bluuebunny/crossref_metadata_2025 huggingface dataset:
Unstable ion-temperature-gradient modes in the Wendelstein 7-X stellarator configuration
Authors: Rafiq T. , Kleiber R. , Nadeem M. , Persson M.
Published: November 2002

The linear stability of the ion-temperature-gradient modes (ITG) in the electrostatic limit is examined in the short wavelength region by using a two fluid reactive model in fully three-dimensional Wendelstein 7-X (W7-X) stellarator [G. Grieger et al., Plasma Physics and Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 525] geometry. The spectrum of stable and unstable modes and their real frequencies and eigenfunctions are calculated. The effects of density gradients, temperature gradients, temperature ratios, wavevector, ballooning angle, curvature and local magnetic shear on the ITG mode are also investigated. The frequency and growth rate of the most unstable ITG mode is calculated and visualized for a specific magnetic flux surface. For the equilibrium under investigation both localized and extended eigenmodes are found. The effect of small and large temperature ratios, small and large density gradients as well as large local magnetic shear are all found to be stabilizing. The highest growth rates are found at the outer part of the surface where the local magnetic shear is small and normal curvature is unfavorable.
Resistive stability of magnetic dipole and other axisymmetric closed field line configurations
Authors: Simakov Andrei N. , Catto Peter J. , Ramos Jesus J. , Hastie R. J.
Published: November 2002

The stability of axisymmetric plasmas confined by a closed poloidal magnetic field is investigated using magnetohydrodynamic equations with anisotropic resistivity and sound waves retained. It is shown that when the system is axially and up–down symmetric and the plasma beta=(plasma pressure/magnetic pressure) is finite, a resistive instability with a growth rate proportional to the cube root of the resistivity exists at the ideal stability boundary for up–down antisymmetric modes. Both the ideal and resistive stability of a Z-pinch equilibrium and the point dipole equilibrium of Krasheninnikov, Catto, and Hazeltine [Phys. Rev. Lett. 82, 2689 (1999)] are studied in detail. For a Z pinch, ideal instabilities are found to always dominate over resistive instabilities. For the point dipole, ideal up–down antisymmetric modes are always stable, and the only resistive instabilities permitted have a growth rate proportional to the resistivity times the square of the azimuthal mode number.
Confined liquid: Simultaneous observation of a molecularly layered structure and hydrodynamic slip
Authors: Sun Gexiao , Bonaccurso Elmar , Franz Volker , Butt Hans-Jürgen
Published: November 2002

The force profile between a glass microsphere and mica in 1-propanol has been measured with the colloidal probe technique. Oscillatory solvation forces indicate a layered structure of the confined propanol for at least three layers. In the same experiment, hydrodynamic forces were measured at high approaching velocity. Comparing measured force curves with calculations we found a significant effective slip, which could be described by a slip length of 10–14 nm.
Improved confinement through internal transport barrier formation with lower hybrid current drive in the Hefei Tokamak-7
Authors: Ding Bojiang , Kuang Guangli , Liu Yuexiu , Shan Jiafang , Shen Weici , Yu Jiawen , Liu Shengxia , Shi Yuejiang , Jie Yinxian , Wan Baonian , Zhao Yanping , Li Jiangang
Published: November 2002

An improved confinement through internal transport barrier formation with lower hybrid current drive (LHCD) is obtained on the Hefei Tokamak-7 [J. K. Xie et al., Proceedings of the 16th International Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 1, p. 685]. The experimental energy confinement time increases from 14.6 ms (Ohmic discharge phase) to 24.5 ms (LHCD phase), which is consistent with the value predicted by the high confinement mode scaling law of ITER93 (edge localized mode free). The confinement factor H (H=τE/τEITER89P) up to 1.42 is obtained during the improved confinement phase. The analysis shows that the modification of the plasma current profile and E×B are two possible reasons for the confinement improvement.
First principles electrochemistry: Electrons and protons reacting as independent ions
Authors: Llano Jorge , Eriksson Leif A.
Published: November 2002

We here present a first principles approach to calculate standard Gibbs energies and the corresponding observables (standard electrode potentials in the hydrogen scale ESHE0 and pKa values) of stoichiometric reactions involving electrons and/or protons as independent species in solution, from absolute electrochemical potentials defined according to quantum and statistical mechanics. In order to pass from the conventional electrodic and thermodynamic descriptions of electrochemistry to the first principles approach based on estimating absolute electrochemical potentials, we revisit the problem of the absolute and relative electrochemical scales from the macroscopic and microscopic viewpoints. A microscopic definition of the absolute electrochemical potential is presented in order to enable an identical thermodynamic treatment of any species in a given phase, i.e., electrons, protons, atoms, molecules, atomic and molecular ions, and electronically excited species. We show that absolute standard chemical potentials in the mole fraction scale can be easily computed with wave function and density functional theories in conjunction with self-consistent reaction field models. Based on Boltzmann and Fermi–Dirac statistics and experimental solvation data, we estimate an internally compatible set of absolute standard chemical and electrochemical potentials of protons and solvated electrons in the molality and molarity scales in aqueous solution at 298 K and 1 atm, within an absolute error of ±0.5 kcal/mol. This scheme enables a consistent and simultaneous description of the Gibbs energy changes and the observables (ESHE0 and pKa ’s) of electron, proton, and proton-coupled electron transfer reactions in aqueous solution at 298 K and 1 atm.
A new model of N2O quantum yield in the UV photolysis of O3/O2/N2 mixtures: Contributions of electronically excited O3 and O3⋅N2
Authors: Prasad Sheo S.
Published: November 2002

Electronically excited O3 and O3⋅N2 dimer are proposed as contributors to the N2O quantum yield, φN2O, following UV photolysis of O3 in O3/O2/N2 mixture. At 100 to 900 torr N2 pressures, φN2O is dominated by the electronically excited O3. In this pressure regime φN2O in the 310⩽λ⩽340 nm region could, potentially, exceed φN2O in the λ<310 nm region. The “classical” O(1D), N2 association predominates above 10 atm. The O3⋅N2+hν may dominate at high pressures (≳500 atm) if the temperature is very low (≲50 K). The atmospheric importance of N2O production via the classical mechanism is well known to be insignificant. In contrast, the production from excited O3 appears to have the potential to significantly affect our current understandings of stratospheric coupled NOx–O3 chemistry and the climatologically important N2O source-sink budget. It is therefore critical to determine the wavelength variation of φN2O in the 310⩽λ⩽340 nm region by gas phase experiments. Theoretical studies are needed to understand, at the quantum chemistry level, the mechanism of the suggested N2O formation from ultra-short-lived electronically excited O3.
Realizing intrinsic piezoresponse in epitaxial submicron lead zirconate titanate capacitors on Si
Authors: Nagarajan V. , Stanishevsky A. , Chen L. , Zhao T. , Liu B.-T. , Melngailis J. , Roytburd A. L. , Ramesh R. , Finder J. , Yu Z. , Droopad R. , Eisenbeiser K.
Published: November 2002

We report on the out-of-plane piezoelectric response (d33), measured via piezoresponse scanning force microscopy, of submicron capacitors fabricated from epitaxial PbZrxTi1−xO3 thin films. Investigations on 1 μm2 and smaller capacitors show that the substrate-induced constraint is dramatically reduced by nanostructuring. At zero field, the experimentally measured values of d33 for clamped as well as submicron capacitors are in good agreement with the predictions from thermodynamic theory. The theory also describes very well the field dependence of the piezoresponse of clamped capacitors of key compositions on the tetragonal side of the PbZrxTi1−xO3 phase diagram as well as the behavior of submicron PbZr0.2Ti0.8O3 (hard ferroelectric) capacitors. However, the field-dependent piezoresponse of submicron capacitors in compositions closer to the morphotropic phase boundary (soft ferroelectrics) is different from the behavior predicted by the theoretical calculations.
Comment on “Stationary equilibria of self-gravitating quasineutral dusty plasmas” [Phys. Plasmas 8, 4740 (2001)]
Authors: Pandey B. P. , Roy Subrata
Published: November 2002

It is pointed out that the recently published study on the stationary equilibria of a self-gravitating quasineutral dusty plasma is not correct. The claim of Rao et al. that a “closed form equation for the dust flow speed” [their Eq. (19)] is derived is misleading since a term proportional to the electrostatic potential φ0 has erroneously been left out. Further, the claim of Rao et al. that the singularities displayed by their Eqs. (19) and (22) at the characteristic speed are due to the inhomogeneity of the self-gravitating potential is devoid of any mathematical merit or physical reasoning.
Equilibrium of field reversed configurations with rotation. III. Two space dimensions and one type of ion
Authors: Qerushi Artan , Rostoker Norman
Published: November 2002

A two-dimensional equilibrium model for field reversed configurations (FRCs) with rotation is presented. In a previous paper [N. Rostoker and A. Qerushi, Phys. Plasmas 9, 3057 (2002)] it was shown that a complete description of equilibria for FRCs with rotation is provided by a generalized Grad–Shafranov equation for the plasma flux function. In this paper it is shown how to solve that fundamental equation for the case of two space dimensions and one type of ion. Periodic boundary conditions and a Green’s function are used to convert the original differential equation to an equivalent integral equation. The integral equation is solved by iteration. An iteration algorithm is described which converges to a solution of the generalized Grad–Shafranov equation starting with a one-dimensional trial function. Analytic one-dimensional solutions are shown to be a limiting case of two-dimensional solutions when the applied magnetic field is constant. In addition to rapid convergence for a complex nonlinear problem, the Green’s function method guarantees that the boundary conditions are satisfied in every iteration.
A Monte Carlo study of the influence of molecular flexibility on the phase diagram of a fused hard sphere model
Authors: McBride Carl , Vega Carlos
Published: November 2002

A study of a rigid fully flexible fused hard sphere model [C. McBride, C. Vega, and L. G. MacDowell, Phys. Rev. E 64, 011703 (2001)] is extended to the smectic and solid branches of the phase diagram. Computer simulations have been performed for a completely rigid model composed of 15 fused hard spheres (15+0), a model of 15 fused hard spheres of which 2 monomers at one end of the model form a flexible tail (13+2), and a model consisting of 15 fused hard spheres with 5 monomers forming a flexible tail (10+5). For the 15+0 model the phase sequence isotropic–nematic–smectic A–columnar is found on compression, and the sequence solid–smectic A–nematic–isotropic on expansion. For the 13+2 model the phase sequence isotropic–nematic–smectic C is found on compression, and the sequence solid–smectic A–nematic–isotropic on expansion. For the 10+5 model the phase sequence isotropic–glass is found on compression. The expansion runs displayed the phase sequence solid–smectic A–isotropic. The introduction of flexibility was seen to stabilize the smectic A phase at the expense of the nematic phase.
Orbital nonrelaxed coupled cluster singles and doubles with perturbative triples corrections calculations of first-order one-electron properties
Authors: Hald Kasper , Halkier Asger , Jørgensen Poul , Coriani Sonia
Published: November 2002

The calculation of first-order one-electron molecular properties is discussed for an orbital nonrelaxed CCSD(T) (coupled cluster singles and doubles with perturbative triples corrections) wave function model. The conventional CCSD(T) triples amplitude equations have been generalized to contain terms that depend explicitly on the perturbation to compensate for the fact that the Hartree–Fock molecular orbitals are not allowed to relax. Results of sample calculations are presented, including the molecular electric quadrupole moment of benzene in the d-aug-cc-pVTZ basis set, which contains 564 contracted basis functions.
Thermochemistry of small cationic iron–sulfur clusters
Authors: Koszinowski Konrad , Schröder Detlef , Schwarz Helmut , Liyanage Rohana , Armentrout P. B.
Published: November 2002

The kinetic energy dependences of the reactions of Fen+ with COS (n=2–6) and CS2 (n=2–5) are studied in a guided-ion beam tandem mass-spectrometer. The main products arise from sulfur transfer and subsequent losses of Fe atoms. In the case of CS2, this reactant also formally replaces one Fe atom of the cluster to form Fen−1CS2+ with losses of further Fe atoms at elevated energies. In addition, the kinetic energy dependences of the reactions of FenS+ (n=2–4) with Xe and CS2 are studied. The former system yields collision-induced dissociations, whereas the latter reagent effects sulfur transfer accompanied by subsequent losses of Fe atoms. Analyses of the cross sections for endothermic reactions yield the bond energies D0(Fen+–S), n=2–5, D0(SFen−1+–Fe), n=2–5, D0(SFen+−S), n=1–3, and D0(S2Fen−1+–Fe), n=2, 3, as well as the ionization energy IE(Fe2S2). These values are derived with explicit consideration of the lifetimes of the energized reaction intermediates. The binding between sulfur and the cluster core strengthens as the cluster size increases, which is rationalized by simple structural arguments.
Vibrational structures of predissociating methylamines (CH3NH2 and CH3ND2) in Ã states: Free internal rotation of CH3 with respect to NH2
Authors: Baek Sun Jong , Choi Kyo-Won , Choi Young S. , Kim Sang Kyu
Published: November 2002

Resonantly-enhanced one-color two-photon (1+1) ionization spectra of jet-cooled methylamines (CH3NH2 and CH3ND2) reveal the vibrational structures of these molecules in predissociative Ã states. Rotational fine structure is clearly resolved for CH3ND2 at the origin and first wagging vibrational level in the excited state. The spectral linewidth becomes homogeneously broadened to give only vibrationally resolved spectral features for the higher vibrational energy levels of CH3ND2 (Ã). From the spectral analysis of the Ã–X transition of CH3ND2, it is found that the methyl moiety rotates nearly freely about the C–N axis with respect to the amino group in the Ã state, indicating that the removal of an electron from the nonbonding orbital of N is responsible for the free internal rotation. Vibrational levels are only barely resolved in the Ã–X excitation spectrum of CH3NH2 due to severe homogeneous line-broadening, indicating ultrashort lifetimes of ∼0.4 ps for predissociating CH3NH2 molecules in the Ã state. Spectral interpretation of the Ã–X excitation spectrum of CH3NH2 is carried out by the comparison with that of CH3ND2, giving the confirmative vibrational assignment of methylamines in Ã states for the first time. The dramatic difference of CH3NH2 and CH3ND2 in their lifetimes in Ã states suggests that the major dissociation channel of the excited methylamine may be the N–H (or D) bond dissociation.
Noise driven configuration of dust clusters by molecular dynamics simulation
Authors: Xie Bai-Song , Yang Zhi-An
Published: November 2002

Molecular dynamics simulations are employed to study the effect of noise on the stationary configuration formation of clusters of two-dimensional dusty plasma. It is found that the noise can induce a new stable cluster configuration that is significantly different from that without noise. And, the tunneling crisis phenomena of nonlinear dynamical system is also exhibited in few body dusty clusters studied under noise. Some implication of results to experiments is also discussed.
A systematic search for new kinetic structures in collisionless current-carrying plasmas
Authors: Luque A. , Schamel H. , Grießmeier J.-M.
Published: November 2002

The parameter space of nonlinear electrostatic structures in one-dimensional current-carrying plasmas is surveyed. The region available for physically meaningful solutions is precisely marked out and several new kinds of trapped particle structures, such as an alternating train of electron and ion holes, are found.
Experimental and theoretical study of intramultiplet transitions in collisions of C(3P) and Si(3P) with He
Authors: Le Picard S. D. , Honvault P. , Bussery-Honvault B. , Canosa A. , Laubé S. , Launay J.-M. , Rowe Bertrand , Chastaing D. , Sims I. R.
Published: November 2002

We present a combined theoretical and experimental study of intramultiplet transitions in collisions of C(3P) and Si(3P) with He. Relaxation rate constants have been measured using the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) technique at 15 K for C(3P)+He and over the 15–49 K temperature range for Si(3P)+He. Three sets of interaction potentials have been generated for the ∑−3 and ∏3 electronic states of CHe and SiHe systems. Quantum-mechanical calculations have been performed on these potential curves for the spin–orbit relaxation and excitation. Cross sections and rate constants are very sensitive to the interaction potential. For the Si(3P)+He system, an overall good agreement between the theoretical and experimental rate constants is found with the best quality interaction potential, while for the C(3P)+He system experimental rate constants are much smaller than the theoretical ones.
Nonlinear phase of the compressional m=1 diocotron instability: Saturation and analogy with geophysical fluid dynamics
Authors: Delzanno G. L. , Finn J. M. , Lapenta G.
Published: November 2002

The nonlinear dynamics of a Penning trap plasma, including the effect of the finite length and end curvature of the plasma column, is studied. A new cylindrical particle-in-cell code, called KANDINSKY, has been implemented by using a new interpolation scheme. The principal idea is to calculate the volume of each cell from a particle volume, in the same manner as is done for the cell charge. With this new method, the density is conserved along streamlines and artificial sources of compressibility are avoided. The code has been validated with a number of tests. The code is then used to compare the dynamics of three different models: the standard Euler or drift-Poisson model, the modified drift-Poisson model [J. Finn et al. Phys. Plasmas 6, 3744 (1999); Phys. Rev. Lett. 84, 2401 (2000)] with compressional effects, and the quasigeostrophic model of geophysical fluid dynamics in the limit of the γ-plane approximation. The results of this investigation show that Penning traps can be used to simulate geophysical fluids. Moreover, the results for the m=1 diocotron instability reproduce qualitatively the experiments [C. F. Driscoll, Phy. Rev. Lett. 64, 645 (1990); C. F. Driscoll et al. Phys. Fluids B 2, 1359 (1990)]: The instability turns the plasma “inside-out” resulting at the end in a stable, monotonic profile.
Models for the pedestal temperature at the edge of H-mode tokamak plasmas
Authors: Onjun T. , Bateman G. , Kritz A. H. , Hammett G.
Published: November 2002

Predictive models are developed for the temperature at the top at the edge of type 1 ELMy (edge localized mode) H-mode (high-confinement mode) plasmas. Theory-motivated models are used for the pedestal width and pressure gradient, while the pedestal density is obtained from experimental data in this study. The pedestal pressure gradient is assumed to be limited by the ballooning mode instability and is expressed in terms of the magnetic shear and geometrical factors. The effect of the bootstrap current, which reduces the magnetic shear in the steep pressure gradient region at the edge of the H-mode plasma, is included in the determination of the magnetic shear. Approaches for calculating the magnetic shear, combined with proposed models for the pedestal width, are used to determine the pedestal temperature. The computed pedestal temperatures are compared with more than 500 measured pedestal temperatures for type 1 ELMy H-mode discharges in four tokamaks. Some of the uncertainties in these results are discussed, and directions for future work to improve edge pedestal scalings are described.
Kelvin–Helmholtz instability in Beltrami fields
Authors: Ito A. , Yoshida Z. , Tatsuno T. , Ohsaki S. , Mahajan S. M.
Published: November 2002

The stability of Beltrami flows has been analyzed. The model equation represents the coupling of the Kelvin–Helmholtz (KH) instability with Alfvén waves. In a single Beltrami flow that parallels a force-free magnetic field, the magnetic field reduces the growth rate of the KH instability, while the marginally stable wave number is unchanged. Calculating the marginally stable eigenfunction of a magnetohydrodynamic flow, the necessary and sufficient condition for the exponential stability has been derived. The stability of double Beltrami flows has also been analyzed, which is represented by linear combinations of two Beltrami flows. Coupling of two vortices yields both stabilizing and destabilizing effects depending on the amplitudes and the eigenvalues of two Beltrami functions.
Stability analysis of hollow electron columns including compressional and thermal effects: Integrability condition and numerical simulations
Authors: Delzanno G. L. , Pariev V. I. , Finn J. M. , Lapenta G.
Published: November 2002

The diocotron spectrum for a simplified fluid model of Malmberg–Penning traps that includes compressional effects due to end curvature with finite temperature is investigated. A class of length profiles for which the linearized eigenvalue equation for perturbations can be integrated by quadratures (integrable cases) has been found. In such cases, there is only algebraic growth when the effective angular frequency has a maximum away from the axis (hollow profile), and the model is mathematically equivalent to the zero curvature (two-dimensional Euler) case. Furthermore, profiles that are slightly nonintegrable (the difference being characterized by a small parameter ε) have been studied, finding that the complex frequency of the unstable l=1 mode scales as ε2/3. Analytical calculations (to be presented in a companion paper) and numerical simulations are found in agreement. For the density profile used, the growth rate of the unstable mode has a minimum at the plasma temperature of about 5 eV, which might be tested experimentally.
Escape patterns, magnetic footprints, and homoclinic tangles due to ergodic magnetic limiters
Authors: da Silva Elton C. , Caldas Iberê L. , Viana Ricardo L. , Sanjuán Miguel A. F.
Published: November 2002

The action of a set of ergodic magnetic limiters in tokamaks is investigated from the Hamiltonian chaotic scattering point of view. Special attention is paid to the influence of invariant sets, such as stable and unstable manifolds, as well as the strange saddle, on the formation of the chaotic layer at the plasma edge. The nonuniform escape process associated to chaotic field lines is also analyzed. It is shown that the ergodic layer produced by the limiters has not only a fractal structure, but it possesses the even more restrictive Wada property.
Nonlinear dust Bernstein–Greene–Kruskal modes in charge-varying dusty plasmas
Authors: Tribeche Mouloud , Zerguini Taha Houssine , Houili Hocine
Published: November 2002

Nonlinear dust Bernstein–Greene–Kruskal (BGK) modes are investigated in a charge-varying dusty plasma. It is found that highly localized structures, solely due to the dust charge variation, can exist. The dust BGK soliton suffers the well-known anomalous damping, the importance of which is roughly proportional to the dust grain velocity. This dissipation causes the soliton amplitude to decay algebraically and the conservation of “soliton mass” leads to the development of a noise tail.
A molecular-dynamics simulation study of dielectric relaxation in a 1,4-polybutadiene melt
Authors: Smith Grant D. , Borodin Oleg , Paul Wolfgang
Published: November 2002

We have carried out atomistic molecular dynamics simulations of a melt of 1,4-poly(butadiene) from temperatures well above the experimentally observed merging of the primary α process and secondary β process down to temperatures approaching the experimentally observed bifurcation temperature. The relaxation strength and maximum loss frequency and its temperature dependence for the combined α-β dielectric relaxation process from simulations were in good agreement with experiment. The maximum loss frequency, melt viscosity, chain normal-mode relaxation times and torsional autocorrelation times were found to exhibit nearly identical non-Arrhenius temperature dependencies well represented by a Vogel–Fulcher fit with parameters in good agreement with experimental values obtained from dielectric and viscosity measurements. The dielectric susceptibility showed increasing intensity at high frequency for the lower temperatures investigated, indicative of a breakdown in time-temperature superposition due to an emerging β process. Comparison of time scales for the chain normal-mode dynamics and dielectric relaxation revealed that the latter is associated with motions on the segmental length scale. The correspondence of time scales and temperature dependence for the dielectric relaxation and the torsional autocorrelation function further confirmed the localized nature of the dielectric relaxation and indicated that the combined α-β dielectric process is fundamentally tied to microscopic conformational dynamics of individual dihedrals. However, the mean conformational transition rates were found to exhibit Arrhenius temperature dependence, leading to a divergence of time scales between the torsional, dielectric, chain and mechanical relaxation processes and the rates of conformational transitions with decreasing temperature. This divergence was associated with the increasingly heterogeneous character of conformational dynamics in the melt with decreasing temperature. Hence, the time scale of the principal (α) relaxation in the melt is fundamentally correlated with the time scale for homogenization of conformational dynamics, and not to the time scale of the conformational transitions themselves.
The diffusion–spin relaxation time distribution function as an experimental probe to characterize fluid mixtures in porous media
Authors: Hürlimann M. D. , Venkataramanan L. , Flaum C.
Published: November 2002

The distribution function between diffusion and spin relaxation time is shown to be a powerful tool to characterize fluid mixtures in porous media. We discuss the nuclear magnetic resonance measurements using diffusion-editing sequences that were developed to measure this quantity. A recently developed two-dimensional inversion routine is used to extract the distribution function from the data. We show both theoretically and experimentally that the technique is suitable for ex situ applications in the presence of grossly inhomogeneous fields. We apply this technique to characterize sedimentary rocks saturated with oil–water mixtures. From the measured diffusion–relaxation time distribution function it is possible to quantify the oil and water saturation, to get a direct indication of the wettability of the porous media, to characterize the bulk properties of the oil and estimate the oil viscosity, and to get information about the geometrical arrangement of the fluid phases in the pore space. We have also identified effects due to susceptibility induced gradients in a sandstone and effects of strongly restricted diffusion in a limestone.
Modeling the NO+H2 reaction on a Pt field emitter tip: Mean-field analysis and Monte Carlo simulations
Authors: De Decker Y. , Baras F. , Kruse N. , Nicolis G.
Published: November 2002

A minimal model for the NO+H2 reaction on a Pt emitter tip is proposed, with emphasis placed on surface explosions observed in field ion microscopy experiments. The model is first studied in the mean-field approximation, where it is shown to exhibit bistability and associated explosive phenomena. Using kinetic Monte Carlo simulations, irreversible phase transitions, fluctuation-induced dynamics and reaction front propagation are observed, which are not predicted by the mean-field approach. This comparative study allows to shed some light on the origins of the surface explosions, and to investigate the influence of the discrete nature of the support on the dynamics.
Effects of chemical and physical modifications on the electronic transport properties of molecular junctions
Authors: Luo Yi , Wang Chuan-Kui , Fu Ying
Published: November 2002

We have shown an accurate theoretical description for electronic transport properties of molecular junctions. By applying the elastic scattering Green’s function theory approach in combination with the frontier molecular orbital theory for describing the surface–molecule coupling and the hybrid density-functional theory for geometrical and electronic structures calculations, reliable predictions are obtained about the chemical and physical modifications on the current–voltage characteristics of molecular junctions, due to molecular geometrical relaxation, the choice of terminal atoms, the effect of donor/acceptor substitutions, as well as their thermal rotations.
Orientational structures of a phase-separating system under oscillatory particles
Authors: Zhu Yue-jin , Ma Yu-qiang
Published: November 2002

We investigate the ordering mechanism of the striped pattern of a binary mixture in the presence of periodic oscillatory particles with a special addictive to one of the two immiscible phases. Due to the presence of oscillatory particles, the phase separation of the system will be significantly perturbed, and composition fluctuations are suppressed anisotropically. By changing the oscillatory frequency ω and amplitude γ, we observe the striped domain structures either parallel or perpendicular to the oscillatory direction, and obtain a γ−ω phase diagram describing the orientational ordering of the phase-separating structures. The results suggest that our model system may provide a novel and simple way to control the ordering morphology of soft materials.
Picosecond structural dynamics in photoexcited Si probed by time-resolved x-ray diffraction
Authors: Kishimura Hiroaki , Yazaki Akio , Kawano Hidetaka , Hironaka Yoichiro , Nakamura Kazutaka G. , Kondo Ken-ichi
Published: November 2002

Direct observation of structural dynamics of a 300 ps laser irradiated silicon crystal is performed by means of picosecond time-resolved x-ray diffraction. Change in x-ray diffraction profiles corresponds to propagation of a strain pulse inside the sample with sound velocity. The strain profiles are simulated by considering carrier dynamics and thermoelastic treatment and well explain the experiments.
An L-shaped equilibrium geometry for germanium dicarbide (GeC2)? Interesting effects of zero-point vibration, scalar relativity, and core–valence correlation
Authors: Sari Levent , Peterson Kirk A. , Yamaguchi Yukio , Schaefer Henry F.
Published: November 2002

The ground state potential energy surface of the GeC2 molecule has been investigated at highly correlated coupled cluster levels of theory. Large basis sets including diffuse functions and functions to describe core correlation effects were employed in order to predict the true equilibrium geometry for GeC2. Like the much-studied valence isoelectronic SiC2, the linear (1∑+), L-shaped (1A′), and T-shaped structures (1A1) must be investigated. The L-shaped Cs geometry is found to have real harmonic vibrational frequencies along every internal coordinate, and the linear stationary point has an imaginary vibrational frequency along the bending mode at every level of theory employed. The T-shaped geometry is found to have an imaginary vibrational frequency along the asymmetric stretching mode. At the coupled cluster with single and double excitations and perturbative triple excitations [CCSD(T)]/correlation consistent polarized valence quadrupole-ζ (cc-pVQZ) level, the nonrelativistic classical relative energies of the T-shaped and linear structures with respect to the L-shaped minimum are 0.1 and 2.8 kcal/mol, respectively. Including zero-point vibrational energy, scalar relativistic, and core-valence corrections, the T-L energy separation is shifted to 0.4 kcal/mol and the relative energy between the L-shaped and linear structures is still 2.8 kcal/mol. All nonrelativistic and relativistic computations predict that the L-shaped (1A′) structure is most favored for the ground state. The linear structure is predicted to be a transition state, as the case of SiC2.
Comparing simulation of plasma turbulence with experiment. II. Gyrokinetic simulations
Authors: Ross David W. , Dorland William
Published: November 2002

The direct quantitative correspondence between theoretical predictions and the measured plasma fluctuations and transport is tested by performing nonlinear gyrokinetic simulations with the GS2 code. This is a continuation of previous work with gyrofluid simulations [D. W. Ross et al., Phys. Plasmas 9, 177 (2002)], and the same L-mode reference discharge in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] is studied. The simulated turbulence is dominated by ion temperature gradient (ITG) modes, corrected by trapped-electron, passing-electron and impurity effects. The energy fluxes obtained in the gyrokinetic simulations are comparable to, even somewhat higher than, those of the earlier work, and the simulated ion thermal transport, corrected for E×B flow shear, exceeds the experimental value by more than a factor of 2. The simulation also overestimates the density fluctuation level. Varying the local temperature gradient shows a stiff response in the flux and an apparent up-shift from the linear mode threshold [A. M. Dimits et al., Phys. Plasmas 7, 969 (2000)]. This effect is insufficient, within the estimated error, to bring the results into conformity with the experiment.
The naphthalene-benzene anion: Anion complex of aromatic hydrocarbons with the smallest electron affinity
Authors: Song Jae Kyu , Lee Nam Ki , Han Sang Yun , Kim Seong Keun
Published: November 2002

The mixed cluster anions between naphthalene and benzene were studied by mass spectrometry, photoelectron spectroscopy, and theoretical calculations. We found clear evidence for a stable anion of the 1:1 complex, [(Np)1(Bz)1]−, which has the smallest electron affinity measured so far for complexes of aromatic hydrocarbons. The photoelectron spectrum of [(Np)1(Bz)1]− was identical in shape with those of (Np)2− and [(Np)1(H2O)1]−, implying the presence of a common ion core, (Np)−, in these systems. Thus [(Np)1(Bz)1]− is viewed to be (Np)1−(Bz)1, where Bz stabilizes the core anion, (Np)−, as a neutral solvent. The geometry of (Np)1−(Bz)1 was calculated to be T-shaped, with a strong character for the π-hydrogen bonding. An adiabatic electron affinity of 0.01 eV was determined from the measured vertical detachment energy of 0.03 eV and the theoretical estimate of the energy difference between the two neutral structures involved.
The Jahn–Teller effect in the lower electronic states of benzene cation. I. Calculation of linear parameters for the e2g modes
Authors: Johnson Philip M.
Published: November 2002

Jahn–Teller (JT) coupling parameters can be simply derived from features of the potential energy surfaces of the JT active vibrational modes of a molecule subject to that effect. Potential energy curves representing cuts of D2h symmetry have been calculated using density functional theory for each of the four active e2g modes of benzene cation in each of the lowest three degenerate electronic states. From these curves and the vibrational frequencies, the JT coupling parameters can be found (vibrational numbering follows Wilson’s notation, coupling parameters are in Moffit’s notation). For the C6H6+X̃ 2E1g state the largest JT coupling parameter is provided by mode 6 (D=0.48), followed closely by mode 8 (0.30) and then mode 9 (0.10). Mode 7 provides almost no stabilization. For the B̃ 2E2g state, calculated coupling parameters are very large, particularly for modes 8 (1.36) and 6 (0.93). Modes 7 (0.10) and 9 (0.07) are smaller but finite. For the D̃ 2E1u state, mode 6 has an impressive linear coupling parameter of 4.12, able to support several vibrations below the JT cusp. Indications of a substantial quadratic stabilization for that mode result in the possibility of a completely static distortion in that state. Similar, but not identical, results are obtained for C6D6+.
The Jahn–Teller effect in the lower electronic states of benzene cation. II. Vibrational analysis and coupling constants of the B̃ 2E2g state
Authors: Johnson Philip M.
Published: November 2002

The vibrational structure seen in photoinduced Rydberg ionization spectra of the B̃ 2E2g state of benzene cation (C6H6+ and C6D6+) has been analyzed by fitting the vibrational patterns to energy levels derived from multimode Jahn–Teller calculations. Most of the structure can be ascribed to various combinations of modes 6 and 16, with minor contributions from 4, 17, and 18 (using Wilson’s numbering convention). In qualitative agreement with parameters derived from electronic structure calculations, the linear coupling parameter for mode 6 is very large (D=1.39 in C6H6+ and 1.28 in C6D6+). This raises questions about certain angular momentum selection rules in the classical Jahn–Teller model.
Accuracy of the centrifugal sudden approximation in the H+H2O reaction and accurate integral cross sections for the H+H2O→H2+OH abstraction reaction
Authors: Zhang Dong H. , Yang Minghui , Lee Soo-Y.
Published: November 2002

The initial state selected time-dependent wave packet method has been extended to calculate the total reaction probability for atom-triatom reactions with total angular momentum J>0 by treating both bonds in the triatom reagent reactively. The total exchange and abstraction reaction probabilities for the title reaction with J=15 calculated with 2 K-blocks (the projection of the total angular momentum on the body-fixed axis) show that one has to treat both OH bonds in the H2O reagent reactively for the exchange reaction, but for the abstraction reaction one can treat one OH bond as a spectator bond to get accurate results. This is in accord with what had been found for the total reaction probabilities for J=0 [Phys. Rev. Lett. 89, 103201 (2002)]. The J=15 reaction probabilities also show that the CS (centrifugal sudden) approximation is inadequate for the title reaction, in particular for the abstraction reaction. The integral cross sections for the abstraction reaction, calculated without the CS approximation but by freezing one OH bond in the H2O reagent for the ground and first vibrationally excited states, are found to be substantially larger than the corresponding CS integral cross sections. However, the integral cross section for the ground vibrational state is still much smaller than the existing experimental result. Since the treatment of freezing OH bond is not expected to introduce any noticeable error, it is conceivable that the experiments overestimated the integral cross section for the abstraction reaction.
Specific heat anomaly at the glass transition
Authors: Odagaki Takashi , Yoshidome Takashi , Tao Toshiaki , Yoshimori Akira
Published: November 2002

A general frame work is devised to obtain the specific heat of nonequilibrium systems described by the energy-landscape picture, where a representative point in the phase space is assumed to obey a stochastic motion which is governed by a master equation. The specific heat depends on the observation time and becomes quenched one for short observation time and annealed one for long observation time. In order to test its validity, the frame work is applied to a two-level system where the state goes back and forth between two levels stochastically. The specific heat is shown to increase from zero to the Schottky form as the observation time is increased from zero to infinity. The anomaly of specific heat at the glass transition is reproduced by a system with a model energy-landscape, where basins of the landscape form a one-dimensional array and jump rate between adjacent basins obeys a power-law distribution. It is shown that the glass transition can be understood as a transition from an annealed to a quenched system and that the glass transition temperature becomes lower when the observation time is increased.
Lattice polymers with hydrogen bondlike interactions
Authors: Buzano C. , Pretti M.
Published: November 2002

We study the phase behavior of two lattice polymer models (self-avoiding walks) incorporating attractive short-range interactions between parallel chain sections, attempting to mimick hydrogen bonding between monomers. The investigation is carried out in the framework of the Bethe approximation on a hypercubic lattice. The former model, which includes attraction between parallel sections longer than one chain segment, has been recently studied in full detail in the two-dimensional case, by means of accurate numerical transfer matrix techniques, and displays a first-order transition from a swollen (“coil”) state to a collapsed ordered (“solid”) state. Our investigation on this model is mainly meant to show that the Bethe approximation is able to recover such a behavior, and to extend the result to three dimensions. The latter model is similar, but takes into account attractive interactions also between one segment long parallel sections, which have no reason to be neglected in principle. We obtain, both in two and three dimensions, two phase transitions: An ordinary theta-collapse from the coil state to an isotropic compact (“globule”) state, and a first-order transition from the globule to the solid phase. Such results are compared with those of previously investigated polymer models, and their relevance to describe the formation of protein secondary structure is also briefly discussed.
Influence of the hydrophobic face width on the degree of association of coiled–coil proteins
Authors: Magaña Susana , Rubio Ana M. , Rey Antonio
Published: November 2002

A very simplistic computational model for the simulation of the association of dimeric coiled–coil proteins has been extended to permit the formation of higher order quaternary structures. The main parameter of the new model is ω, the width of the potential well representing the attraction among hydrophobic residues. For values of ω ranging from 0.5 to 3.0 Å, our Monte Carlo simulations predict the formation of trimers at low temperatures. The different structures (monomers, dimers, and trimers) which populate the simulated system at various temperatures allow us to discuss the optimum value of ω from the point of view of the thermal unfolding transition. On the other hand, the detailed analysis of the trimer structure provides a different point of view about the performance of the model, when compared with the structural information experimentally available for trimeric coiled–coils.
Effect of By in three-dimensional reconnection at the dayside magnetopause
Authors: Lee E. , Min K. W. , Lee D.-Y. , Seon J. , Hwang K. J.
Published: November 2002

Asymmetric magnetic reconnection with a non-zero By component is studied using a three-dimensional magnetohydrodynamic simulation and the results are interpreted in terms of the dayside reconnection at the magnetopause. The By component is implemented by rotating the magnetosheath field. It is seen that a magnetic bulge is formed in the magnetosheath region but displaced in the y direction due to the tilted magnetic field line geometry of reconnection. The bulge moves away from the reconnection region along the magnetopause with a speed significantly slower than that of the symmetric case. In fact, the reconnection rate is seen much slower than the symmetric case. It is also seen that the reconnection rate decreases as the angle of rotation increases. The discontinuity associated with magnetic reconnection geometry is identified as a slow shock with Alfvén waves attached to it on the magnetospheric side, while on the magnetosheath side an intermediate shock is seen, which evolves into a slow shock and a rotational discontinuity near the center of the bulge and near the y-directional edges of the current disruption region. A large By component is generated on the open field lines, exceeding the initial sheath By near the center of the bulge. The inflow mass flux into the reconnection region from the magnetosheath is seven times larger than that from the magnetosphere in the present study, while the outflow mass flux into the magnetosphere is five times larger than that into the magnetosheath, implying that a significant mass transfer occurs from the magnetosheath into the magnetosphere. The outflow plasmas move mostly along the magnetic field lines in the downstream and transported toward the polar region.
Energetic particle mode stability in tokamaks with hollow q-profiles
Authors: Zonca F. , Briguglio S. , Chen L. , Dettrick S. , Fogaccia G. , Testa D. , Vlad G.
Published: November 2002

A thorough analysis of energetic particle modes (EPM) stability and mode structures is presented for tokamaks with hollow q profiles. Focusing on the region near the minimum-q surface, EPM gap modes and resonant EPMs are shown to exist as solutions of the same dispersion relation. By controlling the fast ion distribution function, or, equivalently, their fundamental dynamical properties, a smooth transition between these two classes of modes is obtained within the EPM dispersion relation. When toroidal coupling becomes important, it is demonstrated that EPMs may have either single or double hump radial structures. The local analyses of EPM stability and mode structures near the minimum-q surface are put in the broader framework of EPM stability and EPM induced transport in tokamaks with hollow q profiles and a brief summary is also given of present understanding of such problems based on results of three-dimensional nonlinear hybrid magneto–hydrodynamic–gyrokinetic simulations. Possible implications of present results are discussed in terms of experimental observations and possibilities of designing novel experimental setups to probe, at least conceptually, the complex predictions of theory.
Flow-small angle neutron scattering measurements of colloidal dispersion microstructure evolution through the shear thickening transition
Authors: Maranzano Brent J. , Wagner Norman J.
Published: November 2002

The shear induced microstructure for electrostatic and Brownian suspensions are compared using in situ small angle neutron scattering (SANS). The dispersions consist of 75 nm Stöber silica coated with 3-(trimethoxysilyl) propyl methacrylate (TPM) and have a zeta potential of −42.6±4.7 mV. Neutralizing the surface charge with 0.066 M¯ nitric acid yields stable hard-sphere dispersions. SANS is conducted over a range of shear rates on the charge-stabilized and Brownian suspensions to test the order–disorder transition and hydrocluster mechanisms for shear thickening, and demonstrate the influence of stabilizing forces on the shear induced microstructure evolution. Through treatment of the colloidal micromechanics, shear induced changes in the microstructure are correlated to the hydrodynamic component of the shear stress and the thermodynamic component of the normal stress, i.e., the method of “Rheo-SANS” is developed. The results demonstrate that hydrocluster formation accompanies the shear thickening transition.