bluuebunny/crossref_metadata_2025 huggingface dataset:
Ion acoustic instability driven by a temperature gradient in laser-produced plasmas
Authors: Brantov A. V. , Bychenkov V. Yu. , Rozmus W.
Published: July 2002

The return current instability excited in laser-produced plasmas by a temperature gradient has been studied using a nonlocal theory of electron transport. The transport model is applicable for an arbitrary ratio of the temperature inhomogeneity scale length to the collisional mean free path. It is demonstrated that nonlocal thermal effects have a significant impact on the ion acoustic instability growth rate, threshold and angular distribution of excited waves that can be important for the interaction of smoothed laser beams with a plasma. A nonlinear dependence of the return current instability growth rate on the gradient length has been discovered. The particularly important example of ion acoustic instability due to inhomogeneous plasma heating as a result of inverse bremsstrahlung absorption in a hot spot has been considered.
Vibrational mode analysis of isomorphous hydrogen-bonded guanidinium sulfonates with inelastic neutron scattering and density-functional theory
Authors: Pivovar A. M. , Ward M. D. , Yildirim T. , Neumann D. A.
Published: July 2002

The inelastic neutron scattering spectra of a set of homologous lamellar molecular crystals containing two-dimensional hydrogen bonded sheets of guanidinium ions and sulfonate moieties of organomonosulfonates have been measured. The spectra were collected in the 35–105 meV range using a filter analyzer spectrometer and in the 0–35 meV energy range using a time-of-flight instrument. Parallel and perpendicular vibrational modes, associated with hydrogen displacements with respect to the hydrogen-bonded sheets, were analyzed between 35 and 105 meV by employing different scattering geometries. Comparison of the filter analyzer spectra of the isomorphous guanidinium salts of methanesulfonate, triflate and dithionate enabled assignment of specific modes while providing direct unambiguous measurement of the effect of the organic substituent on the strength of the (guanidinium)N–H⋯O–S(sulfonate) hydrogen bonds. Comparison of the time-of-flight spectra of the three compounds revealed that a mode observed at 7 meV corresponded to rotational motion of the methyl group. Density functional theory reproduced the experimental spectrum of guanidinium methanesulfonate, enabling assignment of the lattice vibrational modes. The scattering data compare well with a previously reported vibrational mode analysis of structurally homologous guanidinium nitrate.
Salt dependence of compression normal forces of quenched polyelectrolyte brushes
Authors: Tamashiro M. N. , Hernández-Zapata E. , Schorr P. A. , Balastre M. , Tirrell M. , Pincus P.
Published: July 2002

We obtained mean-field expressions for the compression normal forces between two identical opposing quenched polyelectrolyte brushes in the presence of monovalent salt. The brush elasticity is modeled using the entropy of ideal Gaussian chains, while the entropy of the microions and the electrostatic contribution to the grand potential is obtained by solving the nonlinear Poisson–Boltzmann equation for the system in contact with a salt reservoir. The interplay between these distinct contributions upon variations of the ionic strength determines the brush thickness. For the polyelectrolyte brush we considered both an isotropic charged slab as well as a longitudinally heterogeneous charge profile obtained using a self-consistent field theory. Using the Derjaguin approximation, we related the planar-geometry results to the realistic two-crossed cylinders experimental setup. Theoretical predictions are compared to experimental measurements of the salt dependence of the compression normal forces between two quenched polyelectrolyte brushes formed by the adsorption of diblock copolymers poly(tert-butyl styrene)-sodium poly(styrene sulfonate) onto an octadecyltriethoxysilane hydrophobically modified mica surface.
Influence of intermolecular interaction on the dynamics of good solvent–polymer solutions
Authors: Harnau Ludger
Published: July 2002

A formalism to study the collective dynamics of good solvent–polymer solutions is presented. The approach takes into account both intramolecular and intermolecular interaction as well as hydrodynamic interaction. Using input from a generalized Ornstein–Zernike equation, a Langevin equation for the dynamic structure factor is solved. Good agreement with experimental data is found.
Augmented Lagrangian method for order-N electronic structure
Authors: Adhikari Satrajit , Baer Roi
Published: July 2002

Molecular electronic ground-state theories, whether ab initio, or semiempirical are most often formulated as a variational principle, where the electronic ground-state energy, considered a linear or nonlinear functional of a reduced density matrix, obtains a constrained minimum. In this communication, we present a Lagrangian analysis of the self-consistent-field electronic structure problem, which does not resort to the concept of orthogonal molecular orbitals. We also develop a method of constrained minimization efficiently applicable to nonlinear energy functional minimization, as well as to linear models such as tight-binding. The method is able to treat large molecules with an effort that scales linearly with the system size. It has built-in robustness and leads directly to the desired minimal solution. Performance is demonstrated on linear alkane and polyene chains.
Electrospinning and electrically forced jets. I. Stability theory
Authors: Hohman Moses M. , Shin Michael , Rutledge Gregory , Brenner Michael P.
Published: July 2002

Electrospinning is a process in which solid fibers are produced from a polymeric fluid stream (solution or melt) delivered through a millimeter-scale nozzle. The solid fibers are notable for their very small diameters (<1 μm). Recent experiments demonstrate that an essential mechanism of electrospinning is a rapidly whipping fluid jet. This series of papers analyzes the mechanics of this whipping jet by studying the instability of an electrically forced fluid jet with increasing field strength. An asymptotic approximation of the equations of electrohydrodynamics is developed so that quantitative comparisons with experiments can be carried out. The approximation governs both long wavelength axisymmetric distortions of the jet, as well as long wavelength oscillations of the centerline of the jet. Three different instabilities are identified: the classical (axisymmetric) Rayleigh instability, and electric field induced axisymmetric and whipping instabilities. At increasing field strengths, the electrical instabilities are enhanced whereas the Rayleigh instability is suppressed. Which instability dominates depends strongly on the surface charge density and radius of the jet. The physical mechanisms for the instability are discussed in the various possible limits.
Effects of strain on the dielectric properties of tunable dielectric SrTiO3 thin films
Authors: Hyun S. , Char K.
Published: July 2002

We fabricated an epitaxial Au/SrTiO3(001)/SrRuO3 (or CaRuO3)(001)/SrTiO3 (or LaAlO3)(001) heterostructure to investigate the effect of the strain on the epitaxial tunable dielectric thin films. SrTiO3 thin films showed very different dielectric properties depending on the bottom electrode with an opposite lattice mismatch. The SrTiO3 thin films grown on the CaRuO3 bottom electrodes showed nearly a two times larger tunability than that on SrRuO3. We think this is due to the different strain on the SrTiO3 thin films. The tensile strain along the applied electric field in the parallel plate capacitor enhances the dielectric constant and the tunability, while the compressive strain decreases them. We believe this is consistent with the hardening of the soft mode phonon due to the compressive strain.
CC3 triplet excitation energies using an explicit spin coupled excitation space
Authors: Hald Kasper , Hättig Christof , Olsen Jeppe , Jørgensen Poul
Published: July 2002

Triplet excitation energies are derived in the approximate triples model CC3 using an explicit spin coupled triplet excitation space. The explicit spin coupled excitation space gives considerable computational savings compared to the spin–orbital approach. Sample calculations are performed on the Be2 and CH2 systems and the performance of the CC3 results are evaluated from a comparison with full configuration interaction (FCI) results.
Simplified model for inelastic acoustic phonon scattering of holes in Si and Ge
Authors: Bufler F. M. , Schenk A. , Fichtner W.
Published: July 2002

An averaging procedure is applied to inelastic acoustic–phonon scattering which leads to lattice-temperature-dependent constants for the phonon energy and the square of the phonon wave vector. The resulting scattering rate depends on energy only thus facilitating the search of after-scattering states in full-band Monte Carlo simulations. The model still accurately reproduces the velocity–field characteristics over a wide range of lattice temperatures, but in silicon the hot-hole tail of the energy distribution is strongly enhanced compared with the elastic equipartition approximation.
Surface morphology and structural changes in insulators induced by high-current 60 keV Cu− implantation
Authors: Lee C. G. , Takeda Y. , Kishimoto N. , Umeda N.
Published: July 2002

Surface morphology and structural changes of amorphous (a-), crystalline (c-) SiO2 and MgAl2O4 spinel, implanted with high-current Cu− of 60 keV, were studied using atomic force microscopy. Kinetic variation of the surface morphology was evaluated by changing dose rate from 1 to 100 μA/cm2. In the cases of a-SiO2 and c-SiO2, the surface morphology significantly varied with dose rate: dome-type textures in the lower dose-rate region caused radiation-induced roughening, while net-type ones in the higher region occurred with radiation-induced smoothing. Step-height measurements showed radiation-induced expansion and compaction for c-SiO2 and a-SiO2, respectively. Radiation-induced amorphization is responsible for the expansion processes of c-SiO2. Contrarily, no amorphization was observed in the spinel up to dose rates of about 100 μA/cm2, although radiation-induced swelling, due to defect accumulation, was discernible by the step-height measurement. The surface morphology and the roughness of the spinel were not very dependent on dose rate. The results of c-SiO2 and a-SiO2 indicate that both of them are subjected to enhanced changes in surface morphology and microstructures and that high-current implantation causes highly dynamic states in the near-surface region. On the other hand, the results for the spinel show pronounced stability and that the spinel may be used as an effective substrate for optical modification, even for high-current ion implantation.
Analytical solitons in nonlinear transmission lines loaded with heterostructure barrier varactors
Authors: Oriols X. , Martı́n F.
Published: July 2002

The propagation of solitons in transmission lines periodically loaded with nonlinear capacitors is analyzed. It is demonstrated that the dependence of soliton amplitude on propagating velocity can be computed with a simple equation, valid for any arbitrary nonlinear capacitance. By means of an approximation to the nonlinear reactance, an analytical expression that completely describes soliton profiles is presented. This can be applied to obtain moderate to high-amplitude soliton wave forms in nonlinear transmission lines (NLTLs) loaded with heterostrucure barrier varactors, where standard approaches do not hold. This model can be of help in understanding harmonic generation in the terahertz range using monolithic NLTLs as frequency multipliers.
Visible electroluminescence in hydrogenated amorphous silicon oxynitride
Authors: Kato Hiromitsu , Masuzawa Akira , Sato Hidefumi , Noma Takashi , Seol Kwang Soo , Fujimaki Makoto , Ohki Yoshimichi
Published: July 2002

The mechanism of electroluminescence in hydrogenated amorphous silicon oxynitride was investigated. The luminescence can be observed only in the samples with high nitrogen content and annealed at high temperatures. It depends on the direction of the applied electric field, and its peak photon energy decreases from 2.3 to 1.8 eV as the nitrogen content increases. From the measurements of conduction current and Fourier transform infrared absorption spectroscopy, it was found that the electrical conduction in the electric field region where the luminescence was observed is governed by the Poole–Frenkel process at the defect centers induced by the high temperature annealing. The electroluminescence is considered to be caused by electronic transition between the band-tail states, at least one of which is related to N or Si–N bonds.
Structural and optical properties of Ge islands grown in an industrial chemical vapor deposition reactor
Authors: Loo R. , Meunier-Beillard P. , Vanhaeren D. , Bender H. , Caymax M. , Vandervorst W. , Dentel D. , Goryll M. , Vescan L.
Published: July 2002

The use of Si based materials for optoelectronic applications is hampered by the indirect nature of the band gap. One possible solution by which to improve the radiative light emission is three-dimensional Stranski–Krastanow growth of Si1−xGex or pure Ge on top of Si. In this article we give a detailed overview about the growth kinetics observed for Ge growth in a standard production oriented chemical vapor deposition system. With increasing deposition time, we observed the usual changeover from monomodal to bimodal island distribution. The island morphology and density can be controlled by varying the growth conditions or by applying a thermal anneal after island growth. Island densities up to 2.3×1010 cm−2 have been obtained for depositions at 650 °C. A Si cap layer is needed for photoluminescence measurements as well as for some device structures. However, Si capping at 700 °C leads to nearly total dissolution of small islands and truncation of bigger dome-shaped islands. This can be prevented by reducing the deposition temperature and by changing the Si gas source. Photoluminescence measurements demonstrate the high layer quality of Si capped islands by the clear separation between the no-phonon line and the transversal optical (TO) replica and the high peak intensities. The spectral range of the island luminescence is between 1.35 (920 meV) and 1.50 μm (828 meV) and depends on the growth conditions. At 20 K, we found up to 70 times higher values for the integrated no-phonon and the TO luminescence from the islands, compared to the integrated intensity from the Si TO peak. Nevertheless, the high photoluminescence intensity can be further enhanced by a thermal treatment in a H2 plasma. Clear island luminescence up to 200 K has been observed after such thermal treatment, which shows the potential of this material system for optoelectronic device applications.
Heating rate dependence of melting of silicon: An in situ x-ray topography study
Authors: Wang Yuren , Kakimoto Koichi
Published: July 2002

In situ observations of the melting processes of dislocation-free silicon crystals were carried out using x-ray topography technique. Heating procedures with various heating rates were used in the experiments. Dot contrasts were observed during the melting process at a high heating rate, while they could not be found at a low heating rate. It was found that the melting process at a high heating rate is spatially inhomogeneous, while that at a low heating rate is homogeneous. It was also found that the oxygen concentration in the sample plays an important role in the formation of dot contrasts during the melting process. The dot contrasts were assumed to originate from the dislocation loops around the oxygen precipitates.
Rotating magnetic field oscillator system for current drive in the translation, confinement, and sustainment experiment
Authors: Tobin S. J. , Reass W. A. , Schrank L. S. , Wurden G. A. , Guo H. Y. , Hoffman A. L. , Lotz D.
Published: July 2002

The experimental setup and test results for the ∼125 MW rotating magnetic field current drive system of the Translation, Confinement and Sustainment Experiment at the University of Washington are described. The oscillator system, constructed at Los Alamos National Laboratory, drives two tank circuits (15 kVpeak potential, 8.5 kApeak maximum circulating current in each tank to date) operated 90° out of phase to produce a 54 G rotating magnetic field with a frequency of 163 kHz (ω=1.02×10−6 s−1). Programmable waveform generators control “hot deck” totem pole drivers that are used to control the grid of 12 Machlett 8618 magnetically beamed triode tubes. This setup allows the current to be turned on or off in less than 100 ns (∼6°). Both tank circuits are isolated from the current source by a 1:1 air core, transmission line transformer. Each tank circuit contains two saddle coils (combined inductance of 1.6 μH), and radio frequency capacitors (580 nF). Test results are presented for three conditions: no external load, a resistive external load, and a plasma load. A SPICE model of the oscillator system was created. Comparisons between this model and experimental data are given.
X-ray absorption spectroscopy and magnetic circular dichroism of the Mn-ferrite nanocrystalline thin films
Authors: Štichauer L. , Mirone A. , Turchini S. , Prosperi T. , Zennaro S. , Zema N. , Lama F. , Pontin R. , Šimša Z. , Tailhades Ph. , Bonningue C.
Published: July 2002

The nanocrystalline MnxFe3−xO4 (x=0, 1.18, 1.56, and 1.9) spinel ferrite thin films were investigated by means of the x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn and Fe L2,3 edges. The cationic distributions of thin spinel ferrite films for x=1.18 and 1.56 were determined using a crystal field atomic multiplets scheme for arbitrary symmetry. The results are compared with the distribution of cations obtained from the differential thermogravimetry analysis of fine powders of corresponding compositions.
Temperature dependence of growth morphology of sputtered (FePt/Pt) films on MgO (100) substrate
Authors: Kim Mu-Gyeom , Shin Sung-Chul
Published: July 2002

We have investigated growth morphology of FePt/Pt films prepared by sputtering on a MgO (100) substrate in a temperature range of 100–600 °C. The L10 ordered structure appeared at a low substrate temperature of 200 °C and became a dominant phase via a second-order type transformation. A transition of FePt film growth morphology from continuous two-dimensional (2D) layer-by-layer mode into 3D island growth mode was observed at a substrate temperature of about 400 °C. When the film grew in continuous mode the stress from lattice misfit played an important role in governing the growth morphology; while growing in island mode the thermal stress increasingly influenced it. The island structure revealed eightfold symmetry following the preferred process of elongating the channel length to broadening its width.
Creation of long lasting luminescence in transparent aluminas
Authors: Ishizaka T. , Kurokawa Y.
Published: July 2002

We have examined long lasting luminescence of transparent alumna and Tb3+–Zn2+(Ca2+)-codoped alumina films prepared by the aqueous sol-gel method. A strong luminescence of 300–500 nm was observed for nondoped alumina, and was shown to depend on the treat temperature of gel. The luminescence was not observed for alumina treated at temperatures higher than 600 °C. The luminescence band was composed of plural components due to oxygen defect sites in alumina. The lasting luminescence of Tb3+–Zn2+(Ca2+)-codoped alumina were observed after irradiation for several seconds using a UV lamp (15 W). The results indicated that the oxygen defect sites of alumina serve as a photoelectron trap center and that the AlV site serves as a hole trap center, which can be thermally released at room temperature.
Nonadiabatic interactions between the ground and low-lying excited electronic states: Vibronic states of the Cl–HCl complex
Authors: Žďánska Petra , Nachtigallová Dana , Nachtigall Petr , Jungwirth Pavel
Published: July 2002

The Cl–HCl radical complex is investigated by a combination of accurate ab initio quantum chemical methods for the evaluation of the three lowest electronic potential energy surfaces and nonadiabatic couplings between them, and quantum evaluation of vibronic states using wave function propagation in imaginary time within a close coupling scheme. The sensitivity of the vibronic energies on the quality of the potential surfaces is clearly demonstrated. Moreover, it is shown that nonadiabatic couplings between the three lowest electronic states play an important role, especially for highly excited vibronic states. Since under experimental conditions the complex is prepared in a superposition of excited vibronic states close to the dissociation limit, the inclusion of nonadiabatic effects is crucial for a quantitative interpretation of future higher resolution spectroscopic experiments.
Technique for measuring the ballistic-limit velocity for small mass fragments impacting thin metal plates
Authors: Mock Willis , Holt William H.
Published: July 2002

A new gas gun technique has been developed in which a small mass fragment is impacted in a precise orientation by a thin metal disk and then recovered. The metal disk is placed inside a hollow sabot that also contains a miniature soft-recovery chamber. After impact, the sabot with the fragment inside is recovered in rags. The fragment will be found either in front of the impactor disk or behind it in the soft-recovery chamber, depending upon whether the disk was perforated. This technique can be used to determine accurate ballistic-limit velocity measurements for fragments with masses less than 0.6 gm. Thirty-six experiments have been performed using this technique for masses in the 0.2–0.6 gm range. Results are presented for several experiments.
Rotationally resolved spectra of transitions involving motion of the methyl group of acetaldehyde in the system Ã 1A″−X̃ 1A′
Authors: Chou Yung-Ching , Huang Cheng-Liang , Chen I-Chia , Ni Chi-Kung , Kung A. H.
Published: July 2002

Fluorescence excitation spectra, at resolution 0.02 cm−1, in the system Ã 1A″−X̃ 1A′ were recorded for acetaldehyde in a supersonic jet. We performed full rotational analysis of bands 1400+150n and 1400−150n, for n=0–4, in which 140+ and 140− denote the two inversion tunneling components of the aldehyde hydrogen out of plane bending, in the vibrational ground state of Ã 1A″. Torsional levels from near the methyl torsional barrier to beyond that barrier are assigned. Sublevels of torsional symmetry A below the barrier are fitted as an asymmetric rotor, but the resulting values of the rotational parameter A are affected significantly by the torsional motion. For the E sublevels, K doublet states split significantly with torsional quantum number vt. Anomalous transitions (ΔKa=0, ΔKc=0) to A sublevels are observed for bands 1400+1504 and 1400−1503, which cannot be simply explained from the Coriolis-type interaction of torsion and rotation. The positions of A and E sublevels in 140−15n cannot be fitted with a program involving only interaction of torsion and rotation, because for n=0–2 states the A/E splitting is reversed from those in 140+15n. Interaction with inversion evidently varies the splitting of torsional sublevels and the K structure.
Electronic decay in weakly bound heteroclusters: Energy transfer versus electron transfer
Authors: Zobeley Jürgen , Santra Robin , Cederbaum Lorenz S.
Published: July 2002

Inner-valence ionized states of weakly bound systems like van der Waals clusters can efficiently decay by electron emission. The mechanism of the decay, which does not occur in the isolated monomer units constituting the clusters has recently been shown to be of intermolecular/interatomic nature. This intermolecular/interatomic Coulombic decay (ICD) mechanism prevails in many systems ranging from hydrogen-bonded molecular clusters to atomic rare gas clusters. In the present paper we extend our previous studies to weakly bound heteroclusters built up of monomer units of largely differing energetics. It is shown that, as soon as the double ionization potential of a monomer unit is lower in energy than the ionization potential of the initially created inner-valence vacancy on a neighboring monomer unit, an additional electronic decay process can take place. In contrast to the ICD mechanism, which involves an efficient energy transfer between the monomer units, this second process is essentially based on an electron transfer process. It is therefore termed electron-transfer mediated decay (ETMD). We have analyzed the mechanisms of the electronic decay processes taking place following inner-valence ionization in weakly bound heteroclusters in an exemplary study of the NeAr dimer. The involved electronic states have been calculated using ab initio Green’s function techniques. The lifetime of the inner-valence Ne(2s−1)Ar vacancy has been estimated and partitioned according to the contributions of the two decay channels based on a perturbation-theoretical description of the decay process. As a result, the lifetime of the inner-valence resonance state is estimated to be of the order of 10–100 fs, the specific value strongly depending on the internuclear separation of the monomers. The ICD process is shown to be by far the dominant decay channel at distances corresponding to bound states of the dimer. With decreasing internuclear separation the ratio of the ETMD and ICD decay widths quickly increases over several orders of magnitude.
Simulation of bulk, confined, and polydisperse systems. II. Application to chain systems
Authors: Escobedo Fernando A.
Published: July 2002

By employing the Massieu formalism presented in the preceding paper, Monte Carlo methods and extrapolation techniques are combined to simulate polymers with chain-length polydispersity. Semigrand canonical and grand canonical ensembles supplemented by identity exchange moves and expanded-ensemble moves have been tailored to simulate both thermodynamic properties and the structure of various coarse-grained model systems. It is demonstrated that the proposed methods can be used to simulate systems with arbitrary molecular weight distribution (e.g., to match experimental data), bulk-pore partitioning of polymer solutions, and liquid–liquid equilibrium of polydisperse polymer melts.
Indication of ferromagnetism in molecular-beam-epitaxy-derived N-type GaMnN
Authors: Overberg Mark E. , Abernathy Cammy R. , Pearton Stephen J. , Theodoropoulou Nikoleta A. , McCarthy Kevin T. , Hebard Arthur F.
Published: July 2002

Growth by molecular-beam epitaxy of the dilute magnetic alloy GaMnN is reported. The GaMnN contains 7.0% Mn as determined by Auger electron spectroscopy, and is single phase as determined by x-ray diffraction and reflection high-energy electron diffraction. Both magnetic and magnetotransport data are reported. The results show the anomalous Hall effect, negative magnetoresistance, and magnetic hysteresis at 10 K, indicating that Mn is incorporating into the GaN and forming the ferromagnetic semiconductor GaMnN. At 25 K the anomalous Hall term vanishes, indicating a Curie temperature between 10 and 25 K.
Measurements of the electron energy distribution function in an Ar/CF4 inductively coupled plasma
Authors: Bowden M. D. , Tabata R. , Suanpoot P. , Uchino K. , Muraoka K. , Noguchi M.
Published: July 2002

Measurements of the electron energy distribution function (eedf) of a low-pressure inductively coupled plasma operated in a mixture of Ar and CF4 are reported. The measurement method was laser Thomson scattering. Extensive test were performed in order to verify that any perturbations caused by the laser did not affect the measurements. The eedf was measured for different concentrations of CF4 gas, and the results indicated that it was non-Maxwellian when even small amounts of CF4 gas were present. This dependence was attributed partially to the effect of electron-molecule vibrational excitation collisions.
Biased-electrode operation of electron cyclotron resonance ion sources
Authors: Mironov V. , Runkel S. , Stiebing K. E. , Hohn O. , Schmidt L. , Schmidt-Böcking H. , Schempp A. , Shirkov G.
Published: July 2002

In order to supplement our investigations on the biased electrode in electron cyclotron resonance ion sources, we have carried out dedicated measurements with a special double structure (ring plus disk) electrode. This geometry allows to separate two mechanisms contributing to the “biased disk effect”: the creation of secondary electrons at the disk and the reflection of plasma loss electrons by the potentials of the disk. At sufficiently high ring-electrode potentials the axial electron losses can be blocked completely. From the experimental results, it is suggested that electrostatic blocking is the main contribution to the biased electrode effect.
A compact point focusing spatial filter for x-ray fluorescence and inelastic x-ray scattering studies
Authors: Behne E. A. , Feng Yejun , Seidler G. T.
Published: July 2002

Appropriately selected absorbers combined with spatial filters are frequently used as x-ray low-pass filters in inelastic x-ray scattering studies and x-ray fluorescence spectroscopies. We describe here a compact point focusing spatial filter with a large collection solid angle and excellent rejection of fluorescence from an absorber when used as an x-ray low pass filter. We expect this instrument to have regular application in x-ray microscopies where momentum resolution is irrelevant, such as x-ray fluorescence holography or fluorescence-mode x-ray absorption fine structure measurements using micro-focused beams. As an additional application, we demonstrate that the filter can also be used as the key component in a filter-based analyzer capable of measuring the near edge structure in non-resonant x-ray Raman scattering with 3.0 eV energy resolution.
Wide field amplitude and phase confocal microscope with parallel phase stepping
Authors: Sawyer N. B. E. , Morgan S. P. , Somekh M. G. , See C. W. , Cao X. F. , Shekunov B. Y. , Astrakharchik E.
Published: July 2002

We describe a wide field phase sensitive confocal microscope based on correlation between speckles. In this system, parallel optical phase stepping provides for the simultaneous capture of four interferograms using a single camera, giving the system the potential to operate at high speed. The interferograms are mapped to a common frame of reference, removing any relative distortion in the process, so that phase, amplitude, and interference efficiency images can be obtained. We use a new phase stepping algorithm that accounts for the unequal phase steps and unbalanced beam amplitudes that are a consequence of the optical arrangement. Experimental results to show that the system operates as a wide field amplitude and phase confocal system are presented.
Comparison study of semi-crystalline and highly crystalline multiwalled carbon nanotubes
Authors: Endo M. , Kim Y. A. , Fukai Y. , Hayashi T. , Terrones M. , Terrones H. , Dresselhaus M. S.
Published: July 2002

Very different types of straight and long multiwalled carbon nanotubes, ranging from semi-crystalline to highly crystalline, are obtained using a floating reactant method, by merely changing the flow rate of the carrier gas. Graphitization behaviors of these nanotubes are studied comparatively in terms of their microstructure and morphology, showing a large change in structure for the semi-crystalline nanotubes, but little change in the highly crystalline nanotubes with heat treatment.
Morphology of symmetric block copolymer in a cylindrical pore
Authors: Sevink G. J. A. , Zvelindovsky A. V. , Fraaije J. G. E. M. , Huinink H. P.
Published: July 2002

The influence of confinement on morphology formation in copolymer systems is an important area of interest in theoretical research. We apply dynamic density functional theory to investigate the effect of pores on the morphology formation in a symmetric diblock copolymer system. The pore is represented by a perfect cylindrical tube. Porous systems are important in biology and are gaining interest for applications in nanotechnology. We show that for the pore sizes under investigation two equilibrium morphologies are possible depending on the surface interaction: a perpendicular or slab morphology and a parallel or multiwall tube morphology. The latter is referred to in the article as dartboard morphology. In the dynamic pathway towards this morphology an intermediate metastable helical phase is found. An important observation is that, for a wide range of pore radii and variations of polymer chain length, no mixed parallel/perpendicular morphologies were found: All observed morphologies are insensitive to the pore diameter.
Gas phase electronic spectrum of C3H in the visible
Authors: Ding H. , Pino T. , Güthe F. , Maier J. P.
Published: July 2002

The electronic spectrum of C3H has been observed by means of a resonant two color two photon ionization technique sampling a supersonic plasma source. On the basis of ab initio calculations, vibrational and rotational analysis, the complex vibronic system observed in the visible and near UV is assigned to three electronic transitions, Ã 2A′←X 2Π, B̃ 2A″←X 2Π and C̃ 2A″←X 2Π of C3H. Potential curves along the CCH bending coordinate have been obtained because of its role for the electronic spectrum and dynamics of C3H.
New experimental equipment for grazing-exit electron-probe microanalysis
Authors: Tsuji K. , Spolnik Z. , Ashino T.
Published: July 2002

New grazing-exit electron-probe microanalysis (GE-EPMA) equipment is developed. In GE-EPMA, characteristic x rays are measured at the grazing-exit angle. X rays emitted from deep positions in the substrate are reduced under grazing-exit conditions; therefore, surface-sensitive analysis is possible with low background. In previous equipments, the sample holder was tilted to change the exit angle. In this new equipment, the energy-dispersive x-ray detector is moved to change the exit angle, and the analyzed position is stable even if the exit angle is changed. Therefore, this equipment is useful especially for particle analysis. The new GE-EPMA equipment is applied to Pd–Se–Te single-particle analysis. Although it was difficult to measure the Se Kα line at an exit angle of 45° due to the large Au Lβ radiation emitted from the Au substrate, Se Kα was measured without any Au signals at the grazing-exit angle near zero.
Development of an in situ ultra-high-vacuum scanning tunneling microscope in the beamline of the 15 MV tandem accelerator for studies of surface modification by a swift heavy ion beam
Authors: Tripathi A. , Singh J. P. , Ahuja R. , Dutt R. N. , Kanjilal D. , Guha A. , Biswas A. , Raychaudhuri A. K.
Published: July 2002

We report the installation and testing of an ultra-high-vacuum (UHV) scanning tunneling microscope (STM) in the beamline for materials science studies of the 15 MV Pelletron accelerator at the Nuclear Science Center (NSC), New Delhi. This is a new facility for in situ irradiation-induced surface modification studies, available in an accelerator laboratory. The article describes its vibration isolation, in-vacuum sample transfer and other salient features for integrating the STM to the beamline. The UHV STM is tested by obtaining atomically resolved images of highly oriented pyrolytic graphite (HOPG). In situ topographic and spectroscopic studies of defect structures produced by impact of 200 MeV Au ions on HOPG, p-type Si, and 200 MeV Ag-irradiated Y1Ba2Cu3O7+δ are studied.
Construction of a low velocity metastable helium atomic beam
Authors: Woestenenk G. R. , Thomsen J. W. , van Rijnbach M. , van der Straten P. , Niehaus A.
Published: July 2002

We have constructed an atomic beam of metastable helium atoms He(2 3S) with a mean velocity of 300 m/s (15 K) and a yield of 3×1012 atoms/s sr. The metastable atoms are produced in a dc discharge in a cryogenic environment cooled by liquid helium. Using a hexapole magnetic lens, we have increased further the beam intensity by focusing the metastable atoms. Initial studies show a factor of 2.5 increase in the beam flux but more is expected when the hexapole is constructed from permanent hexapole magnets. The He(2 3S) atoms are subsequently loaded into a magneto-optical trap.
Space-charge limited currents in coaxial diodes with electron backscatter
Authors: Oliver B. V. , Genoni T. C. , Rose D. V. , Welch D. R.
Published: July 2002

The effect of backscattered electrons on space-charge limited currents of cylindrical (coaxial) diodes with anode center conductors is studied. The scattered electrons are parametrized by a fractional current density α and fractional energy β of the incident electrons. For bipolar flow, current enhancement factors of 2.5 are calculated for α, β≃0.5. Comparison of the model equations to one-dimensional particle-in-cell simulations with fully integrated Monte Carlo scattering algorithms demonstrates very good agreement for a range of energies and anode materials. In the absence of backscattering, the bipolar diode impedance decreases for increasing ratio of cathode to anode radius rc/ra for ratios greater than about 20.
Growth modes and microstructures of ZnO layers deposited by plasma-assisted molecular-beam epitaxy on (0001) sapphire
Authors: Vigué F. , Vennéguès P. , Deparis C. , Vézian S. , Laügt M. , Faurie J.-P.
Published: July 2002

Transmission electron microscopy and high resolution x-ray diffraction are used to characterize defects in ZnO layers grown by plasma-assisted molecular-beam epitaxy on (0001) sapphire. Two- and three-dimensional types of growth modes are described and the observed mosaic structure is analyzed in each case. It is found that two-dimensional layers exhibit a roughness as low as 6 nm. Their subdomains have small lateral coherence lengths and a mean in-plane misorientation of ±0.4°, leading to an important dislocation density of 1–4×1010 cm−2. On the contrary, it is demonstrated that, through numerous interactions between dislocations, the three-dimensional growth mode leads to a better structural quality with a larger lateral coherence length and a smaller in-plane mosaic spread of ±0.07°. The total dislocation density is consequently reduced by 1 order of magnitude down to 3–5×109 cm−2 and the radical modification of the structure results in a change of the dislocation distribution. Our results thus demonstrate that two-dimensional growth mode and low full width at half maximum for symmetric x-ray diffraction are a not reliable indicator of a good structural quality.
Growth of fullerene-like carbon nitride thin solid films consisting of cross-linked nano-onions
Authors: Czigány Zs. , Brunell I. F. , Neidhardt J. , Hultman L. , Suenaga K.
Published: July 2002

Fullerene-like CNx (x≈0.12) thin solid films were deposited by reactive magnetron sputtering of graphite in a nitrogen and argon discharge on cleaved NaCl and Si(001) substrates at 450 °C. As-deposited films consist of 5 nm diam CNx nano-onions with shell sizes corresponding to Goldberg polyhedra determined by high-resolution transmission electron microscopy. Electron energy loss spectroscopy revealed that N incorporation is higher in the core of the onions than at the perimeter. N incorporation promotes pentagon formation and provides reactive sites for interlinks between shells of the onions. A model is proposed for the formation of CNx nano-onions by continuous surface nucleation and growth of hemispherical shells.
Absolutely calibrated, time-resolved measurements of soft x rays using transmission grating spectrometers at the Nike Laser Facility
Authors: Weaver J. L. , Feldman U. , Seely J. F. , Holland G. , Serlin V. , Klapisch M. , Columbant D. , Mostovych A.
Published: July 2002

Accurate simulation of pellet implosions for direct drive inertial confinement fusion requires benchmarking the codes with experimental data. The Naval Research Laboratory (NRL) has begun to measure the absolute intensity of radiation from laser irradiated targets to provide critical information for the radiatively preheated pellet designs developed by the Nike laser group. Two main diagnostics for this effort are two spectrometers incorporating three detection systems. While both spectrometers use 2500 lines/mm transmission gratings, one instrument is coupled to a soft x-ray streak camera and the other is coupled to both an absolutely calibrated Si photodiode array and a charge coupled device (CCD) camera. Absolute calibration of spectrometer components has been undertaken at the National Synchrotron Light Source at Brookhaven National Laboratories. Currently, the system has been used to measure the spatially integrated soft x-ray flux as a function of target material, laser power, and laser spot size. A comparison between measured and calculated flux for Au and CH targets shows reasonable agreement to one-dimensional modeling for two laser power densities.
Modified braid equations for SOq(3) and noncommutative spaces
Authors: Chakrabarti A.
Published: July 2002

General solutions of the R̂TT equation with a maximal number of free parameters in the specrtal decomposition of vector SOq(3) R̂ matrices are implemented to construct modified braid equations (MBE). These matrices conserve the given, standard, group relations of the nine elements of T, but are not constrained to satisfy the standard braid equation (BE). Apart from q and a normalization factor our R̂ contains two free parameters, instead of only one such parameter for deformed unitary algebras studied in a previous paper [Math. QA/0009178] where the nonzero right hand side of the MBE had a linear term proportional to (R̂(12)−R̂(23)). In the present case the rhs is, in general, nonlinear. Several particular solutions are given (Sec. II) and the general structure is analyzed (Appendix A). Our formulation of the problem in terms of projectors yields also two new solutions of standard (nonmodified) braid equation (Sec. II) which are further discussed (Appendix B). The noncommutative three-spaces obtained by implementing such generalized R̂ matrices are studied (Sec. III). The role of coboundary R̂ matrices (not satisfying the standard BE) is explored. The MBE and Baxterization are presented as complementary facets of the same basic construction, namely, the general solution of R̂TT equation (Sec. IV). A new solution is presented in this context. As a simple but remarkable particular case a nontrivial solution of BE is obtained (Appendix B) for q=1. This solution has no free parameter and is not obtainable by twisting the identity matrix. In the concluding remarks (Sec. V), among other points, generalization of our results to SOq(N) is discussed.
Methyl group dynamics in glassy toluene: A neutron scattering study
Authors: Moreno A. J. , Alegrı́a A. , Colmenero J. , Prager M. , Grimm H. , Frick B.
Published: July 2002

We present a neutron scattering study on methyl group dynamics in glassy toluene. The spectra in the whole temperature range, covering the transition from quantum rotational tunneling to classical hopping, have been successfully analyzed in terms of a potential barrier distribution model. The average barrier in the glass is found to be notably higher than the unique barrier of the crystalline β-phase, whose short-range structure is known to be similar to that of the glass. Due to the mainly intermolecular origin of the interactions on the methyl groups in toluene, it is concluded that the rotational potentials are strongly affected by structural disorder.
Surface-modified CuO layer in size-stabilized single-phase Cu2O nanoparticles
Authors: Balamurugan B. , Mehta B. R. , Shivaprasad S. M.
Published: July 2002

Activated reactive evaporation has been used to grow copper oxide nanoparticles in the size range of 8–100 nm. X-ray diffraction spectra clearly show the presence of a single Cu2O phase. Detailed x-ray photoelectron spectroscopy studies show an increase in the ionicity of the Cu2O system with decreasing particle size. Depth profiling and finger printing of x-ray photoelectron spectra reveal that the Cu2O nanoparticles are capped with a CuO surface layer of thickness ≈1.6 nm. This study strongly suggests that the stabilization of the cubic Cu2O nanophase is enhanced by the formation of a CuO surface layer.
Generalized theory of a free-electron laser in a helical wiggler and guide magnetic fields using the kinetic approach
Authors: Misra K. D. , Mishra P. K.
Published: July 2002

A self-consistent theory of a free-electron laser is developed by the kinetic approach, using the method of characteristics in helical wiggler and guide magnetic fields. The detailed relativistic particle trajectories obtained in wiggler and guide magnetic fields are used in linearized Vlasov–Maxwell equations having variations in perpendicular and parallel momenta to obtain the perturbed distribution function in terms of perturbed electric and magnetic fields deviating from the vector potential approach. The perturbed distribution function thus obtained, having variations in perpendicular and parallel momenta for an arbitrary distribution function, is used to obtain current, conductivity and dielectric tensors. The full dispersion relation (FDR) and Compton dispersion relation (CDR) have been obtained. The dispersion diagram has been obtained and the interaction of the negative longitudinal space charge with the electromagnetic wave has been shown. The temporal growth rates obtained from the full dispersion relation and Compton dispersion relation for the tenuous cold relativistic beam in microwave region have been discussed.
A novel method for measuring the surface tension of free standing smectic films
Authors: Schneider F.
Published: July 2002

A method for measuring the surface tension of free standing smectic films is described which is considerably more precise than the methods used up to now. Data on the precision and the reproducibility of the device are presented. The temperature dependence of the surface tension for the smectic liquid crystal 4-octyl-4′-cyanobiphenyl is discussed.
Torsion–rotation analysis of OH stretch overtone–torsion combination bands in methanol
Authors: Rueda D. , Boyarkin O. V. , Rizzo T. R. , Mukhopadhyay I. , Perry D. S.
Published: July 2002

We report rotationally resolved spectra of jet-cooled methanol for the OH stretch overtones, 2v1 and 3v1, and for the torsional combinations, 2v1+v12, 2v1+2v12, 3v1+v12, and 3v1+2v12. The spectra are obtained by direct excitation from the vibrational ground state with an infrared laser pulse. Population in the resulting upper state levels is detected by infrared laser assisted photofragment spectroscopy (IRLAPS). Global fits of the spectra to the Herbst Hamiltonian yield the torsional and rotational parameters, including F, ρ, V3, and V6, for each OH stretch excited state. For each quantum of OH stretch excitation, we find that the torsional barrier height V3 increases by 40.9±1.9 cm−1 and the torsional inertial F decreases by 0.89±0.02 cm−1. With reference to ab initio calculations, we explain the increase in V3 in terms of changes in the electronic structure of methanol as the OH bond is elongated. For Δv12=1 we observe only transitions with ΔK=±1, and for Δv12=2 we observe only ΔK=0. We present a Franck–Condon model to explain these apparent selection rules and the overall pattern of intensity.