bluuebunny/crossref_metadata_2025 huggingface dataset:
Adsorption of colloidal particles by Brownian dynamics simulation: Kinetics and surface structures
Authors: Gray Jeffrey J. , Bonnecaze Roger T.
Published: July 2002

Careful control of the microstructure of an adsorbed monolayer of colloidal particles is important for creating nanostructured devices through self-assembly processes. We present a computational model study for self-assembly of colloidal or nanoscale particulate systems. We develop a new technique for simulating colloidal adsorption processes, and we examine the kinetics and the structure formation on the surface. The technique allows the simulation of a nonhomogeneous suspension with an open boundary that is in equilibrium with a bulk suspension of known volume fraction, including the mean-field forces from the bulk solution and particle flux between the simulation box and the bulk. Short-time kinetics follow a power law similar to the case of diffusion-limited adsorption. Long-time kinetics fit a 2/3-power law form [P. Schaaf, A. Johner, and J. Talbot, Phys. Rev. Lett. 66, 1603 (1991)] and kinetic coefficients are calculated. The zeta potential of the particles is the dominant parameter controlling the final surface coverage, but the zeta potential of the adsorbing surface is the dominant control for the ordering of the adsorbed system. Particles with larger Debye layers (lower salt concentrations) order more easily. Jamming limit coverages are compared to existing equivalent hard-disk models and an energetic model. Since the process is kinetically frustrated, particle exclusion effects play a major role in determining coverage as well as structure.
Large area flat crystal x-ray spectrometer with high integrated intensity for an electron beam ion trap
Authors: Nakamura Nobuyuki
Published: July 2002

A flat crystal x-ray spectrometer has been constructed for spectroscopic studies of highly charged ions with an electron beam ion trap. It consists of a flat crystal and a position sensitive proportional counter. Employment of a flat crystal yields easy alignment, easy processing of the crystal, and high flexibility. The proportional counter has been designed to have a large effective area, which is needed to compensate for weak focusing power of a flat crystal. The utility of the new spectrometer has been demonstrated with the Tokyo electron beam ion trap.
Quantum mechanical calculation of product state distributions for the O(1D)+H2→OH+H reaction on the ground electronic state surface
Authors: Hankel Marlies , Balint-Kurti Gabriel G. , Gray Stephen K.
Published: July 2002

The real wave packet method is used to calculate reaction probabilities and product quantum state distributions for the reaction O(1D)+H2→OH+H. The method yields the desired quantities over a wide range of energies from a single wave packet propagation. The calculations are performed on the lowest adiabatic electronic potential energy surface for zero total angular momentum (J=0). A capture model is used to estimate reaction probabilities for J>0 based on our J=0 data, and thus permit the approximate calculation of cross sections. Two different ground state surfaces are used and the results from calculations on the two surfaces are compared with each other and with experiment.
Theoretical study of [Na(NH3)n]− (n=1–4)
Authors: Hashimoto Kenro , Kamimoto Tetsuya , Miura Nobuaki , Okuda Rei , Daigoku Kota
Published: July 2002

In connection with the recent photoelectron spectroscopy of negatively charged Na atom in ammonia clusters, the geometries, electronic state, vertical detachment energies, and harmonic frequencies of [Na(NH3)n]− (n=1–4) have been studied by the ab initio MO method. Structures having as many Na–N bonds as possible becomes more stable than other isomers as n grows. The Na 3s electrons are widely spread and delocalized in space outside the [Na(NH3)n]+ core for n⩾2. The dramatic redshifts of the photoelectron band for the 32P-type transition with increasing n reflect the electronic change from an atomic state to one-center Rydberg-type states in the neutrals. The frequencies of the combined vibrations of the NH3 ν1 mode are nearly degenerate and are expected to coalesce into only one strong IR band in the NH stretch region irrespective of n.
Direct ab initio variational calculation of vibrational energies of the H2O⋯Cl− complex and resolution of experimental differences
Authors: Irle Stephan , Bowman Joel M.
Published: July 2002

We report the direct ab initio calculation of vibrational energies of the chloride anion–water complex by interfacing the code MULTIMODE, which does variational calculations of vibrational energies, with GAUSSIAN, which does ab initio calculations of electronic energies. Convergence of the results with respect to the level of mode-coupling considered indicates that the present results are reliable enough to distinguish between two sets of conflicting experimental reports of these vibrational energies.
Numerical model for organic light-emitting diodes
Authors: Tutiš E. , Bussac M. N. , Masenelli B. , Carrard M. , Zuppiroli L.
Published: July 2002

An extensive numerical model recently developed for the multilayer organic light-emitting diode is described and applied to a set of real devices. The model contains a detailed description of electrical contacts including dipolar layer formation, thermionic and tunneling injection, space charge effects, field dependent mobilities and recombination processes. The model is applied to simulate several single layer devices and the family of bilayer devices made in our group. It provides insight into the energy level shifts, internal electric fields and charge distribution (and consequently recombination) throughout the device. Finally, the analysis is extended to the optimization of bilayer device.
Mechanisms of the ultrafast production and recombination of solvated electrons in weakly polar fluids: Comparison of multiphoton ionization and detachment via the charge-transfer-to-solvent transition of Na− in THF
Authors: Martini Ignacio B. , Barthel Erik R. , Schwartz Benjamin J.
Published: July 2002

The processes by which solvated electrons are generated and undergo recombination are of great interest in condensed phase physical chemistry because of their relevance to both electron transfer reactions and radiation chemistry. Although most of the work in this area has focused on aqueous systems, many outstanding questions remain, especially concerning the nature of these processes in low polarity solvents where the solvated electron has a fundamentally different structure. In this paper, we use femtosecond spectroscopic techniques to explore the dynamics of solvated electrons in tetrahydrofuran (THF) that are produced in two different ways: ejection by multiphoton ionization of the neat solvent, and detachment via the charge-transfer-to-solvent (CTTS) transition of sodide (Na−). Following multiphoton ionization of the solvent, the recombination of solvated electrons can be well described by a simple model that assumes electrons are first ejected to a given thermalization distance and then move diffusively in the presence of the Coulombic attraction with their geminate cation. The short-time transient absorption dynamics of the THF radical cation in the visible region of the spectrum do not match the kinetics of the solvated electron probed at ∼2 μm, indicating that caution is warranted when drawing conclusions about recombination based only on the dynamics of the solvent cation absorption. With ∼4 eV of excess energy, geminate recombination takes place on the hundreds of picoseconds time scale, corresponding to thermalization distances ⩾40 Å. The recombination of solvated electrons ejected via CTTS detachment of Na−, on the other hand, takes place on two distinct time scales of ⩽2 and ∼200 ps with kinetics that cannot be adequately fit by simple diffusive models. The fraction of electrons that undergo the fast recombination process decreases with increasing excitation energy or intensity. These facts lead us to conclude that electrons localize in the vicinity of their geminate Na atom partners, producing either directly overlapping or solvent-separated contact pairs. The distinct recombination kinetics for the two separate electron generation processes serve to emphasize the differences between them: multiphoton ionization produces a delocalized electron whose wave function samples the structure of the equilibrium fluid before undergoing localization, while CTTS is an electron transfer reaction with dynamics controlled by the motions of solvent molecules adjacent to the parent ion. All the results are compared to recent experiments on the photodetachment of electrons in aqueous systems where contact pairs are also thought to be important, allowing us to develop a qualitative picture for the mechanisms of electron generation and recombination in different solvent environments.
Micromagnetic behavior of conical ferromagnetic particles
Authors: Ross C. A. , Farhoud M. , Hwang M. , Smith Henry I. , Redjdal M. , Humphrey F. B.
Published: July 2002

Large area arrays of cobalt and nickel particles with truncated conical shapes and diameters of 80–120 nm have been prepared using interference lithography combined with an evaporation and lift-off process. The magnetic hysteresis has been measured and the remanent states of the particles have been compared with a three-dimensional micromagnetic model. The model shows a transition from “flower” to “vortex” magnetization states as the particle size increases. The distribution of switching fields and the magnetostatic interactions between particles have been characterized. Both lead to a slow approach to saturation in the hysteresis loops. The suitability of such arrays for data storage is discussed.
Design strategies for GaAs-based unipolar lasers: Optimum injector-active region coupling via resonant tunneling
Authors: Barbieri Stefano , Sirtori Carlo , Page Hideaky , Stellmacher Max , Nagle Julien
Published: July 2002

Electron injection into the upper state of the laser transition in quantum cascade lasers is studied by investigating the electrical and optical characteristics of a set of electroluminescent devices. These devices exploit the active region of an (Al)GaAs laser based on a diagonal–anticrossed transition scheme with emission wavelength at 9.5 μm, and are identical except for the injection barrier thickness which varies from 3.9 up to 8.0 nm. We find that for thin barriers electron wave functions in the injector are directly coupled with those of the continuum. This leads to a parallel current path which strongly reduces the injection efficiency of electrons into the active region. The current leak is suppressed at low temperatures for samples with the thickest barriers, but it is still observable at high temperatures when electrons are thermally activated from the injector miniband into the continuum.
Proper representation of the subgrid-scale eddy viscosity for the dynamic procedure in large eddy simulation using finite difference method
Authors: Tsubokura Makoto
Published: July 2002

Alternative representation of the dynamic eddy viscosity coefficient to the traditional Smagorinsky model is investigated for the application of the model to the finite difference method. The model is derived by considering the consistency of the numerical error between Lij and Mij in the dynamic procedure. The proposed model is validated in the plane channel flow at the Reτ of up to 590 and is found to be less sensitive to the discretized test filtering operation.
Internal dynamics in azetidine: A microwave and ab initio study
Authors: López Juan C. , Blanco Susana , Lesarri Alberto , Alonso José L.
Published: July 2002

The internal dynamics of interconversion between equivalent conformations due to the coupling between ring puckering and NH inversion in azetidine has been investigated by rotational spectroscopy and ab initio computations. Analysis of the rotational spectra in the 8–220 GHz region has been completed for the ground state and first four excited states of the ring-puckering vibration. Rotational transitions exhibit a characteristic doubling originated by tunneling between equivalent conformations through a C2v barrier, which is related to symmetric (A1) and antisymmetric (B1) inversion states. Additionally, nuclear quadrupole hyperfine structure arising from the N nucleus could be resolved for low-J transitions. Accurate rotational and centrifugal distortion parameters together with the energy difference between inversion states derived from μc-type inversion transitions have been derived for each ring-puckering state using a two-state Hamiltonian. An effective monodimensional reduced potential function for the ring-puckering vibration V(X)=10.82(X4+14.29X−8.93X2−0.28X3) has been found consistent with the observed experimental variation of the rotational and centrifugal distortion constants with ring-puckering. This asymmetric single minimum potential function supports the existence of only one stable equatorial form. The barrier to interconversion between equivalent equatorial conformers, related to the C2v conformation of azetidine in which the ring atoms and the NH group are coplanar, has been estimated to range between 1900 and 2600 cm−1. The strong dependence of the dipole moment and quadrupole coupling constants with ring-puckering vibrational state evidence structural changes that occur along the ring-puckering coordinate.
Identification of stable boron clusters in c-Si using tight-binding statics
Authors: Luo Weiwei , Clancy Paulette
Published: July 2002

As a particularly important p-type dopant, boron exhibits some problematical phenomena during the fabrication of microelectronic devices, especially transient enhanced diffusion (TED) following ion implantation and annealing. TED is due, in large part, to the formation of boron-defect clusters. This article describes a search for particularly stable boron-defect clusters (up to B4I4). A tight-binding method, in conjunction with atomic-scale statics calculations, is used to study boron and boron-defect clusters containing up to four boron atoms and four self-interstitials within a matrix of crystalline silicon. Formation and binding energies are reported for these species. There is a tendency to form a four-atom ring containing two Si self-interstitials and two boron atoms. One guiding principle for the stability of the geometry of the clusters is to maximize the number of unstrained bonds (i.e. with Si-like bond lengths); the higher the extent of unstrained bonds, the lower the formation energy. Symmetry is found to play a smaller role in determining preferred structures.
Adsorption and thermal behavior of CO and NO on Pd{110} and Pd{320}
Authors: Hirsimäki M. , Valden M.
Published: July 2002

Chemisorption and thermal behavior of CO and NO on Pd{110} and its vicinal surface, Pd{320} [Pd(S)[3(110)×(100)]], have been investigated using molecular beam surface scattering (MBSS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS). CO is shown to adsorb molecularly on Pd{320} in the temperature range of 250–500 K. No evidence for long-range ordering of CO adlayers on Pd{320} was observed. Saturation coverage for CO/Pd{320} at 300 K is 0.61 ML. The initial sticking probability of CO is 0.91 and is shown to depend only weakly on the surface temperature. It is also found that the initial sticking probability of CO on Pd{110} exhibits a strong dependence on the kinetic energy of CO molecules and the CO adsorption on Pd{320} may also exhibit this same behavior. CO desorbs molecularly from Pd{320} with desorption features near 390 and 500 K. NO is shown to adsorb molecularly below 420 K with a high initial sticking probability close to 0.9. The initial sticking probability is largely temperature independent up to 450 K. No long-range order was observed from NO/Pd{320} in the temperature range of 300–600 K. The saturation coverage of NO is 0.64 ML at 300 K. It is demonstrated that NO partially dissociates upon heating, yielding N2 and N2O. Desorption of molecular NO occurs on both surfaces between 300–600 K, with N2 and N2O desorbing above 450 K. In contrast to Pd{110}, a high temperature N2 peak near 600 K is seen during NO TPD from Pd{320}, which is attributed to tightly bound, adsorbed nitrogen atoms. The relative TPD yields of N2 and N2O from Pd{110} and Pd{320} are found to be strongly dependent on the initial NO coverage with enhanced dissociation at lower coverages. Pd{320} is found relatively unreactive towards the thermal dissociation of NO. The activity of Pd{110} and Pd{320} towards NO dissociation upon adsorption increases with surface temperature. The relative yields of N2 and N2O reach a maximum near 550 K of ∼40% and ∼20%, respectively. By direct comparison to NO/Pd{110}, the present study shows that the presence of (100) steps on the Pd{320} surface does not significantly enhance its reactive properties with respect to adsorption and thermal behavior of CO and NO.
On the validity of the extended Koopmans’ theorem
Authors: Pernal Katarzyna , Cioslowski Jerzy
Published: July 2002

A theorem that provides a sufficient condition for the validity of the extended Koopmans’ theorem (EKT) is formulated and proven. Although its verification may be difficult in practice, this condition involves only quantities pertaining to the N-particle system. A less restrictive necessary condition remains unknown.
Effective interaction between reverse micelles: A study from the potential of mean force at infinite dilution
Authors: Bouaskarne M. , Amokrane S. , Regnaut C.
Published: July 2002

The effective interaction potential between reverse micelles is computed by adding the potential of mean force at infinite dilution to the direct interaction between micelles. The former is computed from an integral equations approach and the latter is modeled by van der Waals and electrostatic contributions. This model is used to discuss the characteristics of the effective interaction—range, strength, and temperature dependence—deduced in previous work from the effective one-component approach. From a detailed analysis of the various contributions to the effective interaction, it is concluded in agreement with other studies, that the relative long range of the interaction is attributed to electrostatic contributions. The moderate interaction strength is found to result from strong cancellations between direct interactions (short range repulsion plus van der Waals attraction between surfactant films) and indirect ones (“depletion” plus “solvation” effects). In contrast with previous studies which attributed the increase with temperature of the interaction strength only to the extension of surfactant chains (direct interaction), the variation with temperature of the potential of mean force is found to be equally important.
The 583.2 GHz torsional hot-band of (D2O)3
Authors: Keutsch Frank N. , Karyakin Evgeniy N. , Saykally Richard J. , van der Avoird Ad
Published: July 2002

We report the observation of a new c-type band of (D2O)3 at 583.215 92(37) GHz, which we assign to the k=±20←±10 torsional hot-band. The new data includes the first observation of K=0 states for the k=+10 and k=−20 levels and effects a correct assignment of these states. A new perturbation was observed for the K=2 states of the k=+20←−10 subband splitting each transition into two equally spaced equal intensity doublets. Analysis of the band and inclusion into a global fit of all torsional bands produces negligible differences with previous analyses, and confirms the validity of the Hamiltonian developed to treat the coupling between torsional motion and overall rotation. The 583.2 GHz band completes the precise characterization of all (D2O)3 vibrational levels below 100 cm−1.
Wetting layer carrier dynamics in InAs/InP quantum dots
Authors: Hinooda S. , Loualiche S. , Lambert B. , Bertru N. , Paillard M. , Marie X. , Amand T.
Published: July 2002

The electronic coupling between InAs/InP quantum dot (QD) array and its wetting layer (WL) is studied by continuous wave and time resolved photoluminescence. The carrier dynamics is explained by the existence of two regimes in the WL: at low QD density the carrier dynamics is dominated by the diffusion and at high density when the distance between QDs is comparable to the carrier mean free path in the WL the quantum capture into QDs dominates. From the identification of these two regimes the carrier mean free path in the WL is estimated to about 30 nm.
Efficient potential energy surfaces from partially filled ab initio data over arbitrarily shaped regions
Authors: Hollebeek Timothy , Ho Tak-San , Rabitz Herschel
Published: July 2002

The reproducing kernel Hilbert space (RKHS) method has been previously shown to be accurate and efficient in the construction of potential energy surfaces (PES) by interpolating fully gridded high level ab initio data. This paper extends the RKHS method to handle partially filled data calculated over arbitrarily shaped regions, while keeping nearly intact its accuracy and efficiency. The extension permits points or regions to be added to or removed from the grid as needed before doing expensive ab initio calculations, thus enabling the construction of RKHS PESs from the data distributions that are most likely to occur in practice. The utility of the new technique is demonstrated using data from the lowest global RKHS PES for the reaction O(1D)+H2, showing that ignoring the irrelevant regions of the PES does not adversely impact the accuracy of the surfaces if the relevant region is adequately sampled.
Softened C–H modes of adsorbed methyl and their implications for dehydrogenation: An ab initio study
Authors: Michaelides A. , Hu P.
Published: July 2002

To investigate the softening of CH vibrational frequencies and their implications for dehydrogenation of adsorbed hydrocarbons, an issue of scientific and technological importance, density functional theory calculations have been performed on the chemisorption and dehydrogenation of CH3 on Cu(111) and Pt(111) surfaces. By comparing these results with those of Ni(111) we find that the CH bonds of the adsorbate, when close enough, interact with metal atoms of the surface. It is this interaction and its associated lengthening and weakening of CH bonds that is the physical origin of mode softening. We rule out the possibility of a relationship between the mere presence of mode softening and dehydrogenation. We do show, however, that there is a clear relationship between the extent to which a surface can induce mode softening and the activation energy to dehydrogenation. In addition, periodic trends concerning the extent of mode softening are reproduced.
Microscopic theory of linear and nonlinear optical and dielectric response in polymer materials
Authors: Munn R. W.
Published: July 2002

A self-consistent cavity model is introduced to calculate optical and dielectric properties of polymer materials from the microscopic structure and molecular response. The material is treated as discrete molecules inside a cavity (chosen as spherical for convenience) and as a continuum outside. The cavity radius is to be increased progressively until the calculated properties converge within a set precision. Linear response for an isotropic polymer material is treated for three types of molecular polarizability: (a) dipolar response that is local within molecules; (b) dipolar response that is nonlocal within and between molecules; and (c) distributed monopolar and dipolar response that is nonlocal within molecules but local between molecules. In each case a microscopic expression is derived for computing the effective polarizability that gives the exact linear susceptibility from the Lorentz expression. For a poled polymer material of axial symmetry with type (c) response, expressions are derived for the anisotropic effective polarizability, the effective dipole moments under their mutual polarizing influence, the resulting dipole energy, and the poling energy in an external electric field (including quadratic terms that arise from allowing molecules to be polarizable). Nonlinear optical susceptibilities are then derived for a polymer material of arbitrary structure with type (c) response generalized to include first and second hyperpolarizabilities. In algebraic form, the results resemble those for distributed response in molecular crystals, without obvious dependence on the choice of cavity shape.
Unconventional diffusion behaviors of intermolecular multiple-quantum coherences in nuclear magnetic resonance
Authors: Chen Zhong , Zhong Jianhui
Published: July 2002

To date, both the intermolecular multiple-quantum coherence (MQC) and demagnetizing field models have led to fully quantitative predictions of NMR signals in a highly polarized system using the CRAZED and similar sequences. In this paper, measurements of apparent MQC diffusion rates, Dnapp, for a specific apparent coherence order, n, were used to investigate the equivalent between the intermolecular MQC and demagnetizing field treatments. A number of physical effects were analyzed both theoretically and experimentally. These effects include molecular diffusion, variation in dipolar correlation distance, radiation damping, inhomogeneous broadening, and spin relaxation, all of which may alter the NMR signal. Two variations of a two-pulse CRAZED sequence, where the signal attenuation is almost entirely caused by the diffusion weighting, were designed to accurately measure and characterize Dnapp during the evolution period. Apparent diffusion rates were extracted from a least-squares fitting of a series of H1 spectra, measured with varying diffusion weighting factors. Complete theoretical formations were explicitly derived from both the intermolecular MQC and demagnetizing field treatments. Numerical simulations based on the demagnetizing field treatment were performed and it was found that the model can be used to predict the apparent diffusion rates. A novel diffusion model for intermolecular MQC is proposed in which the phase shift of each individual spin on different molecules is considered to be uncorrelated. This model successfully predicts the unconventional diffusion behaviors of intermolecular MQCs, specifically for differences of apparent diffusion rates between inter- and intramolecular MQCs. Our theoretical predictions and experimental confirmation demonstrate, for the first time, that Dnapp for intermolecular MQCs of order n are characterized by Dnapp=nDT for n⩾2 and D0app=2DT for n=0, where DT is the translational molecular diffusion rate of the single quantum coherences. These results do not coincide with Dn=n2DT for n⩾0 which is a general relationship for an intramolecular n-quantum coherence. These works about the apparent diffusion rates during the evolution period of the CRAZED sequences provide additional evidence to support the argument of the equivalence between the intermolecular MQC and demagnetizing field models. The general results derived from both intermolecular MQC and demagnetizing field treatments in this report can reasonably explain new observations of diffusion phenomena in nonlinear spin echoes by Kimmich and co-workers. Even though the theoretical prediction about intermolecular MQC diffusion is verified only with specific experiments using tailor-made pulse sequences, it is demonstrated that the function dependence of diffusion rate on coherence order is general. These results provide independent evidence to support the intermolecular MQC theory proposed by Warren and co-workers.
Properties of electronic traps at silicon/1-octadecene interfaces
Authors: Kar Samares , Miramond Corinne , Vuillaume Dominique
Published: July 2002

Silicon surfaces with (111) orientation were hydrogenated in NH4F solution. Alkyl chain monolayers were self-assembled on the hydrogen-terminated silicon (Si–H) surface by the free-radical reaction of 1-octadecene and SiH, activated by ultraviolet (253.7 nm) illumination. Comprehensive electrical characterization of the metal/1-octadecene/silicon structures yielded experimental data on the (silicon–monolayer) interface trap parameters. The admittance data indicated the realization of a true silicon/organic-monolayer interface. The interface trap density obtained, for p-type silicon, was 1.7–3.0×1011/cm2 V over an energy range of 0.22–0.73 eV over the valence band edge. These interface trap densities can be considered remarkably low for a silicon interface formed around room temperature, and are an order of magnitude lower than what was obtained in the case of alkyl chain organic monolayers on naturally oxidized silicon surfaces. The interface trap density was found to be strongly influenced by the silicon doping type and density. This observation suggests that the position of the surface Fermi level has a strong influence on the quality of the octadecene–silicon interface.
Chaotic advection by two interacting finite-area vortices
Authors: Velasco Fuentes O. U.
Published: July 2002

This article deals with the advection of fluid particles in the velocity field of two identical vortices with various vorticity distributions. The two-dimensional velocity field is aperiodic in the range of parameters studied here, namely, the neighborhood of the critical distance for merger. Ideas and methods from the theory of transport in dynamical systems are used to describe and quantify particle advection. These methods are applied to the numerical representation of the velocity field, which is obtained by solving the Euler equations with the vortex-in-cell method. It is found that the strongest stirring of vortex fluid occurs slightly above the critical distance for merger. In this regime the fluid located between the vortices is subjected to intense stirring, and some vortex fluid may be entrained into the chaotic region depending on the smoothness of the vorticity distribution. Initial conditions below the critical distance lead to stirring of fluid mainly before merger. In this case the flow geometry is used to quantify the efficiency of merger, which is defined as the ratio of the circulation of the resultant vortex to the total circulation of the original vortices. It is found that the vortices with the smoothest vorticity profile have the lowest efficiency. Experimental visualizations in a two-dimensional rotating fluid confirm the intense stretching and folding of fluid elements that occurs before the vortices merge.
Incorporation of nitrogen in nitride-arsenides: Origin of improved luminescence efficiency after anneal
Authors: Spruytte Sylvia G. , Coldren Christopher W. , Harris James S. , Wampler William , Krispin Peter , Ploog Klaus , Larson Michael C.
Published: July 2002

A key to the utilization of nitride-arsenides for long wavelength optoelectronic devices is obtaining low defect materials with long nonradiative lifetimes. Currently, these materials must be annealed to obtain device quality material. The likely defect responsible for the low luminescence efficiency is associated with excess nitrogen. Photoluminescence and capacitance–voltage measurements indicate the presence of a trap associated with excess nitrogen which decreases in concentration upon anneal. Our films are grown by elemental source molecular beam epitaxy and the background impurity concentration is low, thus we have investigated the role of crystalline defects. High resolution x-ray diffraction showed improved crystal quality after anneal. We observed that the lattice parameter does not decrease linearly with nitrogen concentration for levels of nitrogen above 2.9 mol % GaN. The fact that Vegard’s law is not observed, despite theoretical calculations that it should, indicates that nitrogen incorporates in locations other than the group V lattice sites. X-ray photoelectron spectroscopy revealed that nitrogen exists in two bonding configurations in not-annealed material: a Ga–N bond and another nitrogen complex in which N is less strongly bonded to gallium atoms. Annealing removes this second nitrogen complex. A combined nuclear reaction analysis and channeling technique showed that not annealed GaNAs contains a significant concentration of interstitial nitrogen that disappears upon anneal. We believe that this interstitial nitrogen is responsible for the deviation from Vegard’s law and the low luminescence efficiency of not annealed GaNAs and GaInNAs quantum wells.
Magneto-optical properties of a GaAs:MnAs hybrid structure sandwiched by GaAs/AlAs distributed Bragg reflectors: Enhanced magneto-optical effect and theoretical analysis
Authors: Shimizu H. , Miyamura M. , Tanaka M.
Published: July 2002

We have fabricated a multilayer structure consisting of a semiconductor-magnetic hybrid material GaAs:MnAs and GaAs/AlAs distributed Bragg reflectors, that is a one-dimensional semiconductor-based magnetophotonic crystal. Significant enhancement of magneto-optical effect was demonstrated at a controlled wavelength at room temperature. Magneto-optical spectra of this multilayer system were well explained by theoretical calculations, and the mechanism for the enhancement of magneto-optical effect was shown. Also, the required conditions for application to optical isolators are discussed and some potential solutions are indicated. Since our structure is composed of all semiconductor-based materials, the present results potentially lead to semiconductor based magneto-optical devices integrated with III–V-based optoelectronics.
Portable, tunable, high-luminosity spherical crystal spectrometer with an x-ray charge coupled device, for high-resolution x-ray spectromicroscopy of clusters heated by femtosecond laser pulses
Authors: Blasco F. , Stenz C. , Salin F. , Faenov A. Ya. , Magunov A. I. , Pikuz T. A. , Skobelev I. Yu.
Published: July 2002

A portable (200×100×100 mm3), high-luminosity, spherically bent crystal spectrometer was designed for measuring in a wide spectral range of 1.2–19.6 Å very low emissivity x-ray spectra of different clusters heated by 35 fs laser radiation. This spectrometer is associated with a custom design x-ray charge coupled device that features a large sensitive area (24.6×24.6 mm2) and a small pixel size (24×24 μm2). This apparatus provides simultaneous high spectral (λ/δλ∼1000–5000) and spatial (40–80 μm) resolution. A large (30×10 mm2) open aperture mica crystal with R=100 mm is used as the dispersive and focusing element. The large tuneability of the spectrometer makes it possible to record high-resolution spectra of H-like ions of oxygen (CO2 clusters) in a spectral range of 15–17 Å, Ne-like like ions of Kr in a spectral range of 5–5.7 Å, and He-like spectra of Ar in a spectral range of 3.0–3.4 and 3.7–4.4 Å without any adjustment of the spectrometer setup. Thanks to the high luminosity (high collection efficiency) of the spectrometer, high quality spectra were obtained using only 15 mJ 2000 laser shots.
Magnetic properties of LaCr1−xMxSb3 (M=V, Mn, Fe, Cu, and Al)
Authors: Dubenko I. S. , Hill P. , Ali N.
Published: July 2002

The influence of Cr substitution by various metals (M=V, Mn, Fe, Cu and Al) on the magnetic state of the itinerant intermetallics La(Cr,M)Sb3 was studied by magnetization and magnetic susceptibility measurements up to 55 kG at 5 K and from 4.2 to 400 K, in a magnetic field of 1000 G, respectively. It was found that the Curie temperature (TC) and magnetization (M) of these compounds depend nonlinearly on the concentration, remaining in the vicinity of the values of TC and M measured for LaCrSb3. Curie temperatures and magnetization values at 55 kG are suppressed by Mn, Fe, V, and Cu, and have a slight maximum at low Al concentration (about 5%).
Growth and properties of (Ga,Mn)As films with high Mn concentration
Authors: Takamura K. , Matsukura F. , Ohno Y. , Ohno H.
Published: July 2002

(Ga, Mn)As films with high nominal Mn concentration (0<x<0.55) are grown by low temperature molecular beam epitaxy (LT-MBE), growth temperature Ts=180 °C. Reflection high energy electron diffraction patterns indicate epitaxial growth of (Ga, Mn)As for x<0.1, whereas they show spotty patterns for x>0.1, which turn to polycrystalline features when x>0.3. X-ray diffraction shows the formation of MnAs together with the growth of (Ga, Mn)As. The lattice constant of the layers suggests that (Ga, Mn)As with high Mn composition as high as 17% can be grown by LT-MBE.
Caging of I2 in deca-dodecasil 3R: Pump–probe experiments and molecular dynamics modeling
Authors: Ermoshin V. A. , Flachenecker G. , Materny A. , Engel V.
Published: July 2002

We present transients obtained by femtosecond time-resolved spectroscopy on I2 encapsulated in well-defined cages of the crystalline microporous SiO2 modification deca-dodecasil 3R. After pump excitation into the electronic B-state, collision-induced predissociation is observed followed by recombination into A/A′ and X states, where the molecules undergo vibrational relaxation. To explain the complex time-behavior of the transient signals, molecular dynamics simulations are performed which account for the nonadiabatic transitions in I2 and the adsorbate-framework interactions. The theoretical study shows that the total signal contains contributions from four optical transitions which are influenced by predissociation and vibrational relaxation processes. A pronounced dependence of the transient signals on the probe-laser wavelength is determined by the position of the resonance window for the optical transition.
Magnetic tunnel junctions with AlN and AlNxOy barriers
Authors: Schwickert M. M. , Childress J. R. , Fontana R. E. , Kellock A. J. , Rice P. M. , Ho M. K. , Thompson T. J. , Gurney B. A.
Published: July 2002

Nonoxide tunnel barriers such as AlN are of interest for magnetic tunnel junctions to avoid the oxidation of the magnetic electrodes. We have investigated the fabrication and properties of thin AlN-based barriers for use in low resistance magnetic tunnel junctions. Electronic, magnetic and structural data of tunnel valves of the form Ta (100 Å)/PtMn (300 Å)/CoFe20 (20 Å–25 Å)/barrier/CoFe20 (10–20 Å)/NiFe16 (35–40 Å)/Ta (100 Å) are presented, where the barrier consists of AlN, AlNxOy or AlN/AlOx with total thicknesses between 8 and 15 Å. The tunnel junctions were sputter deposited and then lithographically patterned down to 2×2 μm2 devices. AlN was deposited by reactive sputtering from an Al target with 20%–35% N2 in the Ar sputter gas at room temperature, resulting in stoichiometric growth of AlNx (x=0.50±0.05), as determined by RBS. TEM analysis shows that the as-deposited AlN barrier is crystalline. For AlN barriers and AlN followed by natural O2 oxidation, we obtain tunnel magnetoresistance >10% with specific junction resistance Rj down to 60 Ωμm2.
A three-dimensional Gaussian-beam ray-tracing program for designing interferometer/polarimeter plasma diagnostics
Authors: Warr George B. , Howard John
Published: July 2002

We have developed a three-dimensional Gaussian-beam ray-tracing program to aid in the design of infrared, far-infrared, and millimeter wave interferometer and polarimeter diagnostic systems for magnetic confinement fusion relevant plasma physics experiments. An overview of the program is presented along with a description of the ray-tracing algorithm. A model is developed for the case of diffraction of a Gaussian beam off a cylindrical grating and is shown to be in good agreement with experimental measurements. The program has been used to aid the design of the scanning-grating interferometer system for the H-1NF heliac experimental plasma device. The program is written in the Research Systems Inc. Interactive Data Language and, on a typical modern personal computer, is able to trace and render the ∼50 element three-view 44-beam H-1NF interferometer optical system in about one minute.
Ab initio and diatomics in molecule potentials for I2−, I2, I3−, and I3
Authors: Vala Jiri , Kosloff Ronnie , Harvey Jeremy N.
Published: July 2002

The electronic structure of the I3− molecular anion and its photoproducts I2−, I2, and I3 were studied. Ab initio calculations were carried out using the multireference configuration interaction (MRCI) method for the valence electrons together with a relativistic effective core potential. The ab initio wave functions were also used to compute some spin–orbit coupling matrix elements, as well as approximate valence bond wave functions, used as guidelines in the construction of a 108-state diatomics in molecule (DIM) description of the electronic structure of I3−. In the DIM model, spin–orbit coupling was introduced as a sum of atomic operators. For I2− the ab initio and the DIM ground-state potentials show excellent agreement with the experimental results. The results for I2 are also in very good agreement with experimental data. For I3−, the MRCI calculations give a very good description of the spectroscopic constants and agree with the vertical excitation energies, provided spin–orbit coupling is included. The DIM description fails both quantitively by leading to erroneous spectroscopic constants, and qualitatively by not even reproducing the MRCI ordering of the excited-states. The failure of the DIM is attributed to the omission of ionic states. The overall qualitative picture of the excited-state potentials shows a maze of dense avoided crossings which means that all energetically allowed photoproducts will be present in the experiment. The ground electronic state of I3 was calculated to be a collinear and centrosymmetric Πu,3/22. The collinear state is stabilized by spin–orbit coupling relative to a bent configuration. Calculated vertical transition energies from the ground to low-lying excited states of the radical are in excellent agreement with the experimental data. The spin–orbit assignment of these states is provided.
Nonlinear electrostatic waves in a magnetized dust-ion plasma
Authors: Farid T. , Mamun A. A. , Shukla P. K. , Mirza Arshad M.
Published: July 2002

It is shown that the nonlinear equation governing the dynamics of coupled dust-acoustic and dust-cyclotron waves in a magnetized dust-ion plasma can be written in the form of an energy integral. The latter is analyzed analytically as well as numerically to investigate the properties of arbitrary amplitude solitary waves. It is found both analytically as well as numerically that there exist solitary waves only with a negative potential. The implications of these results to some space and astrophysical dusty plasma systems, especially to planetary ring systems and cometary tails, are briefly discussed.
On the eigenproblems of PT-symmetric oscillators
Authors: Shin K. C.
Published: July 2002

We consider the non-Hermitian Hamiltonian H=−d2/dx2+P(x2)−(ix)2n+1 on the real line, where P(x) is a polynomial of degree at most n⩾1 with all non-negative real coefficients (possibly P≡0). It is proved that the eigenvalues λ must be in the sector |arg λ|⩽π/(2n+3). Also for the cubic case H=−d2/dx2−(ix)3, we establish a zero-free region of the eigenfunction u and its derivative u′ and we find some other interesting properties of eigenfunctions.
A quasiclassical trajectory study of reactivity and product energy disposal in H+H2O, H+D2O, and H+HOD
Authors: Troya Diego , González Miguel , Schatz George C.
Published: July 2002

The dynamics of the H+H2O→H2+OH, and some isotopic counterpart reactions has been investigated by quasiclassical trajectory (QCT) calculations, and using a recently developed potential energy surface [Wu et al., J. Chem. Phys. 113, 3150 (2000)] that was derived from high quality ab initio calculations. We make an extensive comparison with QCT and quantum scattering results based on other surfaces, particularly that from Ochoa and Clary, as well as with experimental results. Our results show that, in agreement with earlier theoretical results, the cross sections for the reaction of translationally hot hydrogen atoms with ground state H2O (yielding H2+OH) and with ground state D2O (yielding HD+OD) are significantly smaller than experiment. Our results are in agreement with accurate quantum results on comparably accurate surfaces, thereby showing that the disagreement with experiment is not a problem with either the dynamics method or the potential surfaces. In contrast to this, other properties of the reaction dynamics are in generally excellent agreement with experiment. For example, the role of stretch excitation on the H+D2O cross sections follows the trends observed in the experiments. Bend excitation is found to be more active than was previously thought in enhancing reactivity, but is still within experimental uncertainty. Water rotation is found to play an important role in experiments that sample j(H2O) values of 5 or greater. Our studies of the H+D2O and H+HOD→H2+OD,HD+OH reactions yield isotope branching ratios and product distributions (for both spectator and newly formed diatoms) that are generally in good agreement with experiment. The only exception to this arises with the HD rotational distributions in H+D2O, where the observed distributions show less excitation and broader distributions. The internal distributions of experimentally unresolved products are also discussed. We conclude that the new potential energy surface used here is very accurate for describing the H+H2O→H2+OH reaction and isotopic counterparts, providing significant improvement over previously published results.
Intermolecular potential for the interaction of helium with ammonia
Authors: Hodges Matthew P. , Wheatley Richard J.
Published: July 2002

We develop an intermolecular potential for the interaction between helium and ammonia including flexibility in the ammonia inversion tunneling coordinate. The potential energy surface is generated by fitting to scaled perturbation theory calculations and is shown to be comparable with high-quality ab initio supermolecule calculations. We have characterized the potential energy surface for a number of ammonia geometries from planar to a highly distorted geometry. For all but the most distorted ammonia geometry, the global minimum has the helium atom in an equatorial location, equidistant from the two closest hydrogen atoms. As the ammonia molecule moves away from the planar configuration, the equatorial minima become less strongly bound while the binding energy increases in the axial regions of the potential energy surface. At the most distorted ammonia geometry, the equatorial minimum is a local minimum, and the global minimum has the helium atom on the symmetry axis of the molecule at the hydrogen end.
A new approach for studying the hydrodynamic stability of fluids with microstructure
Authors: Somasi Madan , Khomami Bamin
Published: July 2002

A new methodology for studying the stability of fluids with microstructure has been developed. This technique relies on combining continuum based conservation equations and stochastic simulation techniques to determine the hydrodynamic stability of flows under consideration. To illustrate the capability of the method, the stability of viscoelastic Taylor–Couette flow has been examined.
Secondary breakup of axisymmetric liquid drops. II. Impulsive acceleration
Authors: Han Jaehoon , Tryggvason Grétar
Published: July 2002

The secondary breakup of impulsively accelerated liquid drops is examined for small density differences between the drops and the ambient fluid. Two cases are examined in detail: a density ratio close to unity and a density ratio of 10. A finite difference/front tracking numerical technique is used to solve the unsteady axisymmetric Navier–Stokes equations for both the drops and the ambient fluid. The breakup is governed by the Weber number, the Reynolds number, the viscosity ratio, and the density ratio. The results show that Weber number effects are dominant. In the higher density ratio case, ρd/ρo=10, different breakup modes—oscillatory deformation, backward-facing bag mode, and forward-facing bag mode—are seen as the Weber number increases. The forward-facing bag mode observed at high Weber numbers is an essentially inviscid phenomenon, as confirmed by comparisons with inviscid flow simulations. At the lower density ratio, ρd/ρo=1.15, the backward-facing bag mode is absent. The deformation rate also becomes larger as the Weber number increases. The Reynolds number has a secondary effect, changing the critical Weber numbers for the transitions between breakup modes. The increase of the drop viscosity reduces the drop deformation. The results are summarized by “breakup maps” where the different breakup modes are shown in the We–Re plane for different values of the density ratios.