bluuebunny/crossref_metadata_2025 huggingface dataset:
Deep levels in semi-insulating InP obtained by annealing under iron phosphide ambiance
Authors: Dong H. W. , Zhao Y. W. , Zhang Y. H. , Jiao J. H. , Zhao J. Q. , Lin L. Y.
Published: September 2002

Deep levels in semi-insulating (SI) InP obtained by annealing in iron phosphide (IP) ambiance have been characterized by optical transient current spectroscopy (OTCS). Compared with the OTCS result of the SI InP prepared by annealing in pure phosphorus (PP) ambiance, the IP SI InP presents only two traps with activation energies of 0.20 and 0.63 eV, respectively. The results suggest that the diffusion of Fe-atoms suppresses the formation of a few defects in the IP SI InP. The nature of deep levels in the IP and PP SI InP has been discussed on the basis of these results. The relation between material property and defects in those SI InP has also been revealed.
Electron stimulated desorption of negative ions: A time-of-flight experiment
Authors: Goeden C. , Dollinger G.
Published: September 2002

We present a setup for stimulated desorption experiments of negative ions using low energy incident electrons and time-of-flight identification of the desorbed ions. It consists of a pulsed electron gun, an electrostatic focusing system, and a channel plate detector setup. Electron beams down to sub-eV energies can be used due to electrostatic shielding and the compensation of the earth’s magnetic field by a set of Helmholtz coils. The main advantage is the large acceptance for ions of all masses, energies, and desorption angles at the same time, which keeps measuring time reasonably short and allows us to gain information before degeneration of the irradiated sample occurs, even if weak desorption channels are investigated. We demonstrate the power of our setup with first results from a boron doped, (100)-oriented diamond sample, which is partly oxidized and partly hydrogenated with some water contaminations on it. Different binding states of oxygen are disclosed clearly by different desorption thresholds. The C–O binding on oxidized diamond forms a carbonyl group. The 1b2 orbital of water can be seen in a O desorption threshold. The yield of negative hydrogen desorption shows a linear increase for incident electron energies higher than 13 eV. It results from a dipolar dissociation as has been published previously.
Application of density functional theory to tethered polymer chains: Athermal systems
Authors: McCoy John D. , Ye Yuan , Curro John G.
Published: September 2002

Athermal, tethered chains are modeled with density functional (DFT) theory for both the explicit solvent and continuum solvent cases. The structure of DFT is shown to reduce to self-consistent-field theory in the incompressible limit where there is symmetry between solvent and monomer, and to single-chain-mean-field (SCMF) theory in the continuum solvent limit. We show that by careful selection of the reference and ideal systems in DFT theory, self-consistent numerical solutions can be obtained, thereby avoiding the single chain Monte Carlo simulation in SCMF theory. On long length scales, excellent agreement is seen between the simplified DFT theory and molecular dynamics simulations of both continuum solvents and explicit-molecule solvents. In order to describe the structure of the polymer and solvent near the surface it is necessary to include compressibility effects and the nonlocality of the field.
Small voltage electrocapillary flows in the presence of ionic surfactants
Authors: Casson Kenneth , Johnson Duane
Published: September 2002

Rapid flows were observed by applying a small voltage across an organic-aqueous interface in the presence of an ionic surfactant. These flows are in contrast to the experiments of G. I. Taylor and J. R. Melcher [Annu. Rev. Fluid Mech. 1, 111 (1969); D. A. Saville, Annu. Rev. Fluid Mech. 29, 27 (1997)], where no ionic surfactant was used. In their experiments, a large voltage difference was necessary to induce a surface charge between two leaky dielectric fluids. Here, we intentionally create a surface charge by adding a cationic surfactant. The result is that a much lower voltage is necessary to cause substantial flows. Two experiments are performed to compare these results with classical thermocapillary flow experiments.
Pseudopotential hole–particle formalism for excitations in xenon molecules and clusters. I. Theory, atomic and molecular tests
Authors: Jonin C. , Laporte P. , Spiegelmann F.
Published: September 2002

A one-electron pseudopotential hole–particle formalism is implemented to investigate excitations in xenon molecules and clusters. Within this framework, averaged relativistic electron-Xe and electron-Xe+ pseudopotentials are determined to incorporate the excited particle contributions. A consistent hybrid scheme for spin–orbit coupling is developed, involving an atoms-in-molecules type approximation for the hole and a pseudopotential operator for the particle. The reliability of the one-electron pseudopotential scheme is first checked on the atomic spectrum of xenon and the transferability to high excited states is demonstrated. The molecular behavior of the formalism is also investigated by determining the potential energy curves of the lowest excimer states of Xe2*. The spectroscopic constants (De, ωe, and ωexe, respectively) are found to be 4173, 108, and 1.17 cm−1 for state (1)0u−(6s 3P2), 4197, 109, and 1.11 cm−1 for state (1)1u(6s 3P2), and 4250, 107, and 1.14 cm−1 for state (1)0u+(6s 3P2).
Experimental and theoretical study of the ion–molecule association reaction NH4++NH3(+M)→N2H7+(+M)
Authors: Hamon S. , Speck T. , Mitchell J. B. A. , Rowe B. R. , Troe J.
Published: September 2002

The association reaction NH4++NH3(+M)→N2H7+(+M), leading to the proton-bound dimer of ammonia, was studied by the CRESU flow technique over the temperature range 15–170 K, in the bath gases M=He, Ar, and N2, and over the range of bath gas concentrations (0.5–15)×1016 molecule cm−3. The rate coefficients are shown to depend on the temperature, the pressure, and the nature of the bath gas. Theoretical modelling of the reaction involves a combination of ion–molecule capture and unimolecular reaction rate theory. It is shown that the present experiments all correspond to the intermediate falloff regime of the reaction ranging up to the high pressure bimolecular capture limit, whereas earlier experiments between 200 and 350 K were made close to the low pressure termolecular limit. Temperature- and pressure-dependent rate coefficients over wide ranges of conditions are calculated and compared with experimental results from this and earlier work. For the bath gas N2, under most of the applied conditions, the reaction does not follow the energy transfer mechanism, but proceeds via a radical–complex mechanism. In this case, the reaction rate is determined by the capture of NH3 by NH4+(N2)n complexes. The rate of this process is estimated by modified ion–dipole capture theory.
Radial electric field and plasma confinement in the vicinity of a magnetic island
Authors: Shaing K. C.
Published: September 2002

A theory to determine the radial electric field in the vicinity of a magnetic island in tokamaks based on the island-induced symmetry-breaking transport flux is presented. It is shown that the radial electric field is governed by a nonlinear equation that can have multiple equilibrium solutions as plasma parameters change. This results in a bifurcated state of the radial electric field. It is possible that turbulent fluctuations can be suppressed and improved confinement develops in the vicinity of the island. The evolution of the toroidal momentum equation with the effects of the symmetry-breaking flux included is also presented.
Directional diffusion and void formation at a Si (001) bonded wafer interface
Authors: Esser R. H. , Hobart K. D. , Kub F. J.
Published: September 2002

Low-temperature hydrophobic bonding is an enabling technology allowing the fabrication of device structures. Current research into improvement of hydrophobic bonding has focused on the elimination of thermally generated voids. It has been observed that a regular grid etched into the bonding interface can eliminate the thermally generated voids. By manipulation of patterns etched into the bond interface, it was possible to ascertain that the diffusion of interfacial gasses that form the thermally generated voids is enhanced along the 〈110〉 directions. This is shown by an analysis of the void density at various locations in relation to the etched trenches at the bonded interface. Void density between trenches is shown to be 12% of the void density near trenches but nto along a 〈110〉 direction.
The Gelfand–Zeitlin selection rules and representations of quantum Anq algebras
Authors: Leznov A. N.
Published: September 2002

The problem of construction of irreducible representations of quantum Anq algebras is solved by explicit integration of the linear (inhomogeneous) system of equations in finite differences in the n-dimensional space. The general solution of this system is given explicitly, and particular solutions corresponding to the irreducible representations are selected.
Monte Carlo study of liquid crystal phases of hard and soft spherocylinders
Authors: Cuetos A. , Martı́nez-Haya B. , Rull L. F. , Lago S.
Published: September 2002

We report on a Monte Carlo study of the liquid crystal phases of two model fluids of linear elongated molecules: (a) hard spherocylinders with an attractive square-well (SWSC) and (b) purely repulsive soft spherocylinders (SRS), in both cases for a length-to-breadth ratio L*=5. Monte Carlo simulations in the isothermal–isobaric ensemble have been performed at a reduced temperature T*=5 probing thermodynamic states within the isotropic (I), nematic (N), and smectic A (Sm A) regions exhibited by each of the models. In addition, the performance of an entropy criterion to allocate liquid crystalline phase boundaries, recently proposed for the isotropic–nematic transition of the hard spherocylinder (HSC) fluid, is successfully tested for the SWSC and the SRS fluids and furthermore extended to the study of the nematic–smectic transition. With respect to the more extensively studied HSC fluid, the introduction of the attractive square well in the SWSC model brings the I–N and N–Sm A transitions to higher pressures and densities. Moreover, the soft repulsive core of the SRS fluid induces a similar but quite more significant shift of both of these phase boundaries toward higher densities. This latter effect is apparently in contrast with very recent studies of the SRS fluid at lower temperatures, but this discrepancy can be traced back to the different effective size of the molecular repulsive core at different temperatures.
Correlated orientational and translational motions in supercooled liquids
Authors: Bhattacharyya Sarika , Mukherjee Arnab , Bagchi Biman
Published: September 2002

We have carried out NPT molecular dynamics simulations of isolated ellipsoids in a glass forming binary mixture to gain insight into the nature of orientational relaxation (OR) in a viscous liquid. At high pressures when the liquid is highly viscous, the OR is found to occur mainly via correlated hopping, sometimes involving participation of several neighboring atoms, placed in a ring like tunnel. In the glassy state, hopping is found to be accompanied by larger fluctuations in the total energy and the volume of the system. Both orientational and translational hopping are found to be gated, restricted primarily by the entropic bottlenecks, with the orientational motion becoming increasingly slower than the translation as the pressure is increased. Orientational relaxation is found to occur with a wide distribution of decay times.
Nonlinear optical properties of linear chain phosphazenes, (PN)x
Authors: Chandra Jha Prakash , Krishnan Anu , Das Puspendu K. , Ramasesha S.
Published: September 2002

The correlated Pariser–Parr–Pople model Hamiltonian for interacting π-electrons is employed for calculating frequency dependent linear polarizability as well as first and second hyperpolarizabilities of linear chain phosphazenes (–P=N–)x (x=3–6). The model parameters for phosphorus and nitrogen are obtained by comparing the theoretical excitation energies with experimental spectra of the known phosphazene systems. The optical gap of the phosphazene oligomers extrapolates to 3.7 eV compared to 2.8 eV of their organic analogs, namely, the polyenes. The linear polarizability of the phosphazene systems are comparable to those of the polyenes. However, the third harmonic generation coefficients are smaller at the same excitation energies. The power law exponent for the third harmonic generation coefficient in phosphazenes is also much smaller than that in polyenes. The second harmonic generation coefficients of the phosphazenes are smaller than those of the push–pull polyenes. Introduction of terminal push–pull groups on phosphazenes does not significantly improve the second harmonic generation response of these systems.
Isentropic focusing of supersonic plasma jets for magnetized target fusion
Authors: Winterberg F.
Published: September 2002

It is shown that high energy flux densities can be reached by the isentropic Prandtl–Meyer compression flow of a supersonic plasma jet in a convergent nozzle. The energy flux density thereby increases in proportion to M2/(γ−1) where M is the Mach number of the jet and γ the specific heat ratio. With an axial magnetic field set up inside the nozzle by the thermomagnetic Nernst effect, the jet is magnetically insulated from the nozzle wall, reducing the bremsstrahlung radiation and conveniently magnetizing the target plasma. A sufficiently large number of spherically arranged nozzles can then be used for the ignition and confinement of a magnetized thermonuclear target.
Optimization of neutron scattering instrumentation using neutron spin echo: Application to the discrimination of diffuse scattering in neutron reflectivity experiments
Authors: Pynn Roger , Fitzsimmons Michael R. , Rekveldt M. Theo , Major Janos , Fritzsche Helmut , Weller Dieter , Johns Earl C.
Published: September 2002

We describe a method, based on the neutron spin-echo technique, to achieve good resolution in many neutron-scattering experiments, without sacrificing signal intensity. By “good resolution” we do not mean the ∼10−6 energy resolution usually associated with neutron spin-echo spectrometers, but rather the level of resolution traditionally obtained by collimation or monochromatization of neutron beams, often on specialized instruments, and almost always at some penalty in measured intensity. The method we discuss allows good resolution to be achieved in any chosen direction in the vector space defined by the neutron scattering wave vector, Q, and the energy transfer, E. Although the method has general applicability to many neutron scattering measurements, we discuss in detail its application to the problem of separating diffuse scattering from specular reflection in neutron reflectometry. The technological basis of the method is the availability of thin films of magnetic or easily magnetizable material that are very smooth and of uniform thickness and that do not depolarize neutrons. Enhancing resolution using this method can be as simple as adding a few such films to a spectrometer that already provides polarized neutrons. We have tested magnetizable permalloy films for this application and our experimental results show that components work as intended, although some further work is needed to optimize them. The apparatus we have used, which is inexpensive and simple to construct, will allow both in-plane and out-of-plane diffuse scattering to be effectively removed from the measurement of specular reflectivity with an accuracy comparable to that achievable with milliradian collimation. In contrast to traditional methods of achieving this level of resolution, however, the spin echo method permits the use of relatively poorly collimated beams and hence can provide substantial gains in signal intensity.
Far field modeling of the plasma plume of a Hall thruster
Authors: Boyd Iain D. , Dressler Rainer A.
Published: September 2002

Hall thrusters are an attractive form of electric propulsion that are being developed and implemented to replace chemical systems for many in orbit propulsion tasks on communications satellites. One concern in the use of these devices is the possible damage their plumes may cause to the host spacecraft. Computer models of Hall thruster plumes play an important role in integration of these devices onto spacecraft as the space environment is not easily reproduced in ground testing facilities. In this article, a hybrid particle-fluid model of a Hall thruster plume is applied to model the SPT-100 thrusters used on the Russian Express satellites. The emphasis of the article is on making assessment of the model through direct comparison with measurements of ion current density and ion energy distributions taken on board Express spacecraft. A model for simulating atom–ion collisions is described. The sensitivity of the plume simulation results to various aspects of the physical modeling is investigated. The plume model is able to predict many of the most important characteristics of the measured data.
Crystal growth and characterization of the cubic semiconductor Cu2SnSe4
Authors: Marcano G. , Rincón C. , Marı́n G. , Tovar R. , Delgado G.
Published: September 2002

X-ray powder diffraction study of the p-type semiconductor Cu2SnSe4 shows that this material crystallizes in the cubic structure, space group F4¯3m, with unit cell parameter a=5.6846(3) Å. The temperature variation of the hole concentration between 120 and 300 K, obtained from the Hall effect and electrical resistivity measurements, is due to the thermal activation of an acceptor level with ionization energy of about 0.02 eV. The temperature variation of the hole mobility is explained by considering the scattering of charge carriers by ionized impurities and acoustic phonons. From this analysis, the density-of-states effective mass of the holes is estimated to be about 0.8 me, me being the free electron effective mass. From the optical absorption spectra, the fundamental absorption edge is found to be direct. The value of the lowest energy gap and the spin-orbit splitting were estimated to be about 0.35 and 0.20 eV, respectively. The temperature dependence of the magnetization measurements shows that Cu2SnSe4 is paramagnetic, indicating that most of the copper atoms have the divalent charge state.
Temperature-dependent vibrational dephasing: Comparison of liquid and glassy solvents using frequency-selected vibrational echoes
Authors: Xu Qing-Hua , Fayer M. D.
Published: September 2002

Frequency-selected vibrational echo experiments were used to investigate the temperature dependences of vibrational dephasing associated with the 0-1 transition of the CO stretching mode of RuTPPCOPy (TPP=5,10,15,20-tetraphenylporphyrin, Py=pyridine) in two solvents: polymethylmethacrylate (PMMA) and 2-methyltetrahydrofuran (2-MTHF). In PMMA, a glass, the echo decay is exponential at all the temperatures studied, and the dephasing rate increases linearly with increasing temperature. In 2-MTHF, there is a change in the functional form of the temperature dependence when the solvent goes through the glass transition temperature (Tg). Below Tg, the dephasing rate increases linearly with temperature, while above Tg, it rises very steeply in a nonlinear manner. In the liquid at higher temperatures, the vibrational echo decays are nonexponential. A model frequency-frequency correlation function (FFCF) is proposed in which the FFCF differs for a glass and a liquid because of the intrinsic differences in the nature of the dynamics. At least two motions, inertial and diffusive, contribute to the vibrational dephasing in the liquids. The different temperature dependences of inertial and diffusive motions are discussed. Comparison of the model calculations of the vibrational echo temperature dependence and the data show reasonable, but not quantitative agreement.
Influence of the quench rate and the pressure on the glass transition temperature in selenium
Authors: Caprion D. , Schober H. R.
Published: September 2002

We report a molecular dynamics investigation of the glass transition temperature in selenium at pressures ranging from 0 to 6 GPa as a function of the quench rate, Qr. For moderate pressures the specific volume of the glass depends strongly on the quench rate, whereas the specific enthalpy varies only little. We find for both volume and energy a linear dependence on the quench-rate-dependent glass transition temperature. The slopes of these curves reflect the different energy scales of void formation, inter- and intrachain interactions. The extrapolated glass transition temperatures for quench rates of order K/s agree with the experimental ones within 20%. Applying a pressure of 1 GPa the glass transition temperature is raised by 37 K. For the same Qr, the transition temperature Tg is much higher for simulations using fixed volume conditions (NVT ensemble) than for the ones using fixed pressure (NPT ensemble) when one compares results for equal pressure at T=0.
Electronic structures of HonC60 clusters (n=1–5): High electron acceptability of C60
Authors: Nagao Satoshi , Negishi Yuichi , Kato Akiko , Nakamura Yoshiaki , Nakajima Atsushi , Kaya Koji
Published: September 2002

The electronic structures of gas-phase exohedral holmium (Ho)–C60 clusters, HonC60 with n=1–5, were studied by photoionization spectroscopy of the neutrals and photoelectron spectroscopy of their anions. Both spectroscopic analyses reveal that each Ho atom donates its three valence electrons to the unoccupied molecular orbitals of C60. The electron filling through the charge transfer allows us to observe the unoccupied levels of C60 due to high electron acceptability.
Boundary conditions and trajectories of diffusion processes
Authors: Schumaker Mark F.
Published: September 2002

This article constructs trajectories associated with various boundary conditions for the Smoluchowski equation on an interval. Single-particle diffusion processes are first constructed by taking the diffusion limits of random walks. The diffusion limit gives both boundary conditions which enforce the single-particle constraint and properties of underlying trajectories at those boundaries. Mean-field diffusions are then obtained as limits of sums of single-particle processes. The results help to interpret the application of diffusion models to both ion channels and wider pores that facilitate molecular transport across membranes. Potential applications to Brownian dynamics simulations are discussed.
Measurements of ρR asymmetries at burn time in inertial-confinement-fusion capsules
Authors: Séguin F. H. , Li C. K. , Frenje J. A. , Kurebayashi S. , Petrasso R. D. , Marshall F. J. , Meyerhofer D. D. , Soures J. M. , Sangster T. C. , Stoeckl C. , Delettrez J. A. , Radha P. B. , Smalyuk V. A. , Roberts S.
Published: September 2002

Recent spectroscopic analysis of charged particles generated by fusion reactions in direct-drive implosion experiments at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] show the presence of low-mode-number asymmetries in compressed-capsule areal density (ρR) at the time of fusion burn. Experiments involved the acquisition and analysis of spectra of primary (14.7 MeV) protons, from capsules filled with deuterium and helium-3, and secondary (12.6–17.5 MeV) protons, from cryogenic deuterium capsules. The difference between the birth energy and measured energy of these protons provides a measure of the amount of material they passed through on their way out of a capsule, so measurements taken at different angles relative to a target provide information about angular variations in capsule areal density at burn time. Those variations have low-mode-number amplitudes as large as ±50% about the mean (which is typically ∼65 mg/cm2); high-mode-number structure can lead to individual pathlengths through the shell that reach several times the mean. It was found that the observed ρR asymmetries are often similar for contiguous implosions, but change when the laser beam energy balance is significantly changed, indicating a direct connection between drive symmetry and implosion symmetry.
Characterization of Mg doped GaN by positron annihilation spectroscopy
Authors: Moxom Jeremy , Xu Jun , Suzuki R. , Ohdaira T. , Brandes George , Flynn Jeffrey S.
Published: September 2002

Metalorganic vapor-phase epitaxially grown GaN films have been studied using beam positron annihilation spectroscopy. Positron lifetime and Doppler broadening of annihilation radiation measurements indicate that defects, probably Ga vacancies, are reduced in concentration when films are doped with Mg, as previously observed by other workers. In lightly doped films a homogeneous layer with a minimum defect concentration is present from just below the surface, to a depth of around 0.1 μm. For heavily doped films, there is evidence of a further substantial decrease in the defect concentration following an anneal to 900 °C. The effects of impurities and defects are discussed.
Pretilted and grating alignment of liquid crystals by oblique ultraviolet irradiation of polyimide
Authors: Newsome C. J. , O'Neill M.
Published: September 2002

We discuss the formation of pretilt by oblique irradiation of photosensitive polymers that homogeneously align liquid crystals in the direction corresponding to the maximum density of unreacted polymer. We show that a tilted polymer distribution is formed by oblique illumination with one beam of unpolarized or partially polarized light. When unpolarized radiation is used, a small azimuthal anisotropy is generated and the polymer tilt angle increases with incident angle. Pretilted liquid crystal alignment is possible up to a threshold angle at which the azimuthal alignment direction changes. Irradiation with partially polarized light gives a higher azimuthal anisotropy but smaller polymer tilt angles. We obtain liquid crystal pretilt angles up to 4° by oblique irradiation of polyimide with light from an excimer laser operating below the threshold for ablation. A simple two step process is used to obtain pretilted alignment for liquid crystals using surface gratings: a grating etched into polyimide by laser ablation is subsequently irradiated at oblique incidence to give planar liquid crystal alignment with a pretilt angle of 3°.
Electrical damage of an ultrathin Si oxynitride layer induced by scanning tunneling spectroscopy
Authors: Miyata Noriyuki , Ichikawa Masakazu
Published: September 2002

Ultrathin Si oxynitride layers were examined by using scanning tunneling microscopy (STM) and spectroscopy (STS). These techniques revealed that a structural change from an intrinsic defect (Si–Si bond) to a damaged structure (Si cluster) takes place under conventional STM/STS conditions. Comparison of the damaged structures formed in the oxynitride with those in the oxide indicated that nitrogen atoms suppress the expansion of the damaged regions. It was also found that nitrogen incorporation enhances both the defect density and the atomic-scale roughness at the oxynitride/Si interface. We suggested that this degradation is related to a local strain produced by the N≡Si3 structures at the oxynitride/Si interface. On the contrary, a normal oxynitride structure had a higher resistance to an electrical stress than an intrinsic defect, but, when the constant electrical stress was applied, the normal oxynitride structure was also damaged. This damage proceeds in two steps: creation of charge traps, and then formation of Si cluster. From these STM/STS results, we proposed that the electrical breakdown of the conventional gate-oxide film proceeds as a four-step process: (1) formation of Si clusters by the damage of intrinsic defects, (2) creation of traps in the normal structure, (3) formation of Si clusters in the normal structure, and (4) complete local breakdown when the Si clusters become connected.
Dependence of luminescence efficiency on dopant concentration and sintering temperature in the erbium-doped Ba0.7Sr0.3TiO3 thin films
Authors: Kuo Shou-Yi , Chen Chin-Sheng , Tseng Tseung-Yuen , Chang S.-C. , Hsieh Wen-Feng
Published: September 2002

We found the dependence of luminescence efficiency on Er3+ concentration and sintering temperature in the Er-doped Ba0.7Sr0.3TiO3 (BST) thin films is governed by crystallinity and ion–ion interaction. X-ray diffraction and Raman studies of the sol-gel prepared samples show that the BST polycrystalline phase occurred when the sintering temperature reaches 700 °C, whereas, it becomes worse for temperature above 700 °C resulting from phase separation and the Er3+ concentration exceeding 3 mol % due to charge compensation mechanism. The observed green emission reaches maximum at sintering temperature 700 °C and 3 mol % Er3+ ions concentration. We also showed the Er dopant does not affect the dielectric property of BST thin films in C–V measurement and the Ba0.7Sr0.3TiO3 films doped with Er3+ ions may have potential use for electroluminescence devices.
A model of phase transitions in the system of electro-optical dipolar chromophores subject to an electric field
Authors: Pereverzev Yuriy V. , Prezhdo Oleg V. , Dalton Larry R.
Published: September 2002

An analytical model for the nonlinear behavior of the electro-optic (EO) coefficient in chromophore–polymeric materials is developed. The sharp decline of the EO coefficient above a threshold chromophore concentration is attributed to a second order phase transition transforming the chromophore dipolar system into an antiferroelectric state. The rise of antiferroelectric correlations between chromophore dipoles deteriorates the efficiency of the poling process aimed at achieving a noncentrosymmetric chromophore ordering by application of an electric field. The location of the phase transition and the magnitude of the EO coefficient are investigated as functions of molecular and thermodynamic parameters. Particularly remarkable observations are made regarding the dependence of the EO coefficient on the macroscopic shape of samples used for poling. Slab shaped samples that are common in practice are least efficient for the poling process. Any degree of sample elongation in the direction of the poling field shifts the antiferroelectric phase transition towards higher chromophore concentrations and radically increases the maximum value of the EO coefficient. The theory is applied to two chromophore systems that are typical of materials used in EO devices. Fine agreement with the experimental data is achieved with little adjustment.
Infrared spectra of N2O–4He, N2O–3He, and OCS–3He complexes
Authors: Tang Jian , McKellar A. R. W.
Published: September 2002

Infrared spectra of the weakly bound complexes N2O–4He, N2O–3He, and OCS–3He have been observed using a tunable diode laser to probe a pulsed supersonic jet expansion. The rotational structure of the bands was analyzed using a conventional asymmetric rotor Hamiltonian. The N2O–3He and OCS–3He spectra are mostly a type (ΔKa=0) in structure, with very weak b-type (ΔKa=±1) transitions, but for N2O–4He the a- and b-type components are both prominent. The fitted rotational parameters are consistent with roughly T-shaped structures with intermolecular separations around 3.4–3.5 Å for N2O–He and 3.8–3.9 Å for OCS–He. The angle between the N2O or OCS axis and the He position is about 80° for N2O–He and 65° for OCS–He. The vibrational band origins are slightly blueshifted from those of the free molecule, with the N2O–He shifts (+0.2 cm−1) being about twice the magnitude of the OCS–He shifts (+0.1 cm−1). The results are of particular interest since N2O and (especially) OCS have both been used as probes in experiments on ultracold helium nanodroplets.
Channel-specific angular distributions of HCl and CH3 products from the reaction of atomic chlorine with stretch-excited methane
Authors: Kim Zee Hwan , Bechtel Hans A. , Zare Richard N.
Published: September 2002

A beam containing methane and molecular chlorine is expanded into a vacuum where the methane is excited with two quanta of C–H stretching (one quantum each in two of the four C–H bonds). The reaction is initiated by fast Cl atoms generated by photolysis of Cl2 at 355 nm, and the resulting CH3 and HCl products are detected in a state-specific manner using resonance-enhanced multiphoton ionization. Speed-dependent spatial anisotropies (βprod) of HCl and CH3 products allow identification of three major product channels. They are in order of importance: (a) HCl (v=0)+CH3 [ν1 (symmetric stretch) or ν3 (asymmetric stretch)=1]; (b) HCl (v=1)+CH3 [ν2(umbrella bend)=1)]; and (c) HCl (v=1)+CH3 (ν1=1). The CH3 (v=0) product cannot be detected, and the HCl (v=2) product is minor. Channels (a) and (c) proceed in a vibrationally adiabatic manner, whereas channel (b) appears to involve the nonadiabatic interaction involving the low frequency bending mode in methane that correlates to the bending mode in the methyl radical product. The angular distributions differ markedly for the three product channels. This behavior is explained by the propensity for reactive collisions involving H-atom transfer along the line of centers and the difference in the cones of acceptance. The rotational angular momentum vector of the HCl (v=1, J=1) product is aligned perpendicular to the line of centers, which is consistent with an impulsive energy release along the line of centers. Our results clearly demonstrate that the direct and local mode picture of the chemical reaction remains largely valid, which connects vibrational excitation to the scattering dynamics.
Characterization of a high-density, direct-current reflex discharge plasma source operating in Ar and N2
Authors: Toader E. I. , Graham W. G. , Mahony C. M. O. , Steen P. G.
Published: September 2002

The characterization of a direct current, low-pressure, and high-density reflex discharge plasma source operating in argon and in nitrogen, over a range of pressures 1.0–10−2 mbar, discharge currents 20–200 mA, and magnetic fields 0–120 G, and its parametric characterization is presented. Both external parameters, such as the breakdown potential and the discharge voltage–current characteristic, and internal parameters, like the charge carrier’s temperature and density, plasma potential, floating potential, and electron energy distribution function, were measured. The electron energy distribution functions are bi-Maxwellian, but some structure is observed in these functions in nitrogen plasmas. There is experimental evidence for the existence of three groups of electrons within this reflex discharge plasma. Due to the enhanced hollow cathode effect by the magnetic trapping of electrons, the density of the cold group of electrons is as high as 1018 m−3, and the temperature is as low as a few tenths of an electron volt. The bulk plasma density scales with the dissipated power. Another important feature of this reflex plasma source is its high degree of uniformity, while the discharge bulk region is free of electric field.
Effects of slight misorientation of GaN templates on molecular-beam-epitaxy growth of ZnO
Authors: Kato Hiroyuki , Sano Michihiro , Miyamoto Kazuhiro , Yao Takafumi
Published: September 2002

Both the growth mechanism and material properties of ZnO epilayers are found to be strongly dependent on slight misorientation of GaN templates. The observation by in situ reflection high-energy electron diffraction and atomic force microscopy reveals that the growth mode changes from a three-dimensional to a two-dimensional growth mechanism, as the misorientation of GaN templates is varied from [0001] just to 0.2 degree toward the a axis. However, ZnO grows under a three-dimensional mode for misorientation toward the m axis. The observed changes in the growth mechanism are discussed in terms of the different surface migration length along the m and a axis, which is caused by the different surface atomic arrangement along the a and m axis. With an increase in the misorientation angle from 0 to 0.2° toward the a axis, the linewidth of (0002) x-ray rocking curve shows a dramatic narrowing from 1768 to 277 arcsec, while the photoluminescence intensity of exciton emission shows a remarkable enhancement by more than one order of magnitude. Such improvement in material properties can be ascribed to the observed change of the growth mode from three- to two-dimensional growth.
Ultrafast trapping times in ion implanted InP
Authors: Carmody C. , Boudinov H. , Tan H. H. , Jagadish C. , Lederer M. J. , Kolev V. , Luther-Davies B. , Dao L. V. , Gal M.
Published: September 2002

As + and P+ implantation was performed on semi-insulating (SI) and p-type InP samples for the purpose of creating a material suitable for ultrafast optoelectronic applications. SI InP samples were implanted with a dose of 1×1016 cm−2 and p-type InP was implanted with doses between 1×1012 and 1×1016 cm−2. Subsequently, rapid thermal annealing at temperatures between 400 and 700 °C was performed for 30 sec. Hall-effect measurements, double-crystal x-ray diffraction, and time-resolved femtosecond differential reflectivity showed that, for the highest-annealing temperatures, the implanted SI InP samples exhibited high mobility, low resistivity, short response times, and minimal structural damage. Similar measurements on implanted p-type InP showed that the fast response time, high mobility, and good structural recovery could be retained while increasing the resistivity.
Overall rate constants for diffusion and incorporation in clusters of spheres
Authors: Lu Shih-Yuan , Yen Yi-Ming , Tseng Chin-Yao , Tsao Heng-Kwong
Published: September 2002

Three numerical schemes and one approximate model are developed to compute the overall rate constants for diffusion and incorporation of small entities in clusters of spheres. These include the Brownian dynamic simulation, multipole expansion, boundary collocation, and a model linking diffusion-limited (DL) and nondiffusion-limited (NDL) data. The Brownian dynamic simulation is speeded up with a first-passage technique and is capable of taking the finite surface incorporation rate into account. The multipole expansion truncated at the dipole moment gives an excellent approximation while the second order boundary collocation is satisfactorily accurate. The DL to NDL model offers a quick and reasonably accurate estimate of the rate constant. Clusters of Euclidean dimensions, including 1D strings, 2D squares, and 3D cubes, are particularly investigated. The screening effect arising from the long range nature of the disturbance concentration field is found responsible for the variation in the overall rate constant due to structural variation in clusters, and becomes less pronounced as P increases. Here, P measures the relative dominance of surface incorporation over the diffusion. Also, the rate constants for the Euclidean clusters are found to obey the similar scaling laws as those confirmed by Tseng et al. [Phys. Rev. Lett. 86, 5494 (2001)] for the translational drag coefficient of clusters of spheres in the low Reynolds number flow regime.
Mapping technique for stellarators
Authors: Kasilov S. V. , Kernbichler W. , Nemov V. V. , Heyn M. F.
Published: September 2002

The Stochastic Mapping Technique (SMT), a highly efficient method to solve the five-dimensional drift kinetic equation in the long-mean-free-path regime, is presented in an application to stellarators. Within this method, the dimensionality of the problem is reduced to four dimensions through a discretization in one dimension. Instead of tracing test particles in the whole phase space, test particles are followed on particular Poincaré cuts. With this approach, the computation time is reduced by a large factor compared to direct Monte Carlo methods. The SMT is applicable to stellarators with arbitrary magnetic field geometries and topologies. It can be used for any problem where currently conventional Monte Carlo methods are applied. In particular, it is well suited for modeling the distribution function of supra-thermal particles generated by auxiliary heating methods, for studies of stellarator transport properties and for a fast survey of a specific configuration in the whole phase space necessary for an estimation of α-particle confinement.
The convergence condition of the successive approximation process in Compton scattering tomography
Authors: Wang Jiajun , Huang Xianwu , Zhong Xingrong
Published: September 2002

Compton scattering can be used to determine the electron densities of tissues for medical applications and those of materials for industrial applications. Much work has been devoted to solving the reconstruction problem. Hussein proposed a successive approximation process (SAP) for the reconstruction of the density image from the Compton scattered energy spectral data [IEEE Trans. Nucl. Sci., 42, 2155 (1995)]. This algorithm is adapted directly from the neutron scattering tomography reconstruction where a convergence condition for such an algorithm was presented [Hussein and Meneley, Int. J. Multiphase Flow, 12, 1 (1986)]. However, we found that this condition is not correct in general cases and we cannot obtain such a simple form of convergence condition in terms of the maximum length(in unit of mean-free path) a neutron or a photon travels in any pixel in the test section. A detailed discussion of the convergence problem of SAP will be presented in this paper.
Electron spin resonance study of interface defects in atomic layer deposited hafnium oxide on Si
Authors: Kang A. Y. , Lenahan P. M. , Conley J. F. , Solanki R.
Published: September 2002

We report electron spin resonance (ESR) observation of interface defects at the HfO2/(111)Si boundary for HfO2 films deposited via atomic layer chemical vapor deposition using Hf(NO3)4 as a precursor. We observe several signals, dominated by one due to a silicon dangling bond at the Si/dielectric interface. This center is somewhat similar to, but not identical to, Si/SiO2 interface silicon dangling bonds. Comparison between ESR and capacitance versus voltage measurements suggests that these dangling bond centers play an important role in HfO2/Si interface traps.
Visible thermal emission from sub-band-gap laser excited cerium dioxide particles
Authors: Robinson Richard D. , Spanier Jonathan E. , Zhang Feng , Chan Siu-Wai , Herman Irving P.
Published: September 2002

Cerium dioxide particles excited in air with sub-band-gap radiation emit very broad radiation in the visible spectrum above a threshold intensity that decreases with increasing ambient temperature. Concomitant with this emission is the near disappearance of the Stokes and anti-Stokes Raman scattering peaks. Both phenomena are reversible in air up to just above threshold, and are seen for nanoparticles and several-micron-diameter particles with particle diameter comparable to or smaller than the laser focus. Temperature estimates using the Stokes/anti-Stokes scattering intensity ratio suggest there is laser heating due to small intragap absorption and possible nonlinear processes, given the very slow thermal conduction. The broad emission in this loose powder may well be due to thermal emission, on the basis of spectral fitting of the high-energy part of the spectrum to a blackbody radiator at ∼1200–1400 °C, although luminescence from a new phase is a possibility. The sudden decrease in Raman scattering and increase in emission in air are consistent with a transition to a new, possibly luminescent, phase, as is the continued disappearance of the Raman peaks in forming gas when the laser power is reduced below the upstroke threshold. Oxygen point defects and their complexes may play an important role in many of these processes.
Medium perturbations on the molecular polarizability calculated within a localized dipole interaction model
Authors: Jensen Lasse , Swart Marcel , van Duijnen Piet Th. , Snijders Jaap G.
Published: September 2002

We have studied the medium effects on the frequency-dependent polarizability of water by separating the total polarizability of water clusters into polarizabilities of the individual water molecules. A classical frequency-dependent dipole–dipole interaction model based on classical electrostatics and an Unsöld dispersion formula has been used. It is shown that the model reproduces the polarizabilities of small water clusters calculated with time-dependent density functional theory. A comparison between supermolecular calculations and the localized interaction model illustrate the problems arising from using supermolecular calculations to predict the medium perturbations on the solute polarizability. It is also noted that the solute polarizability is more dependent on the local geometry of the cluster than on the size of the cluster.
Molecular simulation of thin polymer films with functional endgroups
Authors: Izumisawa Satoru , Jhon Myung S.
Published: September 2002

An off-lattice Monte Carlo simulation using a coarse-grained bead-spring model was developed to analyze nanoconfined polymer films. The beads in the polymer chains are connected via finitely extensible, nonlinear elastic springs. In addition to nonpolar van der Waals interactions, functional endbeads have a short-range exponential interaction characteristic of endgroup coupling. Our simulated results qualitatively agree with experimental data for perfluoropolyether molecules with functional endgroups in ultrathin films. The chains exhibit an oblate conformation near the wall, but recover a spherical shape as they move farther away from it. The density profile of functional endbeads as a function of distance from the wall shows characteristic oscillation originating from endbead coupling and orientation near the wall. We examined the molecular layering of films via the analysis of an anisotropic radius of gyration for the chains, which is affected by the endgroup interactions.
Insight into protein topology from Monte Carlo simulations
Authors: Derreumaux Philippe
Published: September 2002

Understanding the sequence elements coding for protein topologies is an important step toward determining structures from sequences. Here we use a Monte Carlo approach to generate the equilibrium conformations of the 56 amino acid B1 domain of protein G and several fragments, corresponding to progressive elongations from its N-terminus. This method, which is not biased towards any particular topology and starts from random conformations, yields α/β topologies deviating by 3 Å rms from the experimental structure. It is found that this simple protein model helps clarify the diversity of rate limiting steps that is observed in proteins sharing 15%–80% sequence identities with protein G.