bluuebunny/crossref_metadata_2025 huggingface dataset:
Uncertainty of path integral averages at low temperature
Authors: Whitfield T. W. , Straub John E.
Published: July 2002

Burghardt, Eicke, and Stolze [J. Chem. Phys. 108, 1562 (1998)] have recently presented analytical results for the coherent state path integral (CSPI) approximation to the harmonic oscillator thermal density matrix in a generalized representation. In this work, the variance of the position and momentum operators for the more common Feynman path integral approximation to the density matrix is examined and compared with the results of the generalized CSPI approximation. Both path integral approaches are found to predict minimum uncertainty states at low enough temperatures. Particular attention is given to estimates of internal energy, which can place limits upon the temperature range over which path integral approximations are valid.
The charge of glass and silica surfaces
Authors: Behrens Sven H. , Grier David G.
Published: July 2002

We present a method of calculating the electric charge density of glass and silica surfaces in contact with aqueous electrolytes for two cases of practical relevance that are not amenable to standard techniques: surfaces of low specific area at low ionic strength and surfaces interacting strongly with a second anionic surface.
An injection seeded narrow bandwidth pulsed optical parametric oscillator and its application to the investigation of hyperfine structure in the PF radical
Authors: Fitzpatrick James A. J. , Chekhlov Oleg V. , Elks John M. F. , Western Colin M. , Ashworth Stephen H.
Published: July 2002

We describe the construction of an all solid-state, narrow bandwidth, pulsed optical parametric oscillator (OPO) based on β-barium borate nonlinear crystals. The OPO was injection seeded by an external cavity diode laser in the range 755–855 nm to generate high power narrow bandwidth tunable light in this range and simultaneously at 606–669 nm. The bandwidth of the visible light was ∼130 MHz, and after frequency doubling or sum frequency mixing with the second harmonic of the pump Nd:YAG laser, sub-Doppler spectra with an overall resolution of 450 MHz were taken in the UV. The system is demonstrated by taking high-resolution spectra of the v′=2–3 and 5–7 bands of the A 3Π–X 3Σ−(v′,0) progression and the v′=4–v″=0 band of the d 1Π–a 1Δ transition in PF. These spectra show clear hyperfine structure, and an analysis of this structure is presented and interpreted in terms of the electronic structure of the molecule. As a prelude to this high-resolution study, the first ten members of the A–X band system and the first five members of the d–a band system were recorded at the moderate resolution provided by a pulsed dye laser.
Conformational kinetics in liquid n-butane by transition path sampling
Authors: Ramı́rez Jorge , Laso Manuel
Published: July 2002

A recent method for the study of transition states, transition path sampling, is applied to a classical problem of first order dynamics, the conformational transitions trans–gauche in pure liquid n-butane. Not only can the rate constant be calculated, but an ensemble of transition states can be extracted and analyzed as well. The results of the rate constant and the different distributions at the transition state are compared to those obtained from an equilibrium molecular dynamics simulation of the same system.
Dielectric and electromechanical properties of ferroelectric-relaxor 0.9 Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 thin films
Authors: Kighelman Zian , Damjanovic Dragan , Setter Nava
Published: July 2002

Pyrochlore free 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 thin films were prepared from alkoxide-based solution precursors. Preferential (111) crystallographic orientation was obtained on TiO2/Pt/TiO2/SiO2/Si substrates by spin coating. Dielectric, electrostrictive, and piezoelectric properties of the films were characterized in detail. Films show relaxor-like behavior, but with dielectric permittivity which is low (around 4300 at peak and at 340 Hz with Eac=1.6×106 V/m) compared to bulk ceramics and single crystals. Several parameters which might be responsible for this lower permittivity are suggested. The ac and dc field dependences of the dielectric response have been investigated. Electrostrictive coefficients, M11(7.76×10−18 m2/V2) and Q11(1.9×10−2 m4 C−2), were determined by measuring strain and polarization as a function of the electric field (Eac). The maximum field induced piezoelectric d33 coefficient is 100 pm/V and electrostrictive strains up to 1.2×10−3 (with an ac electric field of 140 kV/cm) were measured.
Simulation of photoluminescence excitation spectra of InGaN epitaxial layers
Authors: Zheng Ruisheng , Taguchi Tsunemasa
Published: July 2002

We present a numerical study of radiative recombination in InGaN epitaxial layers by using a modified rate equation in which the disorder effect of nanostructures is taken into consideration. The photoluminescence excitation (PLE) spectra of the InGaN layers are simulated numerically. The results show that the disordered nanostructure model can be used to interpret the features of PLE spectra observed in many InGaN epilayers. The present theoretical work supports that the self-formed nanostructures with compositional and structural disorder may exit in some InGaN epitaxial layers.
Aggregation-volume-bias Monte Carlo simulations of vapor-liquid nucleation barriers for Lennard-Jonesium
Authors: Chen Bin , Siepmann J. Ilja , Oh Kwang J. , Klein Michael L.
Published: July 2002

A combination of the aggregation-volume-bias Monte Carlo algorithm and the umbrella sampling technique is applied to investigate homogeneous vapor–liquid nucleation. This combined approach is simple, general, and robust. Its efficiency is demonstrated for nucleation of Lennard-Jonesium, for which the precise calculation of the nucleation barriers takes only a few minutes at higher supersaturations to a few hours at lower supersaturations. Comparison of the simulation results to the classical nucleation theory (CNT) shows that CNT overestimates the barrier heights by a value nearly independent of the supersaturation, but provides a reasonable description of the critical cluster sizes.
Properties of atoms in molecules: Construction of one-density matrix from functional group densities
Authors: Hernández-Trujillo J. , Bader R. F. W.
Published: July 2002

The demonstrated transferability of functional groups defined as proper open systems within the theory of atoms in molecules is used to iteratively construct a one-electron density matrix P and its derived electron density distribution. The initial guess at the density used in the fitting procedure is obtained from the addition of the density distributions of groups defined in parent molecules by the maximal matching of their interatomic surfaces. The method thus takes advantage of the observation that the “zero-flux” boundary condition defining a proper open system maximizes the transferability of the density distribution of a given group between molecules, one that is accompanied by a paralleling transferability in all of its properties. The construction is subject to the constraints that P be idempotent and normalized. The method is applied to the construction of P for the molecules HCH2|CH2X, with X=CH3, NH2, OH, and F, where the vertical bar denotes the new C–C interatomic surface, the new zero-flux boundary. The densities for the groups HCH2| and |CH2X are defined in their dimer molecules, HCH2|CH2H and XCH2|CH2X.
An excess electron bound to urea oligomers. II. Chains and ribbons
Authors: Skurski Piotr , Simons Jack
Published: July 2002

The possibility of electron binding to chain- and ribbon-like urea oligomers was studied at the second-order Møller–Plesset perturbation theory level as well as at the coupled cluster level with single, double, and noniterative triple excitations. It was found that all the chains form stable dipole-bound anions whose electron binding energies grow rapidly with chain length, while ribbon-type oligomers bind an excess electron only when they contain an odd number of urea monomer units. Moreover, the chain oligomers support bound excited anionic states of Σ and Π symmetry.
Size-induced lattice relaxation in CeO2 nanoparticles
Authors: Zhou X.-D. , Huebner W.
Published: July 2002

Size-induced lattice relaxation was observed for nanoscale CeO2 single crystals with an average size from 4 to 60 nm. Results showed the finest crystallites exhibited no strain-induced line broadening, while high temperature annealing resulted in larger grain sizes and significant strains. The observed shift in the x-ray diffraction lattice parameters was assumed to be due to the formation of defects on the lattice, specifically oxygen vacancies. Modeling revealed that the oxygen vacancy concentration ([VO••]) was found to be ≈4×1020/cm3 for the 4 nm crystallites, and decreased two orders of magnitude for larger 60 nm single crystals.
Three-dimensional transmission coefficients for neutrons and photons in amorphous materials
Authors: Godoy Salvador , Fujita Shigeji
Published: July 2002

We use Boltzmann’s transport equation to calculate the 3D incoherent Landauer equation for neutrons and photons diffusing in a slab of amorphous material. We use the P1 approximation to calculate the multiple-scattering transmission coefficient as a function of the diffusion coefficient. The 3D transmission coefficient obtained is smaller than the 1D case. The P1 result, valid only for thick slabs, is proved to be an excellent approximation to the exact solution.
Electronic relaxation dynamics of carbon cluster anions: Excitation of the C̃ 2Πg←X̃ 2Πu transition in C6−
Authors: Frischkorn Christian , Bragg Arthur E. , Davis Alison V. , Wester Roland , Neumark Daniel M.
Published: July 2002

Anion femtosecond photoelectron spectroscopy (FPES) has been used to monitor intramolecular electronic relaxation dynamics following the excitation of the C̃ 2Πg←X̃ 2Πu 000 electronic transition in C6−. The time-dependent photoelectron spectra provide a detailed picture of the relaxation dynamics in which the initially excited C̃ 2Πg (v=0) level evolves into highly vibrationally excited C6− in its ground electronic state. The spectra show evidence for a two-step relaxation mechanism: internal conversion (IC) to vibrationally excited B̃ 2Σu+ and Ã 2Σg+ states, occurring on a time scale of 730±50 fs, followed by IC from these intermediate states to highly vibrationally excited levels in the X̃ 2Πu ground state with a time constant of 3.0±0.1 ps.
Self-organized carbon nanostrips with a new LiC10 structure derived from carbon nanotubes
Authors: Yazami Rachid , Gabrisch Heike , Fultz Brent
Published: July 2002

Single walled carbon nanotubes (SWNTs) were reacted with molten lithium at 220 °C for two weeks. This induced dramatic changes in their structure as shown by x-ray and electron diffractometry and Raman spectroscopy. A significant fraction of the initial SWNTs transformed into flat nanostrips having intercalated lithium in between them. Lithium forms a superlattice commensurate with that of the graphitelike nanostrips with √7×√3 in-plane distribution. This new structure corresponds to the LiC10 composition.
Polarization switching and induced birefringence in InGaAsP multiple quantum wells at 1.5 μm
Authors: Marshall D. , Mazilu M. , Miller A. , Button C. C.
Published: July 2002

We analyze the 1.5μm wavelength operation of a room temperature polarization switch based on electron spin dynamics in InGaAsP multiple quantum wells. An unexpected difference in response for left and right circularly polarized pump light in pump–probe measurements was discovered and determined to be caused by an excess carrier induced birefringence. Transient polarization rotation and ellipticity were measured as a function of time delay.
Structure and stability of small copper clusters
Authors: Jug Karl , Zimmermann Bernd , Calaminici Patrizia , Köster Andreas M.
Published: July 2002

The structure and stability of small copper clusters with up to ten atoms has been determined both for the neutral and the ionic clusters with density functional calculations. The calculations were of all-electron type. The structure optimization and frequency analysis were performed on the local density approximation level with the exchange correlation functional by Vosko, Wilk, and Nusair. Subsequently improved calculations for the stability were based on the generalized gradient approximation, where the exchange correlation functional of Perdew and Wang was used. Finally, the binding energies, ionization potentials, electron affinities, and separation energies were calculated. The results show that the trends are in agreement with available experimental data.
Tuning the two-photon absorption response of quadrupolar organic molecules
Authors: Zojer E. , Beljonne D. , Kogej T. , Vogel H. , Marder S. R. , Perry J. W. , Brédas J. L.
Published: July 2002

We apply correlated quantum-chemical techniques to study the origin of the large two-photon absorption (TPA) cross sections, δ, in stilbene derivatives in which electron-donating and electron-withdrawing substituents provide a quadrupolar charge-transfer arrangement. An additional field created by a set of point charges is used to systematically modify the ground-state polarization to determine its consequences for the TPA response. The effect on the molecular structure can be quantified by the evolution of the π-bond-order alternation (π-BOA) of the conjugated backbone. For moderate ground-state polarizations, a marked increase of the TPA response occurs; for large polarizations, δ peaks and then drops dramatically. Insight into the origin of this evolution is gained by comparing the values of δ obtained via the converged sum-over-states approach to the results of a simple three-state model.
Two Krylov space algorithms for repeated large scale sparse matrix diagonalization
Authors: Skone J. H. , Curotto E.
Published: July 2002

Two simple algorithms for the diagonalization of a set of sparse symmetric matrices of the form A+{Δk}k=1L for large values of L are proposed and investigated. The numerical strategies economize computer resources by requiring the reconstruction of the Lanczos basis for a small number of times compared to L. Each member of the set {Δk}k=1L is assumed to have a smaller number of nonzero elements compared to A. Both numerical procedures are derived from the Lanczos algorithm and use periodically a recursion to obtain the Lanczos vectors. Tests are conducted with both random symmetric matrices and with DVR Hamiltonians containing parametric potentials. The performance of the algorithms in terms of numerical accuracy, stability, and CPU time is studied as a function of two properties of the matrix set {Δk}k=1L.
Atomic force microscopy observation of insulated molecular wire formed by conducting polymer and molecular nanotube
Authors: Shimomura Takeshi , Akai Tomonori , Abe Takumi , Ito Kohzo
Published: July 2002

Inclusion complex formation between a conducting polymer, polyaniline (PANI) with emeraldine base, and a molecular nanotube synthesized from α-cyclodextrin (α-CD) has been studied by atomic force microscopy. We observed a rodlike inclusion complex of PANI and the molecular nanotube on mica substrate at room temperature. The height of this structure is nearly equal to the outside diameter of α-CD and almost uniform along the whole length of the structure, which indicates that a conducting wire of PANI is fully covered by molecular nanotubes as insulator. Accordingly, this inclusion complex can be regarded as insulated molecular wire.
Mesoscopic description of a chemical bistable state
Authors: Zhu Rui , Li Qian Shu
Published: July 2002

The effects of intrinsic fluctuations on bistable dynamics of a chemical system are investigated within the framework of a chemical master equation. Calculations are carried out for a three-variable mesoscopic model of the chemical Chua reaction, a mass-action model that displays the bistable state consisting of a period-1 and chaotic states. Contrary to the deterministic description, the chaotic state can be stable or unstable strongly depending on the system size, indicating a new concept of stability concerning multistable states in mesoscopic dynamics. In addition, the intensity of intrinsic fluctuations for one variable of the system is quite larger than those for the other two variables, implying that the data information obtained by measuring one variable in experiments may not reconstruct the real dynamic behavior of the system. Their implications for ecological and microscopic biological systems are also pointed out.
From thin-flux-tube approximation to two-mode approximation
Authors: Zhugzhda Y. D.
Published: July 2002

The well-known mathematical formalism for the reduction of the magnetohydrodynamic equations to an infinite set of coupled one-dimensional equations in the coefficients of the variable expansions in terms of the tube radius is explored. A way to truncate the set at an arbitrary order is shown. A comparison of the approximation with the exact solution for linear waves reveals that the approximation obtained is not restricted to thin flux tubes, contrary to what one would expect. The approximation is a two-mode approximation, and is valid for tubes of arbitrary diameter. The shortcomings of the approximation and the limits of its validity are discussed. The two-mode approximation can be applied to nonlinear waves in coronal loops and to chromospheric filaments in the solar atmosphere.
Effects of magnetic layer thickness on noise in advanced double-layer metal particle tape
Authors: Bissell Philip R. , Mercer Tim , Ardeleanu Paul-Claudiu , Stancu Alexandru , Stoleriu Laurentiu
Published: July 2002

A series of advanced metal particle tapes with identical particles but different magnetic coating thicknesses from 300 to 150 nm have been taken through a dc-demagnetization remanent process from positive to negative saturation. Magnetization was achieved with an electromagnetic device, which produced uniform magnetization in the longitudinal direction of the tape. Spectral noise power maps have been generated for the tapes following these demagnetization processes. The thickest tape showed characteristics similar to those of conventional thick tapes, such as video, with demagnetized noise lower than the saturation noise, whereas the thinnest was characteristic of a thin film with demagnetized noise greater than that at saturation. A representation of the experimental measurement process, based on a Landau–Lifshitz–Gilbert model of the medium and a simulation of the “reading” process, generated many of the experimental features. The changes in the noise maps with magnetic coating thickness were attributed to the increased contributions from the free surfaces of the tape as the magnetic coating thickness was reduced.
Polarization induced by external noise in irreversible electrode processes: A stochastic measure of thermodynamic effects driven by external noise in electro-chemical reaction systems
Authors: Zhao Nanrong , Luo Jiuli
Published: July 2002

A stochastic thermodynamics of finite electrochemical reaction systems affected by external noise has been established in this article. Based on a general stochastic model of the irreversible electrode processes driven by parameter noise, both the fluctuation and the dissipation in this kind of important physico-chemical system are discussed. It shows that an additional polarization induced by external noise in irreversible electrode processes is inevitable. Furthermore, a perfect formalism to estimate quantitatively both the concentration polarization and the activation polarization induced by a Gaussian white noise in the steady irreversible electrode process has been proposed. It turns out that the order of this noise-driven polarization could arrive at the level as noise intensity even in thermodynamic limit. Some illustrations to calculate the polarizations driven by the additive noise and the multiplicative noise in different cases are also given categorically. The formalism as a stochastic measure of the polarization induced by Gaussian white noise proposed in this article is helpful to further study and utilize generally this electrochemical effect driven by noise in physical chemistry.
Effect of disorder on the tunnel magnetoresistance: Lattice Green’s function method
Authors: Li Yun , Chang Ching-Ray , Yao Y. D.
Published: July 2002

The influences of random disorder in the barrier on tunnel conductance (TC) and magnetoresistance (TMR) in ferromagnet (FM)/insulator (semiconductor) (I(S))/FM junctions are investigated theoretically taking into account spin-orbit (SO) interaction. The TMR decreases significantly due to the spin-flip scattering (SF) caused by the SO interaction. We have found that the SF scattering is much stronger for anti-parallel configuration of magnetization and the TMR can even be inverted when the SF scattering is beyond some critical value. The numerical calculations are performed within the single-orbital tight-binding model using the recursive Green’s function method based on the extended Landauer–Büttiker formula.
Infrared reflectance and magnetorefractive effects in metal–insulator CoFe–Al2O3 granular films
Authors: Bozec D. , Kravets V. G. , Matthew J. A. D. , Thompson S. M. , Kravets A. F.
Published: July 2002

The infrared reflectance of (Co50Fe50)x–(Al2O3)1−x granular films which exhibit tunneling magnetoresistance has been measured in the wavelength range 2–25 μm for compositions both above and below the critical percolation volume fraction. The modifications of the Al2O3 Restrahlen reflection region are well accounted for in an effective medium model. In addition, frequency dependent changes of the reflection have been observed under the application of magnetic fields up to 12 kOe. At the longitudinal optic reflection edge around 9 μm distinctive magnetic signatures are found below and above the percolation threshold φc at x=0.17.
Matrix isolation FTIR spectroscopic and density functional theoretical studies of the O2SiCO and O2Si(CO)2 molecules
Authors: Miao Lei , Shao Limin , Wang Wenning , Fan Kangnian , Zhou Mingfei
Published: July 2002

The O2SiCO and O2Si(CO)2 molecules have been produced by reactions of SiO2 with CO in solid argon. Based on isotopic substitution experiments, infrared absorptions at 2206.8, 1390.1, and 989.4 cm−1 are assigned to the C–O and antisymmetric and symmetric SiO2 stretching vibrations of the O2SiCO molecule, and absorptions at 2195.7, 2191.3, and 1367.3 cm−1 are assigned to the O2Si(CO)2 molecule. Density functional theory calculations have been performed for the aforementioned species, which lend strong support to the experimental assignments of the infrared spectra. The O2SiCO molecule has a T-shaped C2v structure, while the O2Si(CO)2 molecule has a twisted C2 structure. The C–O stretching vibrational frequencies are higher than that of free CO due to electrostatic interaction. Bonding analysis showed that besides σ donation and π backdonation that are common to transition metal carbonyls, σ backdonation and π donation also contribute to the bonding between SiO2 and CO in the O2SiCO and O2Si(CO)2 molecules.
Interfacial net charge density of nonintimate Schottky junctions
Authors: Cvikl B. , Korošak D.
Published: July 2002

The origin of the excess capacitance of nonideal Schottky junctions on Si substrates is attributed to the external bias voltage dependent net charge density induced at an appropriate interface for which a functional dependence, in terms of empirical parameters, is explicitly suggested. This conjecture, resulting in the introduction of an additional term in the depletion layer differential capacitance, is verified on frequency dependent, room temperature, C–U data of our ionized cluster beam (ICB) deposited Ag/Si and Pb/Si Schottky junctions as well as on various other relevant published measurements. It is found, that the envelope of the calculated effective density of gap states of our ICB samples exhibits dense almost standing wave-like oscillations across the energy gap at the interface for which the envelope amplitude and number of nodes are sensitively dependent on the external bias voltage.
Accurate correlation potentials from integral formulation of density functional perturbation theory
Authors: Ivanov Stanislav , Levy Mel
Published: July 2002

A general perturbation theory and its integral equation formulation are presented in the context of density functional theory. The integral equation approach can be used to generate potentials yielding very accurate approximate ground-state densities and orbital energies within the framework of Kohn–Sham theory. This formulation eliminates the necessity of directly taking cumbersome functional derivatives in order to arrive at the components of the unknown Kohn–Sham potential, including the correlation potential. The asymptotic properties of the second-order correlation potentials are investigated. Consistent with the work by Facco Bonetti et al. [Phys. Rev. Lett. 86, 2241 (2001)], it is shown that the second-order potentials are divergent, and the source of the incorrect long range decay is identified. An approximate second-order potential with corrected long range properties is proposed.
Microstructure of a Co–CoO obliquely evaporated magnetic tape
Authors: Ito Takuya , Iwasaki Yoh , Tachikawa Hiroshi , Murakami Yasukazu , Shindo Daisuke
Published: July 2002

Microstructure of a Co–CoO obliquely evaporated tape for magnetic recording was studied by transmission electron microscopy with an energy-dispersive x-ray spectrometer and an energy filter. Columnar structure of Co-rich region spanning through the film thickness was observed. The diameters of the columns increased from 20 nm at the bottom to 40 nm at the top of the film. Within a column, several chains of Co crystallites lying along the column axis were observed. Individual Co crystallite constituting a chain had a diameter between 5 and 10 nm. In the lower part of the column, several chains assembled to form a bundle. In the upper part, the Co chains were separated by CoO crystallites in the enlarged diameter of the column. A typical diameter of a Co crystallite was smaller in the upper part of the film than that observed in the lower part. CoO crystallites were found to be densely packed between Co-rich columns whereas previous studies had interpreted the regions between the Co columns to be voided or less dense.
Monte Carlo simulation of europium telluride under hydrostatic pressure
Authors: Scott S. M. , Giebultowicz T. M.
Published: July 2002

EuTe is a type II antiferromagnetic semiconductor that exhibits a constant Néel temperature under high pressures with a lattice constant varying between 6.25 and 6.6 Å [I. N. Goncharenko and I. Mirebeau, Phys. Rev. Lett. 80, 1082 (1998); Europhys. Lett. 37, 633 (1997)]. Because mean field theory predicts a Néel temperature that is independent of J1, these results were interpreted to suggest a J2 independent of interstitial distances. Monte Carlo simulations of bulk EuTe with J2 held fixed and J1 varying from −.4(J2) to −.6(J2) using a Metropolis algorithm with a Hamiltonian comprised of nearest neighbor (NN) and next-nearest neighbor (NNN) terms have been performed. The Néel Temperature (TN) is observed to vary by 4% for a 20% change in the strength of J1, suggesting that interpretations of the behavior of EuTe based on mean field theory should be reevaluated. A linear relation between J1 and TN suggests that changes in NN and NNN interactions exactly cancel for EuTe under hydrostatic pressure.
A drift kinetic approach to stationary collisionless magnetic reconnection in an open cusp plasma
Authors: Egedal J.
Published: July 2002

A set of solutions to the drift kinetic equations is presented describing collisionless magnetic reconnection in a linear magnetic cusp. Effects not accounted for in fluid models cause strong anisotropic features in the velocity distribution function. A mechanism responsible for breaking the “frozen-in law” is identified. This mechanism has its origin in curvature and ∇B drifts and is consistent with fast reconnection.
Integral equations for polymers in quenched random media
Authors: Yethiraj Arun
Published: July 2002

Integral equations are derived for the self-consistent evaluation of intramolecular and intermolecular correlation functions of polymers in quenched random media. The theory uses the replica trick to average over the quenched disorder using the Given–Stell procedure. The equations derived by Chandler in a heuristic fashion are recovered under certain approximations.
Reactions of Ta+ and W+ with H2, D2, and HD: Effect of lanthanide contraction and spin–orbit interactions on reactivity and thermochemistry
Authors: Zhang Xiao-Guang , Rue Chad , Shin Sae-Young , Armentrout P. B.
Published: July 2002

A guided ion beam tandem mass spectrometer is used to examine the kinetic energy dependence of reactions of the third-row transition metal cations, Ta+, and W+, with molecular hydrogen and its isotopologs. A flow tube ion source produces Ta+ and W+ ions in their electronic ground state term and primarily in the lowest spin–orbit level. Corresponding state-specific reaction cross sections are obtained. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies in eV (kJ/mol) of D0(Ta+–H)=2.38±0.06 (230±6) and D0(W+–H)=2.27±0.05 (219±5). The experimental thermochemistry is consistent with ab initio calculations, performed here and from the literature, which also provide the electronic structures of these species and details about the reaction surfaces. Results from reactions with HD provide insight into the reaction mechanisms and indicate that these early metal ions, Ta+ and W+, react largely via insertion mechanisms. Results for these third-row transition metal systems are compared with the first-row and second-row congeners and found to have higher reactivity towards dihydrogen and stronger M+–H bonds. These differences can be attributed to the lanthanide contraction, relativistic effects, and efficient spin–orbit interactions among surfaces of different spin.
Ab initio calculations of magnetoresistance and interlayer exchange constants of the ferromagnet–semiconductor layered systems Fe/Ge
Authors: Perlov Alexander , Popescu Voicu , Ebert Hubert
Published: July 2002

We report on the results of calculations of the electronic, magnetic, and transport properties of Fe/Ge multilayer systems that are meant to model ferromagnetic (FM)/semiconductor (SC)/FM trilayer systems. On the base of the generalized Bloch theorem we have calculated the electronic structure of a set of spin-spiral structures, which allows us to extract averaged interlayer exchange parameters and their dependence on the width of the semiconductor spacer. The dependence of the dc conductivity and magnetoresistance on the width of the semiconductor layer and the relative angle between the moments of adjacent FM layers has also been investigated. The transport properties are studied on the basis of the Drude model.
Interfaces between silicalite surfaces and liquid hexadecane: A molecular dynamics simulation
Authors: Webb Edmund B. , Grest Gary S.
Published: July 2002

Interfaces between liquid hexadecane and low index surfaces of the zeolite silicalite were modeled using molecular dynamics. Pores on the zeolite surface provide access to the interior such that hexadecane molecules diffuse inside. The three surfaces studied differ significantly in the rate of molecular diffusion from the surface into the zeolite bulk. An equilibrium concentration of hexadecane inside the zeolite is established as a function of external liquid pressure. Molecules are partially adsorbed inside the zeolite at the solid/liquid interface and a nonuniform distribution of adsorption length is observed, the shape of which is determined by the subsurface channel structure of the zeolite. Near-surface intersections between channels inside the zeolite greatly influence the partial adsorption distribution as molecules preferentially avoid adsorption in intersections. The lifetime of partially adsorbed molecules varied greatly between the three surfaces. The critical adsorption length, at and beyond which molecules are more likely to be adsorbed into the zeolite than desorb into the liquid, also differed for the three interfaces. This too was a function of subsurface channel structure. In the range examined, temperature and pressure are less influential than channel structure.
Microwave and millimeter-wave spectrum of the He⋯H2+ long-range complex
Authors: Gammie David I. , Page Josephine C. , Shaw Andrew M.
Published: July 2002

The microwave and millimeter-wave spectrum of HeH2+ is reported over the frequency range 6–170 GHz. The observation of hyperfine structure in the spectra of some transitions made a clear distinction between the ortho- and para-hydrogen in the molecular ion. The hyperfine structure and double resonance Zeeman studies have enabled estimates of the quantum numbers involved in the transitions to be made. The Zeeman pattern in a 21.8 GHz doublet has been analyzed using an effective spin Hamiltonian with a case (B) basis set. This suggests an assignment of ΔN=+1, N=10←N=11. A similar analysis, in an extended hyperfine basis set, for the 15.2 GHz transition suggests an assignment of either ΔN=0, N=3, or N=4 as appropriate assumption from which to refine the potential.
Influence of laser writing of polyimides on the alignment of liquid crystals
Authors: Versteeg Dennis J. , Bastiaansen Cees W. M. , Broer Dirk J.
Published: July 2002

The influence of direct laser writing in polyimides on the alignment of organic liquid crystals was investigated. Laser writing was performed below the ablation threshold, which resulted in local melting of the polyimide. A surface relief structure was obtained as a result of the fast heating and cooling cycle during laser writing. No alignment of organic liquid crystals in the vicinity of the surface relief structure was observed as a direct result of the laser writing process. However, it appeared to be possible to wipe out the alignment originating from the rubbing process. This property proved to be useful to generate complex patterns in twisted nematic liquid crystal cells with locally uniaxial alignment. The contrast between the nonpatterned and patterned areas ranged between 40 and 50, which illustrates that the earlier described phenomena are potentially useful in, for instance, personalized security features.
Photo-induced refractive index change in hydrogenated amorphous silicon oxynitride
Authors: Kato Hiromitsu , Fujimaki Makoto , Noma Takashi , Ohki Yoshimichi
Published: July 2002

Refractive index change is shown to be induced by the irradiation of ultraviolet photons in hydrogenated amorphous silicon oxynitride films prepared by plasma-enhanced chemical vapor deposition. The mechanism of the index change and its dependence on the nitrogen content were investigated by electron spin resonance and scanning electron microscopy. It is concluded that the index change is due mainly to densification, and that the contribution of the formation of paramagnetic defects is only slight. To demonstrate the versatility of this refractive index change, a planar diffraction grating was fabricated.
A comparison of drift wave stability in stellarator and tokamak geometry
Authors: Anderson J. , Rafiq T. , Nadeem M. , Persson M.
Published: July 2002

The influence of plasma geometry on the linear stability of electrostatic ion-temperature-gradient driven drift modes (ITG or ηi=Ln/LTi modes) is investigated. An advanced fluid model is used for the ions together with Boltzmann distributed electrons. The derived eigenvalue equation is solved numerically. A comparison is made between an H–1NF [Fusion Technol. 17, 123 (1990)] like stellarator equilibrium, a numerical tokamak equilibrium and the analytical ŝ−α equilibrium. The numerical and the analytical tokamak are found to be in good agreement in the low inverse aspect ratio limit. The growth rates of the tokamak and stellarator are comparable whereas the modulus of the real frequency is substantially larger in the stellarator. The threshold in ηi for the stellarator is found to be somewhat larger. In addition, a stronger stabilization of the ITG mode growth is found for large εn(=Ln/R) in the stellarator case.
Large N limit of SO(N) gauge theory of fermions and Bosons
Authors: Toprak Erdal , Teoman Turgut O.
Published: July 2002

In this paper we study the large Nc limit of SO(Nc) gauge theory coupled to a Majorana field and a real scalar field in 1+1 dimensions extending ideas of Rajeev [Int. J. Mod. Phys. A 9, 5583 (1994)]. We show that the phase space of the resulting classical theory of bilinears, which are the mesonic operators of this theory, is OSp1(H|H)/U(H+|H+), where ℋ|ℋ refers to the underlying complex graded space of combined one-particle states of fermions and bosons and H+|H+ corresponds to the positive frequency subspace. In the begining to simplify our presentation we discuss in detail the case with Majorana fermions only [the purely bosonic case is treated in Toprak and Turgut, J. Math. Phys. 43, 1340 (2002)]. In the Majorana fermion case the phase space is given by O1(H)/U(H+), where ℋ refers to the complex one-particle states and H+ to its positive frequency subspace. The meson spectrum in the linear approximation again obeys a variant of the ’t Hooft equation. The linear approximation to the boson/fermion coupled case brings an additonal bound state equation for mesons, which consists of one fermion and one boson, again of the same form as the well-known ’t Hooft equation.
Theoretical studies of cross sections and photoelectron angular distributions in the valence photoionization of molecular oxygen
Authors: Lin Ping , Lucchese Robert R.
Published: July 2002

Photoionization from ground state of molecular oxygen leading to a number of 1πg−1, 1πu−1, 3σg−1, and 2σu−1 ion states have been investigated for photon energies up to 50 eV using the multichannel Schwinger method with configuration interaction. The use of a multichannel approach and correlated targets not only enables us to obtain results in good agreement with experimental data, but also leads to a better understanding of electron correlation effects in these photoionization processes. In addition to numerous Rydberg-type autoionization states that have been identified in the multichannel study, substantial effects of the open-channel coupling have also been found in almost every channel. These effects are evidences of the strong electron correlation in the photoionization dynamics of this system. The most significant interchannel coupling effect exists in the photoionization to the 3σg−1 channels, in which a complete failure of the single-channel photoionization description is found in the shape resonance process. In contrast to earlier theoretical studies, we found only one shape resonance in the photoionization to the 3σg−1 channels instead of two separated multiplet-specific shape resonances. A full description of the resonant state requires the inclusion of coupling to other valence photoionization channels. The photoelectron angular distributions have also been computed and compared to the existing experimental data. Good agreement between experiment and theory has been achieved. In this study, we also calculated molecular orientation parameters under the fixed-nuclei approximation, which contain symmetry information about the resonances and outgoing waves and which can be used to characterize the dynamics of the photoionization.
Beta-limiting instabilities and global mode stabilization in the National Spherical Torus Experiment
Authors: Sabbagh S. A. , Bell R. E. , Bell M. G. , Bialek J. , Glasser A. H. , LeBlanc B. , Menard J. E. , Paoletti F. , Stutman D. , Fredrickson E. , Garofalo A. M. , Gates D. , Kaye S. M. , Lao L. L. , Maingi R. , Mueller D. , Navratil G. , Ono M. , Peng M. , Synakowski E. , Zhu W. ,   NSTX Research Team
Published: July 2002

Research on the stability of spherical torus plasmas at and above the no-wall beta limit is being addressed on the National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 40, 557 (2000)], that has produced low aspect ratio plasmas, R/a∼1.27 at plasma current exceeding 1.4 MA with high energy confinement (TauE/TauE_ITER89P>2). Toroidal and normalized beta have exceeded 25% and 4.3, respectively, in q∼7 plasmas. The beta limit is observed to increase and then saturate with increasing li. The stability factor βN/li has reached 6, limited by sudden beta collapses. Increased pressure peaking leads to a decrease in βN. Ideal stability analysis of equilibria reconstructed with EFIT [L. L. Lao et al., Nucl. Fusion 25, 1611 (1985)] shows that the plasmas are at the no-wall beta limit for the n=1 kink/ballooning mode. Low aspect ratio and high edge q theoretically alter the plasma stability and mode structure compared to standard tokamak configurations. Below the no-wall limit, stability calculations show the perturbed radial field is maximized near the center column and mode stability is not highly effected by a nearby conducting wall due to the short poloidal wavelength in this region. In contrast, as beta reaches and exceeds the no-wall limit, the mode becomes strongly ballooning with long poloidal wavelength at large major radius and is highly wall stabilized. In this way, wall stabilization is more effective at higher beta in low aspect ratio geometry. The resistive wall mode has been observed in plasmas exceeding the ideal no-wall beta limit and leads to rapid toroidal rotation damping across the plasma core.
Stochastic heat and Burgers equations and their singularities. I. Geometrical properties
Authors: Davies Ian M , Truman Aubrey , Zhao Huaizhong
Published: July 2002

Arnol’d and Thom’s beautiful classification of caustics (shockwaves) for the Burgers equation suggests a similar one for the wavefronts of the corresponding heat equation. We give here a general theorem for Hamiltonian systems characterizing how the level surfaces of Hamilton’s principal function (wavefronts) meet the caustic surface in both the deterministic and stochastic cases. We further show how these results can be applied to the stochastic Burgers equation by using earlier results of Truman and Zhao. The generic example of a caustic, appearing in the two-dimensional case, is the semicubical parabolic cusp with the corresponding zero level surface being a combination of a generalized hypocycloid and a line pair. We refer to these as the cusp and tricorn. The analogous butterfly caustic, in the three-dimensional case, has a cusped zero level surface, the fish, which meets the butterfly caustic in three cusped curves and touches it along a straight line. Our results explain in terms of classical mechanics the properties of the caustic and wavefront for these two archetypal examples and characterize the caustic-wavefront intersection for the general stochastic case. We discuss the application of these results to turbulence for the Burgers velocity field.
Using secondary-proton spectra to study the compression and symmetry of deuterium-filled capsules at OMEGA
Authors: Séguin F. H. , Li C. K. , Frenje J. A. , Hicks D. G. , Green K. M. , Kurebayashi S. , Petrasso R. D. , Soures J. M. , Meyerhofer D. D. , Glebov V. Yu. , Radha P. B. , Stoeckl C. , Roberts S. , Sorce C. , Sangster T. C. , Cable M. D. , Fletcher K. , Padalino S.
Published: July 2002

With new measurement techniques, high-resolution spectrometry of secondary fusion protons has been used to study compression and symmetry of imploded D2-filled capsules in direct-drive inertial-confinement-fusion experiments at the 60-beam OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Data from target capsules with ∼15 atmospheres of D2 fuel, in CH shells 19–27 μm thick, were acquired with a magnet-based, charged-particle spectrometer and with several new “wedge-range-filter”-based spectrometers incorporating special filters and CR39 nuclear track detectors. Capsules with 19-μm shells, imploded with similar laser energies (∼23 kJ) but different methods of single-beam laser smoothing, were studied and found to show different compression characteristics as indicated by the fuel areal density (determined by the ratio of secondary-proton yield to primary-neutron yield) and the total areal density (determined by the energy loss of protons due to slowing in the fuel and shell). In going from 0.3-THz SSD (smoothing by spectral dispersion) to 1-THz SSD and PS (polarization smoothing), the fuel areal density increased by at least 30%, while the total areal density increased by 40% (from ∼52 to ∼72 mg/cm2). In addition, significant low-mode-number spatial asymmetries in implosions were indicated by spectra measured at different angles with respect to the target. The mean energies of protons, measured at different angles during the same shot, varied by as much as 1 MeV, implying angular variations in areal density of order 30 mg/cm2. To the best of our knowledge, this is the first experimental demonstration that capsule symmetry can be sensitively studied by measuring the energy loss of charged particles.
A reaction path Hamiltonian defined on a Newton path
Authors: González Javier , Giménez Xavier , Bofill Josep Maria
Published: July 2002

A reaction path Hamiltonian is formulated in terms of a Newton reference path. The defining differential equation for the latter is solved by quadratically expanding the original equation, which in turn leads to an improved formulation of the reaction path Hamiltonian. This new reference path is found to deal properly with bifurcation points, a difficulty not solved by intrinsic reaction coordinate-following algorithms. The Newton path is found, in addition, to be reliable as for the description of chemical rearrangement processes, as it is illustrated with the application to the HCN↔CHN isomerization as well as the 1,2 hydrogen migration between ethyne oxide and the corresponding carbene.
Classical history theory of vector fields
Authors: Noltingk Duncan
Published: July 2002

We consider the extension of classical history theory to the massive vector field and electromagnetism. It is argued that the action of the two Poincaré groups introduced by Savvidou suggests that the history fields should have five components. The extra degrees of freedom introduced to make the fields five dimensional result in an extra pair of second class constraints in the case of the massive vector field, and in an extended gauge group in the case of electromagnetism. The total gauge transformations depend on two arbitrary parameters, and contain “internal” and “external” U(1) gauge transformations as subgroups.
Quantum three-body system in D dimensions
Authors: Gu Xiao-Yan , Duan Bin , Ma Zhong-Qi
Published: July 2002

The independent eigenstates of the total orbital angular momentum operators for a three-body system in an arbitrary D-dimensional space are presented by the method of group theory. The Schrödinger equation is reduced to the generalized radial equations satisfied by the generalized radial functions with a given total orbital angular momentum denoted by a Young diagram [μ, ν, 0,…, 0] for the SO(D) group. Only three internal variables are involved in the functions and equations. The number of both the functions and the equations for the given angular momentum is finite and equal to (μ−ν+1).