bluuebunny/crossref_metadata_2025 huggingface dataset:
Increasing robustness of indirect drive capsule designs against short wavelength hydrodynamic instabilities
Authors: Haan S. W. , Herrmann M. C. , Dittrich T. R. , Fetterman A. J. , Marinak M. M. , Munro D. H. , Pollaine S. M. , Salmonson J. D. , Strobel G. L. , Suter L. J.
Published: May 2005

Targets meant to achieve ignition on the National Ignition Facility (NIF) [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)] have been redesigned and their performance simulated. Simulations indicate dramatically reduced growth of short wavelength hydrodynamic instabilities, resulting from two changes in the designs. First, better optimization results from systematic mapping of the ignition target performance over the parameter space of ablator and fuel thickness combinations, using techniques developed by one of us (Herrmann). After the space is mapped with one-dimensional simulations, exploration of it with two-dimensional simulations quantifies the dependence of instability growth on target dimensions. Low modes and high modes grow differently for different designs, allowing a trade-off of the two regimes of growth. Significant improvement in high-mode stability can be achieved, relative to previous designs, with only insignificant increase in low-mode growth. This procedure produces capsule designs that, in simulations, tolerate several times the surface roughness that could be tolerated by capsules optimized by older more heuristic techniques. Another significant reduction in instability growth, by another factor of several, is achieved with ablators with radially varying dopant. In this type of capsule the mid-Z dopant, which is needed in the ablator to minimize x-ray preheat at the ablator-ice interface, is optimally positioned within the ablator. A fabrication scenario for graded dopants already exists, using sputter coating to fabricate the ablator shell. We describe the systematics of these advances in capsule design, discuss the basis behind their improved performance, and summarize how this is affecting our plans for NIF ignition.
Nonlinear harmonic generation in high-gain free-electron lasers
Authors: Dattoli G. , Ottaviani P. L. , Pagnutti S.
Published: May 2005

We reconsider the derivation of semianalytical expressions providing the most significant aspects of the high-gain free-electron laser dynamics. We obtain new expressions for the growth of the laser power, of the e-beam-induced energy spread, and of the higher-order nonlinearly generated harmonics. The procedure we employ, based on theoretical ansatz and fitting methods, allows the determination of crucial quantities like the expected harmonic output power and its dependences on the e-beam parameters.
Transition to turbulence and effect of initial conditions on three-dimensional compressible mixing in planar blast-wave-driven systems
Authors: Miles A. R. , Blue B. , Edwards M. J. , Greenough J. A. , Hansen J. F. , Robey H. F. , Drake R. P. , Kuranz C. , Leibrandt D. R.
Published: May 2005

Perturbations on an interface driven by a strong blast wave grow in time due to a combination of Rayleigh–Taylor, Richtmyer–Meshkov, and decompression effects. In this paper, results from three-dimensional (3D) numerical simulations of such a system under drive conditions to be attainable on the National Ignition Facility [E. M. Campbell, Laser Part. Beams 9, 209 (1991)] are presented. Using the multiphysics, adaptive mesh refinement, higher order Godunov Eulerian hydrocode, Raptor [L. H. Howell and J. A. Greenough, J. Comput. Phys. 184, 53 (2003)], the late nonlinear instability evolution, including transition to turbulence, is considered for various multimode perturbation spectra. The 3D post-transition state differs from the 2D result, but the process of transition proceeds similarly in both 2D and 3D. The turbulent mixing transition results in a reduction in the growth rate of the mixing layer relative to its pretransition value and, in the case of the bubble front, relative to the 2D result. The post-transition spike front velocity is approximately the same in 2D and 3D. Implications for hydrodynamic mixing in core-collapse supernovae are discussed.
Non-Fickian interdiffusion of dynamically asymmetric species: A molecular-dynamics study
Authors: Yaneva Jacqueline , Dünweg Burkhard , Milchev Andrey
Published: May 2005

We use molecular dynamics combined with dissipative particle dynamics to construct a model of a binary mixture where the two species differ only in their dynamic properties (friction coefficients). For an asymmetric mixture of slow and fast particles we study the interdiffusion process. The relaxation of the composition profile is investigated in terms of its Fourier coefficients. While for weak asymmetry we observe Fickian behavior, a strongly asymmetric system exhibits clear indications of anomalous diffusion, which occurs in a crossover region between cases I (Fickian) and II (sharp front moving with constant velocity), and is close to the case II limit.
Physical models for coupled electromechanical analysis of silicon nanoelectromechanical systems
Authors: Tang Zhi , Xu Yang , Li Gang , Aluru N. R.
Published: May 2005

Nanoelectromechanical systems (NEMS) can be designed and characterized by understanding the interaction and coupling between the mechanical, electrical, and the van der Waals energy domains. In this paper, we present physical models and their numerical simulation for coupled electrical and mechanical analysis of silicon NEMS. A nonlinear continuum elastic model is employed for mechanical analysis. The material properties required in the continuum model are extracted from molecular-dynamics simulations. We present three electrostatic models—namely, the classical conductor model, the semiclassical model, and the quantum-mechanical model, for electrostatic analysis of NEMS at various length scales. The electrostatic models also account for the corrections to the energy gap and the effective mass due to the strain in the silicon nanostructure. A continuum layer approach is introduced to compute the van der Waals forces. The coupling between the mechanical, electrical, and the van der Waals energy domains as well as their numerical implementation is described. Numerical results are presented for several silicon NEM switches to understand the static electromechanical pull-in behavior.
Ab initiomolecular-orbital study of structures and energetics of Si3H3 neutral and anion
Authors: Saitoh Toshiaki , Naoe Toshimasa , Ikuta Shigeru
Published: May 2005

The geometric structures and isomeric stabilities of various stationary points in Si3H3 neutral and its anion are investigated at the coupled-cluster singles, doubles (triples) [CCSD(T)] level of theory. For geometrical surveys, the basis sets used are of the Dunning’s correlation consistent basis sets of triple-zeta quality for the neutral. To the anions, the Dunning’s correlation consistent basis sets of double-zeta quality with diffuse functions are applied. For the three lower-lying anion isomers, the Dunning’s correlation consistent basis sets of triple-zeta quality with diffuse functions (aug-cc-pVTZ) are also used. The final energies for the optimized stationary points are calculated at the CCSD(T) level of theory with the aug-cc-pVTZ basis sets. The basis sets of 6-311++G(3df,2pd) were also used for the lower-lying anion isomers. The Gaussian-2 method was performed only for the lower-lying anion isomers to clarify the relative stabilities. The global minimum neutral 1 (C1:A2) has an unsymmetrical hydrogen-bridged bond; the conformer 2 in Cs symmetry is a saddle point connecting the two equivalent isomers 1. Two lower-lying isomers (3 and 4) are also predicted within the energy range of 20kJ∕mol. In the anion, however, the conformer 4 (Cs:A′1) with five formal valence electrons is a global minimum. Two more isomers (2 and 3) lie within 20kJ∕mol as in the neutral; the conformer 1 converts to the isomer 2. The quartets for the neutrals and diradical triplets for the anions were further studied; lower-lying quartets and triplets, competing with the corresponding doublet and singlet, respectively, were not found in the present systems. The vertical and adiabatic electron affinities of the global minimum neutral 1, producing the second lowest-lying anion isomer 2, amount to 2.18 and 2.35 eV, respectively, at the CCSD(T)/aug-cc-pVTZ level of theory. The electron addition to the third lowest-lying neutral isomer 4 produces the largest vertical electron affinities of 2.48 eV. The D3h structure, being the global minimum in the corresponding Si3H3+ cation (trisilacyclopropenyl cation), converts to the isomer 8 (Cs) or 11 (C2) due to the Jahn–Teller effect in the Si3H3 neutral.
Calculation of the τ dependence of the vibration–internal rotation–overall rotation interactions in CH3OH from molecular structure and molecular dynamics
Authors: Quade C. Richard
Published: May 2005

The Guan and Quade theory for vibration–large-amplitude internal-motion-rotation interactions has been applied to the internal rotation problem in CH3OH. Through the molecular dynamics, the cos3τ and sin3τ dependence of the torsional–rotational coefficients in the effective Hamiltonian have been calculated from molecular structure. The internal rotation coordinate τ′ for the vibrationally distorted molecule is shown to have the necessary threefold symmetry for all values of τ′. For the methyl deformation modes, the vibrational dependence of the internal rotation potential energy is shown to have a threefold symmetry. The St and StSt dependence of the inertia tensor and Coriolis coupling coefficients has been developed in terms of curvilinear internal coordinates. The T transformation separating rotation from vibrations in zeroth order is then applied, the kinetic-energy tensor inverted to momentum space, and finally the effective torsion–rotation coefficients are calculated by Van Vleck perturbation theory. When compared to the empirical results, the kinetic-energy contributions to the cos3τ and sin3τ dependence of the coefficients are as follows: 54% of Pa2 is accounted for, 28% of PaPb, 16% of PaPc, and 91% of the asymmetry. The calculation is inadequate to account for the Pb2,Pc2, and PbPc coefficients, ranging from factors of 20–70, even with the incorrect sign for some of the terms. Anharmonic force contributions from the vibrations have not been used in the calculation since these forces are not known at this time.
Ionization energy measurements and electronic spectra for ThO
Authors: Goncharov Vasiliy , Han Jiande , Kaledin Leonid A. , Heaven Michael C.
Published: May 2005

The ionization energy (IE) for ThO has been determined using photoionization efficiency and mass-analyzed threshold ionization measurements. An IE of 6.6038(12) eV was obtained, which was appreciably higher than the result from previous appearance potential measurements [6.1(1) eV]. The revised IE is 0.3 eV greater than that of atomic Th, indicating that neutral ThO is more tightly bound than ThO+. The one-color two-photon resonant ionization spectrum of ThO was examined in the range of 315–370 nm. Rotationally resolved bands were recorded for three new electronic states (designated as E′,F′, and G′). In addition, transitions to the A′(v=1,2,3) levels and the N(v=2) level were observed for the first time. Ligand field theory predictions [L. A. Kaledin, J. E. McCord, and M. C. Heaven, J. Mol. Spectrosc. 164, 27 (1994)] were used to propose configurational assignments for 20 electronically excited states.
Thickness dependence of properties of excimer laser crystallized nano-polycrystalline silicon
Authors: Adikaari A. A. D. T. , Silva S. R. P.
Published: May 2005

Excimer laser crystallization is used to produce layered nanocrystalline silicon from hydrogenated amorphous silicon, using a partial melting process. Three types of hydrogenated amorphous silicon samples, 100, 300, and 500 nm thick, were laser treated in order to investigate the changes to the structural, optical, and electrical properties as a function of amorphous silicon thickness with excimer laser crystallization. The resulting nanocrystalline thin films were characterized using Raman spectroscopy, optical absorption measurements, atomic force microscopy, forward recoil spectrometry, and current–voltage measurements. The relationship of crystalline volume and laser energy density was established, along with the behavior of the optical gap and its relationship to hydrogen content. Surface roughness effects are discussed in the context of photovoltaic applications. The effect of increased mobility on photoconductivity after excimer laser crystallization is also examined.
The calculation of excitation energies based on the relativistic two-component zeroth-order regular approximation and time-dependent density-functional with full use of symmetry
Authors: Wang Fan , Ziegler Tom , van Lenthe Erik , van Gisbergen Stan , Baerends Evert Jan
Published: May 2005

In the present work, we propose a relativistic time-dependent density-functional theory (TDDFT) based on the two-component zeroth-order regular approximation and a noncollinear exchange-correlation (XC) functional. This two-component TDDFT formalism has the correct nonrelativistic limit and affords the correct threefold degeneracy of triplet excitations. The relativistic TDDFT formalism is implemented into the AMSTERDAM DENSITY FUNCTIONAL program package for closed-shell systems with full use of double-group symmetry to reduce the computational effort and facilitate the assignments. The performance of the formalism is tested on some closed-shell atoms, ions, and a few diatomic molecules containing heavy elements. The results show that the fine structure of the excited states for most atoms and ions studied here can be accurately accounted for with a proper XC potential. For the excitation energies of the molecules studied here, the present formalism shows promise and the error encountered is comparable to that of nonrelativistic TDDFT calculations on light elements.
Ab initio multireference configuration-interaction theoretical study on the low-lying spin states in binuclear transition-metal complex: Magnetic exchange of [(NH3)5Cr(μ-OH)Cr(NH3)5]5+ and [Cl3FeOFeCl3]2−
Authors: Wang Bingwu , Wei Haiyan , Wang Mingwei , Chen Zhida
Published: May 2005

The magnetic exchange interaction behavior and energy spectrum of low-lying spin states are investigated by using ab initio multireference configuration-interaction method for the representative binuclear transition-metal complexes [(NH3)5Cr(μ-OH)Cr(NH3)5]5+ and [Cl3FeOFeCl3]2−. Our calculations for the nonmodeling real title complexes found that under the appropriate basis sets and active space, ab initio method at multireference configuration-interaction level of theory is able to give accurate energy spectrum of low-lying spin states within reachable computation demand nowadays and the deviation of magnetic exchange interaction to Lande interval rule can be described by the biquadratic correction in terms of Heisenberg spin Hamiltonian. As a methodology comparison, density-functional theory combined with broken-symmetry approach provides an alternative yet efficient approach to produce accurate numerical results, but there are dependences on the particular chosen exchange-correlation functionals and system dependent. The spin population analyses at complete active space self-consistent-field level of the theory provide an instructively understanding and prediction for the magnetic interaction mechanism.
Ab initio potential-energy surface for the reaction Ca+HCl→CaCl+H
Authors: Verbockhaven Gilles , Sanz Cristina , Groenenboom Gerrit C. , Roncero Octavio , van der Avoird Ad
Published: May 2005

The potential-energy surface of the ground electronic state of CaHCl has been obtained from 6400 ab initio points calculated at the multireference configuration-interaction level and represented by a global analytical fit. The Ca+HCl→CaCl+H reaction is endothermic by 5100cm−1 with a barrier of 4470cm−1 at bent geometry, taking the zero energy in the Ca+HCl asymptote. On both sides of this barrier are potential wells at linear geometries, a shallow one due to van der Waals interactions in the entrance channel, and a deep one attributed to the H−Ca++Cl− ionic configuration. The accuracy of the van der Waals well depth, ≈200cm−1, was checked by means of additional calculations at the coupled-cluster singles and doubles with perturbative triples level and it was concluded that previous empirical estimates are unrealistic. Also, the electric dipole function was calculated, analytically fitted in the regions of the two wells, and used to analyze the charge shifts along the reaction path. In the insertion well, 16800cm−1 deep, the electric dipole function confirmed the ionic structure of the HCaCl complex and served to estimate effective atomic charges. Finally, bound rovibrational levels were computed both in the van der Waals well and in the insertion well, and the infrared-absorption spectrum of the insertion complex was simulated in order to facilitate its detection.
Growth and field emission properties of vertically aligned carbon nanofibers
Authors: Poa C. H. P. , Henley S. J. , Chen G. Y. , Adikaari A. A. D. T. , Giusca C. E. , Silva S. R. P.
Published: May 2005

Vertically aligned carbon nanofibers (VACNFs) were synthesized on Ni-coated Si substrates using a dc plasma-enhanced chemical-vapor deposition system. The size of the Ni islands used as catalyst to grow the VACNFs was formed by both thermal annealing and laser processing on thin metal layers. It was observed that the diameter of the carbon nanofibers is strongly dependent on the initial Ni island dimension. By varying the laser power from 228 to 279mJ∕cm2, the size of these Ni islands can be controlled independent of the initial Ni film thickness. Electron field-emission results show that the emission threshold field is dependent on both the height and radius of these VACNFs and also field shielding effects. Threshold fields as low as 2V∕μm was obtained from the sample with the largest height over radius ratio.
Novel method for detecting weak magnetic fields at low frequencies
Authors: González-Martínez S. , Castillo-Torres J. , Mendoza-Santos J. C. , Zamorano-Ulloa R.
Published: May 2005

A low-level-intensity magnetic field detection system has been designed and developed based on the amplification-selection process of signals. This configuration is also very sensitive to magnetic field changes produced by harmonic-like electrical currents transported in finite-length wires. Experimental and theoretical results of magnetic fields detection as low as 10−9T at 120Hz are also presented with an accuracy of around 13%. The assembled equipment is designed to measure an electromotive force induced in a free-magnetic-core coil in order to recover signals which are previously selected, despite the fact that their intensities are much lower than the environment electromagnetic radiation. The prototype has a signal-to-noise ratio of 60dB. This system also presents the advantage for using it as a portable unit of measurement. The concept and prototype may be applied, for example, as a nondestructive method to analyze any corrosion formation in metallic oil pipelines which are subjected to cathodic protection.
Experimental and theoretical investigations of the reactions NH(XΣ−3)+D(S2)→ND(XΣ−3)+H(S2) and NH(XΣ−3)+D(S2)→N(S4)+HD(XΣg+1)
Authors: Qu Z.-W. , Zhu H. , Schinke R. , Adam L. , Hack W.
Published: May 2005

The rate coefficient of the reaction NH(XΣ−3)+D(S2)→k1products (1) is determined in a quasistatic laser-flash photolysis, laser-induced fluorescence system at low pressures. The NH(X) radicals are produced by quenching of NH(aΔ1) (obtained in the photolysis of HN3) with Xe and the D atoms are generated in a D2/He microwave discharge. The NH(X) concentration profile is measured in the presence of a large excess of D atoms. The room-temperature rate coefficient is determined to be k1=(3.9±1.5)×1013cm3mol−1s−1. The rate coefficient k1 is the sum of the two rate coefficients, k1a and k1b, which correspond to the reactions NH(XΣ−3)+D(S2)→k1aND(XΣ−3)+H(S2) (1a) and NH(XΣ−3)+D(S2)→k1bN(S4)+HD(XΣg+1) (1b), respectively. The first reaction proceeds via the A″2 ground state of NH2 whereas the second one proceeds in the A″4 state. A global potential energy surface is constructed for the A″2 state using the internally contracted multireference configuration interaction method and the augmented correlation consistent polarized valence quadrupte zeta atomic basis. This potential energy surface is used in classical trajectory calculations to determine k1a. Similar trajectory calculations are performed for reaction (1b) employing a previously calculated potential for the A″4 state. The calculated room-temperature rate coefficient is k1=4.1×1013cm3mol−1s−1 with k1a=4.0×1013cm3mol−1s−1 and k1b=9.1×1011cm3mol−1s−1. The theoretically determined k1 shows a very weak positive temperature dependence in the range 250⩽T∕K⩽1000. Despite the deep potential well, the exchange reaction on the A″2 ground-state potential energy surface is not statistical.
Formation of Liesegang patterns in the presence of an electric field
Authors: Bena I. , Droz M. , Rácz Z.
Published: May 2005

The effects of an external electric field on the formation of Liesegang patterns are investigated. The patterns are assumed to emerge from a phase separation process in the wake of a diffusive reaction front. The dynamics is described by a Cahn–Hilliard equation with a moving source term representing the reaction zone, and the electric field enters through its effects on the properties of the reaction zone. We employ our previous results [I. Bena, F. Coppex, M. Droz, and Z. Rácz, J. Chem. Phys. 122, 024512 (2005)] on how the electric field changes both the motion of the front, as well as the amount of reaction product left behind the front, and our main conclusion is that the number of precipitation bands becomes finite in a finite electric field. The reason for the finiteness in case when the electric field drives the reagents towards the reaction zone is that the width of consecutive bands increases so that, beyond a distance ℓ+, the precipitation is continuous (plug is formed). In case of an electric field of opposite polarity, the bands emerge in a finite interval ℓ−, since the reaction product decreases with time and the conditions for phase separation cease to exist. We give estimates of ℓ± in terms of measurable quantities and thus present an experimentally verifiable prediction of the “Cahn–Hilliard equation with a moving source” description of Liesegang phenomena.
Vibrational spectroscopy study of Ar+-ion irradiated Si-rich oxide films grown by plasma-enhanced chemical vapor deposition
Authors: Mariotto G. , Das G. , Quaranta A. , Della Mea G. , Corni F. , Tonini R.
Published: May 2005

SiO x thin films with different stoichiometry degree were obtained by plasma-enhanced chemical vapor deposition on crystalline silicon substrates from SiH4 and N2O gas mixtures. Two twin sets of samples were irradiated by 380 keV Ar+ ions at a fluence of 5×1016ions∕cm2 at room temperature and at 500 °C, respectively, and then annealed in vacuum at different temperatures, between 500 and 1100 °C. A set of unirradiated samples has been annealed in the same conditions in order to discriminate the contribution of ion irradiation and of thermal treatments to the changes of the film microstructure. The structural modification of the oxide network and the growth of Si nanoclusters have been studied by vibrational spectroscopy techniques. Fourier transform infrared absorption spectra evidenced that ion irradiation induces a hydrogen loss of about 50%, and that postirradiation thermal treatments lead to the recovery of the irradiation defects and to the out diffusion of the residual hydrogen. After heating at 800 °C, irradiated and unirradiated samples exhibit substantially the same structure from the point of view of infrared-absorption spectra. In the meanwhile, the Si–O–Si stretching peak blue shifts, but never reaches the wavenumber value of pure silica owing to the presence of nitrogen into the network. Raman spectra of as-irradiated films reveal the presence of an amorphous silicon phase within the damaged layer of the oxide matrix. Raman spectra of irradiated samples undergoing thermal treatments at high temperature indicate a rearrangement of the film microstructure with the progressive clustering of the amorphous silicon phase. However, no clear spectroscopic evidence is gained about the crystallization of silicon nanoclusters, even after annealing at the highest temperature. In fact, the Raman scattering from silicon nanocrystals is partially hidden by the Raman peak of the c-Si substrate.
e-beam-referenced work-function evaluation in an x-ray photoelectron spectrometer
Authors: Cohen H. , Nogues C. , Zon I. , Lubomirsky I.
Published: May 2005

A method for work-function evaluation is proposed, based on recording the shift of x-ray photoelectron signals from a surface irradiated by low-energy electrons. The method is capable of measuring samples with very low conductivity, poor back contacts, and high dielectric constants. The method is also applicable to magnetic materials and can be particularly effective for studies of multilayer and heterogeneous systems.
Optical detection of chemical warfare agents and toxic industrial chemicals: Simulation
Authors: Webber Michael E. , Pushkarsky Michael , Patel C. Kumar N.
Published: May 2005

We present an analysis of optical techniques for the detection of chemical warfare agents and toxic industrial chemicals in real-world conditions. We analyze the problem of detecting a target species in the presence of a multitude of interferences that are often stochastic and we provide a broadly applicable technique for evaluating the sensitivity, probability of false positives (PFP), and probability of false negatives (PFN) for a sensor through the illustrative example of a laser photoacoustic spectrometer (L-PAS). This methodology includes (1) a model of real-world air composition, (2) an analytical model of an actual field-deployed L-PAS, (3) stochasticity in instrument response and air composition, (4) repeated detection calculations to obtain statistics and receiver operating characteristic curves, and (5) analyzing these statistics to determine the sensor’s sensitivity, PFP, and PFN. This methodology was used to analyze variations in sensor design and ambient conditions, and can be utilized as a framework for comparing different sensors.
Theoretical characterization of structures and energies of benzene–(H2S)n and (H2S)n (n=1–4) clusters
Authors: Hermida-Ramón Jose M. , Cabaleiro-Lago Enrique M. , Rodríguez-Otero Jesús
Published: May 2005

An ab initio study was performed in clusters up to four H2S molecules and benzene using calculations at MP2∕6-31+G* and MP2/aug-cc-pVDZ levels. Differences between both sets of calculations show the importance of using large basis sets to describe the intermolecular interactions in this system. The obtained binding energies reflect that benzene has not the same behavior in H2S as in water, pointing to a higher solubility of this molecule in H2S than in water. The Bz-cluster binding energy was fitted to an asymptotic representation with a maximum value of the energy of −8.00 kcal/mol that converges in a cluster with 12 H2S molecules. The obtained intermolecular distance in the Bz–H2S dimer is similar to the experimental value; however, the difference is much larger for the angles defining the orientation. The influence of benzene produces a distortion of the (H2S)n clusters, so the intermolecular distances change with regard to the (H2S)n isolated clusters. Frequency shifts are larger in clusters with benzene than without it. In the smallest clusters the shift associated to the stretching of the S–H bonded to benzene is the largest one, but for the cluster with three H2S molecules this stretching is combined with the other S–H stretching of the molecule so the resulting shift is not the largest one.
Predicting equilibrium structures in freezing processes
Authors: Gottwald Dieter , Kahl Gerhard , Likos Christos N.
Published: May 2005

We propose genetic algorithms as a new tool that is able to predict all possible solid candidate structures into which a simple fluid can freeze. In contrast to the conventional approach where the equilibrium structures of the solid phases are chosen from a preselected set of candidates, genetic algorithms perform a parameter-free, unbiased, and unrestricted search in the entire search space, i.e., among all possible candidate structures. We apply the algorithm to recalculate the zero-temperature phase diagrams of neutral star polymers and of charged microgels over a large density range. The power of genetic algorithms and their advantages over conventional approaches is demonstrated by the fact that new and unexpected equilibrium structures for the solid phases are discovered. Improvements of the algorithm that lead to a more rapid convergence are proposed and the role of various parameters of the method is critically assessed.
Kinetics of fluorescence quenching for electron transfer and for energy transfer: Molecular dynamics tests for spherical molecules
Authors: Litniewski Marek , Gorecki Jerzy
Published: May 2005

The Smoluchowski approach to description of fluorescence quenching is tested by comparing the theory with computer simulations for the case of spherical molecules. The distance dependent sink terms describing the electron transfer mechanism and the Forster model for the energy transfer are considered. It is shown that the agreement between the rate coefficient from the model and from simulations depends on the strength of the solute-solvent interactions as well as on the speed of reaction itself. Comparing results of simulations for different quencher concentrations we estimate the strength of quencher concentration dependence effect and the range of times the effect may be significant. In the long time limit the increase in quencher concentration decreased the rate coefficient.
Low temperature pressure broadening of OCS by He
Authors: Ross Kenneth A. , Willey Daniel R.
Published: May 2005

We report experimentally measured cross sections for pressure broadening of OCS by He from 4.2 to 23 K. These measurements were made in a quasiequilibrium cell using the collisional cooling technique. Cross sections were obtained for the broadening of the J=2←1, J=3←2, and J=4←3 rotational transitions of OCS. Theoretical cross sections were also calculated using a recent He–OCS potential surface. While at the higher temperatures, ∼20K, there is only modest disagreement between experiment and theory, this disagreement increases steadily with decreasing temperature, with predicted cross sections rising steeply while the experimental data remains constant or decreases slightly. Comparisons to similar recent experimental studies are made and reasons for the observed discrepancy between experiment and theory are considered.
Comparison of the device physics principles of planar and radial p-n junction nanorod solar cells
Authors: Kayes Brendan M. , Atwater Harry A. , Lewis Nathan S.
Published: May 2005

A device physics model has been developed for radial p-n junction nanorod solar cells, in which densely packed nanorods, each having a p-n junction in the radial direction, are oriented with the rod axis parallel to the incident light direction. High-aspect-ratio (length/diameter) nanorods allow the use of a sufficient thickness of material to obtain good optical absorption while simultaneously providing short collection lengths for excited carriers in a direction normal to the light absorption. The short collection lengths facilitate the efficient collection of photogenerated carriers in materials with low minority-carrier diffusion lengths. The modeling indicates that the design of the radial p-n junction nanorod device should provide large improvements in efficiency relative to a conventional planar geometry p-n junction solar cell, provided that two conditions are satisfied: (1) In a planar solar cell made from the same absorber material, the diffusion length of minority carriers must be too low to allow for extraction of most of the light-generated carriers in the absorber thickness needed to obtain full light absorption. (2) The rate of carrier recombination in the depletion region must not be too large (for silicon this means that the carrier lifetimes in the depletion region must be longer than ∼10ns). If only condition (1) is satisfied, the modeling indicates that the radial cell design will offer only modest improvements in efficiency relative to a conventional planar cell design. Application to Si and GaAs nanorod solar cells is also discussed in detail.
Chemisorption and diffusion of hydrogen on surface and subsurface sites of flat and stepped nickel surfaces
Authors: Bhatia Bhawna , Sholl David S.
Published: May 2005

Plane-wave density functional theory calculations were performed to investigate the binding and diffusion of hydrogen on three flat Ni surfaces, Ni(100), Ni(110), and Ni(111), and two stepped Ni surfaces, Ni(210) and Ni(531). On each surface, the favored adsorption sites were identified by considering the energy and stability of various binding sites and zero-point energy corrections were computed. Binding energies are compared with experimental and theoretical results from the literature. Good agreement with experimental and previous theoretical data is found. At surface coverages where adsorbate-adsorbate interactions are relatively weak, the binding energy of H is similar on the five Ni surfaces studied. Favorable binding energies are observed for stable surface sites, while subsurface sites have unfavorable values relative to the gas phase molecular hydrogen. Minimum energy paths for hydrogen diffusion on Ni surfaces and into subsurface sites were constructed.
Molecular sieve valves driven by adsorbate-adsorbate interactions: Hysteresis in permeation of microporous membranes
Authors: Snyder M. A. , Vlachos D. G.
Published: May 2005

A recently derived mesoscopic framework describing activated micropore diffusion is employed to explore system criticality in microporous membranes under nonequilibrium conditions. Rapid exploration of parameter space, possible with this continuum framework, elucidates a novel temperature-induced ignition and extinction of the molecular flux under a macroscopic gradient in pressure (chemical potential). Deviation from equilibrium like phase behavior (i.e., shifting and narrowing of phase envelopes and double hysteresis) derives from asymmetry of the coupled boundaries of the nonequilibrium membrane. We confirm this new phase behavior, akin to “opening” and “closing” of a molecular valve, via gradient kinetic Monte Carlo simulations of thin one-dimensional and three-dimensional systems. The heat of adsorption, strength of adsorbate-adsorbate intermolecular forces, and chemical potential gradient are all shown to control ‘valve’ actuation, suggesting potential implications in chemical sensing and novel diffusion control.
Skeletal silica characterization in porous-silica low-dielectric-constant films by infrared spectroscopic ellipsometry
Authors: Takada Syozo , Hata Nobuhiro , Seino Yutaka , Fujii Nobutoshi , Kikkawa Takamaro
Published: May 2005

Porous-silica low-dielectric-constant (low-k) films were prepared using a sol-gel method based on the self-assembly of surfactant templates. No change in the refractive index at 633 nm nor in the infrared-absorption intensities of C–H and O–H stretching vibrations at around 2900 and 3400cm−1 of porous-silica low-k films were observed after annealing at each temperature from 523 to 723 K. On the other hand, the Young’s elastic modulus and hardness increased with the increase of annealing temperature. The structure in the complex dielectric function of porous-silica low-k films observed in between 1000 and 1400cm−1 is assigned as the asymmetric stretching vibration mode of the Si–O–Si bond. By applying the effective-medium theory by Bruggeman to the experimental results from infrared spectroscopic ellipsometry, we analyzed the skeletal silica structures. The peak positions of transverse (ωTO) and longitudinal (ωLO) vibration modes for Si–O–Si network in the silica skeleton of porous-silica films changed from 1061 to 1068cm−1 and from 1219 to 1232cm−1, respectively, with the annealing temperature. It is shown that the ωLO2∕ωTO2 of skeletal silica correlates with Young’s elastic modulus of porous-silica low-k films.
Improved importance sampling distribution for rate constant calculation
Authors: Mella Massimo
Published: May 2005

An efficient method to compute the thermal rate constant for rare events within the correlation function C(t) approach is presented. This method, which is based on a modification of the sampling function used to evaluate the dynamical correlation function C(t), can be applied to high-dimensional systems having a rough energy landscape without previous knowledge on the transition states location. In this work, the sampling of a Boltzmann-like distribution for the linear momenta with a lower inverse temperature (β*=1∕kT*) than the correct one (β=1∕kT) is proposed as a way to increase the number of reactive trajectories. The mismatch between the β* and β distributions is then corrected by a reweighting procedure which allows one to obtain the exact correlation function C(t). The efficiency of this scheme in computing the rate of a particle jumping across the barrier of a simple 2D double well potential is improved by a factor 4–25 depending on the relative value of the original β and modified β* temperatures. When merged with the “puddle potential” method [S. A. Corcelli, J. A. Rohman, and J. C. Tully, J. Chem. Phys., 118, 1085 (2003)], the new importance sampling function improves the efficiency of the correlation function approach by a factor 16–800 with respect to the unbiased sampling. To test the method in a more challenging case, the previous model system was extended by adding six harmonically restrained particles, each one interacting with the diffusing particle. This model introduces both the possibility of energy exchange and a rougher energy landscape. The new sampling function alone is found to produce an improvement in efficiency of, at least, an order of magnitude when compared with the unbiased case; when merged with the puddle potential method, a 400-fold saving in computer time is found.
An electrode probe for high-throughput screening of electrochemical libraries
Authors: Jiang Rongzhong , Chu Deryn
Published: May 2005

A pen-shaped O2 electrode probe is designed for high-throughput screening of electrochemical libraries. The electrode probe consists of a large-area O2 electrode and a cylindrical electrolyte sponge with a short cone tip for screening. This type of design can easily minimize the probe resistance contributed by the electrolyte. A zinc electrode library is generated using a nonautomated method to deposit metal zinc on a graphite plate. The zinc electrode library and the O2-electrode probe form an electrochemical library containing 128 micro zinc/air batteries. High-throughput screening of the zinc/air batteries are carried out by moving the tip of the electrode probe under constant potential (1.0V) and measuring the current. A Gaussian distribution is used for statistical analysis of the experimental data. These data obtained with the combinatorial method have a relative standard deviation of 8.9% based on a nonautomated coating procedure. The O2 electrode probe is used to study the effect of addition of Cu in the anode on the performance of the zinc/air battery.
Operational distance and fidelity for quantum channels
Authors: Belavkin Viacheslav P. , D’Ariano Giacomo Mauro , Raginsky Maxim
Published: May 2005

We define and study a fidelity criterion for quantum channels, which we term the minimax fidelity, through a noncommutative generalization of maximal Hellinger distance between two positive kernels in classical probability theory. Like other known fidelities for quantum channels, the minimax fidelity is well defined for channels between finite-dimensional algebras, but it also applies to a certain class of channels between infinite-dimensional algebras (explicitly, those channels that possess an operator-valued Radon-Nikodym density with respect to the trace in the sense of Belavkin-Staszewski) and induces a metric on the set of quantum channels that is topologically equivalent to the CB-norm distance between channels, precisely in the same way as the Bures metric on the density operators associated with statistical states of quantum-mechanical systems, derived from the well-known fidelity (“generalized transition probability”) of Uhlmann, is topologically equivalent to the trace-norm distance.
Dielectric characterization in a broad frequency and temperature range of SrBi2Nb2O9 thin films grown on Pt electrodes
Authors: Guilloux-Viry M. , Duclère J. R. , Rousseau A. , Perrin A. , Fasquelle D. , Carru J. C. , Cattan E. , Soyer C. , Rèmiens D.
Published: May 2005

Ferroelectric and dielectric characteristics of two types of SrBi2Nb2O9 thin films grown by pulsed laser deposition on Pt electrodes were determined: a randomly oriented SrBi2Nb2O9 film (sample A), grown on a so-called polycrystalline Pt∕Ti∕SiO2∕Si (Pt∕Si) and an epitaxial SrBi2Nb2O9 film (sample B) grown on a (110)Pt electrode epitaxially grown on (110)SrTiO3. Some h00 texturation was suggested by the strong intensity of the 200 diffraction peak in sample A whereas the epitaxial sample B presents a strongly (116) preferential orientation as evidenced by x-ray diffraction and electron channeling patterns. Remanent polarization (Pr) determined from the hysteresis loops recorded on several electrodes on samples A and B was typically in the range of 5–8.5μC∕cm2 for both samples. The Curie temperature (TC) determined from dielectric permittivity measurements versus temperature was 705K, for both samples, which differ, in fact, in structural characteristics but not in composition. The Curie–Weiss temperature was found lower than TC, suggesting that the transition is of first order. The evolution of dielectric permittivity and dissipation factor was recorded versus frequency at different temperatures. At room temperature and low frequency (100Hz), ε′ is close to 150 on both films. When increasing the frequency up to 1MHz, the ε′ value decreased down to 137 for sample A and 129 for sample B. This evolution corresponds to a weak dispersion effect in relation with the high crystallization quality of the ferroelectric films. At the same time, dissipation factor tgδ increased from 1.5% to 1.8% and from 3.3% to 5.5% for samples A and B, respectively, when increasing the frequency from 100Hzto1MHz. Further measurements at higher frequency were performed on the epitaxial film. As expected, dielectric permittivity decreased to about 100 when increasing the frequency to 0.5GHz at room temperature, whereas tgδ increased up to about 9%. Decreasing the temperature down to 110K contributes to decrease ε′ from 150 at room temperature to 122 at 110K at 1kHz and from 100 to 87 at 0.5GHz. Simultaneously, tgδ decreases from 8.7% to 5.6% at 0.5GHz.
Anomaly of gas drag force on liquid droplets in a turbulent two-phase flow produced by a mechanical jet sprayer at intermediate Reynolds numbers
Authors: Simakov Nikolai N. , Simakov Andrei N.
Published: May 2005

Hydrodynamic properties of a turbulent two-phase flow (water droplets in the air) produced by a mechanical jet sprayer were studied experimentally and the drag force on the droplets was found to be anomalously weak; the drag coefficient is four to seven times smaller than predicted by standard expressions. Several hypotheses are suggested in this article and discussed in an attempt to explain the phenomenon. A two-dimensional model of the flow with the drag anomaly is proposed, which provides excellent agreement between the numerical and experimental results. Taking the drag anomaly into account allows an explanation of a number of features of the two-phase flow that are hard to explain otherwise, in particular, the fact that only about one-half of the initial water jet momentum was transferred to the gas during the experiment.
Investigation of surface acoustic wave fields in silicon crystals by x-ray diffraction: A dynamical theory approach
Authors: Tucoulou R. , Mathon O. , Ferrero C. , Mocella V. , Roshchupkin D. V. , Kumon R. E.
Published: May 2005

X-ray diffraction spectra at different x-ray energies from a Si crystal subjected to a deformation produced by surface acoustic wave propagation have been modeled using the general framework of dynamical diffraction theory. The simulations have been successfully compared with the corresponding experimental results confirming the accuracy of the elastic model describing the acoustic wave fields inside the crystal.
Design, fabrication, and application of a massively parallel single-bead microreactor system for high-throughput experimentation in heterogeneous catalysis
Authors: Zech Torsten , Bohner Gunilla , Laus Oliver , Klein Jens , Fischer Michael
Published: May 2005

We present a unique approach to high-throughput catalyst testing that is based on using individual beads in a massively parallel arrangement of microreaction chambers on a chip and describe its excellent scalability to very high numbers of parallel reaction chambers that have not been realized before in continuous flow applications. The reactor design benefits to a large extent from the principle of using individual, independent flow restrictors for equi-distribution of the fluid flow to the parallel reaction chambers. As a model reaction for the comparison of results obtained with a 384-parallel and a 625-parallel single-bead reactor, the selective CO oxidation in hydrogen is used. The same relative catalyst performances and catalytic trends can be measured in both reactor systems.
A parallel high-throughput approach to liquid crystal screening
Authors: Cull Tobias , Goulding Mark , Bradley Mark
Published: May 2005

A technique has been developed for the high-throughput (HT) parallel screening of liquid crystal (LC) phase transitions, using optical anisotropic effects observed only in the LC state to allow the determination of melting and clearing points. A camera was used to monitor optical changes observed in arrays of LC samples upon heating, when illuminated with plane polarized light and viewed through a second polarizer aligned perpendicular to the plane of polarized light. Data was processed using Image Pro Plus, extracting values for the melting point and clearing point of each sample in the array. When compared with literature and conventional differential scanning calorimetry the new technique was found to be in excellent agreement, establishing the technique as a viable HT alternative. The method was used to analyze a 75 formulation library prepared using liquid handling for HT clearing point determination, showing the scope and power of the technique in the analysis of larger libraries.
Development of a spatially controllable chemical vapor deposition reactor with combinatorial processing capabilities
Authors: Choo J. O. , Adomaitis R. A. , Henn-Lecordier L. , Cai Y. , Rubloff G. W.
Published: May 2005

Most conventional chemical vapor deposition (CVD) systems do not have the spatial actuation and sensing capabilities necessary to control deposition uniformity or to intentionally induce nonuniform deposition patterns for single-wafer combinatorial CVD experiments. In an effort to address these limitations, a novel CVD reactor system has been developed that can explicitly control the spatial profile of gas-phase chemical composition across the wafer surface. This paper discusses the construction of a prototype reactor system featuring a three-zone, segmented showerhead design. Experiments are performed to assess the ability of this reactor system to deposit tungsten films by the hydrogen reduction process; segment-to-segment process recipes are controlled to deposit spatially nonuniform W films. The capabilities of this reactor system for materials discovery research are discussed.