bluuebunny/crossref_metadata_2025 huggingface dataset:
Conditionally filtered scalar dissipation, scalar diffusion, and velocity in a turbulent jet
Authors: Wang Danhong , Tong Chenning
Published: July 2002

Conditionally filtered conserved scalar (temperature) dissipation (CFD), diffusion, and conditionally filtered velocity are studied experimentally in the fully developed region of a turbulent jet with a jet Reynolds number Rej=UjDj/ν of 40 000. These variables are the unclosed terms in the transport equation of the conserved scalar filtered density function. One-dimensional box filters of widths Δ ranging from 30 to 496 scalar dissipation scales (ηφ) as well as a two-dimensional box filter (Δ/ηφ=90) which consists of three discrete sensors are used to obtain filtered variables. Taylor’s hypothesis is used to perform the streamwise filtering. The means of these conditionally filtered variables conditional on the resolvable-scale scalar fluctuations 〈φ〉L and the subgrid-scale (SGS) variance 〈φ″2〉L indicate two regimes of the SGS scalar mixing. For large SGS variance (〈φ″2〉L/〈φ″2〉>1) the SGS scalar exhibits similarities to scalars in initially binary mixing at early times, suggesting existence of diffusion-layer-like structure and nonequilibrium SGS scalar. The nonequilibrium is further evidenced by the observed large SGS mechanical-to-scalar time scale ratio and large SGS variance production-to-dissipation ratio. These characteristics have not been previously observed in fully developed flows. For small SGS variance the conditionally filtered variables indicate well-mixed SGS scalar. The results also show that the SGS advection is only significant at large SGS variance. Our measurements also suggest that the dependence of the scalar dissipation (and the filtered dissipation) on large-scale scalar fluctuations is caused primarily by the fluctuations in the spectral transfer and that the conditional CFD is directly affected mostly by scales near the filter scale. The present study suggests that the nonequilibrium effects related to sheet-like structures are important for modeling the conditionally filtered variables.
On the mean rate of energy transfer in turbulence
Authors: Wyngaard John C.
Published: July 2002

We derive a physical-space expression for the mean rate of transfer of kinetic energy from wave numbers below k to those above. We show that in stationary, homogeneous turbulence our expression is equivalent to Kraichnan’s Π(k). In inhomogeneous turbulence it differs slightly from the form commonly used in connection with large-eddy simulation. We attribute this difference to the use of an incorrect expression for the turbulent transport of kinetic energy in wave numbers below k. We suggest that our two-term, physical-space expression could be useful in modeling energy transfer in nonstationary, inhomogeneous flows.
Nonequilibrium excited-state distribution of atoms in dense mercury plasmas
Authors: Karabourniotis Dimitrios
Published: July 2002

The nonequilibrium excited-state distribution of mercury atoms up to the ionization limit in high-pressure discharges is deduced through comparison of the actual level-population densities with those calculated from the Saha equation at the plasma-electron temperature. Emission data obtained from two ac (50 Hz) mercury discharges at 2 and 5 bar are used to deduce the absolute excited-state population densities and the electron temperature. It is found that at the maximum emission phase (5 ms), the excited states deviate from the Saha equilibrium, the levels are overpopulated, the overpopulation decreases with the level-excitation energy, the deviation is larger for the higher-pressure discharge, and the two discharges are closer to the equilibrium at the minimum emission phase (0.5 ms). Possible causes of the observed deviations from the Saha equilibrium are suggested. The plasma temperature and densities are also found assuming equilibrium conditions and compared with the results obtained initially.
Structural and electrical characterization of AuTiAlTi/AlGaN/GaN ohmic contacts
Authors: Fay M. W. , Moldovan G. , Brown P. D. , Harrison I. , Birbeck J. C. , Hughes B. T. , Uren M. J. , Martin T.
Published: July 2002

AuTiAlTi/AlGaN/GaN ohmic contact structures rapid thermal annealed at 650, 750, 850, and 950 °C have been analyzed using complementary transmission electron microscopy and electrical characterization techniques. The relationship between annealing temperature, interfacial microstructure, and contact resistance is examined. Annealing temperatures of 750 °C or higher are required to produce an ohmic contact. Contacts annealed at 750 and 850 °C show a planar interface between contact and the AlGaN layer, with minimal consumption of the AlGaN and the formation of a thin TiN interfacial layer. Annealing at 950 °C produces the lowest contact resistance, with a structure showing inclusions through the AlGaN/GaN layer. These inclusions are also shown to be a Ti-nitride, having an Al/Au-rich metallurgical barrier layer surrounding them. However, this metallurgical layer does not produce an electrical barrier.
Shape effects on scattering in three-dimensional quantum wires
Authors: Vargiamidis Vassilios , Valassiades O.
Published: July 2002

We study the effects of the shape of the cross section of a three-dimensional quantum wire on electron scattering from a single point defect in the wire. The confinement of electrons is modeled by both hard- and soft-wall potentials. We find that as the degree of anisotropy of the cross section of the wire is increased intersubband electron scattering is enhanced and intrasubband transmission is suppressed making it appear as though the defect has stronger impact on electron scattering for asymmetric cross sections. Also, increasing the anisotropy of the cross section results in a decrease of the values of the conductance. Furthermore, for the soft-wall confinement the conductance as a function of Fermi energy rises faster than the conductance for the hard-wall confinement. We use the Lippmann–Schwinger equation of scattering theory in order to calculate analytically the transmission coefficients.
Luminescence associated with copper in ZnGeP2
Authors: Wang Lijun , Bai Lihua , Stevens K. T. , Garces N. Y. , Giles N. C. , Setzler S. D. , Schunemann P. G. , Pollak T. M.
Published: July 2002

Bulk ZnGeP2 crystals were diffusion doped with copper and investigated using photoluminescence (PL) spectroscopy, photoluminescence excitation (PLE), and Fourier-transform-infrared spectroscopy. A PL band at 1.3 eV at 5 K was observed. The temperature dependence of the 1.3-eV PL band gives a thermal activation energy of 0.11±0.01 eV. PLE data show a resonance in intensity occurred at 0.1 eV below the conduction band when monitoring the 1.3-eV band. The 1.3-eV emission band is found to be unpolarized in contrast to PL bands observed in nominally undoped ZnGeP2 crystals. The copper-diffused samples have an absorption band at 10 K peaking at about 0.62 eV. We suggest that the 1.3-eV luminescence band is donor-acceptor-pair recombination with the donor energy level about 0.1 eV below the conduction band and a Cu acceptor energy level about 0.62 eV above the valence band. This transition model is also supported by optical quenching experiments.
Resonance dynamics and partial averaging in a restricted three-body system
Authors: Haghighipour Nader
Published: July 2002

Based on the value of the orbital eccentricity of an object and also its proximity to the exact resonant orbit in a three-body system, the pendulum approximation [S. F. Dermott and C. D. Murray, Nature (London) 319, 201 (1983)] or the second fundamental model of resonance [M. H. Andoyer, Bull. Astron. 20, 321 (1903); J. Henrard and A. Lemaı̂tre, Celest. Mech. 30, 197 (1983)] are commonly used to study the motion of that object near its resonant state. In this paper, we present the method of partial averaging as an analytical approach to study the dynamical evolution of a body near a resonance. To focus attention on the capabilities of this technique, a restricted, circular and planar three-body system is considered and the dynamics of its outer planet while captured in a resonance with the inner body is studied. It is shown that the first-order partially averaged system resembles a mathematical pendulum whose librational motion can be viewed as a geometrical interpretation of the resonance capture phenomenon. The driving force of this pendulum corresponds to the gravitational attraction of the inner body and its contribution, at different resonant states, is shown to be proportional to es, where s is the order of the resonance and e is the orbital eccentricity of the outer planet. As examples of such systems, the cases of (1:1), (1:2), and (1:3) resonances are discussed and the results are compared with known planetary systems such as the Sun–Jupiter–Trojan asteroids.
Varicose instabilities in turbulent boundary layers
Authors: Skote M. , Haritonidis J. H. , Henningson D. S.
Published: July 2002

An investigation of a model of turbulence generation in the wall region of a turbulent boundary layer is made through direct numerical simulations. The model is based on the varicose instability of a streak. First, a laminar boundary layer disturbed by a continuous blowing through a slot is simulated in order to reproduce and further investigate the results reported from the experiments of Acarlar and Smith [J. Fluid Mech. 175, 43 (1987)]. An isolated streak with an inflectional profile is generated that becomes unstable, resulting in a train of horseshoe vortices. The frequency of the vortex generation is equal to the experimental results. Comparison of the instability characteristics to those predicted through an Orr–Sommerfeld analysis are in good agreement. Second, a direct numerical simulation of a turbulent boundary layer is performed to point out the similarities between the horseshoe vortices in a turbulent and a laminar boundary layer. The characteristics of streaks and the vortical structures surrounding them in a turbulent boundary layer compare well with the model streak. The results of the present study show that one mechanism for the generation of horseshoe vortices in turbulent boundary layers is related to a normal inflectional instability of the streaks.
Generalized Weiss molecular-field basis for a phenomenological polarization model of lead magnesium niobate
Authors: Piquette Jean C. , McLaughlin Elizabeth A. , Ren Wei , Mukherjee Binu K.
Published: July 2002

A simple physical picture, useful for extended applications of a well-demonstrated phenomenological polarization model [J. C. Piquette and S. E. Forsythe, J. Acoust. Soc. Am. 101, 289 (1997)] is presented. The Weiss model of interacting dipoles (orginally suggested by Weiss for magnetic dipoles but applied here to electric dipoles) is generalized for high field levels. The generalization permits the model to describe the polarization measurements well, across a significant temperature range extending well below Tmax, for applied electric field levels as high as 1.35 MV/m. The measurements reported here were acquired from various polycrystalline samples of lead magnesium niobate (PMN) in a solid solution with lead titanate (PT) doped with lanthanum (La), having Tmax values ranging from −11 to +41 °C. Agreement between theory and data is good. (The overall rms error of fit is less than 0.5% in all cases considered.) The new model, when combined with a suitable model of hysteresis, is useful for describing and controlling the behavior of PMN, which is a material that is useful in actuator and transducer applications where a relatively large strain is desired. The theory, in its present form, does not account for relaxor (frequency-dependent) aspects of the material behavior, and is applied here only to data acquired quasistatically. Moreover, only the case in which no external stress is applied to the material is considered.
Stability of the nematic phase of 4-n-pentyl-4′-cyanobiphenyl studied by computer simulation using a hybrid model
Authors: Cacelli Ivo , Campanile Silvio , Prampolini Giacomo , Tani Alessandro
Published: July 2002

Lengthy molecular dynamics simulations are performed to verify the reliability of a model of the 4-n-pentyl-4′-cyanobiphenyl (5CB) molecule appeared in the literature [C. W. Cross and B. M. Fung, J. Chem. Phys. 101, 6839 (1994)]. At 300 K, the time evolution of the orientational order parameter shows clearly that, when thermodynamic equilibrium is reached, the system is in the isotropic phase. At lower temperatures, however, this model appears to be capable of yielding a nematic phase. A new set of parameters for the intermolecular potential is proposed, such that the system shows a stable nematic phase at 300 K. Orientational properties, correlation functions, and translational diffusion coefficients have been calculated for the new model.
Effects of solute properties and concentrations on liquid crystals: N-(4-ethoxybenzylidene)-2,6-dideutero-4-n-butylaniline
Authors: Syvitski Raymond T. , Pau Monita Y.-M. , Burnell E. Elliott
Published: July 2002

The effects of the nature and concentration of the solutes 1,3,5-trichlorobenzene (TCB), benzene, chlorobenzene, o-dichlorobenzene, m-dichlorobenzene, and p-dichlorobenzene on orientational order in the nematic liquid crystal N-(4-ethoxybenzylidene)-2,6-dideutero-4-n-butylaniline are investigated using NMR spectroscopy. Seven separate samples containing differing concentrations of all solutes are investigated at constant temperature, at constant reduced temperature, at constant TCB orientational order, and at constant liquid crystal orientational order. Various schemes for comparison of orientational order among the samples are examined. For the experiments performed here, the most consistent method to compare solute orientational order in the different samples is to multiply results from the experiments performed at constant temperature by the factor needed to equate the TCB order parameters.
Plasma diffusion across inhomogeneous magnetic fields
Authors: Santhosh Kumar T. A. , Mattoo S. K. , Jha R.
Published: July 2002

This paper reports experimental studies on plasma diffusion across inhomogeneous magnetic fields. Plasma created in a multicusp bucket source is allowed to diffuse across inhomogeneous magnetic fields formed by two different arrangements of permanent magnets. Two-dimensional measurements near the magnetic sheath shows the existence of a shear in electric field and plasma turbulence. The plasma turbulence is localized in the region of a magnetic filter. The observed features of turbulence near the magnetic field can be best explained by electron–ion hybrid modes at the lower hybrid frequencies, which exploit free energy from electron E×B drift, in the background of unmagnetized ions. These results lead to a clear model for the earlier observation of a decrease in electron temperature of the plasma across an inhomogeneous magnetic field.
Binary reaction of nonpassively advected particles immersed in turbulent flows
Authors: Reigada Ramon , Sagués Francesc , Sancho José M.
Published: July 2002

We consider the binary diffusion-controlled reaction A+B→0 between nonpassively advected particles. This reactive system under a two-dimensional synthetic stationary turbulent flow leads to a diversity of kinetic regimes that result from the interplay between reaction, advection, and drag forces. Inertial bias collects particles preferentially in certain regions of the flow depending on their density, and this fact strongly determines the overall kinetic behavior and the spatial organization of the reactive mixture. A Eulerian scheme for the equations that describe this problem is developed and implemented, so that we can numerically follow the decay behavior as well as the patterned evolution of the system. Two-point correlation functions are used in order to characterize the aggregation versus segregation phenomena taking place in the system.
Transport diffusion of liquid water and methanol through membranes
Authors: Zhang Qinxin , Zheng Jie , Shevade Abhijit , Zhang Luzheng , Gehrke Stevin H. , Heffelfinger Grant S. , Jiang Shaoyi
Published: July 2002

In this work, we carried out dual-control-volume grand canonical molecular dynamics simulations of the transport diffusion of liquid water and methanol to vacuum under a fixed chemical potential gradient through a slit pore consisting of Au(111) surfaces covered by −CH3 or −OH terminated self-assembled monolayers (SAMs). Methanol and water are selected as model fluid molecules because water represents a strongly polar molecule while methanol is intermediate between nonpolar and strongly polar molecules. Surface hydrophobicity is adjusted by varying the terminal group of −CH3 (hydrophobic) or −OH (hydrophilic) of SAMs. We observed for the first time from simulations the convex and concave interfaces of fluids transporting across the slit pores. Results show that the characteristics of the interfaces are determined by the interactions between fluid molecules and surfaces. The objective of this work is to provide a fundamental understanding of how these interactions affect transport diffusion.
Effect of polydispersity on Brownian-particle trapping by clusters of traps
Authors: Makhnovskii Yurii A. , Sheu Sheh-Yi , Yang Dah-Yen , Lin Sheng Hsien
Published: July 2002

Brownian-particle survival is considered in the case when killing traps are gathered in spherical clusters distributed in space in a noncorrelated manner. Recently, we [J. Chem. Phys. 111, 711 (1999)] have proposed a mean-field theory of trapping by identical clusters of traps, in which the cluster radius R and the number of traps in a cluster n are the same for all of the clusters. Here we focus on a more realistic model, where the parameters R and n vary from one cluster to another, according to a given probabilistic law. For the model of polydisperse clusters, general expressions are obtained for the particle survival probability and the time-dependent rate coefficient. To exhibit the effect of polydispersity, we compare the trapping kinetics for polydisperse clusters with that for the reference system of identical clusters. It is shown, in particular, that fluctuations in n always lead to a process slowdown. An explicit solution is derived for slightly polydisperse nonoverlapping clusters.
Interfacial gravity currents. I. Mixing and entrainment
Authors: Sutherland B. R.
Published: July 2002

Experiments are performed to examine mixing and transport due to an intrusive gravity current in a two- and three-layer fluid. Successive passages of the intrusion act to increase the depth of the middle layer. The observed broadening rates and the deposition of potential energy are compared with theories that neglect mixing and entrainment processes. As the middle layer widens, a transition in the mass transport by the intrusion is observed: in a two-layer fluid the intrusion transports mass along the entire length of the interface; in a three-layer fluid with sufficiently deep middle layer the intrusion transports mass over a limited distance and simultaneously excites large-amplitude internal wave modes and solitary internal waves.
Measurement of an unexpectedly large shear-induced self-diffusivity in a dilute suspension of spheres
Authors: Zarraga Isidro E. , Leighton David T.
Published: July 2002

We report the measurement of unexpectedly large shear-induced diffusivities for various sized tracers in a dilute suspension of noncolloidal spheres in simple shear. The suspension was sheared in a narrow gap Couette device at low Reynolds number, and the tracer diffusivities parallel to the velocity gradient D were obtained using an orbit-time technique. It is shown that the presence of even a dilute concentration φ of particles renders an otherwise smooth tracer trajectory strongly stochastic and the resulting diffusivity is linear in φ in this limit, indicating irreversible displacements at the pair interaction level. The measured values of the diffusivity, however, are at least an order of magnitude larger than that predicted by current theories. In a previous study, Beimfohr et al. [Proc. DOE/NSF Workshop, Ithaca, NY (1993)] likewise obtained values for D larger than that predicted by theory, but the discrepancy was attributed to the large eccentricity (average aspect ratio=1.19) of the particles used. In the present study, very nearly spherical ground acrylic particles were used, yet diffusivities of the same order of magnitude as that obtained by Beimfohr et al. were measured. Various possible causes for this anomalous diffusivity were explored and tested in the course of our investigation, including the effect of inertial lift and possible non-Newtonian properties of the base fluid, but none proved sufficiently large to account for the observed behavior.
Conditions for the alignment of the principal null directions of two Weyl-like tensors
Authors: Sneddon G. E.
Published: July 2002

A possible means to classify the interaction between the Weyl and Ricci tensors is to look at the number of principal null directions that the Weyl and Plebanski tensors have in common. This paper presents algebraic conditions that can be used to determine this number without explicitly calculating the principal null directions themselves.
Structure and magnetic properties of SiO2-coated Co nanoparticles
Authors: Wu Mingzhong , Zhang Y. D. , Hui S. , Xiao T. D. , Ge Shihui , Hines W. A. , Budnick J. I.
Published: July 2002

SiO 2 -coated Co nanoparticles in a size range of 10 to 50 nm were synthesized by a wet chemical approach, and their structure and magnetic properties were investigated using x-ray diffraction, high-resolution transmission electron microscopy, and a superconducting quantum interference device magnetometer. The structure of the synthesized nanoparticles varied with calcination temperature. When the calcination temperature was as high as 900 °C, the nanoparticles had a core/shell structure: the core was fcc Co and the shell was amorphous SiO2. When the calcination temperature was 800 °C or below, the nanoparticles had a nano-onion structure: the shells from the exterior to the interior were amorphous SiO2, fcc Co, and CoO, and the innermost core was Co3O4. The SiO2 shell had the ability of hindering Co from particle growth during the synthesis procedure and protecting Co against oxidation after the synthesis procedure. The nanoparticles were ferromagnetic. At both low and room temperatures, the saturation magnetization increased with increasing calcination temperature, while the coercivity decreased with increasing calcination temperature. For the nanoparticles calcined at 800 °C or below, the low temperature coercivity was found to be notably higher than the room temperature one due to Co/CoO exchange coupling. For the nanoparticles calcined at 900 °C, the coercivity was relatively low and the saturation magnetization reached the expected values.
Rayleigh–Bénard flow of a rarefied gas and its attractors. II. Chaotic and periodic convective regimes
Authors: Stefanov S. , Roussinov V. , Cercignani C.
Published: July 2002

This second part of our study of the Rayleigh–Bénard flow of a rarefied gas deals with the analysis of the convection regimes for near-continuum conditions at a Knudsen number Kn=0.001. Here a set of qualitatively new regimes and final states have been found for different Froude (Fr) numbers and a fixed temperature ratio Tc/Th=0.1. In contrast to paper I [Phys. Fluids 14, 2255 (2002)] where we were mainly interested in the long time behavior, or, in other words, in the flow attractors, in this paper we extend the analysis of the most interesting cases to the full transition period from the initial state to the established flow regime. Two transient periods (to a chaotic attractor found for Fr=1.0 and to a stable two-roll configuration for Fr=2.0) are computed by using two different approaches: a direct simulation Monte Carlo method (molecular approach) and a finite difference method solving the system of equations for a compressible viscous heat-conducting gas (continuum approach). The comparison of the spatiotemporal structures shows that both approaches exhibit the basic properties of the transition to a chaotic fluid motion.
Self-similar solution to the Fokker–Planck equation in inhomogeneous plasma
Authors: Bychenkov V. Yu. , Rozmus W. , Teshima R.
Published: July 2002

A self-similar solution to the stationary electron Fokker–Planck equation has been found in an inhomogeneous plasma with a given input of energy flux. This solution describes a reduction in the number of bulk electrons and the formation of a superthermal tail with a power law dependence on velocity. Significant modifications in the electron plasma wave Landau damping and return current instability growth rate have been discussed.
Structure, electronic properties, and vibrational spectra of the water octamer with an extra electron: Ab initio study
Authors: Lee Han Myoung , Kim Kwang S.
Published: July 2002

The structure of the electron–water octamer has been investigated for the first time, using ab initio calculations. The lowest-energy conformer and the next lowest energy conformer are predicted to be the major and minor isomers with small and large vertical electron detachment energies. The binding energies and vibrational spectra are investigated.
A transformation of the point vortex equations
Authors: Aref Hassan
Published: July 2002

A transformation of the point vortex equations is introduced and studied. The transformation substitutes two point vortices for each vortex used in the calculation of induced velocity at a point, in particular in the calculation of the velocity of another vortex in the system. For certain symmetric configurations, such as the vortex polygons, this substitution simplifies the calculation of velocity very considerably. The case of three vortices is also considered, where the circumcircle of the vortex triangle may be used for the transformation. The transformation suggests a new differential relation for the vortex velocities in a many-vortex system. Applications of this result are given.
Modified neural networks for rapid recovery of tokamak plasma parameters for real time control
Authors: Sengupta A. , Ranjan P.
Published: July 2002

Two modified neural network techniques are used for the identification of the equilibrium plasma parameters of the Superconducting Steady State Tokamak I from external magnetic measurements. This is expected to ultimately assist in a real time plasma control. As different from the conventional network structure where a single network with the optimum number of processing elements calculates the outputs, a multinetwork system connected in parallel does the calculations here in one of the methods. This network is called the double neural network. The accuracy of the recovered parameters is clearly more than the conventional network. The other type of neural network used here is based on the statistical function parametrization combined with a neural network. The principal component transformation removes linear dependences from the measurements and a dimensional reduction process reduces the dimensionality of the input space. This reduced and transformed input set, rather than the entire set, is fed into the neural network input. This is known as the principal component transformation-based neural network. The accuracy of the recovered parameters in the latter type of modified network is found to be a further improvement over the accuracy of the double neural network. This result differs from that obtained in an earlier work where the double neural network showed better performance. The conventional network and the function parametrization methods have also been used for comparison. The conventional network has been used for an optimization of the set of magnetic diagnostics. The effective set of sensors, as assessed by this network, are compared with the principal component based network. Fault tolerance of the neural networks has been tested. The double neural network showed the maximum resistance to faults in the diagnostics, while the principal component based network performed poorly. Finally the processing times of the methods have been compared. The double network and the principal component network involve the minimum computation time, although the conventional network also performs well enough to be used in real time.
Optical path length control in plasma absorption measurements
Authors: Cruden Brett A. , Rao M. V. V. S. , Sharma Surendra P. , Meyyappan M.
Published: July 2002

An inductively coupled Gaseous Electronics Conference (GEC) cell with modified viewing ports has been used to measure in situ absorption in CF4 plasmas via Fourier transform infrared spectroscopy, and the results compared to those obtained in a standard viewport configuration. The viewing ports were modified so that the window boundary is inside, rather than outside, of the GEC cell. Because the absorption obtained is a line of sight integrated absorption, measurements made represent an averaging of absorbing species inside and outside of the plasma. This modification is made to reduce this spatial averaging and thus allow a more accurate estimation of neutral species concentrations and temperatures within the plasmas. By reducing this path length, we find that the apparent CF4 consumption increases from 65% to 95% and the apparent vibrational temperature of CF4 rises by 50–75 K. The apparent fraction of etch product SiF4 decreases from 4% to 2%. The data suggest that these density changes may be due to significant temperature gradients between the plasma and chamber viewports.
Low-k organosilicate films prepared by tetravinyltetramethylcyclotetrasiloxane
Authors: Lubguban J. , Rajagopalan T. , Mehta N. , Lahlouh B. , Simon S. L. , Gangopadhyay S.
Published: July 2002

Low-k films with k of 2.5–2.9 were deposited under different conditions of pressures and temperatures using a plasma-enhanced chemical vapor deposition (PECVD) system. These films were prepared using a new liquid precursor, tetravinyltetramethylcyclotetrasiloxane (TVTMCTS) and H2 carrier gas. The rf power was kept as low as possible to maintain the original ring structure in the films. The as-deposited films were annealed and the dielectric and optical properties were investigated. Identification of the absorption bands in the IR spectra for as-deposited films reveals a broadband around 950–1200 cm−1 arising from the Si–O stretching mode of the ring (1065 cm−1) and chain structure (1000 cm−1), respectively; a band at 750–900 cm−1 due to Si–O bending (790 cm−1); Si–CH3 rocking mode (760 cm−1); a sharp band centered at 1260 cm−1 due to a Si–CH3 bending mode; and a broadband at 2800–3000 cm−1 due to the CH group. A comparison of the IR spectra of the PECVD film and TVTMCTS liquid reveals that vinyl vibrations (Si–CH=CH2) at 960, 1410, and 3030–3095 cm−1 for CH2 and at 1598 cm−1 for C=C present in the liquid were not detected in the CVD films. Hence C=C bonds were broken in the plasma polymerization process. As the pressure and the deposition temperature (TD) increased, the intensity of the Si–O vibration arising from the ring structure increased and decreased, respectively. Thus by tuning the pressure and TD we can control the structure of the film. There is a good correlation found between the Si–CH3 and Si–O ring intensities and k values; the increasing Si–CH3 and Si–O ring is accompanied by decreasing k. The films were thermally stable up to 400 °C annealing temperature.
Diffusion-assisted long-range reaction between the ends of a polymer: Effective sink approximation
Authors: Barzykin A. V. , Seki K. , Tachiya M.
Published: July 2002

We report a Brownian dynamics (BD) simulation study of the Förster energy transfer in a dye-labeled Rouse polymer chain. The simulation method is based on the normal mode BD propagation and numerical path integration of the survival probability. It is shown that a properly constructed truncated normal-mode approximation (TNMA) can speed up the simulations considerably, without essential loss of accuracy. In particular, an effective-sink TNMA scheme is found to be quite efficient. The idea is based on a standard time scale separation ansatz, where all the normal modes are separated into slow and fast, in terms of the corresponding relaxation times. The fast normal modes are assumed to be equilibrated in the course of reaction and thus can be integrated out. Their effect is to modify the reaction sink for the slow modes. The first-order approximation can be handled most easily, without a simulation. Even this simple approximation can be preferable to the well-known Wilemski–Fixman approximation, if the reaction sink is wide, i.e., when the Förster radius exceeds the polymer mean bond length, the condition often chosen in experiments on polymer folding.
An automatic attenuator device for x-ray detectors at high counting rate
Authors: Alvarez J. , Paiser E. , Capitan M. J.
Published: July 2002

In this article we describe an attenuator device for reducing/controlling the pulse detector counting losses at a high counting rate. The electronics are based on a direct measure of the detector dead time from the analog output signal at the end of the detection chain. Taking into account this parameter the attenuator device decides to reduce/enhance the number of photons that arrive at the detector by inserting/extracting the necessary number of attenuation foils in the x-ray beam path. In that way the number of events in the incoming signal are reduced and the “apparent dynamic range” of the detector is increased.
Contactless excitation and measurement method for inspection of microstructures and thin films
Authors: Kobayashi Toshihiro , Hara Tamio , Ohsawa Jun , Yamaguchi Naohiro
Published: July 2002

A specially designed resonator to avoid the change of the resonance characteristics by thermal influences of the laser beam was fabricated on a silicon on insulator wafer. This resonator design enables one to apply a lateral driving force to generate lateral vibration. The resonator was excited using a laser diode which was driven with sinusoidal current, and the vibration was detected by measuring intensity fluctuation of the He–Ne laser beam reflected on the resonator mass. Four evident resonance frequencies were successfully observed in the range from 100 Hz to 100 kHz.
Cryogenic InSb detector for radiation measurements
Authors: Kanno Ikuo , Yoshihara Fumiki , Nouchi Ryo , Sugiura Osamu , Nakamura Tatsuya , Katagiri Masaki
Published: July 2002

The energy spectra of Am241 alpha particles were measured by a detector employing the compound semiconductor InSb at an operating temperature below 4.2 K. The fabrication method and current–voltage curves are shown. Though the energy resolution of the detector is not discussed in this article, this is the first report on an InSb radiation detector.
Fiberoptic in-vessel viewing system for the International Thermonuclear Experimental Reactor
Authors: Heikkinen Veli , Aikio Mauri , Keränen Kimmo , Wang Minqiang
Published: July 2002

A viewing system was designed and a prototype realized for the in-vessel inspection of the International Thermonuclear Experimental Reactor. The viewing is based on the line scanning principle, and the system consists of ten identical units installed on top of the reactor at 36° intervals. Each device contains a laser, beam steering mirrors, and viewing probe with insertion mechanics. The probe has an outside diameter of 150 mm and a length of 14 m. The illumination design applies frequency-doubled Nd: yttrium–aluminum–garnet lasers whose beams are guided through hermetically sealed windows into the vacuum vessel. The diffuser optics creates a vertically oriented light stripe onto the vessel surface that is viewed by the imaging optics, consisting of 16 modules altogether covering horizontal and vertical field-of-views of 2° and 162°. The optical images are transferred to charge coupled device cameras via coherent fiber arrays. The multifocus design uses stacked fiber rows whose ends are assembled into different axial positions. The viewing probes rotate at a constant angular speed of 1°/s and pictures are taken at 0.01° intervals. The complete picture of the vessel interior is generated in 6 min producing 5.8×109 image pixels. The image processing and analysis of possible defects in the vessel surfaces are performed off-line after the viewing procedure. A full-scale prototype of the viewing probe was constructed to demonstrate the feasibility of the design. Its illumination optics utilizes a diffractive optics element that transforms the collimated input beam into a rectangular output lobe with uniform intensity. The prototype has horizontal and vertical imaging optics field-of-views of 2° and 12°. The test results showed that the prototype can take pictures of good quality applying a continuously rotating probe having an angular speed of 0.08°/s. Under optimum conditions, the minimum resolvable feature size at a 3 m distance is smaller than 1 mm, which satisfies the requirement specification. Further development is needed to increase the illumination power to improve the imaging speed and to develop linear fiber arrays that are compatible with the vacuum and high-flux radiation environment of the primary vacuum vessel.
Three-particle contribution to sedimentation and collective diffusion in hard-sphere suspensions
Authors: Cichocki B. , Ekiel-Jeżewska M. L. , Szymczak P. , Wajnryb E.
Published: July 2002

The virial expansion of the collective mobility (sedimentation) coefficient is considered for hard sphere suspensions at equilibrium. The term of the second order in volume fraction, which involves three-particle hydrodynamic interactions, is calculated with high accuracy. To achieve that we represent the collective mobility coefficient as the sum of convergent integrals over particle configurations. In this way the short-wave-number limit k→0 is avoided. Moreover, an efficient numerical procedure is applied to evaluate the hydrodynamic interactions. The algorithm is based on the multipole expansion, corrected for lubrication. The method allows us to analyze contributions to the collective mobility coefficient from different configurations of three particles and to select the dominant part. This suggests a general approximation scheme.
Electron transfer in the D–B–A model system: A vibronic analysis
Authors: Villani Giovanni
Published: July 2002

The electron transfer (ET) process has been studied in a three sites system: the donor and the acceptor of electrons separated by a bridge. We have considered an isolated system in order to understand the characteristics of the process itself without introducing a bath. The ET process has been studied considering both the electronic and the vibrational part. This complete vibronic analysis has been done in a three electronic-n vibrational model. Two questions are put into evidence. First of all we have analyzed the relevance of the vibrational part in modifying a pure electronic description both in the electron transfer time and in the site populations. A second aspect underlined is the difference between a tight-binding system (donor and acceptor without a direct coupling) and a full coupling one. The drastic difference between these two cases has been rationalized.
Complete sets of invariants for dynamical systems that admit a separation of variables
Authors: Kalnins E. G. , Kress J. M. , Miller W. , Pogosyan G. S.
Published: July 2002

Consider a classical Hamiltonian H in n dimensions consisting of a kinetic energy term plus a potential. If the associated Hamilton–Jacobi equation admits an orthogonal separation of variables, then it is possible to generate algorithmically a canonical basis Q, P where P1=H, P2,…,Pn are the other second-order constants of the motion associated with the separable coordinates, and {Qi,Qj}={Pi,Pj}=0, {Qi,Pj}=δij. The 2n−1 functions Q2,…,Qn,P1,…,Pn form a basis for the invariants. We show how to determine for exactly which spaces and potentials the invariant Qj is a polynomial in the original momenta. We shed light on the general question of exactly when the Hamiltonian admits a constant of the motion that is polynomial in the momenta. For n=2 we go further and consider all cases where the Hamilton–Jacobi equation admits a second-order constant of the motion, not necessarily associated with orthogonal separable coordinates, or even separable coordinates at all. In each of these cases we construct an additional constant of the motion.
Depletion interactions in suspensions of spheres and rod–polymers
Authors: Chen Y.-L. , Schweizer K. S.
Published: July 2002

Liquid-state integral equation methods are employed to study the thermodynamic and structural properties of ideal and repelling rigid rods mixed with hard spheres in the limits when one of the species is dilute. The role of rod aspect ratio and sphere/rod size asymmetry is explored over a wide range of system parameters encompassing the colloid, nanoparticle, and crossover regimes. Novel predictions are found for the polymer (sphere) mediated depletion potentials and second virial coefficients of particles (rods) in dense polymer (sphere) suspensions. The adequacy of the closure approximations employed is tested by comparison with available numerical calculations and more rigorous theories in special limits. The liquid-state theory appears to be accurate for all properties in the nanoparticle regime and for the insertion chemical potential of needles and spherocylinders. However, it significantly underestimates depletion attractions effects in the colloidal regime of short rods and large spheres due to nonlocal entropic repulsion effects between polymers and particles not captured by the classic Percus–Yevick approximation.
Density functional generalized gradient calculations using Slater basis sets
Authors: Cohen Aron J. , Handy Nicholas C.
Published: July 2002

The most common form of density functional calculations on molecular systems used generalized gradient approximation exchange-correlation functionals (such calculations can be applied to larger systems because no exact exchange is included). The most efficient and fastest such codes use an auxiliary basis set to fit the density so that only three-center integrals need to be evaluated. The codes DGAUSS and TURBOMOL use Gaussian basis sets, whereas the long-established ADF code uses Slater basis sets. We here examine the use of Slater basis sets. Our new code evaluates all required integrals numerically by quadrature. We report calculations on the G2 molecular set, contrasting them with similar calculations using Gaussian basis sets. Our conclusion, as far as energetics and structure are concerned, is that very similar predictions may be obtained from basis sets of the same size, and at approximately the same cost.
Experimental and theoretical studies on vacuum ultraviolet absorption cross sections and photodissociation of CH3OH, CH3OD, CD3OH, and CD3OD
Authors: Cheng Bing-Ming , Bahou Mohammed , Chen Wei-Cheng , Yui Chin-hui , Lee Yuan-Pern , Lee L. C.
Published: July 2002

Absorption cross sections of CH3OH, CH3OD, CD3OH, and CD3OD are measured in a 107–220 nm spectral region using synchrotron radiation. Spectra of improved quality for four deuterated isotopomers, coupled with extensive calculations on low-lying excited states of methanol using time-dependent density functional theory with a large cc-pV5Z basis set, enable us to improve assignments of observed spectral features and to better understand the nature of these electronic transitions. Energies and oscillator strengths of all transitions predicted with calculations are consistent with experimental results. Observed isotopic shifts clearly indicate that absorption features in the 163–220 nm region (transition 1 1A″–X 1A′) are associated mainly with breaking of the O–H bond, consistent with theoretical predictions. In the 151–163 nm region (transition 2 1A″–X 1A′), observed small vibrational spacings (806 cm−1 for CH3OH) associated with the C–O stretching mode can be rationalized with a broad double-well-like potential-energy curve resulting from avoided crossing of Rydberg states 11A″(2a″→3s) and 2 1A″(2a″→3p); with isotopic data, further vibrational progressions are identified. Absorption lines in the 140–151 nm region with regular vibrational spacing (∼1046 cm−1 for CH3OH), likely associated with the CH2 twisting mode, are assigned to nearly degenerate transitions 3 1A″–X 1A′ and 3 1A′–X 1A′; the 3 1A″ and 3 1A′ states are associated with excitations 2a″→3p′ and 2a″→3p″, respectively. Progressions associated with the torsional mode of the excited state are observed for the first time. For wavelengths smaller than 140 nm, Rydberg transitions with n⩾3 are tentatively assigned in accord with their quantum defects that are identical for all isotopomers in each Rydberg series.
Quantum beats and Zeeman spectra of glyoxal from superposition of singlet and triplet states
Authors: Chang Chushuan , Chen I-Chia
Published: July 2002

Decays of individual rovibronic levels of trans-glyoxal in its first excited singlet state are investigated in supersonic-jet conditions. Several rotational levels display oscillatory decay from coherent excitation of superposition of singlet and triplet states. Analysis yields a lower bound for the state-dependent coupling matrix elements υST=0.08–62 MHz for the S1–T1 interaction. These matrix elements of the lowest singlet and triplet states show no simple systematic dependence on vibrational states, but the singlet state at greater K quantum number couples to a few triplet states with υST larger than for the state at lower K. The complex beat pattern is explored on varying the polarization of the laser beam relative to an external magnetic field. For state NKaKc=312 at excitation energy 25 254.36 cm−1, the Landé g factor of its coupling triplet hyperfine levels is estimated. From comparison with the theoretical g value for the Hund’s case (b), we found that for this state rotational angular momentum is conserved during singlet–triplet coupling. Irregular MF splittings and correlated beating frequencies resulting from resonance coupling between the singlet and triplet states are observed in the Fourier-transform spectra under the weak magnetic field condition.
Generation of high-amplitude plasma waves for particle acceleration by cross-modulated laser wake fields
Authors: Sheng Z.-M. , Mima K. , Sentoku Y. , Nishihara K. , Zhang J.
Published: July 2002

A scheme is proposed to amplify the wake field of an ultrashort laser pulse by a second laser pulse co-propagating behind with a duration of a few plasma wavelengths or longer for particle acceleration. It is found that, due to the cross-phase modulation by the wake field, the second laser pulse is modulated quickly and split into a multipulse train, where the durations of subpulses are around a plasma oscillation period. This pulse train amplifies the wake field in turn to a high level until wave breaking. With this scheme, a large-amplitude plasma wave can be generated with laser pulses only at moderate intensities. For example, the effective acceleration field for trapped electrons injected from wave-breaking exceeds 100 GeV/m for incident laser pulses at intensity 3.4×1017 W/cm2 in plasma at one percent of the critical density, as demonstrated by particle-in-cell simulations.