bluuebunny/crossref_metadata_2025 huggingface dataset:
Dissociative chemisorption of CH4 on a cesiated Pt(111) surface studied by supersonic molecular beam scattering techniques
Authors: Kondo Takahiro , Sasaki Takashi , Yamamoto Shigehiko
Published: July 2002

We have clarified the dynamics and mechanism of CH4 dissociative chemisorption processes on a cesiated Pt(111) surface by means of molecular beam scattering techniques. A comparison is made of similar processes on a Pt(111) surface. It is found that the CH4 molecules with enough energy to overcome the activation barrier for dissociation can no longer dissociate on a cesiated surface of Pt(111). Time-of-flight and angular intensity distribution measurements of the reflected CH4 molecules have revealed a similarity in inelastic collision dynamics both on the Pt(111) and the cesiated Pt(111) surfaces. Even a low concentration of the Cs layer as 0.06 of coverage is found to modify the surface electronic structure such that the activation barrier is enhanced as a result of an increased Pauli repulsive potential component in the potential energy surface. It is also found from time-of-flight measurements that direct dissociation is the only pass way and precursor mediated dissociation is not possible both on the Pt(111) and the cesiated Pt(111) surfaces in our experimental conditions.
Collision induced phenomena on the CO/H/Ni(100) surface
Authors: Takaoka T. , Inamura M. , Yanagimachi S. , Kusunoki I.
Published: July 2002

The effect of the collision of atoms to the coadsorbed system of CO and H on Ni(100) has been studied using Xe atoms energy-controlled by supersonic molecular beam. After the surface was irradiated with Xe atoms, the intensity of the CO stretching mode observed at 2100 cm−1 in the FTIR spectrum decreased and the CO stretching mode at 1950 cm−1 appeared. This indicates that CO was shifted from on-top sites to bridge sites. It is considered that the collision of Xe atoms induces desorption of CO and rearrangement of the CO adlayer.
Properties of the path-integral quantum hard-sphere fluid in k space
Authors: Sesé Luis M.
Published: July 2002

The properties of quantum fluids in Fourier space, as the system response functions to weak external fields, are analyzed taking the quantum hard-sphere fluid as a probe. This serves to clarify the physical meaning of the different radial correlation functions that can be defined in a path-integral quantum fluid, since these functions are the r-space counterparts of the response functions. The basic feature of the external field relevant to this discussion is connected with its localizing/nonlocalizing effect on the quantum particles composing the fluid (i.e., a localizing field causes the collapse of the particle thermal packet). Fields that localize the quantum particles reveal the so-called instantaneous quantities (e.g., the conventional static structure factor), which are related with the diagonal elements of the density matrix. Fields that do not localize the quantum particles show the so-called linear response quantities, which are related to the diagonal and the off-diagonal density matrix elements. To perform this study the path-integral formalism is considered from the functional analysis approach. Given that the Gaussian Feynman–Hibbs effective potential picture is known to represent well many structural features of the quantum hard-sphere fluid, the parallel study of the response functions within this picture is also presented. In particular, the latter picture provides an accurate Ornstein–Zernike scheme that can be used for numerical calculations of response functions over a wide range of conditions, and also gives fine estimates for quantities difficult to compute with the path integral. Results for the quantum hard-sphere fluid obtained within the latter scheme are reported, tests of consistency are given, and the possibility of approximating the instantaneous response function by means of the coherent part of the linear response function is assessed.
Symmetry-induced perfect transmission of light waves in quasiperiodic dielectric multilayers
Authors: Peng R. W. , Huang X. Q. , Qiu F. , Wang Mu , Hu A. , Jiang S. S. , Mazzer M.
Published: July 2002

Resonant transmission of light has been observed in symmetric Fibonacci TiO2/SiO2 multilayers, which is characterized by many perfect transmission peaks. The perfect transmission dramatically decreases when the mirror symmetry in the multilayer structure is deliberately disrupted. Actually, the feature of perfect transmission peaks can be considered as general evidence for dielectric multilayers with symmetric internal structure. It opens a unique way to control light propagation.
Phase transitions of one-component fluids adsorbed in random porous media: Monte Carlo simulations
Authors: Brennan J. K. , Dong W.
Published: July 2002

The Gibbs-ensemble Monte Carlo method and the Gibbs–Duhem integration scheme are adapted for the simulation of the phase equilibrium of a one-component fluid confined in random porous media. The validity of these methods in the case of rigid porous samples is established by comparing our results with those obtained previously from a series of adsorption isotherms. It is shown that the Gibbs-ensemble and Gibbs–Duhem integration methods significantly improve the efficiency of the simulation of these systems. Such a gain in efficiency allowed us to carry out a systematic investigation of the influence of several characteristics of disordered porous solids (e.g., porosity, pore size distribution, and solid–fluid interaction) on the phase behavior of the confined fluid. Rich phase behaviors have been observed, e.g., multiple fluid–fluid phase transitions and an extreme sensitivity of phase diagram on the microscopic structure of the porous samples. Efforts were devoted to understanding the origins of such rich behavior by analyzing the simulation results in considerable detail.
Asymmetric solvation revisited: The importance of hydrogen bonding in iodide–acetonitrile clusters
Authors: Timerghazin Qadir K. , Nguyen Tao-Nhân , Peslherbe Gilles H.
Published: July 2002

The concept of asymmetric solvation in I−(CH3CN)2 clusters [C. E. H. Dessent, C. G. Bailey, and M. A. Johnson, J. Chem. Phys. 103, 2006 (1995)] was revisited in light of quantum chemistry calculations of the cluster structures, energetics and charge-transfer-to-solvent transition energies. In addition to the two “classical” linear structures—with a symmetrically and an asymmetrically solvated iodide—that were postulated previously, a nonlinear hydrogen-bonded isomer was also found. The calculated binding energies and vertical excitation energies for the linear symmetric and nonlinear hydrogen-bonded I−(CH3CN)2 isomers are in very good agreement with experimental data, while the linear asymmetric structure appears unlikely to be involved in experiments. The importance of hydrogen bonding in halide–acetonitrile clusters manifests itself, not only in the fact that a stable nonlinear hydrogen-bonded isomer exists, but also in slight geometrical distortions of the “classical” linear cluster structures.
Analysis of the equations-of-state of water in the metastable region at high pressures
Authors: Tchijov Vladimir
Published: July 2002

We analyze the existing equations-of-state of liquid water from the point of view of their applicability to equilibrium and nonequilibrium water–ice phase change problems that involve supercooled water. We show that the equation-of-state of Saul and Wagner [J. Phys. Chem. Ref. Data 18, 1537 (1989)] is most suitable for the description of thermodynamic properties of supercooled water in the range of pressures 0–200 kbar, and find the area of its validity in the metastable region.
Convection in the presence of one or more thermally conducting particles: Resonant wavelength excitation by a planar array
Authors: Davis A. M. J.
Published: July 2002

The effect of small, thermally conducting particles on the flow and its stability of the fluid in which they are present has long been of physical interest. In particular, the classic Rayleigh–Bénard stability problem has features that can be thus modified. A single particle can excite subcritical, weak convection, as expected, while a planar array of such particles can adjust the mean Nusselt number at the walls due to their heat capacity. It is also shown, by a standard weakly nonlinear analysis, that the periodic array induces an imperfect bifurcation at the critical Rayleigh number provided that the particles are asymmetrically placed.
Room-temperature observation of high-spin polarization of epitaxial CrO2(100) island films at the Fermi energy
Authors: Dedkov Yu. S. , Fonine M. , König C. , Rüdiger U. , Güntherodt G. , Senz S. , Hesse D.
Published: July 2002

Epitaxial CrO2(100) island films have been grown on TiO2(100) substrates by a chemical-vapor deposition technique. Well-controlled surface and interface properties of the CrO2(100) films were confirmed by scanning tunneling microscopy and transmission electron microscopy, respectively. Spin- and angle-resolved photoemission spectroscopy at room temperature revealed an energy gap of about 2 eV below Fermi level EF for spin-down electrons and a spin polarization of about +95% at EF. After extended sputtering, the spin polarization can be recovered from about +10% up to +85% upon annealing.
Pesin’s dimension for Poincaré recurrences
Authors: Afraimovich V.
Published: July 2002

A new characteristic of Poincaré recurrences is introduced. It describes an average return time in the framework of a general construction for dimension-like characteristics. Some examples are considered including rotations on the circle and the Denjoy example.
Synchronizing spatiotemporal chaos
Authors: Kocarev Ljupčo , Tasev Žarko , Stojanovski Toni , Parlitz Ulrich
Published: July 2002

We show analytically and numerically that a pair of uni-directionally coupled spatially extended systems can synchronize. For the case of partial differential equations the synchronization can be achieved by applying the scalar driving signals only at finite number of space points. Our approach is very general and can be useful for practical applications since the synchronization is achieved via feeding in the response system only the information from certain (discrete) spatial locations of the drive system. We also stress some open problems in the field of synchronization of spatiotemporal chaos.
Enhancing aperiodic stochastic resonance through noise modulation
Authors: Chow Carson C. , Imhoff Thomas T. , Collins J. J.
Published: July 2002

We show that the conventional stochastic resonance (SR) effect for aperiodic signals in a model neuron can be enhanced by modulating the intensity of the input noise (which could be introduced artificially in bioengineering applications) with either the input signal or the unit’s output rate signal. We analyze SR enhancement theoretically and numerically. We discuss how this work provides the theoretical foundation for the development of an optimal noise-based technique for enhancing sensory function.
Dimensional perturbation theory on the Connection Machine
Authors: Germann Timothy C. , Herschbach Dudley R. , Boghosian Bruce M.
Published: July 2002

A recently developed linear algebraic method for the computation of perturbation expansion coefficients to large order is applied to the problem of a hydrogenic atom in a magnetic field. We take as the zeroth order approximation the D→∞ limit, where D is the number of spatial dimensions. In this pseudoclassical limit, the wave function is localized at the minimum of an effective potential surface. A perturbation expansion, corresponding to harmonic oscillations about this minimum and higher order anharmonic correction terms, is then developed in inverse powers of (D−1) about this limit, to 30th order. To demonstrate the implicit parallelism of this method, which is crucial if it is to be successfully applied to problems with many degrees of freedom, we describe and analyze a particular implementation on massively parallel Connection Machine systems (CM-2 and CM-5). After presenting performance results, we conclude with a discussion of the prospects for extending this method to larger systems.
Generation of tunable laser sidebands in the far-infrared region
Authors: Farhoomand Jam , Blake Geoffrey A. , Frerking M. A. , Pickett Herbert M.
Published: July 2002

Continuously tunable laser sidebands have been generated by mixing radiation from an optically pumped far infrared (FIR) molecular laser, operated at 693, 762, 1627, and 1839 GHz, with that from millimeter-wave klystrons in a Schottky-barrier diode. An enhancement in conversion efficiency over similar systems reported previously is obtained by using a Michelson interferometer to separate the sidebands from the carrier and by placing the Schottky diode in an open structure corner cube mount. With 4 mW of laser power at 693 and 762 GHz the sideband power was measured to be 3.0 μW. This is at least an order of magnitude better than the previously reported results. At higher frequencies, 22 mW of 1627-GHz laser power produced about 2.5 μW of sideband output, while 3mW of 1839-GHz laser power generated about 100 nW of sideband radiation. The lower efficiency at the higher frequencies is due primarily to the mismatch between the laser radiation and the fixed-length diode antenna. To demonstrate the tunability of the generated far-infrared radiation, the laser sidebands were swept through absorption lines of HDO and H2CO near 600 and 800 GHz. The absorption signals were easily seen, using either video or lock-in detection techniques.
Superconducting properties of layered Nb0.53Ti0.47/Ge structures prepared by dc sputtering
Authors: Jin B. Y. , Shen Y. H. , Yang H. Q. , Wong H. K. , Hilliard J. E. , Ketterson J. B. , Schuller I. K.
Published: July 2002

Sandwich and superlattice structures composed of the high critical field superconducting alloy Nb0.53Ti0.47 (where the compositions refer to weight percent) and amorphous Ge were sputter-deposited on sapphire substrates using a novel multi-sputter-gun system. The thicknesses of both the superconducting Nb0.53Ti0.47 and the insulating Ge layers were varied over a wide range in order to study the two-dimensional (2D) and three-dimensional (3D) superconducting properties of these structures. The observed suppression of the measured resistive superconducting transition temperature Tc with increasing sheet resistance is tentatively interpreted using the Maekawa–Fukuyama theory which incorporates contributions from localization and Coulomb interaction effects. The upper critical fields, with the field both normal Hc2⊥ and parallel Hc2∥ to the film, were measured up to 50 kG. Hc2∥ exhibited 2D behavior when the thickness of the superconducting layer was less than 200 Å. The zero temperature values of Hc2∥(0) were estimated using the Rickayzen formula, which is valid in the regime l≪(lξ0)1/2≪Ds (where l, ξ0, and Ds are the mean free path, pure material coherence length, and film thickness, respectively). The estimated Hc2∥(0) peaks at about 400 kG for a superconducting layer thickness of 144 Å. This rather high upper critical field, which is considerably above the bulk value of 145 kG, likely has its origin in flux line accommodation within the insulating Ge layers. Since the Rickayzen expression neglects paramagnetic limiting, the true Hc2∥(0) is expected to be somewhat lower; localization/interaction and spin orbit coupling effects are also neglected in this treatment. The situation is none the less encouraging and suggests multilayer superconductors may have a place in high-field magnet technology.
Lateral stress measurement in shock-loaded targets with transverse piezoresistance gauges
Authors: Rosenberg Z. , Partom Y.
Published: July 2002

We present an analytical model for the calibration of lateral piezoresistance stress gauges in shock-wave experiments. This calibration is based on our experimental data with transverse Manganin gauges in shock-loaded polymethylmethacrylate targets. The experiments included lateral stress measurements under shock loading in the 1–30-kbar range and under complete unloading from stresses in the 5–9-kbar range. These unloading measurements resulted in a negative resistance change for the transverse Manganin gauge which implies that a hydrodynamic tension exists in it. The validity of our analytical calibration is demonstrated by comparing measured and predicted lateral stresses in glass targets.
Preparation and electrochromic properties of rf-sputtered molybdenum oxide films
Authors: Miyata N. , Akiyoshi S.
Published: July 2002

Properties of rf-sputtered molybdenum oxide films for electrochromic display devices have been investigated. This report mainly concerns the dependence of the properties on the oxygen concentration in the sputtering atmosphere. The oxide films were prepared by rf sputtering from a compressed powder MoO3 target under an operating pressure of 4×10−2 Torr using a mixture of Ar-0.5–50% O2. Electrical resistivity of the films formed increases with increasing oxygen concentration in the sputtering atmosphere and ranges from 2.5×1010 to ∼1×1012 Ω cm. Spectral transmittance of the films is about 85% in the visible and near-infrared region. Optical band gap and refractive index of these films are 2.67–2.76 eV and 2.01–2.26, respectively. Electrochemichromic properties of the films were also studied using asymmetric cells, and it was found that a good electrochromic performance was obtained usually by the cells composed of the films with a resistivity of 2.5×1010 Ω cm.
Calibrated mapping of the electrical activity in deformed silicon by means of charge-collection microscopy
Authors: Snyman L. W. , Snyman H. C. , Engelbrecht J. A. A.
Published: July 2002

A method has been developed to obtain, by means of charge-collection microscopy (CCM), a calibrated two-dimensional map of the electrical activity of the minority carriers in a semiconductor crystal over large crystal dimensions. This is achieved by the calibration of the collection current in terms of the minority-carrier diffusion length and a photographic gray scale. The technique is applied to plastically deformed silicon and proves that it has great potential for the evaluation of electrical activity mechanisms in imperfect silicon.
Spatially resolved detection of O atoms in etching plasmas by two-photon laser-induced fluorescence
Authors: Selwyn Gary S.
Published: July 2002

Spatially resolved concentration profiles of ground-state oxygen atoms in O2/Ar plasmas have been obtained under loaded etching conditions through the use of a two-photon laser excitation process. These results provide a quantitative measure of the reactive atom concentration gradient during etching of kapton or graphite on the rf-driven electrode. The effects of load, ion bombardment, and diffusion on the reactive atom concentration may be directly monitored by this in situ, unobtrusive, three-dimensional probe technique.
Electron transport measurements in methane using an improved pulsed Townsend technique
Authors: Hunter S. R. , Carter J. G. , Christophorou L. G.
Published: July 2002

An improved pulsed Townsend technique for the measurement of electron transport parameters in gases is described. The accuracy and sensitivity of the technique have been investigated by performing, respectively, electron attachment coefficient measurements in pure O2 over a wide range of E/N at selected O2 pressures and by determining the electron attachment and ionization coefficients and electron drift velocity in CH4 over a wide E/N range. Good agreement has been obtained between the present and the previously published electron attachment coefficients in O2 and for the drift velocity measurements in CH4. The data on the electron attachment coefficient in CH4 (measured for the first time) showed that with the present improved pulsed Townsend method, electron attachment coefficients up to 10 times smaller than the ionization coefficients at a given E/N value can be accurately measured. Our measurements of the electron attachment and ionization coefficients in CH4 are in good agreement with a Boltzmann equation analysis of the electron gain and loss processes in CH4 using published electron scattering cross sections for this molecule.
Theoretical and experimental studies of LiNbO3 degenerate acoustic elastic convolver
Authors: Cho Yasuo , Yamanouchi Kazuhiko
Published: July 2002

The crystal orientation dependencies of the figure of merit (M value) of different types of LiNbO3 degenerate acoustic elastic convolvers (TVC, THC, BWC) have been theoretically and experimentally investigated. First, for TVC, it is shown that the theoretical M values using the nonlinear constants measured by authors these days have been in good agreement with their measured values. Second, for THC, the new definition of the figure of merit (M value) and practically effective equivalent circuit are proposed and the existence of the crystal cut and the direction of propagation with high efficiency and large electromechanical coupling coefficient is shown. Finally, this paper presents the efficient bulk wave mode for LiNbO3 BWC by defining and calculating the M value for BWC.
Observation of soft x-ray amplification in neonlike molybdenum
Authors: MacGowan B. J. , Rosen M. D. , Eckart M. J. , Hagelstein P. L. , Matthews D. L. , Nilson D. G. , Phillips T. W. , Scofield J. H. , Shimkaveg G. , Trebes J. E. , Walling R. S. , Whitten B. L. , Woodworth J. G.
Published: July 2002

Thin molybdenum coated foils have been irradiated in line focus geometry with from 3 to 8×1014 W cm−2 of 0.53-μm light at the Nova laser. The resulting exploding foil plasma has demonstrated x-ray laser gain at four wavelengths (106.4, 131.0, 132.7, and 139.4 Å), identified as 3s-3p transitions in neonlike Mo. The J=0–1, a 3s–3p transition at 141.6 Å has been identified, but does not show evidence of significant gain in disagreement with the theory.
Soliton spin configurations along the classical anisotropic Heisenberg chain
Authors: Gaulin B. D.
Published: July 2002

The spin configurations of a one-dimensional, weakly anisotropic Heisenberg antiferromagnet were examined by means of an importance sampling Monte Carlo simulation. The spin interactions display weak anisotropies of the same form in each of the orthogonal XY and XZ planes. This allows for two soliton modes, with one in each of the planes. The soliton density within the XZ plane has been determined as a function of the relative strengths of the two anisotropies as well as of temperature. These results are compared with theoretical analysis of similar systems.
Exchange coupling of ferromagnetic films across nonmagnetic interlayers
Authors: Swiatek P. , Saurenbach F. , Pang Y. , Grünberg P. , Zinn W.
Published: July 2002

We have studied exchange coupling between ferromagnetic films across nonmagnetic intermediate layers by means of light scattering from spin waves and microwave absorption. The ferromagnetic films were Ni0.8Fe0.2 (permalloy) and Fe and we chose Au, Ag, Cu, V, Cr, Pd, Ge, Si, Mn, and Bi as nonmagnetic interlayer materials. As expected the effective interlayer exchange decreases when the interlayer thickness d0 increases. Minimum thickness d0 where the ferromagnetic films are exchange decoupled range between 10 and 40 Å. We compare these results with the concentration dependence of the magnetic ordering temperature in alloys of Fe and Ni with other elements, taking this as a measure for the strength of the exchange. There is a good correspondence which indicates that the coupling effects in the layered structures and the alloys have the same origin which is presumably by RKKY interaction.
Observational strategies for three-dimensional synchrotron microtomography
Authors: Flannery Brian P. , Roberge W. G.
Published: July 2002

We describe essential features of a data set of projection measurements suitable for tomographic imaging with given accuracy and resolution, and apply the results to analyze the use of synchrotron radiation to perform three-dimensional microtomography. Previously, Grodzins [Nucl. Instrum. Methods 206, 541 (1983)] showed that the number of photons needed to generate an image could be minimized by adjusting the observational energy such that the target’s optical depth FD=2. We correct and extend his results to consider also the spectral distribution of the source. Observational time is minimized at a (typically) lower observational energy where the source flux is larger, even though FD≥2. The results demonstrate that, in principle, synchrotron sources provide sufficient brightness to image small samples with ≂1-μm resolution.
Linear surface photoelectric effect of gold in intense laser field as a possible high-current electron source
Authors: Farkas Gy. , Horváth Z. Gy. , Tóth Cs. , Fotakis C. , Hontzopoulos E.
Published: July 2002

Investigations were conducted on radiation-induced electron emission processes on a gold target surface with a high-intensity (2 MW/cm2) KrF laser (λ=248 nm). The single photon surface photoelectric emission obtained can be used for high-current density electron sources. The measured polarization dependence of electron current shows the dominance of the surface-type effect over that of the volume type, thereby making it possible to optimize the short, high-density electron current creation conditions. The advantage of the grazing light incidence and the multiphoton photoeffect giving rise to a 500 A/cm2 electron current has been demonstrated.
Defect production in electron-irradiated, n-type GaAs
Authors: Look D. C. , Sizelove J. R.
Published: July 2002

Temperature-dependent Hall-effect measurements have been performed on pure, n-type, vapor-phase epitaxial GaAs, irradiated by 1-MeV electrons at room temperature. The energies and production rates of two dominant defect centers, C2 and C3, are as follows: E2=EC−0.148, E3=EC−0.295 ±0.002 eV, τ2=2.0, and τ3=0.5±0.2 cm−1, in good agreement with deep level transient spectroscopy (DLTS) data. However, the most important result of this study is a very high production rate, τAS ≂4±1 cm−1, for ‘‘shallow’’ acceptors (CAS) lying below E3. In fact, CAS is produced at a much higher rate than all of the DLTS traps observed in this energy range, proving that close to half of the primary defects in electron-irradiated GaAs are evidently not seen by DLTS. The high CAS production rate has important implications for microscopic models of C1 and C2, rendering unnecessary the assumption that one of these centers must be an acceptor in order to explain the Hall-effect results. Finally, we show that all available Hall-effect and DLTS data are consistent with CAS representing Ga-sublattice damage, which has not been observed before.
A surface laser-light scattering spectrometer with adjustable resolution
Authors: Dorshow R. B. , Hajiloo Azardokht , Swofford Robert L.
Published: July 2002

We describe a surface laser-light scattering (SLLS) spectrometer, which uses optical heterodyne detection to measure the temporal behavior of the thermally generated capillary waves at a fluid/fluid interface. From the measurement the interfacial tension and one of the bulk fluid viscosities are calculated. An instrument resolution model is described that characterizes the resolving power of the SLLS spectrometer based on the optical parameters measured at the photodetector. The instrument resolution is defined and measured, and the resolution is adjusted to optimize the signal-to-noise ratio. The model was verified using light-scattering measurements with wave numbers in the range 250–1750 cm−1 and with instrument resolution varied by more than a factor of 2. Measurements were made on several liquids whose viscosities vary over an order of magnitude. Data reduction used the analysis of thermally generated capillary waves recently published [A. Hajiloo and J. C. Slattery, J. Colloid Interface Sci. 112, 325 (1986)]. The values calculated for both surface tension and liquid bulk viscosity were in good agreement with the literature.
Generalized analysis of the spherical lens as a launcher of electromagnetic missiles
Authors: Shen Hao-Ming , Wu Tai Tsun , King Ronold W. P.
Published: July 2002

It is found that, like the directly transmitted radiation field previously investigated, an internally multiply reflected radiation field can also become an electromagnetic missile under certain different electromagnetic-missile conditions. The analysis is carried out in a general way, i.e., for the point source located either inside or outside the dielectric sphere. The electromagnetic-missile conditions are obtained for general multiply reflected radiation fields.
Computer modeling of negative ion beam formation
Authors: Whealton J. H. , Bell M. A. , Raridon R. J. , Rothe K. E. , Ryan P. M.
Published: July 2002

Sources of root-mean-square (RMS) emittance growth are described for negative ion extractors in both volume and surface negative ion sources and plasma low-energy beam transport systems. For surface negative ion sources of the Lawrence Berkeley Laboratory or Los Alamos National Laboratory [on Los Alamos Meson Particle Facility (LAMPF)], attention is paid to the nonlinear transverse emittance growth mechanism of the beam/warm plasma interaction. In some cases this is a large effect. In addition, non-normal sheath fields at a convertor are examined as a source of emittance growth. For volume sources, attention is paid to aberration production due to field penetration from the extractor, RMS emittance growth enhancement by a negatively biased plasma extraction electrode, and emittance growth caused by transverse extraction across a magnetic field. For both volume and surface sources, RMS emittance growth due to nonlinear aberration at the plasma extraction electrode is analyzed. Time-dependent contributions to emittance growth are also examined. Nonlinear saturation of ion acoustic waves caused by ion extraction from a warm plasma and beam transversal through a plasma are studied.
Ferromagnetic resonance in ultrahigh vacuum of bcc Fe(001) films grown on Ag(001)
Authors: Urquhart K. B. , Heinrich B. , Cochran J. F. , Arrott A. S. , Myrtle K.
Published: July 2002

Ferromagnetic resonance studies carried out in ultrahigh vacuum at 16.88 GHz on bcc Fe (001) films 5–14.2 monolayers (ML) thick grown on Ag (001) substrates indicate that an ultrathin Fe film 5 ML thick should be magnetized perpendicular to the specimen plane at room temperature. Covering the bare Fe specimens with Ag causes a substantial reduction in the uniaxial surface anisotropy for all Fe film thicknesses and would put the moment of a 5-ML film back into the plane. For a given Fe film thickness, the maximum obtainable uniaxial surface anisotropy depends on both the amount of oxygen contamination in the film and on the surface roughness.
Mechanical properties of mixed silver-halide crystals and polycrystalline optical fibers
Authors: Barkay N. , Levite A. , Moser F. , Katzir A.
Published: July 2002

Mechanical properties of mixed AgClxBr1−x (0≤x≤1) crystals and extruded polycrystalline optical fibers were investigated as a function of composition. The microhardness of as-grown crystals, annealed samples, and fiber-end faces show solid-solution hardening in agreement with the Kataoka–Yamada model [Jpn. J. Appl. Phys. 16, 1119 (1977)]. The fibers have a tensile strength of 50–90 MPa with similar solid-solution strengthening. The infrared transmission of 0.9-mm-diam fibers is not reduced significantly upon bending them 90° on a 3-mm radius. Such mixed-crystal-based silver-halide fibers are suitable for many infrared optical applications.
Quantitative depth profiling with photoacoustic spectroscopy using a new approximation method based on inversion of the Laplace transform
Authors: Harata Akira , Sawada Tsuguo
Published: July 2002

A new and simple approximation method for quantitative depth profiling using photoacoustic spectroscopy (PAS) is described. In the method, an approximate calculation of the inversion of the Laplace transform is adopted in order to simplify the calculation of the depth-dependent optical-absorption coefficient from photoacoustic frequency responses. Some results of simulations shown demonstrate the effectiveness of this method. The method has been applied to the depth profiling of the practical sample in which the optical-absorption coefficient varies continuously with the depth from the surface. The result is qualitatively consistent with microscopic observations of the cross section of the specimen. This approximation could be useful not only for the depth profile with PAS but also for the general depth profile calculations based on the Laplace transformation.
High electron density effects in electron spectroscopies: Consequences for picosecond photoemission and electron-beam sampling
Authors: Clauberg Rolf , Blacha Armin
Published: July 2002

High electron density effects on photoyield and electron energy distributions have become important in several electron spectroscopies and in electron microscopy since the availability of high-intensity photon and electron sources. In this work we show that such effects also lead to limitations in voltage sensitivity and time resolution in photoemission and electron-beam sampling measurements of voltage signals if picosecond time resolution is required. The influence of high electron densities in picosecond pulses for these methods is analyzed theoretically and compared with experimental results in photoemission sampling. It is shown that the space-charge limitation to the number of electrons per pulse which can be emitted from a sample and the energy broadening of the pulse owing to the Coulomb repulsion between the electrons impose significant limitations on the voltage sensitivity of photoemission sampling and of electron-beam sampling with a pulsed high-intensity cathode. Also the time resolution is degraded by the space-charge limitation in photoemission sampling and by the Coulomb broadening in electron-beam sampling. The theoretical analysis of these effects is done within a simple electrostatic model for the space-charge and Coulomb effect and a derivation of a generalized brightness condition from the Liouville theorem of statistical physics which includes the high electron density effects.
Diamond formation on platinum
Authors: Harris Stephen J. , Belton David N. , Weiner Anita M. , Schmieg Steven J.
Published: July 2002

We have made in situ mass spectral measurements and x-ray photoelectron spectroscopy (XPS) measurements at the surface of a diamond film growing on a platinum substrate. We measured the concentrations of CH4, C2H4, and C2H2 and detected additional species with from three to ten carbon atoms. The gas-phase chemical kinetics controlling the concentrations of the C1 and C2 species was modeled, and agreement between the calculated and measured concentrations was good. The presence or absence of the platinum foil had no effect on the measured concentrations, showing that heterogeneous chemistry on platinum did not affect the gas-phase environment. XPS spectra were taken during the course of the diamond growth without exposing the platinum foil to air. After exposure to a room-temperature CH4/H2 mixture but before any growth, the platinum surface was mostly covered with graphitic carbon. Once growth was initiated the graphitic layer was gradually replaced by 1–3 monolayers of hydrocarbon material, which did not thicken with time. Finally, the hydrocarbons were replaced or covered by a diamond film. CO titration experiments demonstrated that practically no active platinum atoms were exposed to the gas-phase reactants during growth. This fact explains the lack of activity of the platinum foil.
A model of Si diffusion in GaAs based on the effect of the Fermi level
Authors: Yu Shaofeng , Gösele Ulrich M. , Tan Teh Y.
Published: July 2002

We propose a quantitative model for Si diffusion in GaAs. In this model we incorporate the experimental result that Si is an amphoteric impurity which constitutes a shallow donor when occupying the Ga site, Si+Ga, and a shallow acceptor when occupying the As site, Si−As. At high concentrations Si+Ga and Si−As coexist. The amphoteric behavior of Si may be viewed as an effect of the Fermi level. We assume that Si+Ga and Si−As diffuse respectively on the Ga and As sublattices. That is, the diffusion of Si+Ga and Si−As is governed by the variously charged Ga vacancies and As vacancies (or self-interstitials), respectively. The experimentally observed Si diffusivity is concentration dependent which results from the Si amphoteric nature as well as from another effect of the Fermi level, which is its influence on the concentrations of the charged point-defect species. Satisfactory quantitative descriptions of available experimental results are obtained. An analysis of the data on Si diffusion into a Sn-doped GaAs substrate has led us to believe that the previously proposed Si-pair diffusion model may no longer be favored.
Defining the quality of a high-T
c superconductor
Authors: Mehdaoui A. , Loegel B. , Bolmont D.
Published: July 2002

We compare our experimental results obtained for the complex susceptibility of YBaCuO compounds with other results available in the literature concerning YBaCuO as well as other high-temperature superconducting oxides (LaBaCuO, BiSrCaCuO) or other inhomogeneous compounds like the Chevrel phase. We discuss a phenomenological model based on Clem’s formulation [Physica C 50, 153 (1988)]. We present a plot of all the experimental collected results, which allows a classification in terms of quality for a large variety of samples.
Magnetic hysteresis and Mössbauer studies in ultrafine iron particles
Authors: Papaefthymiou V. , Kostikas A. , Simopoulos A. , Niarchos D. , Gangopadyay S. , Hadjipanayis G. C. , Sorensen C. M. , Klabunde K. J.
Published: July 2002

The magnetic hysteresis behavior of ultrafine Fe particles has been examined by SQUID magnetometry and Mössbauer spectroscopy. Ultrafine Fe particles have been prepared by vapor deposition under argon atmosphere in pressures from 1 to 6 Torr and passivated by exposure to oxygen at low pressure. Particles with size below 10 nm showed a small coercivity (∼100 Oe) at room temperature, which increased drastically upon cooling to cryogenic temperatures (1000–1500 Oe). The larger particles had the highest coercivity at room temperature (∼1000 Oe) which increased by 100% at 10 K. Mössbauer studies revealed that the Fe particles are surrounded by an Fe oxide layer composed of ultrafine Fe3O4 or γ-Fe2O3 grains. At room temperature the oxide component gives a very broad absorption superimposed on an α-Fe spectrum. The enhanced coercivity at room temperature may be attributed to anisotropy induced by exchange interaction between the ferromagnetic core and the ferri- or antiferromagnetic oxide layer.
Sources of multiply charged ions for heavy-ion fusion
Authors: Humphries S. , Rutkowski Henry
Published: July 2002

We report experiments on methods to raise the proportion of multiply charged ions for high-flux-ion extraction from vacuum-arc plasma sources. Beams of doubly and triply charged ions have application to induction linacs for heavy-ion fusion. We confirmed that the arc cathode material strongly influenced the ion-charge-state proportions. We believe the electrostatic potential of the plasma streaming from the arc played a major role in the transport of multiply charged ions. Reduction of electron losses from the plasma increased the observed fluxes of doubly charged ions. We inhibited electron loss and enhanced the flux of multiply charge ions by applying a bias voltage to the plasma source and by adding an axial magnetic field. We also investigated a method to increase the proportion of multiply charged ions by time-of-flight separation following rapid switching of the expanding plasma. We developed a new arc-source geometry for the experiments. The source had a hollow anode and an axial magnetic field to anchor the position of emission spots on the cathode.
Time-dependent Boltzmann equation in a self-sustained discharge XeCl laser: Influence of electron-electron and superelastic collisions
Authors: Gorse C. , Capitelli M. , Dipace A.
Published: July 2002

Electron energy distribution functions (EEDF) in a XeCl laser have been obtained by solving the time-dependent Boltzmann equation coupled to the circuit equations, plasmachemistry, and photon kinetics. The results show that in some zones of temporal evolution, EEDF does not instantaneously follow the alteration of electric field. Moreover, electron-electron and superelastic collisions play a non-negligible role in affecting EEDF and related quantities.
Reduction of induced damage in GaAs processed by Ga+ focused-ion-beam-assisted Cl2 etching
Authors: Sugimoto Y. , Taneya M. , Hidaka H. , Akita K.
Published: July 2002

Damage in GaAs induced by Ga+ focused-ion-beam-assisted Cl2 etching is studied by photoluminescence (PL) intensity measurements as functions of ion energy, ion dose, and substrate temperature. By decreasing the ion energy from 10 to 1 keV, the damage depth decrease to 1/5, where damage depth is taken as the thickness at which the PL intensity recovers by wet etching. The damage depth is shallower when the etching yield is larger with the same ion energy. By increasing the ion dose, the normalized PL intensity decreases, but damage depth is nearly constant. Over 1015 ion dose, the normalized PL intensity shows a constant value. By increasing the sample temperature, the damage depth becomes shallower. At 150  °C with ion energy of 1 keV, the damage depth is less than 0.5 μm, which is the detection limit of the PL measurement in GaAs substrate.
Model for the above-threshold characteristics and threshold voltage in polycrystalline silicon transistors
Authors: Fortunato G. , Migliorato P.
Published: July 2002

A new theory of polycrystalline silicon thin-film transistors is proposed, based on a continuous trap state density. Three different regimes are predicted: subthreshold, transitional, and crystallinelike. Two characteristic voltages are identified: the threshold voltage, corresponding to the condition of equal trapped and free charge concentration at the oxide/semiconductor interface and the on-voltage, corresponding to the condition of equal trapped and free charge in the whole space-charge region. In the case of an exponential distribution of gap states, approximated analytical expressions can be deduced and a simple accurate fitting procedure is presented. A very good agreement with the experiment is obtained, confirming the importance of taking into account the detailed energy dependence of the trap distribution.
Use of thermal energy for surface contact potential gas detection
Authors: Balasubramanian A. , Santiago-Aviles J. J. , Zemel Jay N.
Published: July 2002

In a recent paper, Christofides and Mandelis [J. Appl. Phys. 66, 3975 (1989)] postulated the existence of a hydrogen-induced modulation of the pyroelectric coefficient of polyvinylidene difluoride when a Pd film is exposed to hydrogen gas. Their evidence was a photothermally induced signal. By examining the theory of the hydrogen effect on Pd, we established that a more likely cause is the thermal modulation of the contact potential difference between the PD and the counter-electrode. We demonstrate that the same behavior is observed when hydrogen gas is exposed to a Pd-mica-Au capacitor as observed by Christofides and Mandelis.
CoSm-based high-coercivity thin films for longitudinal recording
Authors: Velu E. M. T. , Lambeth D. N.
Published: July 2002

In order to produce thin media with high intrinsic coercivity, we studied a number of CoSm films produced by rf-diode sputtering. We determined an optimized set of sputtering parameters for films sputtered both on glass and NiP-coated Al substrates with and without Cr underlayers. While the coercivity of the films depended strongly on argon pressure, the 〈110〉 texture of the Cr underlayer remained unaffected by it. The 〈110〉 texture of Cr changed to 〈200〉 texture at temperatures ≥300 °C, and the coercivity of the film was significantly reduced. Under optimal conditions we obtained intrinsic coercivities ≳2400 Oe for films with thickness <20 nm.
Theoretical equation of state for aluminized nitromethane
Authors: Jones Hermenzo D. , Zerilli Frank J.
Published: July 2002

The simple fluid constituents of the reaction products of an aluminized nitromethane explosive are described by a perturbation technique based on their intermolecular interactions. Liquid metal constituents are treated with a Grover scaling model. Standard solid-state approaches are applied to the solid components of the reaction products. Calculated detonation velocities compared favorably with experimental data.
Simple determination of the width of the slit function in single-shot coherent anti-Stokes Raman spectroscopy
Authors: Heneghan Shawn P. , Vangsness Marlin D. , Pan John C.
Published: July 2002

The determination of temperatures using single-shot coherent anti-Stokes Raman spectroscopy normally requires an accurate measurement of the instrument slit function. This slit function is normally determined at a known temperature, and then assumed to be the applicable function at all temperatures and independent of the optical path which varies with density or temperature gradients. In this communication we show a simple method of determining the width of the slit function from the collected data at the actual temperature and turbulence level. This method depends on local thermodynamic equilibrium and is a generally applicable technique to determine the instrument slit function. It is limited to temperatures in excess of 1100 K, as it requires that there be a nonzero signal in the v=1 vibrational level of N2.
Investigation of the systematics of CMXD at the L2,3 edges in (RE)2Co17
Authors: Fischer P. , Schütz G. , Stähler S. , Wiesinger G.
Published: July 2002

We have performed systematic investigations of the circular magnetic x-ray dichroism, i.e., the spin-dependent absorption of circularly polarized x rays at the L2,3 edges of the 4f elements in the compound (RE)2Co17. In all systems significant CMXD signals with relative large amplitudes of some percent have been found. It is expected, that the spin-dependent absorption profile reflects the spin-density of d-like unoccupied states at the Fermi level and thus contains information on the orientation of the 4f moment. The ratio of the CMXD signal at the L2 and L3 edges depends strongly on the angular momentum of the 4f core and indicates in case of Ce2Co17, that the 4f electrons are of itinerant character with almost quenched angular momentum, as theoretically expected for similar Ce systems.
Relaxation phenomena in polymer nonlinear optical materials
Authors: Singer K. D. , King L. A.
Published: July 2002

The isothermal decay of the second-order nonlinear optical susceptibility of a guest-host azo dye methacrylate film was measured by second-harmonic generation at a series of temperatures. The decay curves were fit to Williams–Watt–Kohlrausch stretched exponentials. The activation energies and pre-exponential factors were found to agree with those derived from nonisothermal electric-field-induced second-harmonic generation measurements on similar films. It is concluded that a common underlying process involving a distribution of local relaxations governs the film behavior.
Determination of the composition of strained InGaAsP layers on InP substrates using photoreflectance and double-crystal x-ray diffractometry
Authors: Flemish J. R. , Shen H. , Jones K. A. , Dutta M. , Ban V. S.
Published: July 2002

Determining the composition of strained InGaAsP films through measurements of the lattice spacings and band-gap energies (Eg) requires converting the measured values to ones which would correspond to unstrained material. In strained layers the lattice constants perpendicular and parallel to the growth plane can vary significantly from the relaxed value, and the optically measured Eg is affected by a strain-induced splitting of the valence band and a shifting of the direct gap energy. By combining double-crystal x-ray data with room-temperature photoreflectance results, we determine the InGaAsP composition accurately using an iterative procedure. Film compositions calculated using strain adjusted values of Eg agree with those determined by energy dispersive spectroscopy to within 1–2 at. %, whereas if energy shifts are not considered, the error approaches 10 at. % for strain on the order of 0.4%.