bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-sharing utilization of a high-power pulsed-laser diode for inducing damage and for detecting its threshold level with high sensitivity
Authors: Yarai Atsushi , Hirai Atsushi , Murahashi Hirotaka , Nakanishi Takuji
Published: July 2002

This article proposes a method to utilize a commercially available high-power pulsed-laser diode (HPLD) in time sharing to induce damage by laser irradiation and to detect the threshold level of the damage. This method features: (1) damage inducement when the HPLD is under the pulsed high-power laser oscillation mode, and (2) surface reflectivity measurement when the HPLD is under the cw light-emitted-diode illumination mode. This technique allows nondestructive measurement of the damage threshold level with high sensitivity, in spite of its extremely simple approach. Accordingly, the technique is appropriate for real-time inspection in a manufacturing process. The practical data for determining the damage threshold are shown. The damage threshold for aluminum (JIS A5052) was estimated to be approximately 62 mJ/mm2. The capability of our proposed method is shown compared with that of more conventional methods. The advantages of our technique are described.
Trends in plasma conditions inferred from an analysis of x-ray data from high wire-number, Z-pinch load implosions
Authors: Whitney K. G. , Pulsifer P. E. , Apruzese J. P. , Thornhill J. W. , Davis J. , Chong Y. K. , Sanford T. W. L. , Mock R. C. , Nash T. J.
Published: July 2002

An analysis of x-ray data from two series of Z-pinch shots taken on the short current-risetime Saturn accelerator at Sandia National Laboratories [Proceedings of 6th International IEEE Pulsed Power Conference, Arlington, VA, edited by P. J. Turchi and B. H. Bernstein (IEEE, New York, 1987), p. 310] is presented. In one series, the array radius was held constant and the array mass was varied; in the other series, the array mass was held constant and its radius varied. In both sets of experiments, large wire-number loads (N⩾93) of aluminum were used in contrast to earlier small wire-number aluminum array experiments on Saturn where N⩽42. Average electron temperatures and ion densities were inferred from the data. In addition, from the measured size of the emission region of K-shell x rays and from the inferred ion density, a fraction of the total array mass that participated in the K-shell emission was inferred and found to be directly correlated to the K-shell yields that were measured. This paper also demonstrates that the yields varied as a function of array mass and radius in much closer agreement with predictions [J. Appl. Phys. 67, 1725 (1990)] than had been observed in the earlier small wire-number experiments. Thus, a serious misperception that the reason for the early disagreement was in the calculations and not in the experiments is corrected. These predictions were made using one-dimensional (1D) magnetohydrodynamics calculations. The density and temperature trends inferred from the data analysis are well-behaved and consistent with the 1D calculations. This data analysis confirms the importance of achieving uniform plasma initial conditions and implosion symmetry when comparing computer code calculations with experiment. When the wire number of an array load is increased, a more uniform shell of plasma is calculated initially as the wires explode and, as the plasma stagnates on axis, the x-ray powers and yields are found experimentally to approach the powers and yields predicted by 1D calculations.
Structure and energetics of the silicon carbide clusters SiC3 and Si2C2
Authors: Rintelman Jamie M. , Gordon Mark S.
Published: July 2002

A comprehensive ab initio study of the four atom silicon carbide clusters SiC3 and Si2C2 using multiconfigurational self-consistent-field wave functions is presented. In contrast to previous studies the global minimum isomer for SiC3 is predicted to be a C∞v linear triplet with a terminal silicon atom. For Si2C2 the global minimum is a rhombic structure, in accordance with previous studies, while the linear triplet Si–C–C–Si is just 1.0 kcal mol−1 higher in energy.
Oxygen driven reconstruction dynamics of Ni(977) measured by time-lapse scanning tunneling microscopy
Authors: Pearl T. P. , Sibener S. J.
Published: July 2002

Time-lapse scanning tunneling microscopy (STM) has been used to observe the oxygen induced reconstruction behavior of Ni(977), a stepped metallic surface. Previous studies using helium atom diffraction resolved the macroscopic kinetics for the reversible step-doubling and -singling of this vicinal surface. Sequential STM imaging recorded at elevated temperature has now elucidated atomic-level mechanistic details for the merging of steps in the presence of small amounts of adsorbed oxygen, less than 2% of a monolayer. Point contact between neighboring steps decorated with chemisorbed oxygen facilitates rapid step coalescence by means of zippering. An optimal oxygen concentration of step edge saturation was found to enable the step merging to proceed most rapidly. Excess oxygen was found to hinder the coalescence of neighboring steps through the possible growth of overlayer structures on the terraces. At sufficiently high temperatures, the surface is driven back to single steps due to oxygen dissolution. The departure of oxygen from the surface through dissolution, as well as the associated presence of oxygen in the selvedge region, may both play a role in destabilizing the double steps. Local step density influences the coalescence behavior by defining the number of available step edge sites. The microscopic details made available by time-resolved STM imaging illuminate some of the mechanistic steps related to the initial stages of metallic oxidation, and the sensitivity of surface morphological transformations to local surface structure and adsorbate coverage.
The gaseous reaction of vinyl radical with oxygen
Authors: Wang Hui , Wang Baoshan , He Yong , Kong Fanao
Published: July 2002

The gaseous reaction of vinyl radical with oxygen has been experimentally investigated. C2H3 radical was produced by laser photolysis of C2H3Br at 248 nm. The vibrationally excited products of the reaction were detected by time-resolved Fourier transform infrared emission spectroscopy. H2CO(ν1), HCO(ν1,ν3), and CO2(ν3) are ascertained as the main emitters. The most favorable product channel is HCO and H2CO. The reaction channel leading to CO2+CH3 has been found for the first time. The minor reactions leading to C2H2+HO2, C2H3O+O, and C2H2O2+H may also occur. A secondary reaction product of CO is observed, which is generated from the primary reaction product HCO. Combining theoretical analysis with the present experimental results, the reaction pathways are clarified. The results are of importance for understanding the combustion processes of hydrocarbon.
Coulomb gap and variable-range hopping in self-organized carbon networks
Authors: Govor L. V. , Bashmakov I. A. , Boehme K. , Pientka M. , Parisi J.
Published: July 2002

Carbon networks fabricated by means of a self-organized process, which is in the focus of our interest, represent disordered porous systems. The degree of disorder and, accordingly, the values of their electric conductivity extending from insulator to metal behavior change via heat treatment under vacuum conditions at process temperatures in the range from 600 to 1000 °C. Upon varying the ambient temperature from 4.2 to 295 K, four transport mechanisms can be observed. For carbon nets whose conductivity is far beyond the metal–insulator transition (MIT), the specific resistivity ρ depends on the temperature T as ρ(T)∝T−b exp([T0/T ]1/p). In the low-temperature range, a Coulomb gap in the density of states located near the Fermi energy level occurs, which means that the characteristic value of the exponent is p=2. At high temperatures, the pre-exponential part ρ(T)∝T−b dominates. In the intermediate temperature range, we disclose Mott’s hopping law with p=3. However, the specific resistivity of the carbon networks subject close to the MIT follows the power law ρ(T)∝T−b with 0≲b≲3 at low temperatures. In the high-temperature range, the specific resistivity is characterized by ρ(T)∝exp(−[T/T1]c−1), where the values for c vary from 1.3 to 1.5. The above four charge transport mechanisms can be explained by the tails in the density of localized states pulled out of the conduction and valence band, as a consequence of disorder and, particularly, by some overlap between these tails.
Casimir and pseudo-Casimir interactions in confined polyelectrolytes
Authors: Podgornik R. , Dobnikar J.
Published: July 2002

We investigate the pseudo-Casimir force acting between two charged surfaces confining a single polyelectrolyte chain with opposite charge. We expand the exact free energy to the second order in the local electrostatic field as well as the replicated polymer density field around the mean-field (saddle-point) solution. The quadratic terms lead to a fluctuation interaction that is partly due to the (thermal) Casimir effect for the confined electrostatic field and partly due to the pseudo-Casimir effect due to the confined replicated polymer density field. We study the intersurface separation dependence of both effects and show that the pseudo-Casimir effect leads to a long range attraction between the surfaces that decays with an anomalous algebraic exponent of ∼1.7, smaller than the standard exponent of 2 in the case of Casimir interactions.
Effect of acceptor and donor dopants on polarization components of lead zirconate titanate thin films
Authors: Majumder S. B. , Roy B. , Katiyar R. S. , Krupanidhi S. B.
Published: July 2002

We have compared the magnitudes of reversible and irreversible polarization components of sol–gel-derived Nd3+- and Fe3+-doped PZT (53/47) thin films on platinized silicon substrates. Beyond the switching field, it was found that the reversible component of the polarization remains almost constant both for donor- (Nd3+) and acceptor- (Fe3+) doped PZT films. The irreversible polarization component reduces with the increase in Nd3+ content, whereas it increases until 3 at. % Fe3+-doped PZT thin films. The dielectric behavior of these films at subswitching fields was analyzed in terms of Rayleigh law. The inverse of the Raleigh coefficient (υ) was considered as a measure of the obstacle for the domain-wall motion. In the case of Fe-doped PZT, the inverse of the Raleigh coefficient (υ) shows a declining linearity with Fe content, which may be found exactly opposite to that observed for Nd-doped PZT. The observed results are explained in terms of the nature of the defect-domain-wall interaction of acceptor-and-donor-doped PZT thin films. Studies indicated that in the case of Fe-doped PZT films, the possible defect interaction initiates only above 2 at. % and it was found to be much lower in magnitude in comparison with the Nd doping.
Alignment of nematic liquid crystals on an electrically poled photopolymer film
Authors: Kim M. W. , Rastegar A. , Olenik I. Drevensek , Kim M. W. , Rasing Th.
Published: July 2002

We have observed liquid crystal alignment on an electrically poled polyvinylcinnamate film in which a polar symmetry was introduced by an in-plane electric field and stabilized by unpolarized ultraviolet light. The azimuthal anchoring energy and pretilt angle was measured in a twisted nematic or planar cell and is explained via the molecular structure of the unreacted cinnamates and of the photoproducts β-truxinates. Cinnamates mainly contribute via a strong flow alignment, while β-truxinate produces the anisotropic properties of the film such as its polar anisotropy, azimuthal anchoring, and pretilt angle.
Ab initio global potential, dipole, adiabatic, and relativistic correction surfaces for the HCN–HNC system
Authors: van Mourik Tanja , Harris Gregory J. , Polyansky Oleg L. , Tennyson Jonathan , Császár Attila G. , Knowles Peter J.
Published: July 2002

Ab initio semiglobal potential energy and dipole moment hypersurfaces for the isomerising HCN–HNC system are computed, using a grid of 242 points, principally at the all-electron cc-pCVQZ CCSD(T) level. Several potential energy hypersurfaces (PES) are presented including one which simultaneously fits 1527 points from earlier ab initio, smaller basis CCSD(T) calculations of Bowman et al. [J. Chem. Phys. 99, 308 (1993)]. The resulting potential is then morphed with 17 aug-cc-pCVQZ CCSD(T) points calculated at HNC geometries to improve the representation of the HNC part of the surface. The PES is further adjusted to coincide with three ab initio points calculated, at the cc-pCV5Z CCSD(T) level, at the critical points of the system. The final PES includes relativistic and adiabatic corrections. Vibrational band origins for HCN and HNC with energy up to 12 400 cm−1 above the HCN zero-point energy are calculated variationally with the new surfaces. Band transition dipoles for the fundamentals of HCN and HNC, and a few overtone and hot band transitions for HCN have been calculated with the new dipole surface, giving generally very good agreement with experiment. The rotational levels of ground and vibrationally excited states are reproduced to high accuracy.
Closed-cycle refrigerator-cooled scanning SQUID microscope for room-temperature samples
Authors: Fleet E. F. , Chatraphorn S. , Wellstood F. C. , Knauss L. A. , Green S. M.
Published: July 2002

We have designed, built, and operated a scanning superconducting quantum interference device (SQUID) microscope that uses a closed-cycle refrigerator to cool a YBa2Cu3O7 (YBCO) dc SQUID to 77 K. The SQUID is mounted in custom vacuum housing that has a thin sapphire window that maintains thermal isolation of the SQUID while allowing samples to be imaged in air at room temperature. Samples are mounted on an x–y scanning table and can be brought to within about 60 μm of the SQUID for magnetic field imaging. The SQUID has an effective pick-up area of 1.2×10−9 m2 and a level of flux noise of 10.5 μΦ0/Hz1/2 in the white noise region (above 500 Hz). We describe the performance of the system and present images of a variety of samples.
Frequency shifts of Rossby waves in the inertial subranges of β-plane turbulence
Authors: Ishihara Takashi , Kaneda Yukio
Published: July 2002

Nonlinear interactions between waves and turbulence cause systematic frequency shifts in Rossby waves. The frequency shifts in the inertial subranges of statistically steady β-plane turbulence were examined theoretically and numerically. The theoretical analysis is based on the Lagrangian closure called the Lagrangian renormalized approximation and predicts that when the β effect is small, the frequency shifts of Rossby waves are proportional to kx/k4/3 in the inverse energy transfer range, while they are proportional to kx with or without a log-correction term in the enstrophy transfer range, depending on the flow conditions, where kx is the wave number in the eastward direction. Numerical simulations using 10242 grid points of forced β-plane turbulence that exhibit the inertial subranges, show fairly good agreement with theoretical predictions.
Surface tension at the vapor/liquid interface in an attractive hard-core Yukawa fluid
Authors: González-Melchor Minerva , Trokhymchuk Andrij , Alejandre José
Published: July 2002

Canonical ensemble molecular dynamics and Monte Carlo simulations have been performed to study the vapor/liquid coexistence in a hard-core fluid with an attractive Yukawa interaction. Coexisting densities and pressure along the vapor/liquid coexistence line for different ranges of attractive interaction have been evaluated and found to agree well with the Gibbs ensemble Monte Carlo data reported in the literature. To obtain surface tension, the normal and tangential components of the pressure tensor have been calculated during simulations by using a hybrid molecular dynamics algorithm (which combines the hard-core and continuous forces) and by using an original numerical algorithm for the hard-core contribution to the virial in Monte Carlo simulations. We found that surface tension is strongly dependent on the range of attractive interaction, i.e., it drops when the attraction becomes short-ranged. The relation of the attractive hard-core Yukawa potential to the spherically-truncated Lennard-Jones potential in terms of the interfacial properties is discussed.
Analysis of centrifugal convection in rotating pipes
Authors: Shtern Vladimir , Zimin Valery , Hussain Fazle
Published: July 2002

New exact solutions, obtained for centrifugal convection of a compressible fluid in pipes and annular pipes, explain axially elongated counterflow and energy separation—poorly understood phenomena occurring in vortex devices, e.g., hydrocyclones and Ranque tubes. Centrifugal acceleration (which can be up to 106 times gravity in practical vortex tubes), combined with an axial gradient of temperature (even small), induces an intense flow from the cold end to the hot end along the pipe wall and a backflow near the axis. To account for large density variations in vortex devices, we use the axial temperature gradient as a small parameter instead of the Boussinesq approximation. For weak pipe rotation, the swirl is of solid-body type and solutions are compact: vz/vza=1−4y2+3y4 and (T−Tw)/(Ta−Tw)=(1−y2)3; where y=r/rw, the subscripts w and a denote values of axial velocity vz, temperature T, and radial distance r, at the wall and on the axis. The axial gradient of pressure, being proportional to 3y2−1, has opposite directions near the wall, y=1, and near the axis, y=0; this explains the counterflow. With increasing pipe rotation, the flow starts to converge to the axis. This causes important new effects: (i) the density and swirl velocity maxima occur away from the wall (vortex core formation), (ii) the temperature near the axis becomes lower than near the wall (the Ranque effect), (iii) the axial gradient of temperature drops from the wall to the axis, and (iv) the total axial heat flux (Nu) reaches its maximum Numax≈4000 and then decreases as swirl increases. These features can be exploited for the development of a micro-heat-exchanger, e.g., for cooling computer chips.
Electro-optic phenomena in nematic liquid crystals studied experimentally and by Monte-Carlo simulations
Authors: Komorowska K. , Pawlik G. , Mitus A. C. , Miniewicz A.
Published: July 2002

In this article we compare results of experiments on light self-diffraction in nematic liquid crystal panels with corresponding results of the Monte-Carlo simulations of a two-dimensional nematic liquid crystal model in the presence of a spatially modulated electric field. In the simulations molecular interactions were described by the Lebwohl–Lasher Hamiltonian. The results obtained on the diffraction efficiency and spatial and temporal behavior of refractive index changes in nematic liquid crystal are satisfactorily reproduced by Monte-Carlo simulations. We discuss the complementarity of both methods in studying and designing systems for optical information processing using liquid crystals.
Vacuum-ultraviolet mass-analyzed threshold ionization spectra of iodobutane isomers: Conformer-specific ionization and ion-core dissociation followed by ionization
Authors: Park Sang Tae , Kim Sang Kyu , Kim Myung Soo
Published: July 2002

Mass-analyzed threshold ionization (MATI) spectra using coherent vacuum ultraviolet radiation have been obtained for t-butyl iodide, iso-butyl iodide, 2-iodobutane, and 1-iodobutane. The ionization energy to the lower spin–orbit state of t-butyl iodide ion has been determined to be 8.9984±0.0006 eV, while the threshold for fragmentation to t-C4H9+ and I has been estimated to be 9.1762±0.0047 eV. Heat of formation of t-butyl cation, ΔfH0(t-C4H9+), has been re-estimated, 733.7±3.3 kJ mol−1. Peaks due to two different conformers of iso-butyl iodide ion, PH and PC, are clearly resolved in the MATI spectra, enabling the measurement of ionization energies to the lower spin–orbit states of respective conformers, 9.1725±0.0006 and 9.1972±0.0006 eV. Corresponding values for the upper spin–orbit states have been determined from the MATI spectra for the C4H9+ fragments generated by dissociation in the ion core of neutral as 9.7394±0.0024 and 9.7649±0.0023 eV. Only two out of three possible 2-iodobutane conformers have been observed in the MATI spectra with ionization energies to their lower spin–orbit states of 9.0883±0.0006 and 9.0913±0.0006 eV, even though conformer identification was not possible. Similarly, it is likely that four distinct peaks observed in the ionization threshold region of the MATI spectra of 1-iodobutane are the origins for the different conformers. Plausible mechanisms for the ion-core fragmentation of iodobutanes are discussed.
Extracting the CH chromophore vibrational dynamics of CHBrClF directly from spectra: Unexpected constants of the motion and symmetries
Authors: Jung Christof , Ziemniak Eva , Taylor Howard S.
Published: July 2002

Recently Quack and co-workers have developed a spectroscopic Hamiltonian that uncovers for Bromochlorofluoromethane vibrational excitation in the electronic ground state, the anharmonicities and resonant interactions between the normal modes that determine and fit the measured spectrum. In this paper using this spectroscopic Hamiltonian (no PES) and simple quantum, semiclassical and nonlinear classical methods of analysis a great deal of further information is extracted. Namely in polyad 5 and 8 the energy levels are disentangled into three and four different types of simple motion respectively. Each type of motion gives rise to simple level series. Each series is the quantization of a simple to visualize intramolecular motion. Quantum numbers which are close to conserved quantities are assigned to every state. The most extraordinary of these motions is a rapid cone like rotation of the HC bond atop the asymmetrical triangular pyramid of the heavy part of the molecule. The spectrum of these states shows an approximate double degeneracy even though the molecule is of C1 symmetry; the rapid circulation of the hydrogen atom causes the dynamics to become insensitive to the differences between the various halogen masses and positions.
Influence of the environment on the excited state deactivation in functionalized quinque-thienyls
Authors: Lanzani G. , Cerullo G. , De Silvestri S. , Barbarella G. , Sotgiu G.
Published: July 2002

The transient photoinduced transmission of quinquethiophene-S,S-dioxide in solution is reported. Stimulated emission is observed in the viscous solvent, but quenched in polar and nonpolar low viscosity solvents. The lifetime of the photoexcited state changes of more than one order of magnitude depending on the solvent properties, pointing out conformational effects. Low-energy coherent transient is observed and assigned to real time conformational dynamics.
Electron drift effects on magnetic islands
Authors: Mikhalovskii A. B. , Konovalov S. V. , Pustovitov V. D. , Tsypin V. S. , Galvão R. M. O. , Nascimento I. C.
Published: July 2002

Electron drift effects on magnetic islands in a hot-electron plasma are studied. The slab approximation is used. The electron temperature gradient is neglected. The authors analyze the properties of the electrostatic potential profile function and the density profile function in the presence of the electron drift effects and the problem of regularization of these profile functions near the island separatrix. It is emphasized that the electron drift frequency depends on the island magnetic flux and, as a result of this dependence, the resonant interaction of magnetic islands with the electron drift waves is possible. It is shown that, allowing for the finiteness of the “effective” ion Larmor radius ρs (the ion sound radius), the polarization current effect proves to be dependent on the electron drift effects.
Velocity shear driven electron skin size vortices
Authors: Vranješ J. , Shukla P. K. , Poedts S.
Published: July 2002

Nonlinear electromagnetic perturbations in a nonuniform plasma are studied in the presence of nonuniform flows that are aligned either perpendicular or parallel to the ambient magnetic field lines. Coherent structures with typical spatial scales of the order of the electron skin depth are found in the form of a stationary tripolar vortex, and for spatial scales of the order of the electron Larmor radius at the ion temperature in the form of a vortex chain. In both cases, the structures are driven and carried by the plasma flow. In the case of a parallel flow, the dipolar vortex propagates perpendicular to the field lines. For plasma in the Earth’s magnetotail, the solutions presented here should be expected at spatial scales of the order of about 10–20 km.
The application of a vacuum ultraviolet Fourier transform spectrometer and synchrotron radiation source to measurements of the III ε(1,0) band of NO
Authors: Rufus J. , Yoshino K. , Esmond J. R. , Thorne A. P. , Murray J. E. , Imajo T. , Ito K. , Matsui T.
Published: July 2002

The ε(1,0) band of NO was measured at high resolution (0.06 cm−1) by the vacuum ultraviolet (VUV) Fourier transform spectrometer from Imperial College, London, using synchrotron radiation at the Photon Factory, KEK, Japan, as a continuum light source. Such resolution facilitates a line by line analysis of the NO ε(1,0) band which yields accurate rotational line positions and term values as well as the photoabsorption cross sections. The molecular constants of the D(1)  Σ2 level are found to be T0=55 570.582±0.055 cm−1, Bv=1.979 66±0.000 19 cm−1, Dv=(5.8±4.7)×10−5cm−1, γ=−0.127±0.008 cm−1 and γD=−(1.03±0.04)×10−3 cm−1. The sum of the line strengths for all rotational transitions of the NO ε(1,0) band is determined as 2.55×10−15 cm2 cm−1, corresponding to a band oscillator strength of 0.002 88±0.000 17.
Ab initio prediction of the vibration-rotation-tunneling spectrum of HCl–(H2O)2
Authors: Wormer P. E. S. , Groenenboom G. C. , van der Avoird A.
Published: July 2002

Quantum calculations of the vibration-rotation-tunneling (VRT) levels of the trimer HCl–(H2O)2 are presented. Two internal degrees of freedom are considered—the rotation angles of the two nonhydrogen-bonded (flipping) hydrogens in the complex—together with the overall rotation of the trimer in space. The kinetic energy expression of van der Avoird et al. [J. Chem. Phys. 105, 8034 (1996)] is used in a slightly modified form. The experimental microwave geometry of Kisiel et al. [J. Chem. Phys. 112, 5767 (2000)] served as input in the generation of a planar reference structure. The two-dimensional potential energy surface is generated ab initio by the iterative coupled-cluster method based on singly and doubly excited states with triply excited states included noniteratively [CCSD(T)]. Frequencies of vibrations and tunnel splittings are predicted for two isotopomers. The effect of the nonadditive three-body forces is considered and found to be important.
Microwave modulation of exciton emission in molecular controlled semiconductor resistor
Authors: Havdala Rachel , Langof Lidia , Lifshitz Efrat , Naaman Ron
Published: July 2002

A GaAs/AlGaAs structure coated with molecules was investigated utilizing microwave modulated photoluminescence (MMPL) spectroscopy. This method enabled to resolve the photoluminescence (PL) band into localized and de-localized excitons. The results indicated a limited in-plan mobility of the excitons, due to electrostatic fluctuations or band bending induced by the adsorbed molecules. The reduction of exciton mobility is correlated with the reduction in the photocurrent observed in former studies. The present study provides an insight into the mechanism which couples molecules with a semiconductor structure and explains its operation as a molecular controlled semiconductor resistor (MOCSER), found to be a sensitive actuator in sensor applications.
Raman free-electron laser with longitudinal electrostatic wiggler and annular electron beam
Authors: Maraghechi B. , Javan N. Sepehri
Published: July 2002

A linear theory for the parametric instability of a free-electron laser with a longitudinal electrostatic wiggler is presented. A space-charge wave and a predominantly transverse magnetic wave guide mode are excited when a relativistic electron beam is passed through a static spatially periodic longitudinal electric wiggler. The thick annular electron beam is contained in a cylindrical metal wave guide and an axial magnetic field of arbitrary magnitude. An expression for the wave-field components is derived and solved numerically along with the dispersion relations for the eigenmodes of a wave guide containing a magnetized annular electron beam. Effects of the variation of the beam radii, the axial magnetic field, and electron beam kinetic energy on the growth rate, efficiency, radiation wavelength, and the required pump wavelength are studied. A suitable range of parameters for the generation of millimeter wave is indicated.
The hyperfine structure of the 1 3Δg state of Na2
Authors: Liu Y. , Ji B. , Cheung A. S.-C. , Stwalley W. C. , Field R. W. , Lyyra A. M. , Li Li
Published: July 2002

The hyperfine spectra of the Na2 1 3Δg state have been recorded with sub-Doppler continuous wave (CW) perturbation facilitated optical–optical double resonance (PFOODR) spectroscopy via A 1∑u+∼b 3∏u mixed intermediate levels. The rotational lines into the N=14–51 levels of the 1 3Δg state observed previously [J. Mol. Spectrosc. 134, 50 (1989)] have four components for transitions between symmetric rotational levels or five components for transitions between antisymmetric rotational levels and the hyperfine coupling belongs to Hund’s case bβS. For the low-N levels observed later, however, the hyperfine spectra are much more complicated. We have worked out the matrix elements of the molecular Hamiltonian in the case bβS basis. After taking into consideration spin–orbit, spin–spin, and spin–rotation interactions, we obtained a set of molecular constants for the Na2 1 3Δg state, with which we can reproduce the hyperfine spectra of both high- and low-N rotational levels.
Resonant and nonresonant electron impact detachment of CN− and BO−
Authors: Andersen L. H. , Bak J. , Boyé S. , Clausen M. , Hovgaard M. , Jensen M. J. , Lapierre A. , Seiersen K.
Published: July 2002

Absolute cross sections for electron-impact detachment and electron-impact dissociation of CN− and BO− were measured for electron energies from threshold to 40 eV. With both ions we see only little dissociation when electron detachment occurs. In the case of CN− a resonance in the detachment cross section is discovered at an energy of ∼10 eV. No resonances were seen in the case of BO−. We argue that the resonance observed for CN− is due to an excited state of the dianion. The nonresonant part of the detachment cross section is found to follow the classical prediction given by Andersen et al. [Phys. Rev. Lett. 74, 892 (1995)].
Equilibrium polymerization of cyclic carbonate oligomers
Authors: Ballone P. , Jones R. O.
Published: July 2002

A model of the polymerization of ring oligomers of bisphenol A polycarbonate (BPA-PC) is used to investigate the influence of dimensionality (2D or 3D), density and temperature on the size distribution of the polymer chains. The polymerization step is catalyzed by a single active particle, conserves the number and type of the chemical bonds, and occurs without a significant gain in either potential energy or configurational entropy. Monte Carlo and molecular dynamics simulations show that polymerization of cyclic oligomers occurs readily at high density and is driven by the entropy associated with the distribution of interparticle bonds. Polymerization competes at lower densities with long range diffusion, which favors small molecular species, and is prevented if the system is sufficiently dilute. Polymerization occurs in 2D via a weakly first order transition as a function of density and is characterized by low hysteresis and large fluctuations in the size of polymer chains. Polymerization occurs more readily in 3D than in 2D, and is favored by increasing temperature, as expected for an entropy-driven process.
Calculation of renormalized viscosity and resistivity in magnetohydrodynamic turbulence
Authors: Verma Mahendra K.
Published: July 2002

A self-consistent renormalization (RG) scheme has been applied to nonhelical magnetohydrodynamic turbulence with normalized cross helicity σc=0 and σc→1. Kolmogorov’s 53 power law is assumed in order to compute the renormalized parameters. It has been shown that the RG fixed point is stable for d⩾dc≈2.2. The renormalized viscosity ν* and resistivity η* have been calculated, and they are found to be positive for all parameter regimes. For σc=0 and large Alfvén ratio (ratio of kinetic and magnetic energies) rA, ν*=0.36, and η*=0.85. As rA is decreased, ν* increases and η* decreases, until rA≈0.25, where both ν* and η* are approximately zero. For large d, both ν* and η* vary as d−1/2. The renormalized parameters for the case σc→1 are also reported.
Monte Carlo simulation of liquid-crystal alignment and chiral symmetry-breaking
Authors: Xu Jianling , Selinger Robin L. B. , Selinger Jonathan V. , Shashidhar R.
Published: July 2002

We carry out Monte Carlo simulations to investigate the effect of molecular shape on liquid-crystal order. In our approach, each model mesogen consists of several soft spheres bonded rigidly together. The arrangement of the spheres may be straight (to represent uniaxial molecules), Z-shaped (for biaxial molecules), or banana-shaped (for bent-core molecules). Using this approach, we investigate the alignment of the nematic phase by substrates decorated with parallel ridges. We compare results for wide and narrow ridge spacing and examine local order near the substrates, and show that our results are consistent with the predictions of Landau theory. We also investigate chiral symmetry-breaking in systems of bent-core molecules. We find a chiral crystalline phase as well as a nonchiral smectic-A phase, but not a chiral smectic-C phase.
Limitations on the stabilization of resistive tearing modes
Authors: Gianakon T. A.
Published: July 2002

A study of the passive stabilization of resistive magnetohydrodynamic (MHD) tearing modes is presented in the context of zero β full-MHD equations with the NIMROD [A. H. Glasser et al., Plasma Phys. Controlled Fusion 41, A747 (1999)] code. The stabilization effect of the current is modeled by a source term in the Ohm’s law. This source term is evaluated based on a diffusive model with anisotropic diffusivities parallel and perpendicular to the perturbed magnetic field lines and driven by a spatially localized source. The effects of spatial localization of the current source are explored that illustrate stabilization down to a combination of source current channel width and parallel and perpendicular diffusion scale lengths.
Interferometric investigation of femtosecond laser-heated expanded states
Authors: Widmann K. , Guethlein G. , Foord M. E. , Cauble R. C. , Patterson F. G. , Price D. F. , Rogers F. J. , Springer P. T. , Stewart R. E. , Ng A. , Ao T. , Forsman A.
Published: July 2002

Simultaneous temporally and spatially resolved measurements of the phase change and reflectivity of S- and P-polarized femtosecond laser probes are obtained from hot expanded states produced by femtosecond laser heating of a solid aluminum target. The combined set of data provides an integral test of equation-of-state models in a regime up to 10 Mbar and densities of 0.01–1 times solid. The results suggest that target stoichiometry at the few Å level should be considered in the analysis of phase and reflectivity measurements in such experiments.
A density functional theory study of carbon monoxide oxidation on the Cu3Pt(111) alloy surface: Comparison with the reactions on Pt(111) and Cu(111)
Authors: Zhang C. J. , Baxter R. J. , Hu P. , Alavi A. , Lee M.-H.
Published: July 2002

Alloying metals is often used as an effective way to enhance the reactivity of surfaces. Aiming to shed light on the effect of alloying on reaction mechanisms, we carry out a comparative study of CO oxidation on Cu3Pt(111), Pt(111), and Cu(111) by means of density functional theory calculations. Alloying effects on the bonding sites and bonding energies of adsorbates, and the reaction pathways are investigated. It is shown that CO preferentially adsorbs on an atop site of Pt and O preferentially adsorbs on a fcc hollow site of three Cu atoms on Cu3Pt(111). It is also found that the adsorption energies of CO (or Oa) decreases on Pt (or Cu) on the alloy surface with respect to those on pure metals. More importantly, having identified the transition states for CO oxidation on those three surfaces, we found an interesting trend for the reaction barrier on the three surfaces. Similar to the adsorption energies, the reaction barrier on Cu3Pt possesses an intermediate value of those on pure Pt and Cu metals. The physical origin of these results has been analyzed in detail.
Measurement of piezoelectric field and tunneling times in strongly biased InGaN/GaN quantum wells
Authors: Jho Y. D. , Yahng J. S. , Oh E. , Kim D. S.
Published: July 2002

We have measured both spectrum- and time-resolved photoluminescence (PL) of InGaN/GaN light-emitting diode structure as a function of an external bias. From spectrum-resolved PL, we observed regions of blueshift and redshift in peak PL energies. From the bias point at which redshift begins, which we attribute to the inversion of electric field due to full compensation of the piezoelectric field (PEF), we estimate PEF to be 2.1±0.2 MV/cm. From time-resolved PL, we found the carrier lifetimes to drastically decrease (2.5 ns–2 ps) with increasing reverse bias. We attribute this decrease to escape tunneling through tilted barriers.
Magnetic field and temperature dependence of terahertz radiation from InAs surfaces excited by femtosecond laser pulses
Authors: Hangyo M. , Migita M. , Nakayama K.
Published: July 2002

The magnetic field (0–5 T) and temperature (10–300 K) dependence of terahertz radiation from InAs surfaces excited by femtosecond laser pulses has been studied in detail. The radiation intensity is strongly enhanced under the magnetic field and at low temperatures, which is explained based on simple classical motion of photoexcited electrons under a built-in surface electric field and Lorentz force undergoing scattering. The radiation spectra are broad even at low temperatures under high magnetic fields, which suggests that strong electron scattering mechanisms exist for photoexcited electrons at InAs surfaces.
Coulomb blockade in multiwalled carbon nanotube island with nanotube leads
Authors: Yoneya Nobuhide , Watanabe Eiichirou , Tsukagoshi Kazuhito , Aoyagi Yoshinobu
Published: July 2002

We propose and demonstrate a technique to form an ultrasmall island for a single-electron transistor in a multiwalled carbon nanotube (MWNT). The two small areas in the MWNT connected to electrodes were etched in oxygen plasma, resulting in the cutting of the MWNT into three pieces. An island and two leads connected the island through the tunneling barriers were made of the MWNT. In this device, we observed the Coulomb blockade effect at 4.5 K, and the estimated charging energy was 38 meV. This result is an essential step for the use of carbon nanotubes in nanoscale electronics circuits.
Transient photocurrent overshoot in quantum-well infrared photodetectors
Authors: Letov V. , Ershov M. , Matsik S. G. , Perera A. G. U. , Liu H. C. , Wasilewski Z. R. , Buchanan M.
Published: July 2002

We report a strongly nonexponential behavior of the transient photocurrent in quantum-well infrared photodetectors (QWIPs) in response to a step-like infrared illumination. The transient photocurrent displays an overshoot on the time scale 0.1–1 ms at low temperatures (T<70 K), exceeding the steady-state photocurrent by as much as ≈50%. The overshoot behavior is attributed to a nonlinearity of responsivity caused by the modulation of the electric field in QWIP under relatively high illumination power, when the photocurrent exceeds the dark current. This explanation is confirmed by the experimental data and numerical simulation. These effects can play an important role when QWIPs operate in nonlinear regimes, such as in a heterodyne mode or in low-temperature and low-background applications.
Formation mechanisms for carbon onions and nanocapsules in C+-ion implanted copper
Authors: Abe Hiroaki , Yamamoto Shunya , Miyashita Atsumi , Sickafus Kurt E.
Published: July 2002

Copper substrates were implanted with carbon ions at temperature ranging from 570 to 973 K. Implantation microstructures were investigated using transmission electron microscopy and high-resolution electron microscopy. Carbon onions and nanocapsules were observed together with amorphous carbon layers. Most of the nanocapsules were found to be hollow and rarely included copper nanoparticles. The encapsulating of Cu nanoparticles with graphene layers, the gradual shrinkage of the encapsulated clusters, and finally the disappearance of the clusters (leaving behind hollow nanocapsules) were observed under electron irradiation at 783 K. Statistics of cluster size as a function of ion fluence, implantation temperature, and substrate crystallinity gave insights into the nucleation processes of onions and nanocapsules. One process involves the formation of graphene layers on grain boundaries to encapsulate copper particles. The other process is the nucleation of graphene cages, probably fullerenes, due to high concentration of carbon atoms and high amount of radiation damage.
Co-implantation of Be+O and Mg+O into GaN
Authors: Kent D. G. , Overberg M. E. , Pearton S. J.
Published: July 2002

Beryllium and magnesium were co-implanted with oxygen into GaN at precise donor-to-acceptor ratios of 0.5–2. High activation efficiency was achieved for Be+O co-implants at a donor–acceptor ratio of 2 and anneal temperatures of 1050–1100 °C. The activation process was not a simple, single-step mechanism. Hall data indicated a relatively low (⩽100 meV) ionization energy for Be in the co-implanted samples. This process may have application in improving p-ohmic contact resistance by selective area Be+O implantation. By sharp contrast Mg+O co-implants produced n-type conductivity under all conditions.
Self-sputtering of silver by mono- and polyatomic projectiles: A molecular dynamics investigation
Authors: Lindenblatt M. , Heinrich R. , Wucher A. , Garrison B. J.
Published: July 2002

The self-sputtering of silver under bombardment with Agm (m=1,2,3) projectiles has been investigated by molecular dynamics (MD) simulation using the many body MD/MC-Corrected Effective Medium Potential developed by DePristo and co-workers. More specifically, the total sputtering yield as well as the mass distribution, i.e., the distribution of monomers and clusters within the flux of sputtered particles were calculated. For di- and triatomic projectiles, we observe a pronounced dependence of the calculated yields on the orientation of the incoming cluster, whereas the internuclear distance (and thus vibrational excitation of the projectile) does not seem to play a significant role. When averaged over the impact orientation, the calculated yields per projectile atom exhibit a distinct nonlinear enhancement when compared to the respective values calculated for monatomic projectiles of the same impact velocity. The abundances of nascent and final Agn clusters (identified immediately above and far away from the surface, respectively) within the sputtered flux are found to be significantly enhanced under polyatomic projectile bombardment, the effect increasing with increasing size of the sputtered cluster. Moreover, clusters produced under polyatomic bombardment appear to be colder, a finding which might be of considerable interest in the light of mass spectrometric surface analysis techniques.
Electromigration in Cu thin films with Sn and Al cross strips
Authors: Michael Nancy L. , Kim Choong-Un
Published: July 2002

Electromigration in Cu thin films is studied in a cross-strip configuration. Cu lines with isolated areas of Cu(Al) or Cu(Sn) are tested between 250 and 390 °C with the following results. The hillock and void marker motion indicates that Sn moves in the direction of electron flow. The marker polarity indicates that it decreases the grain boundary electromigration of Cu, in agreement with previous studies. This study also finds evidence of active surface migration in Cu. During tests in forming gas, hillocks and voids form adjacent to a native Al2O3 layer at all temperatures, indicating the likelihood that Cu migrates faster through the Cu free surface than the interface between the surface layer of Al2O3 and Cu(Al). Active surface migration in Cu thin films is also evidenced by the growth of hillocks with highly developed facets, most of which are attached to the underlying film by narrow necks.
Enhanced ultraviolet emission and optical properties in polyvinyl pyrrolidone surface modified ZnO quantum dots
Authors: Yang C. L. , Wang J. N. , Ge W. K. , Guo L. , Yang S. H. , Shen D. Z.
Published: July 2002

Optical properties of ZnO quantum dots (QDs) capped with polyvinyl pyrrolidone (PVP) molecules have been investigated. It is demonstrated that surface modification by PVP can dramatically change the emission spectra of the ZnO QDs. At the optimized condition with a PVP/Zn2+ ratio of 3:5, the photoluminescence (PL) spectrum of ZnO QDs shows a strong ultraviolet (UV) emission while the low energy green emission is fully quenched. This is a result of the surface passivation of the ZnO QDs by the PVP molecules. The origin of the green emission is attributed to the surface states associated with oxygen vacancies. Temperature and excitation power dependent PL studies suggest that the UV emission is associated with localized states.
Influence of doping on the two-dimensional electron gas distribution in AlGaN/GaN heterostructure transistors
Authors: Chu R. M. , Zhou Y. G. , Zheng Y. D. , Han P. , Shen B. , Gu S. L.
Published: July 2002

A study of two-dimensional electron gas distribution in AlGaN/GaN heterostructure field effect transistors is performed by solving the coupled Schrödinger’s and Poisson’s equation self-consistently. Taking the piezoelectric effect into account, the two-dimensional electron gas concentration is calculated to be as high as 1019 cm−3. To gain an understanding on how the two-dimensional electron gas distribution is influenced by dopant concentration in material, we observed the two-dimensional electron gas concentration and occupation of subbands versus doping level in GaN and in AlGaN layer. The results show that the two-dimensional electron gas concentration depends much more strongly on the doping level in AlGaN than in GaN. And besides, the heavier doping in GaN should weaken the quantum confinement in the AlGaN/GaN heterointerface.
Broadband composite spin-state-selective rotations for NMR spectroscopy on partially aligned molecules
Authors: Schulte-Herbrüggen Thomas , Untidt Thomas S. , Nielsen Niels Chr. , Sørensen Ole W.
Published: July 2002

Composite spin-state-selective rotations are introduced. They are devised to remedy the following problem: When determining scalar plus residual dipolar couplings by conventional NMR experiments in partially aligned molecules, a considerable variation in the effective coupling constant often entails systematic errors compromising the accuracy of the measurements. The concept of composite pulses is extended in order to design spin-state-selective composite rotations (S3CR) robust to variation in effective coupling constants. With the S3CR approach the broadband performance of spin-state-selective excitation improves significantly as is demonstrated both numerically and experimentally.