaces for the electronic states of the CCCH radical correlating at linear nuclear arrangement with the X 2Π state are calculated by means of an extensive ab initio approach. They are used to compute the vibronic and spin–orbit structure of this electronic state by means of a variational approach based on the use of normal bending coordinates. The results of calculations enable a reliable interpretation of the available experimental data and offer explanation for several apparently peculiar features observed.
Copolymers with designed proteinlike sequences obtained by polymeranalogous transformations of homopolymer globules
Authors: Kuchanov Semion I. , Khokhlov Alexei R.
Published: February 2003

In this paper we consider one of the ways to obtain HP-copolymers with specially designed “proteinlike” sequences such that in the dense globular conformation all the hydrophobic H-units form the core of this globule, while polar P-units constitute the envelope of this core. Namely, we assume that some low molecular reagent Z is added to the dilute solution of homopolymer H-globules and that it induces a polymeranalogous reaction H+Z→P. We present the full theoretical analysis of the macrokinetics of the corresponding chemical reaction and show that at a certain interrelation between the diffusion coefficient of the reagent Z and the constant of the chemical reaction it is possible to end up with a copolymer chain with proteinlike statistics in the sense described above. We analyze the statistical properties of the sequences in resulting copolymer chains for the general case. Also we consider the scattering properties and spinodal of the melts of these chains.
Radial structure of shell modulations near peak compression of spherical implosions
Authors: Smalyuk V. A. , Dumanis S. B. , Marshall F. J. , Delettrez J. A. , Meyerhofer D. D. , Regan S. P. , Sangster T. C. , Yaakobi B. , Koch J. A.
Published: February 2003

The structure of shell modulations is measured at peak compression of directly driven spherical implosions using absorption of titanium-doped layers placed at various distances of 1, 5, 7, and 9 μm from the inner surface of 20 μm thick plastic CH shells filled with 18 atm of D3He gas. The modulations are measured using the ratios of monochromatic core images taken inside and outside of the titanium 1s–2p absorption spectral region. Peak-compression, time-integrated areal-density modulations are higher at the inner shell surface, which is unstable during the deceleration phase of an implosion with a modulation level of 59±14%. The perturbations are lower in the central part of the shell, having a modulation level of 18±5%. The outer surface of the shell, which is unstable during the acceleration phase of an implosion, has a modulation level of 52±20%.
On the stabilization of ion temperature gradient modes by finite beta in noncircular toroidal devices
Authors: Jhowry B. , Anderson J.
Published: February 2003

The stability of electromagnetic ion temperature gradient driven (ITG) modes is investigated using an advanced fluid model allowing for arbitrary elongation (κ) and finite beta effects and Shafranov shift. The ITG eigenvalue equation is solved numerically and the stability properties are discussed and compared in both the circular and noncircular geometry. It is found that elongation and finite beta effects reduce the growth rates and that the ITG mode is stabilized for large beta values as elongation is increased. Elongation increases the beta threshold of the ITG mode with larger growth rates as compared with circular geometry.
Nonlinear oscillations in dusty plasmas in the presence of nonisothermal trapped electrons
Authors: Tribeche Mouloud , Zerguini Taha Houssine , Aoutou Kamel
Published: February 2003

The recent analysis of the excitation of new dust Bernstein–Greene–Kruskal (DBGK) modes in a noncollisional dusty plasma [Tribeche et al., Phys. Plasmas 7, 4013 (2000)] is extended to include self-consistently the effects of trapped electrons. A kinetic analysis is presented. Highly nonlinear differential equations are obtained and solved numerically leading to some new and original results. The dust density is found to be highly perturbed in the region of the DBGK soliton localization. The dust particles are locally expelled and pushed out of this region, a phenomenon which can generate intense and spiky electric fields in the medium. Furthermore, the results show that under certain conditions double layers (DLs) as well as localized cusped solitons are found to coexist. Such two-level structures are unique to the influence of dust and reflected particles and usually the DLs and the solitons do not coexist in the same parameter regime.
A class of high εβp equilibria in strongly shaped finite aspect ratio tokamak plasmas
Authors: Lin-Liu Y. R. , Turnbull A. D. , Miller R. L. , Chan V. S. , Politzer P. A.
Published: February 2003

A class of very high poloidal beta (βp) equilibria is exhibited in which εβp (ε is the inverse aspect ratio) exceeds analytic equilibrium limits previously anticipated from simplified large aspect ratio models, as well as previous experimental equilibrium limits. The extension in these limits is shown to be due to a combination of finite aspect ratio effects and strong cross section shaping. In a conventional aspect ratio configuration, equilibria with εβp=3 exist, which is 50% higher than both the previously observed record and the analytically predicted limit. For very low aspect ratio with ε−1=1.4, typical of spherical tori, values of εβp=5 are possible.
Multiresolution analysis in statistical mechanics. I. Using wavelets to calculate thermodynamic properties
Authors: Ismail Ahmed E. , Rutledge Gregory C. , Stephanopoulos George
Published: February 2003

The wavelet transform, a family of orthonormal bases, is introduced as a technique for performing multiresolution analysis in statistical mechanics. The wavelet transform is a hierarchical technique designed to separate data sets into sets representing local averages and local differences. Although one-to-one transformations of data sets are possible, the advantage of the wavelet transform is as an approximation scheme for the efficient calculation of thermodynamic and ensemble properties. Even under the most drastic of approximations, the resulting errors in the values obtained for average absolute magnetization, free energy, and heat capacity are on the order of 10%, with a corresponding computational efficiency gain of two orders of magnitude for a system such as a 4×4 Ising lattice. In addition, the errors in the results tend toward zero in the neighborhood of fixed points, as determined by renormalization group theory.
Multiresolution analysis in statistical mechanics. II. The wavelet transform as a basis for Monte Carlo simulations on lattices
Authors: Ismail Ahmed E. , Stephanopoulos George , Rutledge Gregory C.
Published: February 2003

In this paper, we extend our analysis of lattice systems using the wavelet transform to systems for which exact enumeration is impractical. For such systems, we illustrate a wavelet-accelerated Monte Carlo (WAMC) algorithm, which hierarchically coarse-grains a lattice model by computing the probability distribution for successively larger block spins. We demonstrate that although the method perturbs the system by changing its Hamiltonian and by allowing block spins to take on values not permitted for individual spins, the results obtained agree with the analytical results in the preceding paper, and “converge” to exact results obtained in the absence of coarse-graining. Additionally, we show that the decorrelation time for the WAMC is no worse than that of Metropolis Monte Carlo (MMC), and that scaling laws can be constructed from data performed in several short simulations to estimate the results that would be obtained from the original simulation. Although the algorithm is not asymptotically faster than traditional MMC, the new algorithm executes several orders of magnitude faster than a full simulation of the original problem because of its hierarchical design. Consequently, the new method allows for rapid analysis of a phase diagram, allowing computational time to be focused on regions near phase transitions.