bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron-impact detachment and dissociation of C4− ions
Authors: Le Padellec A. , Rabilloud F. , Pegg D. , Neau A. , Hellberg F. , Thomas R. , Schmidt H. T. , Larsson M. , Danared H. , Källberg A. , Andersson K. , Hanstorp D.
Published: July 2002

CRYRING was used to study collision processes between an electron and a negative ion cluster C4−. The total detachment cross sections for the production of the neutral 4C, 3C, 2C, and C fragments were measured. The cross sections for pure detachment, and for detachment plus dissociation leading to the production of C3+C, 2C2, and C2+2C were extracted using a grid. It was found that the pure detachment process overwhelmingly dominates all other fragmentation processes. The threshold location for the detachment channel is found to be around 6.0 eV. Although the doubly charged negative ion C42− has received little previous attention, a defined near-threshold resonance observed in the detachment cross section curve, has been associated with the short-lived state C42− (0.7 fs lifetime).
Nonlinear magnetohydrodynamical effects in precessional fishbone oscillations
Authors: Ödblom A. , Breizman B. N. , Sharapov S. E. , Hender T. C. , Pastukhov V. P.
Published: July 2002

The role of magnetohydrodynamic nonlinearities in precessional m=n=1 fishbone oscillations has been analyzed analytically and numerically. The work is based on the reduced magnetohydrodynamic (MHD) equations including a linear energetic particle drive model. When the energetic particle pressure is close to the instability threshold, the top-hat linear eigenmode profile of the ideal MHD m=1 radial displacement splits up into a two-step structure around the q=1 flux surface, due to the finite frequency ω of the mode. The width of the individual steps is a factor γ/ω smaller than the distance between them, where γ is the growth rate of the mode. We find that the MHD nonlinearities modify the mode structure further, and produce explosive nonlinear growth, accompanied by frequency chirping, for modes that are near the instability threshold. The results are quite different for fishbone oscillations that are excited well above the stability threshold. The growth rates of these linearly fast growing modes decreases nonlinearly and the MHD nonlinearities are stabilizing in this limit. The nonlinear MHD effects are important when the plasma displacement is comparable to, or larger than, the scale length of the fishbone structure.
Volume-elements of integration: A geometric algebra approach
Authors: Pesonen Janne , Halonen Lauri
Published: July 2002

In this work, geometric algebra is applied to obtain the volume-element of integration for the 3 Cartesian coordinates of the center-of-mass, 3 Euler angles, and 3N−6 shape coordinates needed to describe the position, orientation, and shape of an N-atomic molecule. The volume-element is obtained as a product of N volume-elements, each associated with a set of three coordinates. The method presented has several advantages. For example, one does not need to expand any determinants, and all calculations are performed in the three-dimensional physical space (not in some 3N-dimensional abstract configuration space). Several examples and applications are given.
Response to “Comment on ‘Diffusion measurements with the pulsed gradient nonlinear spin echo method’ ” [J. Chem. Phys. 116, 1204 (2002)]
Authors: Ardelean Ioan , Kimmich Rainer
Published: July 2002

The above comment (in the following called “the Comment”) questions our recent paper on diffusion measurements with nonlinear spin echoes.1 These doubts are unfounded and may even be interpreted in a misleading way. There is no unjustified or “undocumented” approximation in our papers on diffusion measurements with nonlinear spin echoes.
Comparison of simulation codes for the beam dynamics of low-energy ions
Authors: Hanke K. , Heising S. , Probert G. , Scrivens R.
Published: July 2002

Results are presented of the simulation of low-energy ion dynamics with three different codes: KOBRA, IGUN, and CPO. To share work between different codes one needs to be confident that they agree. For test cases, good agreement was found between the codes and analytical solutions. Where possible, results have been compared to experimental data from the CERN laser ion source. These simulations of the real beam line are in general agreement with the data.
Shunting arc plasma generation over a wide range of ambient pressure and ion extraction
Authors: Yukimura Ken , Masamune Sadao
Published: July 2002

This article describes the electrical characteristics of a shunting arc plasma as well as the ion extraction to a target to test the shunting arc as a pulsed ion source for plasma-based ion implantation. The shunting arc plasma is formed around a rod which is connected to a capacitor and contains the ion species of rod materials. A 20 μF capacitor with charging voltage from 1.0 to 2.0 kV was used to ignite and sustain the shunting arc plasma around a 4-mm-long carbon rod of 2 mm in diameter. It has been found that the shunting are could be ignited over a wide range of ambient pressure from 1.3 mPa to 0.1 MPa in a single device without changing the rod size or capacitance. Ions could be extracted from the plasma by applying a negative voltage of −1 kV to a target set 150 mm away from the rod. It has thus been confirmed that the shunting arc would be useful as a pulsed ion source for solid-state materials for plasma-based ion implantation.
Simultaneous phase control of Li2 wave packets in two electronic states
Authors: Stauffer Hans U. , Ballard Joshua B. , Amitay Zohar , Leone Stephen R.
Published: July 2002

State-selective phase control of rotational Li2 wave packets, prepared simultaneously in the E(1∑g+) electronic state by one photon absorption and the A(1∑u+) electronic state by resonant impulsive stimulated Raman scattering, is demonstrated. Following the initial population of a rovibrational launch state on the A electronic potential energy curve with a cw laser, a single sub-picosecond wave packet preparation pulse centered near 800 nm simultaneously creates a two-state rotational wave packet in the E state (νE=18, JE=23 and 25) and a three-state rotational wave packet in the A state (νA=15, JA=22, 24, and 26). A temporally delayed 800 nm probe pulse subsequently ionizes both electronic components of the wave packet to allow measurement of the time-dependent coherence in these two electronic states. Via phase manipulation of resonant transition frequencies contained within the preparation pulse, the phases of the E(18,25) and A(15,26) quantum states are either varied concurrently or individually controlled, whereas the phases of the other rovibronic states of the wave packet are in all cases held essentially constant. This phase manipulation is shown to be more complex than a simple additive effect involving the phases applied to the resonant frequencies. These experimental results are compared with the predictions of second order time-dependent perturbation theory. Although systematic discrepancies exist, most likely due to an additional phase introduced during the two-photon probe process, once these differences are accounted for, good agreement is found between experiment and perturbation theory.
Angle-resolved energy distributions of He ions produced by backscattering of metastable He at low work function metal surface
Authors: Wada M. , Sasao M. , Nishiura M. , Yamaoka H. , Matsumoto Y. , Shinto K.
Published: July 2002

An apparatus capable of measuring the angle-resolved energy distribution function of ions produced by backscattering at metal surfaces is designed and tested. To exclude noise due to other impurity ions and electrons from the signal of target ions, a water-cooled magnetic deflection-type momentum analyzer is installed on a rotating table in a vacuum chamber. In addition to the beam forming and beam diagnostics components, work function monitoring equipment and an ion source for surface cleaning are installed. Preliminary tests of the apparatus was conducted to measure He+ emission from neutral He beam injection onto a Mo surface. With an improvement of the signal-to-noise ratio, the apparatus can be used to measure the angle-resolved energy distribution function of He− produced at a metal surface by He beam injection.
Measurement of velocity distribution functions of neutral particles in ion source plasma
Authors: Kitatani K. , Kasuya T. , Nakayama T. , Yamamoto K. , Wada M.
Published: July 2002

The velocity distribution function of the gas effusing from a 5.5-cm-diam, 5-cm-long small ion source was measured by the mass separated time-of-flight method. The measured distribution function could be fitted to a Maxwellian distribution before the commencement of a discharge, when the deuterium pressure in the ion source was below 0.6 Pa. The effect of discharge upon the distribution function of neutral deuterium below this operating pressure was investigated by changing the arc power. The deuterium gas temperature determined by fitting the measured distribution function to a Maxwellian showed a maximum gas temperature of 800 K at discharge power density of 6 W/cm3. Effects of discharge upon argon and helium gas temperature were also measured, which showed a monotonic increase against the discharge power.
First plasma analysis of the CEA/Saclay ECR hydrogen negative ion source
Authors: Gobin R. , Beauvais P.-Y. , Benmeziane K. , Delferrière O. , Ferdinand R. , Harrault F. , Lagniel J.-M. , Sherman J.
Published: July 2002

Reliable high intensity H− ion source development is now part of the CEA/Saclay work in the field of high intensity linear accelerators. A 2.45 GHz electron cyclotron resonance (ECR) H− ion source and test bench have been built. This new source has been designed taking into account our experience on the French high intensity ECR proton source. For H− ion production, the high-energy electrons created in the ECR zone are trapped by a dipole magnetic filter. A rectangular 210 mm long plasma chamber and an intermediate iron shield are used to minimize the magnetic field in the extraction region. A second magnetic dipole separates electrons and negative ions in a 10 kV extraction system. To reduce the electron/H− ratio, the plasma electrode is biased by a power supply. The first helium plasma allowed us to verify the satisfactory electron separator behavior. Pulsed hydrogen plasma is currently produced. The first plasma characterization is under progress as a function of ion source parameters by using Langmuir probes and optical spectrometer. The first results are presented and possible evolutions toward a higher efficiency source will be discussed.
Thermal equilibrium among ion species into ECR plasma
Authors: Cavenago M.
Published: July 2002

The ion cooling exerted by lighter ions inside an ECR plasma is often credited as an explanation of the well-known gas mixing effect, since the ion temperature is about a constant Tp for all the ion species in the plasma. Here the energy balance equations are written in detail for any ion species, including the improved evaporation cooling in the collisional confinement regime. A formula for Tp and decoupled equations for small corrections around Tp are given, accounting for the colder gas input. The charge state distribution are written in closed form. Gas flow between plasma center and periphery is described. Some equilibria for most common gas mixing cases are compared, showing in particular the efficacy of neon.
High-temperature ion source for elements of platinum-palladium group
Authors: Belokurov A. P. , Kousmine R. N. , Martynov M. I. , Sokolov D. V.
Published: July 2002

This paper is devoted to research and development of an ion source which is able to work by the cw-operation for a long time at the gas-discharge chamber temperature of ∼1500 °C and higher. The problem of discharge chamber heat is solved with the help of special graphite heaters. The method of heat is combined: heat radiation and electronic heating. As a result it was possible to create a palladium ion source with the current density up to 60 mA/sm2 which operates in a steady-state condition of more than 100 h.
Ion sources for radioactive ion beams in ISOL facilities (plenary)
Authors: Schmor P. W.
Published: July 2002

Research requiring radioactive ion beams (RIBs) are rapidly expanding worldwide. On-line-isotope-separation RIB facilities, in particular, present a unique and challenging environment for ion source operation. Ion sources are required to ionize short-lived isotopes, in a very high radiation environment, efficiently, rapidly, reliably, and selectively so that beams with small emittances can be formed, mass separated, and transported to experiments and accelerators. Various types of ion sources have been constructed and modified to satisfy the scientific and technical requirements. This paper reviews some of the ion source challenges and solutions.
Development of high current density surface ionization sources for heavy ion fusion applications
Authors: Chacon-Golcher E. , Baca D. , Kwan J. W.
Published: July 2002

Results are presented on the maximum current density obtained in pulsed (2 μs) contact ionization sources that utilize potassium and cesium atoms on porous tungsten substrates. Maximum values of 89 and 48 mA/cm2 for K+ and Cs+, respectively, were obtained in space charge limited mode for the source geometry used. Similar results were observed for potassium aluminosilicate (K20⋅Al2O3⋅4SiO2) sources. Neutral emission rates were measured in the temperature range of interest to heavy ion fusion (1000–1150 °C). Evidence is presented regarding a diffusion-limited flow of alkali atoms towards the emitting surface, which would be a determining factor in the source lifetime.
Dark soliton generation for the intermediate nonlinear Schrödinger equation
Authors: Matsuno Yoshimasa
Published: July 2002

The generation of dark solitons due to a small change of the initial data is studied within the framework of the intermediate nonlinear Schrödinger (INLS) equation. In particular, we analyze the spectral problem associated with the INLS equation when the potential consists of a small perturbation imposed on a constant background. We derive a criterion for the perturbation to generate a pair of new discrete eigenvalues as well as their explicit expressions in terms of the perturbation. In addition, we demonstrate that the eigenvalues appear without a threshold on the magnitude of the perturbation. We also consider both the shallow- and deep-water limits of various results obtained for the INLS spectral problem. In the former case, the limiting procedure can be performed smoothly whereas in the latter case, the spectral equation exhibits a new feature of bound states, showing that the eigenvalue is exponentially small compared with the magnitude of the perturbation.
H
−
ion source test and development capabilities at ISIS
Authors: Thomason J. W. G. , Sidlow R. , Whitehead M. O.
Published: July 2002

The ISIS ion source is a surface plasma ion source of the Penning type, and routinely produces 35 mA of H− ions during a 200 μs pulse at 50 Hz for uninterrupted periods of up to 50 days. However, because of the constant demands on ISIS from the neutron user community, very little ion source development has ever been possible. It is now necessary to produce sources with enhanced performances for next generation projects such as the European spallation source. Hence a dedicated ion source development rig (ISDR) has been constructed at Rutherford Appleton Laboratory to fully characterize the ISIS ion source and then facilitate appropriate development work. The ISDR has been designed to replicate the beam transport configurations on both the present ISIS preinjector and the proposed ISIS radio frequency quadrupole, while providing additional beam diagnostics equipment. The commissioning of the ISDR will be described, and initial results presented, along with future development plans.
Particle trapping and annihilation within the extraction system of ion sources
Authors: Kellerman Peter L. , Benveniste Victor , Bradley Michael P. , Saadatmand Kourosh
Published: July 2002

Many applications of ion sources (e.g., ion implantation in the semiconductor industry) are very sensitive to particles and have tight specifications on allowable particle number and size. Among the sources of particles are the ion source itself (due to either nucleation in the plasma, or ion bombardment of the surfaces), and the extraction electrodes (due to ion bombardment). This article investigates the processes to which such particles are subjected during their flight through the extraction electrodes. They travel at much lower velocity than the accelerated ions due to their much larger mass, and so are bombarded by these increasingly energetic ions. The processes considered during the trajectory of the particle are: charging, acceleration in the electrode fields, entrapment within the suppression gap, heating from ion bombardment, radiation cooling, melting, vaporization, and Coulomb explosion. These processes are all modeled simultaneously as the trajectory of the particle is followed. A general condition for particle trapping is developed, and it is found that in the case of silicon particles, these particles can be annihilated by a combination of Coulomb disintegration and vaporization.
Three-dimensional investigations of electrical barriers using electron beam induced current measurements
Authors: Gaevski Mikhail , Elfwing Mattias , Olsson Eva , Kvist Anders
Published: July 2002

3D-investigations of electrically active interfaces in ZnO varistor materials have been performed using electron beam induced current (EBIC) contrast in the scanning electron microscope in combination with the focused ion beam workstation (FIB). The EBIC measurements were performed with spatial resolution better than 100 nm and the depth dependence of the signal was studied by variation of the acceleration voltage. The FIB was used for characterization of the subsurface geometry of electrically active grain boundaries. Experimental EBIC profiles, obtained by scanning the electron beam across grain boundaries, were compared with computer simulations using three different models for the electron–hole generation function. The EBIC contrast has been found to depend strongly on the geometric properties, i.e., the tilt of the grain boundary, and on asymmetries in the depletion region at the boundary. Calculations of the EBIC contrast taking these two effects into account show good agreement with experimental data. The “hot electron effect” close to the electrical breakdown voltage has been experimentally observed. The possibility to study asymmetry in the depletion region and the influence of the applied bias on the electrical properties of individual interfaces using the EBIC technique has been demonstrated.
Bandstructure modulation for carbon nanotubes in a uniform electric field
Authors: O’Keeffe James , Wei Chengyu , Cho Kyeongjae
Published: July 2002

A method to electronically modulate the energy gap and bandstructure of semiconducting carbon nanotubes is proposed. We investigate this bandstructure modulation mechanism using tight-binding and density functional theory (DFT). Results show that the energy gap of a semiconducting nanotube can be narrowed, when the tube is placed in an electric field perpendicular to the tube axis. In contrast, Metallic tubes were found to exhibit a screening behavior, whereby free charge redistributes about the tube circumference as a result of the external field. In this case, the bandstructure shows little perturbation in response to an applied electric field.
High-energy heavy ion cocktail beams at the 88 Inch Cyclotron (abstract)
Authors: McMahan M. A. , Argento D. , Gimpel T. , Guy A. , Morel J. , Osborne K. , Siero C. R. , Thatcher R. K. , Wutte D. , Lyneis C.
Published: July 2002

The combination of cyclotron and electron cyclotron resonance (ECR) ion source provides the ability to accelerate “cocktails” of ions, mixtures of ions of near-identical charge-to-mass ratio. This concept was developed soon after the first ECR ion source became operational at the 88 Inch Cyclotron and has become a powerful tool in the field of heavy ion radiation effects testing. The standard 4.5 MeV/nuc cocktail at a charge-to-mass ratio of 0.2 contains ions from B2+ to Bi41+. Copper and cobalt are provided by the direct insertion method, B is produced utilizing the MIVOC technique and Bi is produced with the oven technique. Recently, following upgrades to the AECR and the cyclotron vacuum system, a new high-energy heavy cocktail beam has been developed. This cocktail at a charge-to-mass ration of 0.27 using 400 MeV Ar11+ as a “pilot” beam, contains ions from B3+ to Xe38+ and provides accelerated ions with a range of 100 μm in Si, of great advantage for some of the newer-generation microelectronics. It has joined the 4.5 MeV/u heavy ion cocktail and the 32.5 MeV/u light ion cocktail as standards in the cyclotron’s cocktail repertoire.
Modified electron cyclotron resonance source design for He3 accelerator mass spectroscopy (abstract)
Authors: Pardo R. C. , Moehs D. P. , Scott R. H. , Vondrasek R. C.
Published: July 2002

The 10 GHz ECR-I ion source at ATLAS is being modified to create a small inner volume within the main plasma chamber to reduce the residual helium background gas load. This effort is part of an experiment to measure the residual concentration of He3 in highly isotopically enriched He3 samples. The modification consists of a new extractor electrode on which is integrally mounted a 2.5-cm-diam quartz tube of approximately 8 cm length. Energizing just the extraction coil produces an ECR only inside the quartz volume. The quartz tube is pumped only through the 1-mm-diam extraction hole. rf power is provided by propagation through the main plasma tank acting as a multimode waveguide. The source must operate in the pressure regime of 10–200 mTorr in order to achieve sufficient sensitivity to measure a He3/4He ratio of approximately 1×10−15. Results of these tests and the operation of the source in this highly unusual mode will be presented.
Semiclassics in the lowest Landau band
Authors: Hainzl Christian
Published: July 2002

This article deals with the comparison between the strong Thomas–Fermi theory and the quantum mechanical ground state energy of a large atom confined to lowest Landau band wave functions. Using the tools of microlocal semiclassical spectral asymptotics we derive precise error estimates. The approach presented in this article suggests the definition of a modified strong Thomas–Fermi functional, where the main modification consists in replacing the integration over the variables perpendicular to the magnetic field by an expansion in angular momentum eigenfunctions. The resulting DSTF theory is studied in detail in the second part of the article.
Lattice models of protein folding permitting disordered native states
Authors: Crippen Gordon M. , Chhajer Mukesh
Published: July 2002

Self-avoiding lattice walks are often used as minimalist models of proteins. Typically, the polypeptide chain is represented as a lattice walk with each amino acid residue lying on a lattice point, and the Hamiltonian being a sum of interactions between pairs of sequentially nonadjacent residues on adjacent points. Interactions depend on the types of the two residues, and there are usually two or more types. A sequence is said to fold to a particular “native” conformation if the ground state is nondegenerate, i.e., that native conformation is the unique global energy minimum conformation. However, real proteins have some flexibility in the folded state. If this is permitted in a lattice model, the most stably and cooperatively folding sequences have very disordered native states unless the Hamiltonian either favors only a few specific interactions or includes a solvation term. The result points the way toward qualitatively more realistic lattice models for protein folding.
Ferromagnetic resonance and exchange anisotropy in ferromagnetic/antiferromagnetic bilayers
Authors: Hu Jing-guo , Jin Guo-jun , Ma Yu-qiang
Published: July 2002

In this article, we study the in-plane ferromagnetic resonance (FMR) and exchange anisotropy in a ferromagnetic (FM)/antiferromagnetic (AFM) bilayer. In the semi-infinite limit for the AFM layer, the equilibrium energy for the system is established, and then analytical expressions are derived for FMR frequency, frequency linewidth, exchange anisotropy constant, effective exchange bias, and pinning angle. Three physical parameters are important, i.e., the classical exchange bias, the interface turning coefficient, and the applied field angle. Combining the formulas derived with numerical calculation, it is shown that there are two branches of FMR modes distinguished by critical fields which depend on the interface turning the coefficient and the applied field angle. We find that the effective exchange bias depends on the applied field angle but not on its magnitude. A comparison with previously reported theoretical results is made, and experimental measurements for physical parameters are suggested.
Maximization of capacity and lp norms for some product channels
Authors: King Christopher
Published: July 2002

It is conjectured that the Holevo capacity of a product channel Ω⊗Φ is achieved when product states are used as input. Amosov, Holevo, and Werner have also conjectured that the maximal lp norm of a product channel is achieved with product input states. In this article we establish both of these conjectures in the case that Ω is arbitrary and Φ is a CQ or QC channel (as defined by Holevo). We also establish the Amosov, Holevo and Werner conjecture when Ω is arbitrary and either Φ is a qubit channel and p=2, or Φ is a unital qubit channel and p is integer. Our proofs involve a new conjecture for the norm of an output state of the half-noisy channel I⊗Φ, when Φ is a qubit channel. We show that this conjecture in some cases also implies additivity of the Holevo capacity.
Production of carbon clusters by laser ablation of polymers in vacuum
Authors: Shibagaki K. , Takada N. , Sasaki K. , Kadota K.
Published: July 2002

Time-of-flight mass spectrometry was used for analyzing ionic species produced by Nd:YAG laser ablation of polymers [polypropylene (PP), polyethylene (PE), and polytetrafluoroethylene] in vacuum. The temporal variation of the mass distribution after the laser irradiation was investigated. The target polymers were decomposed to almost atomic species by the laser irradiation. After the laser irradiation, significant temporal evolution of carbon cluster ions (CnHm+, CnHm−, and CnFm−) was observed. This result indicates that the cluster ions are produced from atomic species via gas-phase reactions in the ablation plume with no ambient gases. The mass spectra of Cn− and CnHm− obtained from the PP and PE targets suggest that even carbon clusters (C2k) are hydrogenated more efficiently than odd ones (C2k+1) to produce C2kHm.
Time-resolved reflection surface x-ray diffraction
Authors: Hong Hawoong , Wu Z. , Chiang T.-C. , Zschack P. , Chen Haydn
Published: July 2002

Methods to collect two-dimensional time-resolved x-ray diffraction patterns from surfaces/interfaces were developed. Reflection surface x-ray diffraction utilizing high brilliance x rays and a charge coupled device can achieve a time resolution as good as one second. Also, two-dimensional maps of reflectivity rocking curves can be recorded fast enough to monitor growth processes. These methods were demonstrated for the study of Ag and Pb films on Si (111)−(7×7) surfaces.
Active compensation in combination with weak passive shielding for magnetocardiographic measurements
Authors: Kuriki S. , Hayashi A. , Washio T. , Fujita M.
Published: July 2002

We studied different schemes of noise reduction using active compensation of environmental magnetic field noises in order to develop a high-temperature-superconductor (HTS) magnetometer system for the measurement of magnetocardiographic signals. The active compensation was combined with weak passive shielding of about 20 dB by the use of a magnetically shielded room (MSR) having a single layer of surrounding μ-metal. A novel method using a normal detection coil and compensation coils that were wound around the walls of the MSR to enable magnetic coupling was examined. Effective suppression of environmental field noises of more than 20 dB at 0.5–10 Hz and 10 dB at 10–100 Hz, covering the low-frequency range of biological signals, was obtained. In an alternative method using a reference HTS magnetometer in the MSR for noise detection, a compensation coil was set in the MSR and served as a feedback coil in operating the magnetometer. Residual field noises that were limited by the intrinsic noise of the reference magnetometer were obtained. Using such active compensation and weak passive shielding, measurement of magnetocardiogram was possible with a sensing HTS magnetometer in a wide frequency range of 0.5–100 Hz.
A vacuum-ultraviolet and x-ray linear camera for high-temperature plasmas
Authors: Baciero A. , Zurro B. , McCarthy K. J. , Martı́n P. , de la Fuente M. C. , Rodrı́guez-Barquero L.
Published: July 2002

We describe a prototype linear vacuum-ultraviolet x-ray pinhole camera based on the luminescence of a phosphor screen working in reflection mode. The luminescence emitted by the phosphor, in response to radiation selected by a broadband filter, is detected using a 1024 pixel intensified linear array detector. By operating this detector with integration times from 20 to 50 ms, random plasma fluctuations can be smeared out, so that structures related to the magnetic topology can be searched for.
Generation of semiclassical and delocalized vibrational wave packet motion of HF molecules oriented in an external static electric field
Authors: Hervé Stephen , Le Quéré Frédéric , Marquardt Roberto
Published: July 2002

The quantum dynamics of the HF molecule is investigated in the presence of a static electric field and coherent infrared radiation. The time dependent dynamics is induced by infrared multiphoton excitation and the time evolution of the rovibrational wave packet is calculated in configuration space. Results are given for static field strengths needed to orient molecules such that their internal dynamics can be described by simplified, purely vibrational models. For highly oriented HF molecules, time intervals of approximate duration of 70 fs occur repeatedly during and after the excitation in which the reduced vibrational wave packet motion is nearly semiclassical (“semiclassical windows”). The occurrence of these time intervals can be made more regular after the excitation, if the pulse duration is chosen adequately.
Electromagnetic vortices in electron-positron-ion plasmas with shear flow
Authors: Haque Q. , Saleem H. , Vranješ J.
Published: July 2002

Two coupled nonlinear equations for a perturbed electromagnetic field in an electron-positron-ion plasma with shear flow, embedded in a nonuniform magnetic field, are derived and solved analytically. The nonlinear solutions of these equations are found in the form of tripolar vortices. The linear instability condition is also discussed. It is found that the presence of ions can play a significant role in nonlinear dynamics, even if the density of ions is of the order of 10−3 as compared to the densities of the lighter species.
Intrachain exciton quenching analysis in conjugated polymers by two-photon spectroscopy
Authors: Schroeder R. , Graupner W. , Scherf U. , Ullrich B.
Published: July 2002

Intrachain exciton quenching in ladder-type polyparaphenylene has been quantified for the first time. Two-phonon absorption and subsequent upconverted photoluminescence of ladder-type polyparaphenylene and polyfluorene has been measured and analyzed with a new theoretical framework for the two-photon absorption saturation. This microscopic analysis determines the number of singlet excitons as a function of the excitation intensity, which is compared to the photoluminescence intensity. The absorption saturation alone perfectly describes the photoluminescence saturation in polyfluorene, whereas in ladder-type polyparaphenylene, exciton–exciton quenching occurs. By measuring in solution, the formation of stacked polymer chains is prevented. Therefore, the quenching is due to intrachain interaction.
Polarized edge-incident photovoltage spectroscopy and reflectance characterization of a GaAs/GaAlAs vertical-cavity surface-emitting laser structure
Authors: Liang J. S. , Wang S. D. , Huang Y. S. , Tien C. W. , Chang Y. M. , Chen C. W. , Li N. Y. , Lin D. Y. , Pollak Fred H.
Published: July 2002

Polarized edge-incident photovoltage spectroscopy (EPVS) and normal-incident reflectance (NIR) studies are performed at room temperature on a bare as-grown wafer of GaAs/GaAlAs-based vertical-cavity surface-emitting laser (VCSEL) structure, designed for emitting at a wavelength near 850 nm. In addition to the interference features related to the properties of the mirrors stacks, in the NIR spectrum only the Fabry–Pérot cavity mode is clearly visible, while the polarized EPV spectra are used to identify the wavelength of fundamental and higher-order transitions from the quantum wells (QWs) in the active region. By comparing the experimental results with theoretical calculations, we are able to ascertain the Al composition, structure parameters, and material quality of the QWs in the VCSEL active regions. We demonstrate the potential of polarized EPVS for the contactless and nondestructive characterization of VCSELs at room temperature.
On the structure of covariant phase observables
Authors: Pellonpää Juha-Pekka
Published: July 2002

We study the mathematical structure of covariant phase observables. Such observables can alternatively be expressed as phase matrices, as sequences of unit vectors, as sequences of phase states, or as equivalence classes of covariant trace-preserving operations. Covariant generalized operator measures are defined by structure matrices which form a W*-algebra with phase matrices as its subset. The properties of the Radon–Nikodým derivatives of phase probability measures are studied.
Dynamical benchmarks of the nucleation kinetics of water
Authors: Schenter G. K. , Kathmann S. M. , Garrett B. C.
Published: July 2002

Recently a theory of vapor-to-liquid phase nucleation was developed based on the kinetics of cluster formation and decomposition. The new method used variational transition state theory (VTST) to obtain the evaporation and condensation rate constants needed in the kinetic model of nucleation. VTST provides a means to systematically improve estimates of rate constants involved in the nucleation process. In the current work, we perform dynamical simulations of the condensation process, estimating the effective reactive cross section using a definition of a cluster that is determined from VTST. These calculations allow us to characterize dynamical corrections to the VTST rate constants. We find that for water cluster sizes ranging from 10–40 waters, VTST estimates of the condensation and evaporation rate constants using a spherical dividing surface require dynamical corrections that are approximately a factor of two.
Catalysis on microstructured bimetallic surfaces
Authors: Imbihl R.
Published: July 2002

Microstructured bimetallic Pt/Rh and Pt/Ti surfaces have been employed to study the dynamics of catalytic NO reduction and the O2+H2 reaction at low pressure (p<10−3 mbar). Photoelectron emission microscopy and scanning photoelectron microscopy were used as spatially resolved in situ methods to image the local work function changes and to identify chemical changes in the substrate and in the adsorbate layer. It is shown that diffusional coupling leads to dynamic effects which are dependent on the macroscopic size (μm range). With alkali metals on the surface, stationary patterns form whose mechanism of formation has been studied in detail.
Role of sp2 C cluster size on the field emission properties of sulfur-incorporated nanocomposite carbon thin films
Authors: Gupta S. , Weiner B. R. , Morell G.
Published: July 2002

The electron field emission properties of sulfur-incorporated nanocomposite carbon thin films grown by hot-filament chemical vapor deposition were investigated as a function of film microstructure. The in-plane correlation length (La) of the sp2 C clusters in these films was determined from the intensity ratio of the D and G bands [I(D)/I(G)] in the visible Raman spectra using a phenomenological model. The turn-on field was found to decrease with increasing sp2 C cluster size in the range of 0.8–1.4 nm. The lowest turn-on field found was 4.0 V/μm corresponding to films having sp2 C clusters of around 1.4 nm and conductivity of 30 Ω−1 cm−1. These findings are discussed in terms of a reduced field emission barrier brought about by the incorporation of sulfur and the need for relatively longer conductive paths capable of withstanding the relatively large emission currents.
Particle balance measurements during detachment in a gas-target divertor simulator
Authors: Hollmann E. M. , Pigarov A. Yu. , Seraydarian R. , Whyte D. G. , Krasheninnikov S. I.
Published: July 2002

Particle balance measurements have been performed in low-density [Ne(r=0)<1011 cm−3], low temperature [Te(r=0)<5 eV] hydrogen plasma discharges in a linear gas-target divertor simulator device. Axial density drops of up to a factor 50 were observed at high gas-target pressures (Ptarget≈30 mTorr). For these highly detached plasmas, the largest particle loss term was found to be radial transport to the side walls. These results show that cross-field transport can be important for plasma detachment, which is traditionally thought of as resulting from volume recombination.
Switching field trends in pseudo spin valve nanoelement arrays
Authors: Castaño F. J. , Hao Y. , Ross C. A. , Vögeli B. , Smith Henry I. , Haratani S.
Published: July 2002

Room temperature magnetic properties of arrays of NiFe 6 nm/Cu 3–6 nm Co 4 nm pseudospin valve elements with widths below 100 nm have been investigated as a function of the aspect ratio of the elements. The elements are made from sputtered film stacks patterned using interference lithography and ion milling. The separate magnetic switching of the Co and NiFe layers can be clearly distinguished. The magnetic layers interact through both exchange and magnetostatic coupling. As both the aspect ratio of the element and the Cu spacer thickness increase, the magnetostatic coupling becomes weaker and the magnetization of the layers becomes coupled parallel at remanence.
Four-beam correlation method for local fluctuation measurements in plasmas
Authors: Ejiri A.
Published: July 2002

In order to measure fine scale fluctuations of visible emission in plasmas, the four-beam correlation method is proposed and its feasibility is studied. This is an extension of the conventional cross beam techniques and uses four beams (i.e., four lines of sight), which lie in two parallel planes. While two beams in the same plane provide the fluctuation levels at the intersection, two beams in different planes provide correlation as a function of the distance between the planes. Various cases are considered, and the method feasibility is shown. This method is useful for reconstructing a two- or three-dimensional wave-number spectrum. In addition, it can be used to measure the magnetic field direction assuming that the parallel correlation length is longer than the perpendicular one.
X-ray photoelectron spectroscopy study of the role of nitrogen in FeSiAl(N) reactive sputtered films
Authors: Snyder J. E. , Anderegg J. W.
Published: July 2002

Nitrogen has a profound effect on the stress, nanostructure, and magnetic properties of FeSiAl(N) reactive sputtered films. X-ray photoelectron spectroscopy is an excellent tool to investigate the role of N in these films, since it provides both compositional and chemical bonding information simultaneously. The N is observed to first combine with the Al in the films. For increasing amounts of N, increasing amounts of Al as nitride are observed. Then at the point that the Al is all nitride, the N starts to combine with Si. It is in this region that an abrupt change is observed in magnetic properties, stress, and nanostructure. For low N, the films consist of single phase 100-nm-diam bcc grains; after the transition, films consist of a two phase nanostructure of 10-nm-bcc grains in an amorphous matrix. The N content of the films also increases sharply at this point. The addition of N to the sputtering plasma also causes changes in the apparent relative sputtering yields at these higher N levels: Fe content of the films decreases considerably, while Al and Si percentages show modest increases. This can help explain why the saturation magnetization (Ms) has been observed to decrease with increasing N in the plasma, for these higher N levels.
An ab initio study of the f–f spectroscopy of americium +3
Authors: Tilson Jeffrey L. , Naleway Conrad , Seth Michael , Shepard Ron , Wagner Albert F. , Ermler Walter C.
Published: July 2002

The levels associated with the lowest F7 and D5 terms of Am+3 have been calculated using ab initio spin–orbit configuration interaction techniques. A series of configuration interaction calculations were carried out that include significant amounts of single and double excitations and with two different pseudopotentials available in the literature. Double and single excitations from the 6s, 6p, and 5f subshells are all important in the determination of the level energies. A comparison of the two examined pseudopotentials with increasing amounts of electron correlation indicates that both yield results in qualitative agreement with experiment. More importantly, though, it is estimated that both are in significant quantitative error relative to experimental results, even for very large configuration interaction calculations. The calculations were performed using the new parallel spin–orbit configuration interaction component to the COLUMBUS Program System.
Development of a three-dimensional numerical model of grain boundaries in highly doped polycrystalline silicon and applications to solar cells
Authors: Altermatt Pietro P. , Heiser Gernot
Published: July 2002

We have developed a three-dimensional numerical model of grain boundaries to simulate the electrical properties of polycrystalline silicon with doping densities larger than approximately 5×1017 cm−3. We show that three-dimensional effects play an important role in quantifying the minority-carrier properties of polycrystalline silicon. Our simulations reproduce the open-circuit voltage of a wide range of published experiments on thin-film silicon p-n junction solar cells, choosing a velocity parameter for recombination at the grain boundaries, S, in the order of 105–106 cm/s. The simulations indicate that, although S has been reduced by one order of magnitude over the last two decades, improvements in the open-circuit voltage have mainly been achieved by increasing the grain size. A few options are proposed to further reduce S.
Entropy driven demixing in fluids of rigidly ordered particles
Authors: DuBois Sylviane , Perera Aurélien
Published: July 2002

The entropy driven demixing in binary mixtures of rigidly ordered hard particles is studied herein. To that end, a free energy functional for this type of system, as well as an approximation for the corresponding pair direct correlation function, are proposed. The accuracy of the functional is first tested by computing the pressure for previously studied similar systems. The spinodal of the fluid–fluid separation is studied, and shown to be a loop in the (ρ1,ρ2) density–density phase diagram, with both a lower and an upper critical point. This feature is strikingly different from what is observed in the demixing of disordered mixtures, where only a lower critical point is generally found. The influence of molecular shape is equally studied, by varying geometric properties such as convexity, aspect and breadth ratios as well as by mixing prolate and oblate shapes. Within the present theory, demixing is forbidden for mixtures of hard spheres and mixtures of prolate (oblate) shapes with the same breadth, irrespective of their aspect ratios. Demixing is enhanced for mixtures of prolates and oblates and more generally for large asymmetry in the particles breadths. The proposed functional is built such that it reduces to the correct Onsager limit when very large aspect ratios are considered. However, the Onsager second virial approximation, when applied to cases of moderate to small aspect ratios, differs from the present theory, as it can only exhibit demixing with a lower critical point.
Selective growth of flowerlike carbon structures by Radio Frequency plasma-assisted pulsed-laser deposition
Authors: Yap Yoke Khin , Yoshimura Masashi , Mori Yusuke , Sasaki Takatomo
Published: July 2002

Selective growth of a novel flowerlike carbon structure is described. These structures are grown on Si (100) substrates coated with Fe2O3 particles. Deposition of carbon is performed by a RF-plasma-assisted pulsed-laser deposition (PLD) technique at 800 °C with pure oxygen as the working gas. The flowerlike carbon structures have 2 to 10 branches spread randomly outward from the center. These structures are constructed from nanoscale graphite crystallites, as revealed by Raman spectroscopy. The Fe2O3 particles oxidize the Si surface where selective growth of the flowerlike carbon occurs with Si–O–C bonds at the interface. We believe that similar bonding might have contributed to the selective growth of multiwalled carbon nanotubes on silicon dioxide.
An edge pedestal investigation for high-confinement tokamak plasmas
Authors: Stacey Weston M.
Published: July 2002

A model for the edge pedestal in high-confinement mode tokamak plasmas is described. Separate gradient scale lengths of the density and of the ion and electron temperatures are calculated from transport and atomic physics considerations, and a common pedestal width is calculated from either a magnetohydrodynamic pedestal β-limit or from neutral penetration considerations. Predictions of the model for a representative gas fueled tokamak model problem are discussed vis-à-vis measured values of pedestal gradient scale lengths and widths, ballooning mode limits, and scaling with operational parameters.
Thin film microcalorimeter for heat capacity measurements in high magnetic fields
Authors: Zink B. L. , Revaz B. , Sappey R. , Hellman F.
Published: July 2002

Si–N based membrane calorimeters are a promising technology for the study of thermal properties of small quantities of materials in both pulsed and steady-state magnetic fields to 60 T and beyond. We present results that demonstrate our ability to measure the heat capacity of thin film samples from 2–300 K in steady-state fields up to 8 T. These measurements include the magnetoresistance of the Pt and Nb–Si thermometers and focus on confirming that the thermal conductance of the Si–N membrane does not change significantly in magnetic fields. This means the thermal conductance needs to be measured only in zero field, reducing the measurement time in high field. This is particularly important for future measurements in fields up to 60 T.
Spatial and temporal diagnostics of a high-brightness electron beam by technique of laser Thomson scattering
Authors: Yang Jinfeng , Yanagida Tatsuya , Yorozu Masafumi , Sakai Fumio , Okada Yasushiro , Endo Akira
Published: July 2002

A technique of spatial and temporal diagnostics of an electron beam was investigated experimentally by Thomson scattering of the electron beam with a femtosecond laser beam at a 90° configuration. The spatial structure of the electron beam was measured by scanning the laser across the electron beam in the Thomson scattering, and compared with imaging of the beam profile on a phosphor screen using a charge coupled device. By scanning the femtosecond laser beam in time along the electron beam, we measured the temporal density distributions of short and long electron bunches which were produced in a photocathode rf gun with laser injection phases of 30° and 55°. A subpicosecond time slice of the actual electron beam was achieved in the measurement.