bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure and spectroscopy of the He2Cl2 van der Waals cluster
Authors: Hernández Marta I. , Halberstadt Nadine , Sands William D. , Janda Kenneth C.
Published: July 2002

The rovibrational structure of the He2Cl2 van der Waals cluster in the X and B electronic states is studied by means of full dimensional quantum-mechanical calculations. He2Cl2 is the smallest cluster containing helium for which rotationally state-resolved data are available and for which the effects of Bose statistics are important. The He2Cl2 wave functions exhibit quite large amplitude motions, particularly for the He–He bending mode [associated with the angle formed between the two He–(center of mass of Cl2) bonds]. The preferred geometry of the ground van der Waals state is planar, with the He–He axis perpendicular to the Cl2 axis. It is shown that a reduced dimension model for the He–He bending vibration together with a rigid structural model reproduces well the low-lying energy levels of the complex and allows us to assign proper statistical weights to the asymmetric top transitions of the B←X spectra. In particular, the symmetry under He exchange of the rigid rotor levels is shown to depend on the He–He bending level. The observed excitation spectra are successfully simulated using the proposed model. The effective rigid structure that best reproduces the rotationally excited levels with a rigid rotor analysis is a distorted tetrahedron where the He–He angle is approximately 130°. This difference from the 180° most probable configuration is due to the complex dependence of the moments of inertia on the internal degrees of freedom for such a floppy molecule. Therefore, structural information obtained from observed spectra of this or similar clusters should be carefully analyzed to avoid reaching misleading conclusions. Fragmentation rates for He2Cl2(B,v=10–13) are, for the first time, extracted from experimental data, confirming that the dissociation process is mainly sequential.
Ferroelectric properties of Pb1−3y/2Lay(Zr0.4Ti0.6)O3 structures with La concentration gradients
Authors: Boerasu I. , Pintilie L. , Kosec M.
Published: July 2002

Graded ferroelectric (GF) structures were obtained by successive deposition of Pb1−3y/2Lay(Zr0.4Ti0.6)O3 films with different La contents on Pt/TiO2Si substrates. The method used for deposition was sol gel. The final structure consists of four layers, starting from the Pt electrode to the top gold electrode: (1) no La; (2) 4% La; (3) 8% La; and (4) 12% La. Ferroelectric properties of the GF structure were investigated, and an anomalous shift of the hysteresis loop along the polarization axis was found. The sense of this shift depends on the initial direction of the ferroelectric polarization. The presence of hysteresis displacement “up” or “down” can be explained if the presence of a built-in charge in the GF structure is admitted. The built-in potential produced by this charge is computed using a simple theoretical model.
Energy and spatial resolved measurement of soft x-ray emission with photon counting x-ray charge coupled device camera in compact helical system
Authors: Liang Y. , Ida K. , Kado S. , Nomura I. , Minami T.
Published: July 2002

Two-dimensional profiles (32×16 spatial channels) of energy spectra of x-ray emission are measured with an energy resolution of 0.16 keV using the photon counting soft x-ray charge coupled device (CCD) camera by optimizing the intensity of x ray with attenuation Be filter in the compact helical system. The energy spectra measured with the CCD camera agrees with that estimated from electron temperature and density measured with YAG Thomson scattering.
Chaotic behavior of ion-temperature-gradient driven drift-dissipative modes
Authors: Rafiq Tariq , Qamar Anisa , Mirza Arshad M. , Murtaza G.
Published: July 2002

A new set of nonlinear mode coupling equations for finite amplitude low-frequency electromagnetic waves has been derived for nonuniform, resistive, magnetized electron-ion plasma with sheared flows. At equilibrium, the plasma is assumed to have density, ion-temperature, magnetic field, and velocity gradients. The temporal behavior of the nonlinear mode coupling equations is found to be governed by eight coupled equations, which are the generalization of the Lorenz and Stenflo equations, admitting chaotic trajectories. The linear stability of the generalized Lorenz–Stenflo system of equations is also presented under different approximations. The results of the present investigation should be helpful in understanding the wave phenomena in space and tokamak plasmas.
Penetration depth of energetic F atoms from F2 dissociation in layered rare gas samples
Authors: Dickgießer M. , Schwentner N.
Published: July 2002

Penetration depths of atoms with kinetic energy provided by photodissociation of parent molecules in the top layer of a multilayer sample are determined from the probability to cross a spacer layer of thickness d and to arrive at the interface to a substrate. Top layer growth up to a final thickness s corresponds to a continuous increase of the effective spacer layer thickness. Modeling of growth and comparison with sample-to-sample variation of d allows us to determine separately and in a consistent way the precursors’ dissociation cross section q⋅σ and the mean penetration depth d0 of the fragments together with elimination of contaminated samples. For F atoms with 4.3 eV kinetic energy from F2 dissociation values of q⋅σ=3×10−17 cm2 and d0=2.1 nm (8 to 9 monolayers) are derived for Ar spacers. A strong increase of d0 in the case of unintentional multistep excitation of F fragments is demonstrated.
Resonance radiation transport in plasma display panels
Authors: Hagelaar G. J. M. , Klein M. H. , Snijkers R. J. M. M. , Kroesen G. M. W.
Published: July 2002

In fluid models of the gas discharges in plasma display panels, the trapping of resonance radiation is usually accounted for by a trapping factor. In this work, we present a Monte Carlo model for resonance photons, which gives a much more accurate description. First, we compare the results of this Monte Carlo model with the results of the fluid model trapping factor approach. Although the trapping factor approach does not yield the same spatial distribution for the density of the resonant state atoms, the spatially integrated density is in good agreement with the results of the Monte Carlo model. Next, we compare the results of the Monte Carlo model with measured spectra of emitted resonance radiation. The agreement is very good. Thus we provide, via the Monte Carlo model, experimental support for the widely used trapping factor approach.
Modeling benzene orientational randomization in Na–Y zeolite at finite loadings with kinetic Monte Carlo and master equation methods
Authors: Blanco Cristian , Saravanan Chandra , Allen Melissa , Auerbach Scott M.
Published: July 2002

We have modeled the orientational dynamics of benzene in Na–Y zeolite, motivated by the NMR study of Isfort et al. at loadings of five benzenes per cage [Chem. Phys. Lett. 288, 71 (1998)]. We consider guest-guest interactions in two stages: first, we include only site blocking; next, we consider both site blocking and nearest-neighbor attractions. We calculated orientational correlation functions using kinetic Monte Carlo and also with a mean field master equation (MFME). Both methods produce correlation functions exhibiting biexponential decay in time. Analytically solving the MFME shows that long-time decay is controlled by a composite of intracage and cage-to-cage jumps. The apparent activation energy is greater than the fundamental cage-to-cage barrier when considering only site blocking, but is less than the same fundamental barrier when also including guest-guest attractions. This suggests that the actual cage-to-cage barrier is greater than the 40 kJ mol−1 reported by Isfort et al., which lends credence to previous simulations of benzene in Na–Y.
Optimal two-dimensional models for wake flows
Authors: Balachandar S. , Najjar F. M.
Published: July 2002

In the case of nominally two-dimensional (2D) cylinders of arbitrary cross section in cross flow, the three-dimensionality of the wake manifests in the form of quasi-streamwise vortices. These three-dimensional (3D) features profoundly influence lift and drag forces. However, a two-dimensional projection of such a flow, where the effects of three-dimensionality are modeled, will be computationally very attractive. One can consider the two-dimensional projection as the limiting case of large eddy simulation, where the spanwise direction has been completely averaged out. The transport equation for the span-averaged spanwise component of vorticity, ω̄z, is considered; the 3D effects to be modeled appear as a subgrid scale flux of torque. It is shown that simple minded eddy viscosity type models that assume the flux vector to be proportional to the spatial gradient of ω̄z are inadequate. Here we extend the optimal modeling formalism [Moser, Balachandar, and Adrian, Turbulence and Internal Flow/Unsteady Aerodynamics and Hypersonics Conference, Annapolis, MD, pp. 269–274 (1998); Langford and Moser, J. Fluid Mech. 398, 321 (1999)] to address issues pertaining to complex flows with multiple directions of inhomogeneity. We present optimal closures for subgrid flux modeled in terms of ω̄z distribution, based on linear and quadratic stochastic approximations. These ideas are tested using the database of flow over a flat plate held normal to a cross flow. It is observed that even the optimal model has about 70% normalized error, indicating that the subgrid flux is only about 30% deterministic. Furthermore, it is observed that local models are inadequate, but there exists a region of nonlocality for model dependence, expanding beyond which does not improve the estimate. Higher order nonlinearities however do not seem to improve the model’s predictability.
An assessment of theoretical methods for the study of transition metal carbonyl complexes: [Cl2Rh(CO)2]− and [Cl2Rh(CO)]− as case studies
Authors: Hu Zhenming , Boyd Russell J.
Published: July 2002

A comparative theoretical study of the equilibrium geometries, vibrational frequencies, and reaction energetics of Rh–CO bond activation in the [Cl2Rh(CO)2]− and [Cl2Rh(CO)]− complexes has been performed using the B3LYP density functional method and the traditional correlated second-order Møller–Plesset (MP2), quadratic configuration interaction with single and double substitutions (QCISD) and coupled-cluster single double (triple) [CCSD(T)] methods. CCSD(T) is employed herein as a benchmark method to examine the validity of the B3LYP and MP2 methods in studies of transition metal complexes. The results show that the geometries and energies obtained with the B3LYP method agree quite well with the QCISD and CCSD(T) results. The conventional MP2 method yields poor results for all geometries, vibrational frequencies, and reaction energies. It is noteworthy that the relative reaction energies calculated at the highest correlated CCSD(T) level using the B3LYP and MP2 geometries are comparable even though the B3LYP and MP2 geometries differ significantly from each other. The absolute energies calculated with the QCISD and CCSD(T) methods at the B3LYP geometries are all lower than those obtained with the MP2 geometries, indicating that the B3LYP method could be more reliable for the study of the geometries and energetics of the catalytic reactions. Basis set effects have been examined by B3LYP calculations. The present results provide a comprehensive assessment of the widely used ab initio theoretical methods for the study of transition metal carbonyl complexes.
Electron paramagnetic resonance as a quantitative tool for the study of multiwalled carbon nanotubes
Authors: Coleman J. N. , O'Brien D. F. , Dalton A. B. , McCarthy B. , Lahr B. , Barklie R. C. , Blau W. J.
Published: July 2002

We have described a method that maximizes the phase separation of graphitic particles (GP) and multiwalled carbon nanotubes (MWNT) in solutions of various organic polymeric hosts. This involves the formation of sediment and a solute. These components were characterized for MWNT and GP content using electron paramagnetic resonance (EPR) measurements. All EPR signals could be deconvoluted into nanotube and GP components. When normalized, these components are representative of the mass of MWNT and GP present. This allows us to make quantitative measurements of nanotube and GP content in different environments. The most successful polymer host was poly (m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) (PmPV). In this case the solute contained 63% of the added nanotubes with only 2% of the added graphite remaining.
Persistent photoconductivity study in a high mobility AlGaN/GaN heterostructure
Authors: Elhamri S. , Saxler A. , Mitchel W. C. , Elsass C. R. , Smorchkova I. P. , Heying B. , Haus E. , Fini P. , Ibbetson J. P. , Keller S. , Petroff P. M. , DenBaars S. P. , Mishra U. K. , Speck J. S.
Published: July 2002

We have used the Shubnikov–de Haas and the Hall effects to investigate the effect of subband gap illumination on the transport properties of a very high mobility, μ=54 000 cm2/V s at T=1.2 K, Al0.09GaN0.91/GaN heterostructure. We have found that this illumination resulted in a photocurrent that was persistent at low temperatures. This photocurrent, which led to an increase in the carrier density, was used to study the dependence of the mobility and the quantum scattering time on the carrier density. Unlike in previous studies of persistent photoconductivity in GaN and AlGaN/GaN by other researchers, we have found that the mobility did not always increase with the carrier density as a result of illumination. For small duration of illumination, the mobility increased with the carrier density. However additional illumination resulted in a decrease of the mobility despite the increase in the carrier density. We believe that screening is responsible for the initial increase in the mobility, while ionization of defects in GaN, interface charges, and alloy disorder may be responsible for the eventual decrease in the mobility.
CoCrPtTa single layer perpendicular magnetic recording media with carbon underlayer
Authors: Onoue Takahiro , Takizawa Atsunari , Asahi Toru , Osaka Tetsuya
Published: July 2002

Sputter-deposited amorphous carbon was applied as an underlayer for a Co73Cr18Pt6Ta3 single layer perpendicular magnetic recording medium. It is shown that the amorphous carbon underlayer can control the Co [001] preferred orientation of the Co73Cr18Pt6Ta3 magnetic recording layer and that the microcrystalline structure, magnetic properties, and read–write characteristics of Co73Cr18Pt6Ta3 media are greatly dependent on the carbon underlayer thickness. By x-ray diffraction experiments and transmission electron microscopy observations, it was confirmed that the distribution of the c-axis orientation of Co grains becomes broader in the direction perpendicular to the film surface with increasing carbon underlayer thickness. Furthermore, the Co73Cr18Pt6Ta3 medium with a thick carbon underlayer indicated a relatively high perpendicular coercivity of 2800 Oe and an excellent signal to noise ratio in read–write characteristics.
The effect of postgrowth ion irradiation on the microstructure and the interface properties of amorphous carbon films on silicon
Authors: Patsalas P. , Logothetidis S.
Published: July 2002

We studied the effect of postgrowth low energy (1.5 keV) Ar+ ion irradiation of various amorphous carbon (a-C) films with different microstructure and bonding. Detailed x-ray diffraction was used to identify the existing phases in thin (30–200 nm) a-C films, deposited on Si and subjected to Ar+ ion irradiation and to study the mechanisms of formation of diamond, lonsdaleite (diamond-2H), silicon carbide, and graphite grains. The relative content of each crystalline phase, the grain size, and distribution depend strongly on the microstructure of the as-grown films that is affected by the bias voltage applied during deposition of the films. Ion bombardment promoted the formation of graphite throughout the films and diamond and SiC grains close to the a-C/Si interface. Diamond and SiC nucleation at the a-C/Si interface area occurs beyond the penetration depth of the Ar+ ions and therefore is not attributed to a direct, ballistic ion–solid interaction but rather to a mechanism of charge transfer through conductivity channels. The latter is also considered and discussed in terms of other features of the as-grown a-C films such as density, voids content, electrical resistivity, and surface and interface morphology.
Two-point closures for weakly compressible turbulence
Authors: Bertoglio Jean-Pierre , Bataille Françoise , Marion Jean-Denis
Published: July 2002

The aim of the present paper is to address the problem of homogeneous isotropic compressible turbulence within the framework of two-point statistical closures. In order to simplify the description, weak compressibility assumptions are first introduced in the equations governing the fluctuating field: the fluid is supposed to be barotropic and nonlinear terms involving density fluctuations are neglected. The equations for the two-point two-time correlations are written. They are closed by extending the direct interaction approximation to (weakly) compressible fields. This work is then used as a ground to derive an extension of the eddy damped quasi-normal Markovian closure that accounts for compressibility effects. Both closures reflect the existence of acoustic waves and the complex nature of nonlinear interactions between modes. The EDQNM model is solved numerically for the case of an isotropic turbulent field maintained statistically stationary by injecting energy in the large scales. At low Mach number, the spectrum of the purely compressible velocity is found first to scale as K−5/3, then to evolve, at large time, towards an asymptotic K−11/3 scaling. The same behavior is found for the pressure spectrum, equipartition of energy between the two compressible modes being observed. Results concerning the dissipation show that its dilatational component is proportional to the square of the turbulent Mach number. At higher Mach number, the slope of the compressible spectra appears to be less steep, and discrepancies from the Mt2 scaling are detected. The influence of the Reynolds number is also analyzed and the ratio of the compressible energy to the solenoidal kinetic energy is found to scale as Mt2 Re for Mt2 Re smaller than 10. A simple spectral model reproducing the main features of the EDQNM results at low Mach number is proposed.
Self-normalized photothermal techniques for thermal diffusivity measurements
Authors: Balderas-López J. A. , Mandelis A.
Published: July 2002

Two self-normalized photothermal techniques, to carry out thermal diffusivity measurements of condensed phase materials, are presented. These simple methodologies involve linear fitting procedures of the signal amplitude and phase. These procedures lead to the elimination of the usual requirement for instrumental transfer-function normalization. The thermal diffusivities for two dental resins and two pure liquids are measured with these simple methodologies and very good agreement is found with values reported in the literature, where more involved analysis is usually required.
Photoemission studies of ZnSe epilayers grown on GaAs(111)B surface
Authors: Feng P. X. , Leckey R. C. G. , Riley J. D. , Brack N. , Pigram P. J. , Hollering M. , Ley L.
Published: July 2002

The growth and characterization of ZnSe epilayers on GaAs(111)B was studied. Insight into the formation mechanism of this type of surface, interface, and bulk has been provided by photoemission spectroscopy. When Zn and Se are deposited, Se reacts with As to form Se–As bonds. Therefore, the electron mean free path obtained from the intensity variation of the surface As layer is less than that from the As bulk intensity since its emission is transferred from the surface peak into chemically shifted As peak. Deposition of ZnSe results in a substrate core level shift of 0.59 eV toward lower binding energy. A value of 0.91 eV for the valence band offset, with the valence band maximum of ZnSe below that of GaAs, was obtained.
Ionic dissociation of NaCl on frozen water
Authors: Park Seong-Chan , Pradeep T. , Kang H.
Published: July 2002

Low-energy reactive ion scattering (RIS) experiments show that NaCl dissociates almost completely on condensed ice surfaces prepared on Ru(001) even at 100 K forming solvated ions. NaCl by itself does not dissociate on a Ru(001) substrate; however, submonolayer coverages of water sets-in dissociation. Na+ ions thus created are immobile such that they do not migrate across one water bilayer (BL) over several minutes. Accumulation of positive charge at the ice surface makes Na+ diffuse into the sublayers.
Integral equation study of liquid hydrogen fluoride
Authors: Martı́n C. , Lombardero M. , Anta J. A. , Lomba E.
Published: July 2002

Liquid hydrogen fluoride is a well-known hydrogen bonded substance, in many aspects related to liquid water, and for which a wide variety of interaction models have recently been proposed. We have studied two of these models by means of a reference hypernetted chain equation in order to assess the ability of this latter approach to describe the properties of this highly associative system. Our calculations, when compared with molecular dynamic results, show that the integral equation reproduces quantitatively both the structure and the thermodynamics of liquid hydrogen fluoride over a wide range of thermodynamic states. However, the integral equation approach is apparently unable to produce estimates for the phase diagram since the low-density (gas phase) side of the binodal curve lies inside the nonsolution region of the equation. This failure can be understood as the result of the inability of standard integral equation theories to account for the behavior of low density strongly associative systems like highly charged electrolytes or, in this case, the gaseous phase of hydrogen fluoride.
Coexistence and criticality of fluids with long-range potentials
Authors: Camp Philip J. , Patey G. N.
Published: July 2002

Using mixed-field finite-size scaling simulations, we have investigated the liquid–vapor critical behavior of three-dimensional fluids with algebraically decaying attractive pair interactions, which vary like −1/r3+σ with σ=3, 1, and 0.1. The finite-size scaling analysis was carried out by matching the critical ordering operator distribution, pL(x), against the limiting Ising form, i.e., Ising criticality was assumed. When the potential is short-ranged (σ=3) the simulation results are entirely consistent with the expected Ising critical behavior. When the potential is long-ranged (σ=1, 0.1), however, marked deviations from Ising behavior are observed, particularly in the form of the critical ordering operator distribution, and in the estimated values of β/ν. The results are consistent with non-Ising criticality which is predicted theoretically in fluids with long-range interactions. Some results from Gibbs ensemble simulations are also provided in order to sketch the shape of the liquid–vapor coexistence envelope. We discuss the relevance of our results to the current issue of criticality in ionic fluids.
Diagnostics and analyses of decay process in laser produced tetrakis(dimethyl-amino)ethylene plasma
Authors: Ding Guowen , Scharer John E. , Kelly Kurt L.
Published: July 2002

A large volume (hundreds of cm3) plasma is created by a 193 nm laser ionizing an organic vapor, tetrakis(dimethyl-amino)ethylene (TMAE). The plasma is characterized as high electron density (1013–1012 cm−3) and low electron temperature (∼0.1 eV). To investigate the plasma decay processes, a fast Langmuir probe technique is developed, including detailed considerations of probe structure, probe surface cleaning, shielding, frequency response of the detection system, physical processes in probe measurement, dummy probe corrections as well as noise analysis. The mechanisms for the plasma decay are studied and a delayed ionization process following the laser pulse is found to be important. This mechanism is also supported by optical emission measurements which show that nitrogen enhances the delayed emission from TMAE plasma. A model combining electron–ion recombination and delayed ionization is utilized together with experimental results to order the terms and calculate the relaxation times for delayed ionization. The relaxation times are longer for lower TMAE pressures and lower electron densities.
Micromagnetic predictions for thermally assisted reversal over long time scales
Authors: Xue Jianhua , Victora R. H.
Published: July 2002

A scaling technique is developed to predict micromagnetic properties of magnetic materials subject to thermal fluctuations over long time scales. This theory, based on the equivalence of time and temperature for thermally activated processes, is tested by predicting hysteresis loops for two different thin film magnetic recording media. A comparison to directly calculated values and to experimental hysteresis loop results shows that important features such as the coercivity are predicted with less than 1% error.
Numerical simulation of a gas–liquid flow in a fixed bed
Authors: Koo Sangkyun , Sangani Ashok S.
Published: July 2002

A countercurrent gas–liquid flow through a fixed bed of spherical particles is examined numerically by solving the particle-scale equations governing the gas and liquid flows. The liquid is assumed to flow along the surface of the particles forming a thin film. The case of small gas flow rates is examined in detail first. In this limit the presence of the liquid film increases the gas pressure drop over its value for a dry bed by three mechanisms: The liquid film makes the apparent size of the particles larger, decreases the pore space for the gas flow, and, with its velocity pointing opposite to the mean gas flow, increases the apparent velocity of the gas compared with the particle surface. The excess pressure drop is determined for both periodic and random arrangements of particles. Next, the case of high gas flow rates where the traction exerted by the gas at the gas–liquid interface is comparable to the weight of the liquid film is examined. In this regime the liquid holdup increases with the gas flow rate and the pressure drop-gas velocity relation is nonlinear. The results of numerical simulations are compared with approximate models and it is shown that a simple capillary model yields reasonably accurate predictions for the liquid holdup and gas pressure drop.
Reversible and irreversible processes in donor-doped Pb(Zr,Ti)O3
Authors: Bolten D. , Böttger U. , Schneller T. , Grossmann M. , Lohse O. , Waser R.
Published: July 2002

In this letter, the reversible and irreversible polarization contributions in donor-doped Pb(Zr,Ti)O3 are investigated using conventional capacitance–voltage (C–V) and hysteresis measurements. The dependence of the Rayleigh coefficient in the subcoercive regime on the doping concentration is investigated and shown to be consistent with the assumption of an interaction of the domain walls with randomly distributed defects.
S
1
–S
2
vibronic coupling in cis-1,3,5-hexatriene. I. Electronic structure calculations
Authors: Woywod Clemens , Livingood William C. , Frederick John H.
Published: July 2002

The estimates for the vertical excitation energy of the 2 1A1 state of cis-1,3,5-hexatriene (CHT) vary considerably and provide a good example of the difficulties that can arise in determining transition energies. The great uncertainty is surprising if one considers that this state has already been characterized by high resolution techniques such as resonance enhanced multiphoton ionization (REMPI) and fluorescence excitation spectroscopy in free jet expansions. A theoretical analysis of this problem is clearly needed and the present work, along with the following paper, represents an effort to investigate the nature of the 2 1A1 and 1 1B1 states of CHT. It is shown that a combination of ab initio electronic structure and quantum-mechanical wave packet calculations is required to systematically approach a question as involved as locating the energetical position of the 2 1A1 level. We characterize the energy dependence of the 1 1A1, 2 1A1, and 1 1B1 states of CHT as a function of the in-plane normal coordinates for small displacements from the ground-state equilibrium geometry employing the single-state multiconfigurational second-order perturbation theory (CASPT2) method. This information constitutes the basis for the construction of diabatic harmonic model potential-energy functions associated with the three electronic states in the Franck–Condon region that is essential for the treatment of nonadiabatic dynamics. Five totally symmetric modes with high Franck–Condon and/or tuning activity are identified. Vibronic interaction between the S1 and S2 states is primarily mediated by four vibrations of b1 symmetry, ν26, ν27, ν30, and ν31. ν30 and ν31 are found to be exceptionally powerful interstate coupling modes and the strong nonadiabatic effects induced by these modes in CHT are mainly responsible for the spectroscopic differences observed for the S1 and S2 states of CHT and trans-1,3,5-hexatriene.
Improved trial wave functions in quantum Monte Carlo: Application to acetylene and its dissociation fragments
Authors: Barnett R. N. , Sun Zhiwei , Lester W. A.
Published: July 2002

Recent quantum Monte Carlo (QMC) studies of electronic structure have considered various trial function enhancements directed at improved fixed-node energies. In this study we investigate complete active space self-consistent field (CASSCF) trial functions in the diffusion Monte Carlo (DMC) method. We study longer CASSCF expansions than typically used in QMC studies and optimize correlation function parameters, basis function coefficients, and configuration state function mixing coefficients. To perform a stable, efficient wavefunction optimization, sample points are analytically obtained from an integrable probability density function or a Monte Carlo walk guided by a positive definite function. The approach is applied to acetylene and its dissociation fragments (C, CH, C2, C2H, C2H2). For these systems 70%–90% of the correlation energy is recovered with variational MC and 91%–98% with DMC.
Optical properties of undoped and Co2+-doped Cd2GeSe4 single crystals
Authors: Lee Jeoung Ju , Yang Chang Soo , Park Young Sin , Kim Kun Ho , Jin Moon Seog , Park Hong Lee , Kim Wha Tek
Published: July 2002

Cd 2 GeSe 4 and Cd2GeSe4:Co2+ (Co: 0.2, 0.3, 0.4, and 0.5 mol %) single crystals with a rhombohedral (hexagonal) structure were grown by the vertical Bridgman method. The structural and optical properties of Cd2GeSe4:Co2+ (Co: 0.2, 0.3, 0.4, and 0.5 mol %) were characterized by x-ray diffraction and optical absorption measurements at room temperature (298 K). The lattice constants, a0 and c0, increased when increasing Co2+ in the Cd2GeSe4:Co2+ crystal to 0.3 mol %. They increased sharply when increasing Co2+ to 0.4 and 0.5 mol %. The optical energy band gap of the Cd2GeSe4:Co2+ crystal decreased when increasing Co2+ in the Cd2GeSe4:Co2+ crystal to 0.3 mol % and it also decreased sharply when increasing the amount of Co2+ to 0.4 and 0.5 mol %. Two impurity optical absorption bands were observed at wavelength ranges of 1300–1900 nm and 700–900 nm. These bands originated from the Co2+ ions located at the C3v symmetry point in the Cd2GeSe4:Co2+ crystal.
Experimental investigation of electron oscillation inside the filter of a vacuum arc plasma source
Authors: Kwok D. T. K. , Zhang T. , Chu P. K. , Bilek M. M. M. , Vizir A. , Brown I. G.
Published: July 2002

We report here experimental evidence of electron oscillation within the toroidal-section magnetic duct of a filtered vacuum arc plasma source. Our results clearly demonstrate that electrons can oscillate inside the duct under the combined effects of the electric and magnetic fields. In another experiment, we observe that, under the influence of the electron motion, the trajectories of the plasma ions are more or less unchanged except in the intensity when the Bilek plate is biased. Finally, our time-of-flight experiments show that the effects due to collisional scattering between plasma ions and oscillating electrons are masked by those associated with the metal plasma flow through the duct, and collisional scattering does not give rise to an increase of the mean charge state of the plasma ions. We conclude that the application of a bias voltage to the duct not only perturbs the ions but also influences the plasma electrons. Our results demonstrate that electrons at the central axis are one of the major reasons leading to improved plasma transport through the duct.
Velocity angular dispersion of surface and bulk modes in LiTaO3 at hypersonic frequencies
Authors: Kuok M. H. , Ng S. C. , Zhang V. L. , Fan H. J.
Published: July 2002

The existence of two surface acoustic waves, the Rayleigh wave and the second leaky wave, on X-cut LiTaO3 has been detected using Brillouin scattering. Evidence of dispersion between ultrasonic and hypersonic velocities was found for both surface and bulk acoustic waves. Elastic constant values were determined from a simultaneous least-squares fit of the angular dispersion data for the longitudinal and the two transverse bulk acoustic modes.
Normal modes of bounded whistler produced toroidal plasmas
Authors: Tripathi S. K. P. , Bora D.
Published: July 2002

In tight aspect ratio toroidal devices, characteristics of bounded whistler sustained plasma filled resonant cavities are shown to have significant differences as compared to characteristics of cylindrical cavities in helicon plasma sources. In the present article the analytical-numerical treatment of the problem in electronmagnetohydrodynamic regime is based on wave-field computation and modified dispersive properties of wave for toroidal devices. In the limit of tight aspect ratio, the parallel wavelength of the excited whistler becomes comparable to the system dimension and, thus, strong poloidal mode coupling introduces new features to radially bounded whistlers. Smooth transition of toroidal bounded whistler modes towards conventional helicon modes is observed as the major radius increases.
Schottky contacts on amorphous carbon: A more reliable approach
Authors: Paul S. , Clough F. J.
Published: July 2002

It is considered impossible to form Schottky contacts on amorphous hydrogenated carbon (a-C:H). Taking the lead from our earlier work in which we report the formation of Schottky contacts on radio frequency plasma enhanced chemical vapor deposition (rf PECVD) grown a-C:H films, here we present a way of confining the semiconductor-like parts of a-C:H to particular regions of the films. Thus, the location and formation of Schottky contacts in a-C:H, are rendered more predictable. This was achieved by depositing a-C:H on metal-patterned Si substrates.
On the applicability of local asymptotic stability criteria to stellarator stability
Authors: Carreras B. A. , Lynch V. E. , Ichiguchi K. , Wakatani M. , Tatsuno T.
Published: July 2002

An intrinsic consequence of the three-dimensional nature of the stellarator equilibrium may be the existence of local flattening of the pressure profile at the resonant surfaces. This local flattening of the pressure profile significantly changes the stability properties. The localized interchange modes are stabilized, and a new instability branch controls the critical beta. This instability is strongly stabilized by shear at high poloidal mode numbers. As a consequence, the plasma stability properties change, and the asymptotically derived local stability criteria often used in stellarator design are no longer applicable.
The different dynamical modes of nested wire array Z pinches
Authors: Chittenden J. P. , Lebedev S. V. , Bland S. N. , Ciardi A. , Haines M. G.
Published: July 2002

Magnetohydrodynamic simulations are used to predict which modes of implosion are in operation in nested wire array experiments. Separate r–θ plane simulations of the flux of plasma imploding towards the axis from the outer array and the bombardment of the inner array by this flux are presented. The different implosion modes are distinguished by the level of momentum transfer and magnetic flux compression during collision. For typical configurations, large interwire gaps (>1.5 mm) are required in the inner array in order to access the advantageous mode where the outer array material passes through.
Ballistic electron microscopy study of ultrathin oxidized aluminum barriers for magnetic tunnel junctions
Authors: Rippard W. H. , Perrella A. C. , Buhrman R. A.
Published: July 2002

Ballistic electron emission microscopy has been used to study thin aluminum oxide tunnel junction barriers formed both by magnetron sputter deposition and thermal evaporation. We have found that the barriers made by oxidation of evaporated Al become fully formed at a significantly thinner mean deposited thickness (∼6 Å) than junctions made by sputter deposition. The effective barrier height of the aluminum oxide has been determined to be 1.22±0.05 eV and is independent of the method of deposition, thickness, and oxidation conditions.
Numerical study of global stability of oblate field-reversed configurations
Authors: Belova E. V. , Jardin S. C. , Ji H. , Yamada M. , Kulsrud R.
Published: July 2002

Global stability of the oblate (small elongation, E<1) Field-Reversed Configuration (FRC) has been investigated numerically using both three-dimensional magnetohydrodynamic (MHD) and hybrid (fluid electrons and kinetic ions) simulations. For every nonzero value of the toroidal mode number n, there are three MHD modes that must be stabilized. For n=1, these are the interchange, the tilt and the radial shift; while for n>1 these are the interchange and two co-interchange modes with different polarization. It is shown that the n=1 tilt mode becomes an external mode when E<1, and it can be effectively stabilized by close-fitting conducting shells, even in the small Larmor radii (MHD) regime. The tilt mode stability improves with increasing oblateness, however at sufficiently small elongations the radial shift mode becomes more unstable than the tilt mode. The interchange mode stability is strongly profile dependent, and all n⩾1 interchange modes can be stabilized for a class of pressure profile with separatrix beta larger than 0.035. Our results show that all three n=1 modes can be stabilized in the MHD regime, but the stabilization of the n>1 co-interchange modes still remains an open question.
Efficient evaluation of the Coulomb force in density-functional theory calculations
Authors: Shao Yihan , White Christopher A. , Head-Gordon Martin
Published: July 2002

The Coulomb force in density-functional theory calculations is efficiently evaluated based on a partitioning into near-field (NF) and far-field (FF) interactions. For the NF contributions, a J force engine method is developed based on our previous J matrix engine methods, and offers a significant speedup over derivative electron repulsion integral evaluation, without any approximation. In test calculations on water clusters and linear alkanes, the computer time for the NF force is reduced by a factor of 5–7 with a 3-21G basis set and 6–8 with a 6-31G** basis set. The FF force is treated by a generalization of the continuous fast multipole method, and the FF computational cost is found to be comparable to that of an energy evaluation.
Nature of grain boundaries in laser crystallized polycrystalline silicon thin films
Authors: Christiansen S. , Lengsfeld P. , Krinke J. , Nerding M. , Nickel N. H. , Strunk H. P.
Published: July 2002

The grain boundary populations in laser crystallized polycrystalline silicon thin films are determined by electron microscope analysis, using electron backscattering contrast in the scanning electron microscope, and convergent beam electron diffraction in the transmission electron microscope. The grain boundary populations of the grains larger than 0.5 μm are dominated by first and second order twin boundaries. This result is found to be a general feature of laser crystallization independent of the experimental details of the laser crystallization process. Texture analysis of the laser crystallized poly-Si films shows that under certain experimental conditions a {111}-preferential orientation of the grains perpendicular to the substrate can be obtained.
Dissipation in media with memory: A master equation in the statistical resonance approximation
Authors: Wilkie Joshua
Published: July 2002

A non-Markovian master equation is derived for the reduced probability density matrix of a subsystem interacting with a general reservoir of coupled anharmonic modes. Relaxation of the subsystem is mediated through resonant interactions with the reservoir. These interactions correspond to local vibrational motions with finite lifetimes due to diffusion/dephasing. The derivation assumes that the density of these interaction modes is very large so that they can be treated using statistical methods. The resulting master equation is shown to preserve probability, Hermiticity, and translational invariance, and conditions are stated which guarantee the positivity of the reduced density. At long time the master equation reduces to a Markovian equation of dynamical semigroup type. In the high temperature limit the theory is parameter free, and shows good agreement with the exact master equation of a spin-boson system.
Short-wavelength photolysis of jet-cooled OClO(2A2 ν1>20)→ClO(X 2ΠΩ,v,J)+O(3PJ)
Authors: Delmdahl Ralph F. , Parker David H. , Eppink André T. J. B.
Published: July 2002

Highly inverted vibrational level populations are found for ClO fragments resulting from the UV photodissociation of OClO(2A2 ν1>20) into ClO(X 2ΠΩ,v,J) and O(3PJ) fragments. These distributions depend significantly on the spin–orbit J state of the oxygen O(3PJ) partner atom. In contrast, the ClO rotational excitation is modest. Distinct rotational structure is visible in the O (3PJ) photofragment yield spectrum recorded from the highly excited OClO (2A2 ν1=21) vibronic band, which is indicative for hitherto unforeseen long dissociation lifetimes of very highly excited OClO. The data point towards an unexpected nearly-linear and highly asymmetric dissociation geometry. Carrying out near-threshold fragmentation experiments of OClO (2A20,0,0)→ClO (X 2Π3/2 v=0, J)+O(3P2,1,0) the dissociation energy D0 of OClO has been accurately determined to 247.3±0.5 kJ/mol.
A crystalline orbital study of polydiacetylenes
Authors: Tobita Motoi , Hirata So , Bartlett Rodney J.
Published: July 2002

The electronic and structural properties of the ground and excited states of infinite polydiacetylene chains in acetylenic (PDA) and butatrienic (PBT) structures are studied by a series of ab initio crystalline orbital and linear-combination-of-atomic-orbital periodic density functional theory methods. A complete geometry optimization is performed for PDA and PBT with analytical energy gradient techniques at the Hartree–Fock (HF) and Becke3-Lee-Yang-Parr (B3LYP) levels. The HF/6-31G* and B3LYP/6-31G* reproduce the experimental geometrical parameters of substituted polydiacetylenes with a PDA-like structure. We compute the relative stability and the potential energy curves along the structural transition between PDA and PBT at the HF, B3LYP, and second-order many-body perturbation theory [MBPT(2)] levels. All these calculations predict PDA to be more stable than PBT by 28–87 kJ mol−1. A minimum corresponding to the PBT-like structure is found at the HF level, but not at the B3LYP or MBPT(2) level. We report the frequencies of all the infrared- and Raman-active vibrational modes of PDA at the HF and B3LYP levels. The frequencies of the carbon backbone stretching modes calculated at the B3LYP/6-31G* level are within 60 cm−1 of the measured frequencies of resonance Raman bands, when the former values are scaled by a uniform scale factor of 0.96. The ionization potential (IP), electron affinity (EA), and fundamental band gap (Eg) of PDA are calculated at the HF and B3LYP levels and also at the MBPT(2) level employing the quasiparticle formalism. B3LYP/6-31G* provides the most reasonable IP, EA, and Eg, which are within 0.6 eV of the experimental results. Vertical excitation energies to the lowest singlet and triplet excitons of PDA are obtained by configuration interaction singles and by time-dependent density functional theory employing the B3LYP functional. These treatments properly account for the nonvanishing exciton binding energy. While the CIS/6-31G* excessively overestimates the singlet excitation binding energies, B3LYP/6-31G* provides a value (0.3 eV) that is in good agreement with experiment (0.4 eV).
Hyper-Rayleigh scattering from CH4, CD4, CF4, and CCl4
Authors: Pyatt Robby D. , Shelton David P.
Published: July 2002

First hyperpolarizabilities β were obtained for CX4 molecules in gas and liquid phases from hyper-Rayleigh scattering measurements at λ=1064 nm. The gas-phase results for βxyz (atomic units) are 7.1±0.9 (CH4), 7.0±0.9 (CD4), 5.4±0.6 (CF4), and 11.0±0.9 (CCl4). Ab initio calculations of β are consistent with the experimental results for CF4 and CCl4, but not for CH4 and CD4. The effective liquid-phase β for CCl4 is 3× the gas phase value, but the liquid environment has little effect on β for the other molecules. Thermal-lens and absorption coefficients were also measured for ten liquids.
Field line diffusion and loss in a tokamak with an ergodic magnetic limiter
Authors: da Silva Elton C. , Caldas Iberê L. , Viana Ricardo L.
Published: July 2002

A numerical study of chaotic field line diffusion in a tokamak with an ergodic magnetic limiter is described. The equilibrium model field is analytically obtained by solving a Grad–Schlüter–Shafranov equation in toroidal polar coordinates, and the limiter field is determined by supposing its action as a sequence of delta-function pulses. A symplectic twist mapping is introduced to analyze the mean square radial deviation of a bunch of field lines in a predominantly chaotic region. The formation of a stochastic layer and field diffusivity at the plasma edge are investigated. Field line transport is initially subdiffusive and becomes superdiffusive after a few iterations. The field lines are lost when they collide with the tokamak inner wall; their decay rate is exponential with Poisson statistics.
Exact equivalence between the sequential long-range charge transfer rates and the electric conductances in evaluation of chemical yields
Authors: Yan YiJing , Li Xin-Qi , Zhang Houyu
Published: July 2002

Analytical expressions for calculating the stationary yields of chemical products are derived in a general kinetic scheme for the multistep or sequential hopping rate processes. Established further is an exact mapping between the stationary chemical yields and the normalized electric direct currents.
Transient decay of beam-produced fields in thin-walled tubes
Authors: Ekdahl Carl
Published: July 2002

An electromagnetics problem that arises in long-pulse beam experiments is the decay of image currents (and their fields) resulting from a beam that is offset from the geometric axis of the beam tube. A related problem, which arises in calibration of beam diagnostics, is that of simulating the beam using a current-carrying conductor. These problems are solved analytically in the limit that the tube walls are thin compared with the magnetic-field penetration depth. It is shown that the transient decay time of the fields for these two cases is different.
Instantaneous normal mode analysis of liquid methanol
Authors: Garberoglio G. , Vallauri R.
Published: July 2002

The instantaneous normal modes for liquid methanol at 200 K are calculated and related to the topologically linear hydrogen bond network in this system. A shoulder observed in the spectra of both collective and single molecule correlation functions is explained as arising from the presence of “stretching” modes of the hydrogen bonded network, similarly to what happens in liquid hydrogen fluoride. In the case of methanol, however, the relation between the chain dynamics and the instantaneous normal modes is much less evident, due to the strongest “coupling” of the chain motion to other diffusive processes present in the liquid.