bluuebunny/crossref_metadata_2025 huggingface dataset:
Interplay of ionic and structural heterogeneity on functional action potential duration gradients: Implications for arrhythmogenesis
Authors: Sampson Kevin J. , Henriquez Craig S.
Published: August 2002

Action potential duration (APD) dispersion in the heart is governed by the underlying cellular architecture and the spatial distribution of the membrane properties. Understanding the contribution of each factor is important in designing more effective methods for the control of arrhythmias. Recent experimental studies have shown that the insertion of structural barriers in ionically heterogeneous tissue facilitates the formation of unidirectional block and discordant alternans. In this work, computational modeling is used to examine the effect of internal obstacles on the formation of functional APD gradients in ionically heterogeneous tissue. Intrinsic APD differences are introduced by assigning two discrete cell types to each half of a square domain. The combined effect of structural and ionic heterogeneities is shown to produce gradients in APD that are oblique to both the intrinsic gradients in APD and the physical boundary. Simulation results are presented that show that the magnitude and spatial extent of the subsequent APD gradients are modulated by the size and orientation of the obstacle, the degree of anisotropy, and the location of the pacing site. Long, thin internal obstacles are found to produce the greatest dispersion in APD. The combination of internal obstacles and ionic heterogeneities is shown to produce a substrate for re-entrant excitation following a pair of near threshold point stimuli.
Simulation study of conversion of laser energy into x rays in laser plasmas
Authors: Wang W. , Dong Q. L. , Zhang J.
Published: August 2002

A simulation study of the conversion process of laser energy into x rays from a gold foil target irradiated by a 1 ns laser pulse with 1014 W/cm2 intensity at 0.35 μm wavelength is presented. The thickness of the foil is optimized to achieve high conversion efficiency on its rear side. The optimized foil can provide unique plasmas with a low spatial gradient of temperature and density.
Production of ultracollimated bunches of multi-MeV electrons by 35 fs laser pulses propagating in exploding-foil plasmas
Authors: Giulietti D. , Galimberti M. , Giulietti A. , Gizzi L. A. , Numico R. , Tomassini P. , Borghesi M. , Malka V. , Fritzler S. , Pittman M. , Ta Phouc K. , Pukhov A.
Published: August 2002

Very collimated bunches of high energy electrons have been produced by focusing super-intense femtosecond laser pulses in submillimeter under-dense plasmas. The density of the plasma, preformed with the laser exploding-foil technique, was mapped using Nomarski interferometry. The electron beam was fully characterized: up to 109 electrons per shot were accelerated, most of which in a beam of aperture below 10−3 sterad, with energies up to 40 MeV. These measurements, which are well modeled by three-dimensional numerical simulations, validate a reliable method to generate ultrashort and ultracollimated electron bunches.
Winnerless competition between sensory neurons generates chaos: A possible mechanism for molluscan hunting behavior
Authors: Varona Pablo , Rabinovich Mikhail I. , Selverston Allen I. , Arshavsky Yuri I.
Published: August 2002

In the presence of prey, the marine mollusk Clione limacina exhibits search behavior, i.e., circular motions whose plane and radius change in a chaotic-like manner. We have formulated a dynamical model of the chaotic hunting behavior of Clione based on physiological in vivo and in vitro experiments. The model includes a description of the action of the cerebral hunting interneuron on the receptor neurons of the gravity sensory organ, the statocyst. A network of six receptor model neurons with Lotka–Volterra-type dynamics and nonsymmetric inhibitory interactions has no simple static attractors that correspond to winner take all phenomena. Instead, the winnerless competition induced by the hunting neuron displays hyperchaos with two positive Lyapunov exponents. The origin of the chaos is related to the interaction of two clusters of receptor neurons that are described with two heteroclinic loops in phase space. We hypothesize that the chaotic activity of the receptor neurons can drive the complex behavior of Clione observed during hunting.
Behavior of the shape memory alloy NiTi during one-dimensional shock loading
Authors: Millett J. C. F. , Bourne N. K. , Gray G. T.
Published: August 2002

The response of alloys based on the intermetallic compound NiTi to high-strain-rate and shock loading conditions has recently attracted attention. In particular, similarities between it, and other shape memory materials such as the alloy U–6%Nb in the propagation of the plastic wave in Taylor cylinders are of significant interest. In this article, the Hugoniot is measured using multiple manganin stress gauges, either embedded between plates of the NiTi alloy, or supported with blocks of polymethylmethacrylate. In this way, the shock stress, shock velocity, and details of the shock wave profile have been gathered. An inflection at lower stresses has been found in the Hugoniot curve (stress-particle velocity), and has been ascribed to the martensitic phase transformation that is characteristic of the shape memory effect in this alloy. In a similar way, the variation of shock velocity with particle velocity has been found to be nonlinear, contrary to other pure metal and alloy systems. Finally, a break in slope in the rising part of the shock profile has been identified as the Hugoniot elastic limit in NiTi. Conversion to the one-dimensional stress equivalent, and comparison to quasistatic data indicates that NiTi exhibits significant strain-rate sensitivity.
Origins of substrate-topography-induced magnetic anisotropy in sputtered cobalt-alloy films
Authors: Twisselmann D. J. , Chambers P. G. , Ross C. A. , Khanna G. , Clemens B. M.
Published: August 2002

Cr(001)/CoCrPt (112̄0) films, used in magnetic hard disks, exhibit in-plane magnetic anisotropy when grown over a substrate patterned with shallow grooves. In this experiment, Cr/CoCrPt films are grown over substrates that have been patterned using lithographic techniques to produce well-controlled topography. The in-plane anisotropy, which is of the order of 104 J m−3, increases with the groove height and frequency. This magnetic anisotropy is a result of anisotropic in-plane crystallographic orientation and anisotropic stress. Stress-induced anisotropy accounts for up to 25% of the total in-plane anisotropy, but the major contribution to the anisotropy is a preferential Co c-axis alignment parallel to the groove direction.
Fiber-coupled antennas for ultrafast coherent terahertz spectroscopy in low temperatures and high magnetic fields
Authors: Crooker S. A.
Published: August 2002

For the purposes of measuring the high-frequency complex conductivity of correlated-electron materials at low temperatures and high magnetic fields, a method is introduced for performing coherent time-domain terahertz spectroscopy directly in the cryogenic bore of existing dc and pulsed magnets. Miniature fiber-coupled THz emitters and receivers are constructed and are demonstrated to work down to 1.5 K and up to 17 T, for eventual use in higher-field magnets. Maintaining the submicron alignment between fiber and antenna during thermal cycling, obtaining ultrafast (<200 fs) optical gating pulses at the end of long optical fibers, and designing highly efficient devices that work well with low-power optical gating pulses constitute the major technical challenges of this project. Data on a YBCO superconducting thin film and a high mobility two-dimensional electron gas are shown.
Derivation of bolometer equations relevant to operation in fusion experiments
Authors: Giannone L. , Mast K. , Schubert M.
Published: August 2002

An overview of the bolometer diagnostic in the W7-AS stellarator is presented. Bolometers are used in plasma physics to measure radiated power losses that are mainly produced by impurity lines and neutral particles. With an array of collimated bolometers, the spatial and temporal evolution of these losses can be measured. The accuracy of the inversion of the line integrated measurements into local values necessitates a careful calibration and conversion of the measured signal into an incident power of each foil. Using transmission line theory to determine the equivalent circuit for the sensor and cable, the equations for bolometer calibration and measurement, taking into account the cable resistance and capacitance between amplifier and detector, are derived. The ability to switch the bolometer amplifiers from measurement to calibration mode between discharges allows continuous monitoring of the calibration constants of the bolometer foil. Measurements by bolometer cameras and their Abel inversion for the radial profile of radiated power in W7-AS are presented.
Improved method of data acquisition and processing for the measurement of thermal diffusivity by the laser flash technique
Authors: Krishnaiah M. V. , Seenivasan G. , Murti P. Srirama
Published: August 2002

Generally, in the laser flash technique, thermal diffusivity of any sample is calculated using the t1/2 value obtained by analyzing the transient signal. The t1/2 value, and thus the thermal diffusivity obtained in this way, will have a good amount of scatter in the data at low temperatures due to the weak signal and the pick up of external noise during the measurement. This necessitated suitable modifications in the data acquisition system to enhance the signal-to-noise ratio for the measurements carried out at low temperatures. This article presents the necessary modifications made in the existing data acquisition system. The measurements of thermal diffusivity of stainless-steel samples by the laser flash method and their results are discussed here.
Ionizing beam tracking in gaseous chambers for field mapping
Authors: Jalowy T. , Neugebauer R. , Groeneveld K. O. , Ponciano C. R. , Farenzena L. S. , da Silveira E. F.
Published: August 2002

The combination of the time-of-flight (TOF) technique with two-dimensional position-sensitive (XY) ion detection is employed for tracking projectile-gas collisions. The main goal is to determine the detailed distribution of local electric fields, which accelerate the gas ions produced in the collisions. A low-pressure target gas spectrometer for this goal is described and all necessary XY- TOF expressions concerning the ion dynamics are presented. Experimental results of a 150 keV neutral H beam colliding with a He–Ne–Ar mixture are compared with the calculations. The new method can also be used for mapping magnetic fields or in large collision gas cells, e.g., to increase resolution in mass spectrometry and to perform initial momentum measurements of the produced ions.
Surface plasmon resonance measurement of AlOx insulating barriers in magnetic tunnel junctions formed by remote rf plasma oxidation process
Authors: Park J. H. , Yoon K. S. , Yang J. Y. , Kim E. K. , Lee C. H. , Kim C. O. , Hong J. P.
Published: August 2002

A surface plasmon resonance spectroscopy (SPRS) measurement was carried out to determine dielectric properties, oxidation states, and optimum thickness of AlOx insulating barrier in magnetic tunnel junctions. The insulating barriers were prepared as a function of oxidation time by using a remote rf plasma oxidation process. The experimental results revealed that the dielectric value and optimum thickness of the AlOx insulating barrier in our junctions were found to be 1.79+i1.77 and 11.26 Å, respectively. For comparison, the magnetic tunneling junction was also fabricated using the same oxidation condition. The best magneto-resistance ratio of about 34% was observed at an optimum oxidation time of 70 s, as expected by the SPRS measurement.
Structure and optically pumped lasing from nanocrystalline ZnO thin films prepared by thermal oxidation of ZnS thin films
Authors: Zhang X. T. , Liu Y. C. , Zhang L. G. , Zhang J. Y. , Lu Y. M. , Shen D. Z. , Xu W. , Zhong G. Z. , Fan X. W. , Kong X. G.
Published: August 2002

In this article, we observe the optically pumped lasing from the high-quality nanocrystalline ZnO thin films obtained by thermal oxidation of ZnS thin films, which were grown on SiO2 substrates by low-pressure-metalorganic chemical vapor deposition technique. The x-ray diffraction (XRD) patterns indicate that high-quality ZnS films possess a preferred (111) orientation. ZnS has a transformation to ZnO at an annealing temperature (Ta) of 500 °C, and fully transforms into ZnO at Ta⩾700 °C from the XRD patterns. The obtained ZnO films possess a polycrystalline hexagonal wurtzite structure. The fifth-order Raman scattering is observed in the films, which indicates that a large deformation energy exists in the lattice. In photoluminescence (PL), spectra, for all the samples with different annealing temperatures, the near-band-edge (NBE) PL peak has a pronounced blueshift with increasing annealing temperature, while the full width at half maximum (FWHM) decreases gradually. We think that emissions of the bound excitons play an important role in NBE PL. A strong PL with a FWHM of 62 meV at 3.30 eV from NBE has been obtained at room temperature for the samples annealed at 900 °C. The PL intensity ratio of the ultraviolet emission to the deep-level emission is as high as 98 at room temperature, indicating the high-quality of the nanocrystalline ZnO films.
Chaos in the relativistic two-electron atom
Authors: Matrasulov D. U.
Published: August 2002

Chaotic autoionization of the relativistic two-electron atom is investigated. A theoretical analysis of chaotic dynamics of the relativistic outer electron under the periodic perturbation due to the inner electron, assumed to be on a circular orbit, based on the Chirikov criterion, is given. The diffusion coefficient, the ionization rate, and time are calculated.
Modes of convection in the magnetotail
Authors: Baumjohann Wolfgang
Published: August 2002

The flow of plasma in the Earth’s magnetotail cannot reach a steady state, since adiabatic convection would lead to exceedingly high pressure of the associated magnetic flux tubes closer to the Earth, the so-called pressure catastrophe. The natural way to avoid the pressure catastrophe is to significantly reduce the flux tube volume by reconnection, and observations show a near-Earth reconnection line typically around 20–25 Earth radii down tail. Earthward flows from this reconnection line are rather bursty and typically seen outside of 10 Earth radii. At this point they are strongly braked by the here dominant dipolar magnetic field. The pressure gradients piled up by the flow lead to the substorm current wedge, and possibly other substorm phenomena observed in the Earth’s ionosphere. When more and more flux tubes are piled up, the dipolarization front moves tailward and finally shuts off near-Earth reconnection.
Counterstreaming ions as evidence of magnetic reconnection in the recovery phase of substorms at the kinetic level
Authors: Nagai Tsugunobu , Nakamura Masao , Shinohara Iku , Fujimoto Masaki , Saito Yoshifumi , Mukai Toshifumi
Published: August 2002

Counterstreaming ions embedded in hot isotropic ions are found at the front of fast earthward plasma flows in the recovery phase of substorms in the Earth’s magnetotail. The counterstreaming ions are present only when the northward component of the magnetic field increases in the equatorial plane. Hybrid simulations of magnetic reconnection have been carried out. It is found that counterstreaming ions form in the leading edge of jetting plasmas produced with magnetic reconnection, where the magnetic field lines pile up due to the pre-existing stationary plasmas. These counterstreaming ions originate from cold ions on the northern and southern tail lobe field lines, and earthward transport of the reconnected field lines makes these cold ions flow into the equatorial plane. The present observations provide strong evidence of magnetic reconnection in the recovery phase of substorms at the kinetic level.
Nonlinear dynamics of thin current sheets
Authors: Daughton William
Published: August 2002

Observations indicate that the current sheet in the Earth’s geomagnetic tail may compress to a thickness comparable to an ion gyro-radius prior to substorm onset. In recent years, there has been considerable controversy regarding the kinetic stability of these thin structures. In particular, the growth rate of the kink instability and its relevance to magnetotail dynamics is still being debated. In this work, a series of fully kinetic particle-in-cell simulations are performed for a thin Harris sheet. The ion to electron mass ratio is varied between mi/me=4→400 and careful comparisons are made with a formally exact approach to the linear Vlasov theory. At low mass ratio mi/me<64, the simulations are in excellent agreement with the linear theory, but at high mass ratio the kink instability is observed to grow more rapidly in the kinetic simulations than predicted by theory. The resolution to this apparent discrepancy involves the lower hybrid instability which is active on the edge of the sheet and rapidly produces nonlinear modifications to the initial equilibrium. The nature of this nonlinear deformation is characterized and a simple model is proposed to explain the physics. After the growth and saturation of the lower hybrid fluctuations, the deformed current sheet is similar in structure to a Harris equilibrium with an additional background population. This may explain the large growth rate of the kink instability at later times, since this type of modification to the Harris sheet has been shown to greatly enhance the growth rate of the kink mode.
Geomagnetic transport in the solar wind driven nightside magnetosphere–ionosphere system
Authors: Horton W. , Crabtree C. , Doxas I. , Weigel R. S.
Published: August 2002

A spatially resolved nonlinear dynamical model of the solar wind driven geomagnetic tail plasma is developed for the purpose of space weather predictions. The model represents the fluctuating electromagnetic fields and high pressure central plasma sheet by a large number (2N≲200) of semiglobal coupled current loops ranging from the near-Earth geosynchronous orbit position for substorm dynamics to deep in the geotail. There is a spectrum of dynamical frequencies ranging from 1 h periods to 1 min and shorter periods. The low-frequency modes are global and lead to the dynamics of the low-dimensional (d=6) WINDMI model. The high-frequency dynamics are nonlinear compressional-rarefraction waves propagating up and down the geotail. The localized pulses start from sites of local reconnection set off either by the tearing mode unloading trigger or by localized solar wind disturbances acting on the nightside magnetopause. Larger unloading events lead to nonlinear steepening of the compressional pulsations which act to trigger secondary convection events under certain conditions.
Parallel electric fields in the upward current region of the aurora: Indirect and direct observations
Authors: Ergun R. E. , Andersson L. , Main D. S. , Su Y.-J. , Carlson C. W. , McFadden J. P. , Mozer F. S.
Published: August 2002

In this article we present electric field, magnetic field, and charged particle observations from the upward current region of the aurora focusing on the structure of electric fields at the boundary between the auroral cavity and the ionosphere. Over 100 high-resolution measurements of the auroral cavity that were taken by the Fast Auroral Snapshot (FAST) satellite are included in this study. The observations support earlier models of the auroral zone that held that quasi-static parallel electric fields are the primary acceleration mechanism. In addition to the statistical study, several examples of direct observations of the parallel electric fields at the low-altitude boundary of the auroral cavity are put forth. These observations suggest that the parallel electric fields at the boundary between the auroral cavity and the ionosphere are self-consistently supported as oblique double layers.
Chebyshev real wave packet propagation: H+O2 (J=0) state-to-state reactive scattering calculations
Authors: Zhang Hong , Smith Sean C.
Published: August 2002

In this paper we explore the relative performance of two recently developed wave packet methodologies for reactive scattering, namely the real wave packet Chebyshev domain propagation of Gray and Balint-Kurti [J. Chem. Phys. 108, 950 (1998)] and the Lanczos subspace wave packet approach of Smith et al. [J. Chem. Phys. 116, 2354 (2002); Chem. Phys. Lett. 336, 149 (2001)]. In the former method, a modified Schrödinger equation is employed to propagate the real part of the wave packet via the well-known Chebyshev iteration. While the time-dependent wave packet from the modified Schrödinger equation is different from that obtained using the standard Schrödinger equation, time-to-energy Fourier transformation yields wave functions which differ only trivially by normalization. In the Lanczos subspace approach the linear system of equations defining the action of the Green operator may be solved via either time-dependent or time-independent methods, both of which are extremely efficient due to the simple tridiagonal structure of the Hamiltonian in the Lanczos representation. The two different wave packet methods are applied to three dimensional reactive scattering of H+O2 (total J=0). State-to-state reaction probabilities, product state distributions, as well as initial-state-resolved cumulative reaction probabilities are examined.
Magnetic interactions and interface properties in Co/Fe multilayers
Authors: Agazzi L. , Bennett S. , Berry F. J. , Carbucicchio M. , Rateo M. , Ruggiero G. , Turilli G.
Published: August 2002

Co/Fe multilayers with different layer thickness formed by electron beam evaporation in ultrahigh vacuum have been investigated by grazing incidence x-ray reflectivity (GIXRR) and alternating gradient force magnetometry. The interface thicknesses are lower than GIXRR uncertainty (∼1 nm), favoring a strong magnetic exchange interaction between the layers responsible for their single phase magnetic behavior. The hysteresis loops were interpreted as the result of two different magnetization processes related to the presence of an out-of-plane component of the magnetization.
Monomode, nonleaky planar waveguides in a Nd3+-doped silicate glass produced by silicon ion implantation at low doses
Authors: Chen Feng , Wang Ke-Ming , Wang Xue-Lin , Li Xi-Shan , Lu Qing-Ming , Shen Ding-Yu , Nie Rui
Published: August 2002

Monomode, nonleaky planar waveguides in a Nd3+-doped silicate glass were fabricated by 3.0 MeV Si+ ion implantation at doses ranging from 5×1013 to 6×1014 ions/cm2 at room temperature. The enhancement of the refractive index confined the light propagating in the waveguide without tunneling effects in contrast to the traditional barrier-type waveguide. Annealing at moderate temperatures (about 200 °C) in air was performed, which reduced the propagation loss from 20–30 dB/cm to a minimum of 0.5 dB/cm.
Winding shape optimization for asymmetric confinement magnets
Authors: Ricci Leonardo , Bertoldi Andrea , Bassi Davide
Published: August 2002

We discuss a numerical method for the optimization of the shape of noncircular coil windings for atomic physics applications. The method relies on two main features: first, a piecewise straight modeling of the curve which describes the winding; second, the definition of a step perturbation, based on pursuit curve theory, that changes the shape of the curve while conserving its length. The evolution resulting from a sequence of random steps is piloted towards the optimized shape by means of a Monte Carlo procedure. The method allows for significant improvement of the performance of magnetic configurations and is particularly suitable for dealing with cases in which geometrical constraints play a major role. Finally, as an example, the application of the method to the important case of a clover-leaf trap is discussed.
Zero applied stress rheometer
Authors: Bellon L. , Buisson L. , Ciliberto S. , Vittoz F.
Published: August 2002

In order to test the fluctuation-dissipation relation on rheological properties of soft materials, we built an experiment to measure thermally excited strain in a sample and compare it to the classical response to an external stress. The rheometer is based on a cylindrical Couette geometry. We use differential interferometry to achieve better than 10−10 rad/Hz sensitivity in angular position above 0.5 Hz. The forcing method, based on electrostatic interaction in a capacitor, generates torques comparable to that of thermal noise. Experiments on a calibrated silicon oil show good agreement between response and fluctuation measurements.
Simultaneous in situ detection of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy
Authors: Brown Steven S. , Stark Harald , Ciciora Steven J. , McLaughlin Richard J. , Ravishankara A. R.
Published: August 2002

This article describes the application of cavity ring-down spectroscopy (CaRDS) to the simultaneous concentration measurement of nitrate radical, NO3, and dinitrogen pentoxide, N2O5, in the ambient atmosphere. The sensitivity for detection of both NO3 and N2O5 is 0.5 pptv (2σ) for a 5 s integration, comparable to or better than previous measurements of NO3 (e.g., via DOAS), but with significantly better time resolution. Furthermore, direct measurement of N2O5 represent a previously unavailable capability. Concentrations of both species are measured simultaneously in two separate flow systems and optical cavities pumped by the same pulsed dye laser at 662 nm. One of the flow systems remains at ambient temperature for detection of NO3, while the other is heated to 80 °C to induce thermal decomposition of N2O5 providing a measurement of the sum of the NO3 and N2O5 concentrations. This article outlines a series of laboratory and field tests of the instrument’s performance. Important considerations include signal acquisition, zero measurements, aerosol interference, flow system losses, and the conversion efficiency for N2O5 thermolysis to NO3. We describe the limitations of this method and show how they can be quantified and accounted for in field measurements.
Study of interface states and oxide quality to avoid contrast reversal in scanning capacitance microscopy
Authors: Goghero Didier , Raineri Vito , Giannazzo Filippo
Published: August 2002

We demonstrate that the contrast reversal effect in scanning capacitance microscopy (SCM) is related to the Si/SiO2 interface microroughness. The surface roughness has been associated with the concentration of states at the Si/SiO2 interface and a monotonic behavior of the SCM imaging is preferentially observed for a smooth surface and consequently a low state concentration. Changes in the oxide quality have also been found to strongly influence the measurements. A criterion based on the hysteresis measurements from forward and reverse dC/dV–V curves is discussed to better evaluate the oxide quality and to obtain reproducible SCM data.
Crystal size and oxygen segregation for polycrystalline GaN
Authors: Butcher K. S. A. , Timmers H. , Afifuddin  , Chen Patrick P.-T. , Weijers T. D. M. , Goldys E. M. , Tansley T. L. , Elliman R. G. , Freitas J. A.
Published: August 2002

The grain size for polycrystalline GaN, grown in low-temperature gallium-rich conditions, is shown to be correlated to the oxygen content of the films. Films with lower oxygen content were observed to have larger crystals with an increased tendency to a single-preferred crystal orientation. Elastic recoil detection analysis with heavy ions (i.e., 200 MeV Au 197 ions) was used to determine the composition of the GaN films grown for the study, including the hydrogen, carbon, gallium, nitrogen, and oxygen content. Atomic force microscopy and x-ray diffraction were used to study the sample morphology. From these measurements, the available surface area of the films was found to be sufficient for a significant proportion of the oxygen present in the films to segregate at the grain boundaries. This interpretation is consistent with earlier theoretical studies of the formation and segregation of the VGa-(ON)3 defect complex at dislocation sites in gallium-rich GaN. For this work, however, the defect complex is believed to segregate at the grain boundary of the polycrystalline GaN.
Mobile charge, soft breakdown, and self-healing in hydrogen silsesquioxane based intermetal dielectric
Authors: Devine R. A. B.
Published: August 2002

The electrical characteristics of hydrogen silsesquioxane based flowable oxide (FOx®) films proposed for interconnect isolation applications have been studied. It is demonstrated that negative and positive charges exist in the as-made, cured films with densities of 0.95×1012 and 1.5×1012 cm−2, respectively for thicknesses of 114 nm. The negative charges can be removed from the films by application of modest electric fields (positive or negative, ∼1.75 MV cm−1). The positive charge can be similarly displaced but not removed from the film; this results in time dependent relaxation and redistribution of the positive charge if the films are left unbiased. Time dependent irreversible evolution of the leakage current under positive and negative bias (∼3 MV cm−1) shows a slow breakdown phenomena. An unusual self-healing effect is evidenced in these films.
Characterization of finite-time Lyapunov exponents and vectors in two-dimensional turbulence
Authors: Lapeyre Guillaume
Published: August 2002

This paper discusses the application of Lyapunov theory in chaotic systems to the dynamics of tracer gradients in two-dimensional flows. The Lyapunov theory indicates that more attention should be given to the Lyapunov vector orientation. Moreover, the properties of Lyapunov vectors and exponents are explained in light of recent results on tracer gradients dynamics. Differences between the different Lyapunov vectors can be interpreted in terms of competition between the effects of effective rotation and strain. Also, the differences between backward and forward vectors give information on the local reversibility of the tracer gradient dynamics. A numerical simulation of two-dimensional turbulence serves to highlight these points and the spatial distribution of finite time Lyapunov exponents is also discussed in relation to stirring properties.
Hydrogarnet defect in chabazite and sodalite zeolites: A periodic Hartree–Fock and B3-LYP study
Authors: Pascale F. , Ugliengo P. , Civalleri B. , Orlando R. , D’Arco P. , Dovesi R.
Published: August 2002

The geometric, energetic, and vibrational features of the hydrogarnet defect in chabazite (CHA) and sodalite (SOD) have been studied at the Hartree-Fock and B3-LYP levels by using the periodic ab initio CRYSTAL code based on localized Gaussian basis functions. The geometry of the defective structures (CHA-HG and SOD-HG) has been fully optimized at both levels of theory by fixing the unit cell parameters to the values of the defect-free structures. The local structure of the defect is dictated by the hydrogen bond interaction among the OH groups, the strength of the shortest hydrogen bond in SOD-HG being 47 and 55 kJ/mol at the HF and B3-LYP levels, respectively. The reaction of CHA and SOD with gas phase water has been shown to be exothermic when either orthosilicic acid or α-quartz are formed as a product. On the contrary, the reactions are strongly endothermic with respect to liquid water. The vibrational spectra in the high-frequency OH stretching region for both SOD-HG and CHA-HG have been simulated using the ab initio harmonic OH stretching frequencies and compared to the experimental spectra of the mineral katoite and defective silicalite. Structure, geometry, hydrogen bond strength, defect formation energy, and OH stretching frequencies turn out to be significantly dependent on the adopted Hamiltonian for both CHA-HG and SOD-HG.
On the modulation of optical transmission spectra of thin dye layers by a supporting medium
Authors: Bortchagovsky E. G. , Fischer U. C.
Published: August 2002

This work is devoted to the analysis of the shape of transmission spectra of composite systems including a thin adsorbed layer of absorbing dye molecules. It is demonstrated, that different from the often observed absorption characteristics in the transmission spectra as determined by the imaginary part of the layer’s dielectric function, the transmission spectra are in general determined by coherent light scattering of the molecules in the layer and an interference of this scattered light with the incident light. As a result, the shape of the dispersion of both, imaginary and real parts of the layer’s dielectric function, determines the transmission spectrum. Such spectra cannot be constructed from the transmission spectra of the initial system and the deposited film alone. It is shown that the shape of the transmission spectra of the composite films is described in general by an asymmetrical line like a Fano line and that the origin of such a shape can be associated with the physics underlying a Fano resonance. It is shown experimentally and theoretically that the deposition of a thin absorbing layer can give rise to an increase in the transmission of an initial system.
Atomistic Monte Carlo simulation of strictly monodisperse long polyethylene melts through a generalized chain bridging algorithm
Authors: Karayiannis Nikos Ch. , Giannousaki Ageliki E. , Mavrantzas Vlasis G. , Theodorou Doros N.
Published: August 2002

This work is concerned with the atomistic simulation of the volumetric, conformational and structural properties of monodisperse polyethylene (PE) melts of molecular length ranging from C78 up to C1000. In the past, polydisperse models of these melts have been simulated in atomistic detail with the end-bridging Monte Carlo algorithm [Pant and Theodorou, Macromolecules 28, 7224 (1995); Mavrantzas et al., Macromolecules 32, 5072 (1999)]. In the present work, strictly monodisperse as well as polydisperse PE melts are simulated using the recently introduced double bridging and intramolecular double rebridging chain connectivity-altering Monte Carlo moves [Karayiannis et al., Phys. Rev. Lett. 88, 105503 (2002)]. These algorithms constitute generalizations of the EB move, since they entail the construction of two trimer bridges between two properly chosen pairs of dimers along the backbones of two different chains or along the same chain. In the simulations, a new molecular model is employed which is a hybrid of the united-atom TraPPE model [Martin and Siepmann, J. Phys. Chem. B 102, 2569 (1998)] and the anisotropic united-atom model [Toxvaerd, J. Chem. Phys. 107, 5197 (1997)]. Results are first presented documenting the efficiency of the algorithm in equilibrating long-chain PE melts and its dependence on chain length and polydispersity. Simulation data concerning the volumetric, conformational and structural properties of the monodisperse PE melts, obtained with the new simulation algorithm, are found to be in excellent agreement with available experimental data.
Structures, energetics, and spectra of OH−(H2O)n and SH−(H2O)n clusters, n=1–5: Ab initio study
Authors: Masamura M.
Published: August 2002

OH − ( H 2 O ) n and SH−(H2O)n (n=1–5) clusters have been studied using ab initio calculations. The relative stabilities of various isomers of SH−(H2O)n are quite different from those of the corresponding isomers of OH−(H2O)n (n=3–5) at 0 K, and are almost the same as those of the corresponding isomers of OH−(H2O)n at 298 K. That is, the relative stabilities of various isomers of SH−(H2O)n and OH−(H2O)n at 298 K are quite different from those at 0 K due to the entropy effect. The ionization potential, charge-transfer-to-solvent energy, and OH stretching vibrational spectra are reported to facilitate future experimental work.
Electrons weakly bound to hydrogen bonded clusters: A pseudopotential model including dispersion interactions
Authors: Šindelka Milan , Špirko Vladimı́r , Jungwirth Pavel
Published: August 2002

A pseudopotential model for the description of binding of an excess electron to polar clusters or molecules is presented. In addition to Coulomb, short range repulsion, and polarization interactions between the excess electron and the neutral core, the model also accounts for dispersion within a second order perturbation treatment. The pseudopotential, which should enable future dynamical calculations coupling the excess electron with nuclear motions, is successfully tested against accurate ab initio results for a whole set of geometries of hydrogen fluoride dimer anion. Predictions are made for an electron bound to a collinear hydrogen fluoride trimer for different values of the intermonomer separations. For the optimal and shorter values of this separation two bound states of the excess electron in (HF)3− are predicted to exist.
Symmetry-forcing procedure and convergence behavior of perturbation expansions for molecular interaction energies
Authors: Patkowski Konrad , Jeziorski Bogumil , Korona Tatiana , Szalewicz Krzysztof
Published: August 2002

Symmetry-adapted perturbation theory (SAPT) expansions corresponding to several symmetry-forcing procedures are applied through large order to study the interaction of lithium and hydrogen atoms. The interaction energies predicted by the perturbation theory are compared with the results obtained using the full configuration interaction (FCI) method. Since the ground state of the LiH molecule is submerged in the continuum of Pauli-forbidden states, these calculations are a demanding test for the SAPT approach in which the electrons from different monomers are treated as distinguishable particles. We show that if the symmetry is forced in a rather weak way, characteristic of the Murrell–Shaw–Musher–Amos theory, a divergent perturbation series is obtained. When the symmetry is forced in a strong way, as is done in the Eisenschitz–London–Hirschfelder–van der Avoird theory, one obtains a convergent series, but the interaction energy computed through any finite order exhibits wrong asymptotic behavior at large interatomic distances R. We show that by forcing the symmetry in an appropriate, intermediate way one obtains perturbation series which correctly predict leading terms in the 1/R asymptotic expansion of the interaction energy and, despite the presence of the Pauli-forbidden continuum, converge quickly to the FCI value of the interaction energy.
Rotationally resolved laser-induced fluorescence of biacetyl in A 1Au–X 1Ag
Authors: Huang Cheng-Liang , Liu Hon-Huei , Liu Chen-Lin , Kung A. H. , Ni Chi-Kung
Published: August 2002

Laser-induced fluorescence of jet-cooled biacetyl was recorded in the region of 22 177–22 277 cm−1 with a resolution of 0.012 cm−1. Three bands, 000, 1501, and 2101 with transitions J″⩽3 were observed and analyzed. For the 000 band, E2–E1 and G–G subbands were observed. However, only c-type transitions, ΔKa=odd, ΔKc=even, were observed. Another kind of transition, d-type (ΔKa=even, ΔKc=even), which is allowed by selection rules, was not observed. For the 1501 band, two subbands E3–E3 and G–G were observed. The E3–E3 subband can be described by the rigid rotor model, and it contains only ΔKa=odd, ΔKc=even transitions. On the other hand, the G–G subband cannot be even approximately described by the rigid rotor model. It is dominated by c-type transitions. In addition, a/b-type and d-type transitions were all observed, though the intensities are small. For the 2101 band, E2–E1, G–G, and A3–A1 subbands were observed. The E2–E1, G–G subbands contain all the possible types of transitions, i.e., a/b-and c/d-type transitions. However, the A3–A1 subband has ΔKa=odd, ΔKc=even transitions only. The band origin, rotational constants, and the splitting due to the tunneling in these three bands was accurately measured. Relatively large vibronic transition intensities were observed in both the 1501 and 2101 bands.
An ab initio based model for the simulation of multiple P2 atoms embedded in a cluster of spherical ligands, with application to Al in solid para-hydrogen
Authors: Wang Qian , Alexander Millard H. , Krumrine Jennifer R.
Published: August 2002

We present here a novel approach to the determination of the interaction between two atoms, each in a P2 electronic state, embedded in a cluster of spherical atoms. The model requires accurate ab initio potential energy curves for the M2 system, for all the 36 electronic states which correlate with dissociation into ground-state M(2P) atoms. Consequently, making use of a valence-bond-like model, we transform these 36 molecular orbital states into a set of 36 Cartesian (qaqb) states which correspond to assigning the two p electrons to Cartesian orbitals centered on either atom. It is then easy to use the earlier Balling and Wright model [L. C. Balling and J. J. Wright, J. Chem. Phys. 79, 2941 (1983)] to determine, in this 36 state basis, the matrix elements corresponding to the interaction of each P2 atom with any number of surrounding spherical ligands. The lowest eigenvalue of the resulting 36×36 matrix defines, in an adiabatic approximation, the potential governing the motion of the atoms. We apply this approach to the determination of the interaction of two Al atoms embedded in solid pH2, site-substituted in the center of two adjacent hexagons. We find the interaction between the two Al atoms to be significantly modified by the presence of the intervening pH2 molecules.
A double Rydberg anion with a hydrogen bond and a solvated double Rydberg anion: Interpretation of the photoelectron spectrum of N2H7−
Authors: Ortiz J. V.
Published: September 2002

A double Rydberg anion has two electrons in diffuse orbitals that are bound by a closed-shell, cationic core. Low-energy features in the recently reported photoelectron spectrum of N2H7− are assigned to double Rydberg anions on the basis of electron propagator calculations employing Brueckner doubles, coupled-cluster reference states. The lowest electron detachment energy, 0.415 eV, corresponds to an initial state consisting of a hydrogen-bridged N2H7+ core and two diffuse electrons. A feature at slightly higher energy, 0.578 eV, belongs to a complex that comprises a tetrahedral NH4− double Rydberg anion and an ammonia solvent molecule. The most intense peak in the photoelectron spectrum, which occurs at 1.460 eV, pertains to a complex with a hydride anion and two ammonia solvent molecules. Plots of Dyson orbitals associated with electron detachment energies facilitate qualitative interpretation of electronic structure in the anions and in the neutral final states. Vibrational structure associated with each of these features has been interpreted as well. Previous assignments of electron detachment energies to the hydride–ammonia and tetrahedral isomers of NH4− have been confirmed with the present methods. Vibrationally excited final states have been assigned for this spectrum also.
Reassignment of the O2 spectrum just below dissociation threshold based on ab initio calculations
Authors: van Vroonhoven Mirjam C. G. N. , Groenenboom Gerrit C.
Published: August 2002

Vibrational Herzberg bands of the O2 molecule just below its first O(3P)+O(3P) dissociation limit are since long-known to be perturbed. Jenouvrier et al. [J. Mol. Spectrosc. 198, 136 (1999)] assigned the cause of the perturbations to five vibrational levels supported by the shallow minimum in the 1 3Πu potential energy curve around 5.5a0. Using ab initio potential energy curves and spin–orbit couplings from previous work [J. Chem. Phys. 116, 1954 (2002)] we present a full quantum calculation of all ungerade rotation–vibration–electronic states of oxygen just below the dissociation threshold, through a total angular momentum quantum number of J=19. This calculation shows that the original assignment, based on a Hund’s case (a) model of a regular 1 3Πu multiplet was not correct. Based on our calculation we present a new assignment of the perturbing states: 1 3Πu,Ω=2(v=0), 1 3Πu,1(0), 1 3Πu,2(1), 1 3Πu,1(1), and 1 3Πu,0−(0) in order of ascending term values. We show the new assignment to be consistent with experimental data and we also propose new spectroscopic parameters for the perturbing states.
Error field locked modes thresholds in rotating plasmas, anomalous braking and spin-up
Authors: Lazzaro E. , Buttery R. J. , Hender T. C. , Zanca P. , Fitzpatrick R. , Bigi M. , Bolzonella T. , Coelho R. , DeBenedetti M. , Nowak S. , Sauter O. , Stamp M. ,   Contributors to the EFDA-JET work programme
Published: August 2002

The mechanisms of nonlinear interaction of external helical fields with a rotating plasma are investigated analyzing the results of recent systematic experiments on the Joint European Torus (JET) [A. Gibson et al., Phys. Plasmas 5, 1839 (1998)] that widen the previous data base collected on Compass-D [T. C. Hender et al., Nucl. Fusion 32, 2091 (1992)], Doublet III-D [R. J. La Haye et al., Nucl. Fusion 32, 2119 (1992)] and JET. The empirical scaling laws governing the onset of “error field” locked modes are re-assessed and interpreted in terms of existing driven reconnection theories and with new models. In particular the important mechanisms of plasma rotation braking, and spin up, associated with error fields are analyzed in detail and interpreted.
Model of the plasma discharge in a Hall thruster with heat conduction
Authors: Ahedo E. , Gallardo J. M. , Martı́nez-Sánchez M.
Published: August 2002

The inclusion of heat conduction into a one-dimensional, macroscopic model of the plasma inside a Hall thruster and in the near plume is found to smooth the temperature profile of previous solutions with a nonconductive model. The spatial structure still consists of reverse-flow, ionization, and acceleration regions. Conductive energy flow, being of the same order of convective flow, has significant effects on the rear part of the channel where it can make impossible the establishment of a steady anode sheath. As a result, there is an upper bound on the plasma reverse flow for the existence of stationary solutions. The analysis of inertial effects on the electron dynamics concludes that the main contribution is the azimuthal electron motion, which can produce extra collisionality, mainly in the near plume. The different contributions to the effective axial diffusion of electrons and the ion temperature are evaluated. A parametric investigation yields the basic scaling laws of the thruster stationary performance.
Effective screened potentials of strongly coupled semiclassical plasma
Authors: Ramazanov T. S. , Dzhumagulova K. N.
Published: August 2002

The pseudopotentials of particle interaction, taking into account both quantum-mechanical effects of diffraction at short distances, and also screening field effects at large distances are obtained for a strongly coupled semiclassical plasma. The limiting cases of potentials are considered.
Comparison of a density functional theory and a Hartree treatment of silicon quantum dot
Authors: Sée Johann , Dollfus Philippe , Galdin Sylvie
Published: August 2002

This article presents a self-consistent Poisson–Schrödinger calculation based on either the Hartree or the density functional theory for silicon quantum dots surrounded by silicon dioxide. These models can treat any shape of confining potential and take into account the different effective masses and permittivities of the materials. The energy states and the equivalent capacitance of a single dot are then determined as a function of dot radius and number of confined electrons. Finally, the Hartree approach and the density functional theory are compared.