bluuebunny/crossref_metadata_2025 huggingface dataset:
Dielectric relaxation and underlying dynamics of acetonitrile and 1-ethyl-3-methylimidazolium triflate mixtures using THz transmission spectroscopy
Authors: Asaki M. L. T. , Redondo A. , Zawodzinski T. A. , Taylor A. J.
Published: July 2002

We use terahertz (THz) transmission spectroscopy to obtain the frequency dependent complex dielectric functions for pure acetonitrile, pure 1-ethyl-3-methylimidazolium triflate (emim triflate, a room temperature molten salt), and mixtures of the two liquids. The behavior of the pure liquids is modeled with either two (acetonitrile) or three (emim triflate) Debye relaxations. We then discuss the interactions of the molten salt and solvent based on the modified Debye relaxations evident in the mixtures. We determine that at low molten salt concentrations, the mixtures behave like electrolyte solutions of a crystalline salt dissolved in a solvent. At higher molten salt concentrations, the behavior is that of a mixture of two liquids.
Electrostatic fluctuations in plasmas with distribution functions described by simple pole expansions
Authors: Gunell H. , Skiff F.
Published: July 2002

The spectral densities of electrostatic fluctuations in plasmas where both the ion and electron distribution are described by simple pole expansions are calculated, for both one-dimensional and isotropic three-dimensional plasmas. Examples of electric field and density fluctuations are presented.
Space-time geometries characterized by solutions to the geodesic equations
Authors: Andrew Keith , Fleming Charles G.
Published: July 2002

A novel method for calculating and visualizing the geodesic structure of space-time models is presented. By utilizing the symbolic computational power of Mathematica on a NeXT, and an IBM RS 6000 workstation, the geodesic equations are found analytically for any given metric. Once spliced into a fortran program, the geodesic equations are solved numerically, on a CRAY Y-MP supercomputer, for a given bundle of null geodesics. Here, the null geodesic structure of three singular space-time geometries is examined: the Schwarzchild, Kerr, and Winicour space-times. Using Mathematica software, the numerical data for the geodesic paths is displayed graphically, providing a picture of a given spacetime volume for each set of initial conditions. The parameter dependence of space-time, due to a particular metric, can be observed by sequencing through various parameters, such as the mass and spin. These pictures can then be composed into a video tape which displays the range of behavior as the parameters are varied.
The quantum mechanics of a classically chaotic dissipative system
Authors: Savage C. M.
Published: July 2002

The quantum mechanical model of a dissipative system that is chaotic in its classical limit is solved numerically. The system is nonlinear optical second harmonic generation, and the model is the quantum optical master equation. The steady-state of the master equation is found and compared with the classical chaotic attractor. Computational methods appropriate for vector processors and for the connection machine are described.
Nonlinear piezoelectricity and electrostriction of alpha quartz
Authors: Kittinger Erwin , Tichý Jan , Friedel Wolfgang
Published: July 2002

Starting from the rotationally invariant nonlinear equations of electroelasticity the initial derivative of the propagation velocity of elastic waves with respect to the magnitude of an electric biasing field is derived. This derivative is given by an expression which is linear in third-order material constants. From propagation measurements of ultrasonic waves under the application of a constant electric field, values for the eight independent components of the nonlinear piezoelectric tensor eKABCD and for the eight independent components of the electrostriction tensor lKLAB for alpha quartz are obtained. While some of the electrostriction constants remain rather uncertain, the nonlinear piezoelectric constants are determined with good accuracy.
Microwave properties of YBa2Cu3O7−x thin films studied with coplanar transmission line resonators
Authors: Rauch W. , Gornik E. , Sölkner G. , Valenzuela A. A. , Fox F. , Behner H.
Published: July 2002

The microwave properties of epitaxial YBa2Cu3O7−x thin films with critical temperatures Tc up to 91.5 K and critical current densities Jc up to 6×106 A/cm2 at 77 K have been investigated between 5 and 20 GHz using coplanar transmission line resonators. Information about the surface resistance Rs and the magnetic penetration depth λL has been obtained from temperature- and power-dependent measurements of the quality factors and resonance frequencies of the resonators. The lowest surface resistance was 105 μΩ at 77 K and 38 μΩ at 9 K measured at 6.2 GHz, respectively. The dependence of Rs on the surface magnetic field Brf showed that with decreasing film quality the films exhibit a granular behavior. At high critical current densities Jc≳4×106 A/cm2, the intrinsic properties of the material show up. The magnetic penetration depth was in the range between 160 and 270 nm. Both Rs and λL turned out to be sensitive to the film quality represented by the transition width ΔTc and the critical current density Jc at 77 K.
Enhanced optical properties of metal-coated nanoparticles
Authors: Haus Joseph W. , Zhou H. S. , Takami S. , Hirasawa M. , Honma I. , Komiyama H.
Published: July 2002

Metal-coated, nanometer-size particles are modeled with a realistic distribution of coating thicknesses. General expressions are given for the local-field enhancement, absorption, and nonlinear optical response. In addition, heuristic arguments are used to determine the effects of a diffuse, rather than a sharp interface. The linear and nonlinear optical properties are discussed within the context of the effective medium theory for small volume fractions. An efficient method of solution is used with the flexibility to handle an arbitrary number of coatings.
Quantitative thermomagnetic analysis of CoCr films and experimental determination of the CoCr phase diagram
Authors: Snyder J. E. , Kryder M. H.
Published: July 2002

We have experimentally determined the CoCr phase diagram in a region of great interest for magnetic recording media, by quantitative thermomagnetic analyses (TMA) of CoCr films. We used TMA results, together with CoCr bulk data, film-average measurements of composition and room temperature Ms, and a novel curve-fitting and analysis technique. Our TMA experiments suggested a model in which magnetic (exchange) energy causes the phase separation. There are four different phase diagrams for CoCr in the literature. Our experimentally determined phase diagram shows good qualitative agreement with the theoretical Hasebe phase diagram [J. Jpn. Inst. Met. 46, 577 (1982) (in Japanese)], the only one whose calculation included magnetic (exchange) energy terms. We have also estimated the relative amounts of the different phases present in the films, and found about 40 to 55 vol % to be nonmagnetic at room temperature. For such material, the magnetostatic contributions to magnetic anisotropy could be substantial.
Subnanosecond switching of YBaCuO films between superconducting and normal states induced by current pulse
Authors: Karasik B. S. , Zorin M. A. , Milostnaya I. I. , Elantev A. I. , Gol’tsman G. N. , Gershenzon E. M.
Published: July 2002

A study is reported of the current switching in high-quality YBaCuO films deposited onto NdGaO3 and ZrO2 substrates between superconducting (S) and normal (N) states. The films 60–120 nm thick prepared by laser ablation were structured into single strips between gold contacts. The time dependence of the resistance after application of the voltage step to the film was monitored. Experiment performed within certain ranges of voltage amplitudes and temperatures has shown the occurrence of the fast stage (shorter than 400 ps) both in S-N and N-S transitions. A fraction of the film resistance changing within this stage in the S-N transition increases with the current amplitude. A subnanosecond N-S stage becomes more pronounced for shorter pulses. The fast switching is followed by the much slower change of resistance. The mechanism of switching is discussed in terms of the hot-electron phenomena in YBaCuO. The contributions of other thermal processes (e.g., a phonon escape from the film, a heat diffusion in the film and substrate, a resistive domain formation) in the subsequent stage of the resistance dynamic have been also discussed. The basic limiting characteristics (average dissipated power, energy needed for switching, maximum repetition rate) of a picosecond switch which is proposed to be developed are estimated.
The dynamics of two air bubbles loaded by an underwater shock wave
Authors: Kodama T. , Takayama K. , Nagayasu N.
Published: July 2002

An experimental study has been made on the interaction of an underwater shock wave with two air bubbles attached to a gelatin surface. The shock wave was generated by detonating a microexplosive pellet, and the subsequent behavior of bubble collapse was visualized by high-speed photography. By measuring the directivity and the maximum depth of the liquid jets of the two bubbles penetrating into the gelatin, it was found that when the bubbles are beyond a critical separation distance, the depth of the penetration was no longer affected by the presence of the other bubble, and that the penetration depth for a given bubble in a two-bubble arrangement is a function of the size of the other bubble.
Direct comparison of the pressure-induced band-gap shifts in cubic and hexagonal GaN
Authors: Reimann K. , Steube M. , Brandt O. , Yang H. , Ploog K. H.
Published: July 2002

Low-temperature photoluminescence measurements are performed simultaneously on cubic (zinc blende) and hexagonal (wurtzite) gallium nitride at hydrostatic pressures up to 11.9 GPa. The linear pressure coefficient of the band gap of hexagonal GaN is found to be (0.94±0.04) meV/GPa larger than that of cubic GaN.
Phosphorus and antimony in GaAs as tracers for self-diffusion on the arsenic sublattice
Authors: Scholz R. F. , Gösele U.
Published: July 2002

For investigating arsenic self-diffusion, phosphorus (P) and antimony (Sb) were used. New P and Sb in-diffusion experiments show the same behavior as P and Sb interdiffusion experiments. This is in contradiction to results from the literature. By means of transmission electron microscopy we observed a phase transformation to polycrystalline GaAs1−xPx at the surface under experimental conditions similar to those described in the literature. This leads to a penetration depth much higher than for the P in-diffusion without this phase transformation. The new diffusion coefficients of the P and Sb in-diffusion above 900 °C are in the same range as the interdiffusion coefficients and self-diffusion coefficients from As-tracer diffusion. From this result we conclude that P and Sb have tracer properties for arsenic self-diffusion in GaAs. Measurements of the diffusion coefficients at different As-vapor pressures revealed that As self-diffusion is dominated by As self-interstitials. Furthermore, we performed annealing experiments on differently doped samples. There is no indication of a Fermi-level effect. Hence, As self-diffusion is governed mainly by neutral defects. Between 900 and 1100 °C, we obtain an Arrhenius expression DAsSD=75 exp(−4.4 eV/kT)cm2/s, which describes As self-diffusion. The P in-diffusion values below 900 °C show a weaker temperature dependence, which indicates a changeover to a different diffusion mechanism.
Total cross sections for collisions of O− and S− with hydrogen
Authors: Huq M. S. , Scott D. , Champion R. L. , Doverspike L. D.
Published: July 2002

Absolute total cross sections for electron detachment and H− (D−) production have been measured for collisions of O− and S− with H2 and D2 for relative collision energies ranging from approximately 0.5 to 25 eV. Electron detachment appears to occur via several mechanisms. Isotope effects are observed in all cross sections. Threshold studies indicate that there is no appreciable potential barrier to H− formation via the ion–molecule reaction O−+H2 → H−+OH, whereas a barrier of about 1 eV is observed for H− production in collisions of S− with H2.
Irreversible immobile random adsorption of dimers, trimers, ... on 2D lattices
Authors: Nord R. S. , Evans J. W.
Published: July 2002

Models where pairs, triples, or larger (typically connected) sets of sites on a 2D lattice ‘‘fill’’ irreversibly (described here as dimer, trimer, ... filling or adsorption), either randomly or cooperatively, are required to describe many surface adsorption and reaction processes. Since filling is assumed to be irreversible and immobile (species are ‘‘frozen’’ once adsorbed), even the stationary, saturation state, which is nontrivial since the lattice cannot fill completely, is not in equilibrium. The kinetics and statistics of these processes are naturally described by recasting the master equations in hierarchic form for probabilities of subconfigurations of empty sites. These hierarchies are infinite for the infinite lattices considered here, but approximate solutions can be obtained by implementing truncation procedures. Those used here exploit a shielding property of suitable walls of empty sites peculiar to irreversible filling processes. Accurate results, including saturation coverage estimates, are presented for random filling of dimers, and trimers of different shapes, on various infinite 2D lattices, and for square tetramers on an infinite square lattice.
Nitric oxide adsorption, decomposition, and desorption on Rh(100)
Authors: Villarrubia J. S. , Ho W.
Published: July 2002

Nitric oxide adsorption, decomposition, and desorption were studied on Rh(100) in the temperature range from 88 to 1100 K using electron energy loss spectroscopy (EELS) and temperature programmed desorption (TPD). The EEL spectrometer was equipped with a multichannel detector for fast data acquisition. There are two adsorption states of NO on Rh(100), designated α1NO and α2NO, characterized by vibrational modes at 114 and 196 meV, respectively, and assigned to a lying down or highly inclined species and a vertically adsorbed species. The populations of the two states as functions of the total NO coverage were measured on the clean surface and with coadsorbed oxygen and CO. These coadsorbed species, whether adsorbed before or after the NO, increase the α2 population at the expense of α1. A model that includes an adsorbate–adsorbate interaction (range≈7 Å) which converts α1NO to α2NO and which permits adsorbing NO to diffuse so as to favor α1 adsorption fits the measured populations of the two species on the clean surface and produces a saturation coverage of 0.62 ML (1 ML=1.39×1015 molecule/cm2), in good agreement with the published result. Decomposition and desorption of NO at temperatures >90 K were studied by a series of temperature programmed EELS (TP-EELS) experiments at heating rates from 0.048 to 5.25 K/s and by TPD. At saturation, 62% of the NO decomposes as evidenced by the extent of N2 desorption in TPD peaks at 460 and 770 K. The remaining NO desorbs molecularly near 430 K with an activation energy Ea=28±3 kcal/mol and first order preexponential v=1014±1 s−1, as determined by TP-EELS. The decomposition of α1NO occurs near 170 K with Ea=10.5±0.7 kcal/mol and v=1011.8±0.7 s−1. The extent of the α2NO decomposition and its activation energy are strongly coverage dependent. The temperature at which its decomposition rate is a maximum approaches that of α1NO at low coverages, consistent with a decomposition mechanism involving an α1NO intermediate.
Polarizabilities and hyperpolarizabilities of carbon dioxide
Authors: Maroulis George , Thakkar Ajit J.
Published: July 2002

The dipole (α), quadrupole (C), and dipole–octopole (E) polarizabilities, the dipole–dipole–quadrupole (B) and second dipole (γ) hyperpolarizabilities, and the quadrupole (θ) and hexadecapole (Φ) moments are calculated for the ground state of CO2 at its equilibrium geometry. The values are obtained from fourth-order many-body perturbation theory energies of CO2 in the presence of various configurations of point charges. Electron correlation affects the longitudinal components more than the transverse ones; hence, electron correlation effects are greater for the anisotropies than for the isotropic averages of these properties. Our best vibrationless estimates for the isotropic values are ᾱ≂17.63 e2a20E−1h, C̄≂77.8 e2a40E−1h, B̄≂−2.1×102 e3a40E−2h, γ̄≂1.20×103 e4a40E−3h, and θzz≂−3.24 ea20. The quadrupole moment, mean dipole polarizability and hyperpolarizability are in satisfactory agreement with experiment. On the other hand, the polarizability anisotropy Δα≂14.3 e2a20E−1h agrees with experimental estimates based on Rayleigh scattering depolarization ratios but not with those based on the Kerr and Stark effects. Further theoretical and experimental work on the vibrational contributions to Δα is needed to resolve this discrepancy. The hexadecapole moment is small in magnitude, and is very sensitive to both basis set and electron correlation effects; its accurate prediction will be a challenge for quantum chemical methods.
Classical and quantum continuum percolation with hard core interactions
Authors: Saven J. G. , Skinner J. L. , Wright J. R.
Published: July 2002

We study the classical and quantum percolation of spheres in a three-dimensional continuum. Each sphere has an impenetrable hard core of diameter σ, and two spheres are considered to be directly connected if the distance between their centers is less than d. We calculate the critical percolation density as a function of σ/d. In the classical problem this is the density ρc at which an infinite cluster of connected spheres first forms. In the quantum problem, we study a tight-binding model where the hopping matrix element between two spheres is nonzero only if they are directly connected. In this case the critical density ρq is the density at which the eigenstates of the Hamiltonian first become extended. Our method uses Monte Carlo simulation and finite-size scaling techniques, and for the quantum problem, the concept of quantum connectivity. We find that both ρc and ρq exhibit nonmonotonic behavior as a function of σ/d. We also find that for all values of σ/d, ρq>ρc, although the ratio of the thresholds decreases with increasing σ/d. We argue that a better understanding of this ratio is obtained by considering the average coordination number. We speculate about the nature of both classical and quantum percolation as σ/d approaches 1.
Structure and thermodynamics of the liquid–vapor interface of fluorocarbons and semifluorinated alkane diblocks: A molecular dynamics study
Authors: Hariharan Arvind , Harris Jonathan G.
Published: July 2002

We use molecular dynamics simulations to predict the equilibrium liquid–vapor interface structure and surface tension of two liquids, one comprised of short fluorocarbon–hydrocarbon diblock chains and the other of short fluorocarbon chains. Larger Lennard-Jones diameters and shallower well depths distinguish the perfluoromethyl segments from the methyl ones. In this model, realistic bond angle potentials, torsional potentials, and bond lengths describe the intramolecular interactions. At high temperatures, the density profile of the copolymer melt decays monotonically from the bulk liquid density to the vapor density and the structure of the free surface is similar to that of homopolymer melts. Increasing the chain length or decreasing the temperature causes the fluorocarbon segments to segregate to the free surface. Consequently, the constraint of connectivity between the two blocks results in oscillatory density profiles and a rich structure. Our model predicts that a copolymer can have a lower surface tension than either homopolymer of similar length. We also find that the simple Lennard-Jones based model is deficient in that it fails to explain the surface tension differences between decane and perfluorodecane.
Monte Carlo simulation of confined semiflexible polymer melts
Authors: Yethiraj Arun
Published: July 2002

The behavior of semiflexible polymer melts confined between surfaces is investigated via Monte Carlo simulation. A depletion of chain sites near the wall at low densities and an enhancement of chain sites near the wall at high densities is observed, which can be explained in terms of a competition between packing and configurational entropic effects. An increase in the molecular stiffness results in improved packing of segments against the walls, but increases the loss in configurational entropy suffered by a single molecule near the wall. Changing the molecular stiffness changes the average bond angle and hence alters the way the molecules pack against the surface. The density profiles are a result of these competing effects. One consequence is that for some values of the stiffness, the solvation force (estimated from the density profiles via a superposition approximation) ceases to be an oscillatory function of wall separation even at liquidlike densities. The conformational properties of the molecules are also investigated. If each molecule is modeled in terms of an equivalent spheroid with the same moment of inertia, it is found that conformational changes occur only when the molecular centres of mass are at distances from the surface that are smaller than the thickness of the spheroid.
Molecular rotations and local modes
Authors: Lukka Tuomas , Halonen Lauri
Published: July 2002

Rotational energy level structures of stretching vibrational states have been investigated in the XH2, XH3, and XH4 type symmetrical hydrides. Transformations from standard vibration–rotation normal coordinate Hamiltonians are made to internal coordinate representations which explicitly give the terms responsible for rotational coupling between the different local mode states. It is shown that the local mode relations between the vibration–rotation parameters as given by Lehmann [J. Chem. Phys. 95, 2361 (1991)] and by Halonen and Robiette [J. Chem. Phys. 84, 6861 (1986)] make these Hamiltonians diagonal in the local mode basis. The effective vibration–rotation parameters of overtones are then proved to obey the local mode relations closer and closer as the vibrational excitation increases. A simple vibrational model accounts well for the vibrational dependencies of vibration–rotation constants in the case of SiH4, GeH4, and SnH4.
Helium scattering investigation of the structure of the Ge(100) surface above 600 K
Authors: Meli C. A. , Greene E. F.
Published: July 2002

The scattering of helium atoms from a Ge(100) surface has been studied at temperatures to within 110 K of the Ge melting point (1210 K). Although the elastic scattering is greatly attenuated as the temperature increases, the (1/2 0) and (00) helium diffraction peaks remain detectable up to 1000 and 1100 K respectively. No increased attenuation of these diffraction peaks that would be expected to accompany the appearance of numerous adatoms and vacancies is observed. This makes doubtful the interpretation of the results of an earlier x-ray diffraction study. A slight change in the Debye–Waller attenuation of the (00) diffracted helium peak near 825 K is observed, possibly indicating a weak, dimer-preserving phase transition.
Step site bonding on a vicinal Si(100) surface upon Cl2 adsorption
Authors: Dohnálek Z. , Gao Q. , Choyke W. J. , Yates J. T.
Published: July 2002

Characteristic chlorine bonding sites have been detected on a vicinal Si(100) single crystal surface exposing Si(100) terraces, 7–8 Si atoms in width. These terraces are separated by two-atom layer height steps. Following annealing to 673 K, three Cl+ beams are observed by the electron stimulated desorption ion angular distribution method. Two of the Cl+ beams originate from silicon-dimer dangling bond sites on the Si(100) terraces. The third Cl+ beam is associated with the Si–Cl bond on the step sites, and under zero field conditions it emits Cl+ at a polar angle of 24±2° with respect to the 〈100〉 direction, in the downstairs direction. The direction of Cl+ emission from the step site is qualitatively consistent with the theoretical model of the step reconstruction proposed by Chadi.
Local order and diffusive motions in simple liquids
Authors: González Hernán J. , Cernuschi-Frías Bruno , Barraza Néstor R. , Cernuschi Félix
Published: July 2002

In order to study the properties of the short-range order in simple liquids, a ‘‘moving average filter’’ is applied to the positions and velocities obtained from molecular dynamics simulations of L-J systems, in order to reduce the thermal motions. The fictitious systems that are obtained allow a better analysis, not only of the underlying structure, but also of the time-dependent low-frequency properties, such as diffusive motions. From the analysis of the correlation radial function g(r) and the recently proposed [S. Mazur, J. Chem. Phys. 97, 9276 (1992)] ‘‘neighborship distributions,’’ P(n,r), it is shown that the face-centered-cubic lattice is inadequate to model the local order in the liquid state. To study the nature of the diffusive motions, a spatial-dependent velocity correlation function is computed. It shows significant positive values for the filtered systems. This suggests a cooperative mechanism for diffusion. However, it seems that the collective motions do not involve large clusters, but rather small clusters and linear ‘‘chains’’ of molecules.
Potential energy surfaces for the low-lying 2A″ and 2A′ States of HO2: Use of the diatomics in molecules model to fit ab initio data
Authors: Kendrick Brian , Pack Russell T
Published: July 2002

A DIM (diatomics in molecules) model utilizing a large basis set (34 2A″ and 32 2A′ states) was used to obtain the potential energy surfaces relevant to the chemical reaction H+O2→OH+O. The ground state, 12A″, surface was fitted to 910 accurate ab initio points of Walch et al. [J. Chem. Phys. 94, 7068 (1991)]. The resulting fit accurately describes the C2v conical intersection in the regions for which ab initio data are available, and the linear conical intersection is accurately described in the H+O2 region. It is also an accurate global fit with an rms deviation of 0.096 eV (2.22 kcal/mol). The behavior of the low-lying excited states, 12A′, 22A″, and 22A′, appears to be qualitatively correct everywhere and quantitative near the low-lying conical intersections. The DIM formulation allows the computation of the gauge potential relevant for the description of the geometric phase and non-adiabatic effects in multi-surface reactive scattering calculations.
Effect of the Onsager coefficient and internal relaxation modes on spinodal decomposition in the high molecular isotopic blend polystyrene/deutero-polystyrene studied with small angle neutron scattering
Authors: Müller G. , Schwahn D. , Eckerlebe H. , Rieger J. , Springer T.
Published: July 2002

The early state of spinodal decomposition was studied by small angle neutron scattering in the critical mixture of the isotopic blend deutero-polystyrene/polystyrene (d-PS/PS) of equal molecular volume of 1.42×106 cm3/mol in a temperature range 12 K≤‖Tc−T‖≤82 K. This process can be described by the relaxation between two static structure factors, S(Q) representing the equilibrium values of the system in the mixed state and at the temperature where phase separation occurs. The time evolution of the relaxation process is described by the dynamical structure factor, L(Q,t) which depends on the dynamic properties of the mixture. It will be shown that the static structure factor of a mixed system can also be determined in the unstable two-phase region during the early state of spinodal decomposition. Consistent values for the Flory–Huggins parameter were found in comparison with a lower molecular d-PS/PS sample and, therefore, a lower critical temperature which was even smaller than the phase separation temperatures of the present system.

The observed time evolution of the fluctuation modes is nonexponential. Therefore, it was originally supposed that internal modes of the coil come into play. The analysis of the data with an ansatz by Akcasu, which takes internal modes into account showed, however, that the phase separation in the experimental range of wave number and time is dominated by the centre of mass diffusion as in the C–H–C case and the nonexponential behavior was attributed to a time dependent increase of the ‘‘range’’ of the Onsager coefficient. A range of the Onsager coefficient larger than the radius of gyration of a single coil is predicted in case of entangled polymers. However, no time dependence was predicted so far. The evaluated diffusion constants follow an Arrhenius behavior and are consistent with earlier studies. They show a D0∝N−2 scaling consistent with reptation. A further result is the observation of a second order peak in the structure factor already in the early times of spinodal decomposition. So far, this was only attributed to the late state of spinodal decomposition.
Nucleation during epithermal bombardment of surfaces
Authors: Pflueger Randall J.
Published: July 2002

A kinetic theory of nucleation is modified to describe formation of clusters on surfaces bombarded by epithermal atoms (defined here as having insufficient energy to create sites leading to heterogeneous nucleation), accounting for enhanced (non-thermal) emission of particles from clusters and for formation of vacancy-adatom pairs. A potential function is given for the kinetic Gibbs–Thompson equation which yields the critical cluster size, and closed form expressions are obtained (in the steady-state approximation) for the rate of formation of supercritical clusters, and the non-equilibrium cluster-size distribution. The modified theory is combined with a simple model of a growing 2-D surface to demonstrate conditions for suppressing formation of stable clusters. Some prior applications of nucleation theory to epithermal bombardment are reviewed and extended, and a simple estimate is obtained for the epithermal flux which suppresses the nucleation rate (to a particular arbitrary degree) which is subject to experimental verification. Prospects for realizing improved yield in interface-structure-sensitive electronic devices are discussed.
Investigation of the depth of preferential surface ordering in liquids: A photoelectron spectroscopic investigation of liquid mixtures
Authors: Bindu V. , Pradeep T.
Published: July 2002

Angle resolved x-ray photoelectron spectroscopic studies of a series of liquid mixtures of a perfluoropolyether (Krytox 16350) and a polyphenylether (Santovac-5) show that the former spreads on the latter and the film thickness could even be smaller than the photoelectron mean free path. The molecular ordering in krytox remain as that of the free liquid which is manifested in the enhancement of the relative intensity of specific features in the photoelectron spectrum as the electron take-off angle is decreased. The preferential ordering is limited to the very top and an upper limit of this thickness is estimated to be about 8 Å. This observation is in accordance with molecular dynamics simulations of long chain hydrocarbon liquids.
Scattering dynamics in HF+He, Ne, and Ar: State-to-state cross sections, Dopplerimetry, and alignment measurement via direct infrared laser absorption in crossed supersonic jets
Authors: Chapman William B. , Weida Miles J. , Nesbitt David J.
Published: July 2002

Absolute state-to-state cross sections are reported for rotationally inelastic scattering in crossed jets of HF with He, Ne, and Ar at mean center-of-mass collision energies of 480, 390, and 350 cm−1, respectively. HF seeded in Ar diluent gas is cooled into the J=0 ground rotational state in a pulsed supersonic expansion, followed by single collision rotational excitation with rare gas atoms from a second pulsed supersonic jet. The column-integrated densities of HF in both the initial and final scattering states are probed in the jet intersection region via direct absorption of light from a narrow bandwidth (0.0001 cm−1), continuously tunable, color center laser. Total inelastic cross sections for collisional loss out of J=0 and collisional excitation into J>0 states are determined in absolute units from the dependence of infrared absorption signals on collider gas concentration. Full close coupling scattering calculations are performed on several ab initio and empirical potential energy surfaces for each of the three HF+rare gas systems. Agreement for He+HF and Ar+HF integral cross sections is remarkably good, but significant discrepancies are noted for the less accurately determined Ne+HF surface. Photoelastic polarization modulation of the IR laser is used to probe for rotational alignment in the scattered HF flux; the measurements set an upper polarizance limit for collisionally populated J=1 HF molecules [probed on P(1)] of |P|<2%. High resolution IR laser Dopplerimetry reveals velocity structure in the collisionally excited J=1 Doppler profiles, which is in excellent qualitative agreement with theoretical predictions of rainbow features in the J=1←0 state-to-state differential cross section.
Monte Carlo simulation of self-avoiding lattice chains subject to simple shear flow. I. Model and simulation algorithm
Authors: Xu Guoqiang , Ding Jiandong , Yang Yuliang
Published: July 2002

The efficient on-lattice Monte Carlo (MC) method is extended to simulate, for the first time, the simple shear flow of multiple macromolecular chains with self-avoiding walk (SAW) on a molecular level by introducing a pseudopotential to describe the flow field. This pseudopotential makes sense only for the potential difference associated with each local microrelaxational movement of a bead in the chain strictly defined by the four-site lattice model and bond fluctuation approach. The free-draining bead-spring model is thus investigated at low and high shear rates, and the resultant shear stress and first normal stress difference are obtained by statistics according to the sampled configurational distributions under flow. As the first paper of a corresponding series, the pseudopotential is checked in detail and confirmed by the simulation outputs for both a single SAW chain and multiple SAW chains in two dimensions. The simulated velocity profile is found to greatly satisfy the requirement of the simple shear flow unless the shear rate is unreasonably high. The material functions (apparent viscosity and first normal stress coefficient) show the shear-rate dependence at high shear rate, which is, in turn, explained by the large chain deformation on the microscopic origin. Both Newtonian regime and non-Newtonian regime are found in our on-lattice MC simulation. The nonlinear rheological behaviors at high shear rates revealed in this MC simulation agree with the experimental observations in literature for most polymers. On the other hand, the asymptotic behavior about the chain-length dependence of zero-shear viscosity can be reasonably explained by the present linear viscoelastic theory. Consequently, this paper puts forward a novel and unified simulation approach to study the chain conformation and chain dynamics under shear flow and the nonlinear viscoelasticity of polymeric fluids for both dilute and concentrated solutions or polymer melts.
Nonequilibrium molecular velocity distribution in binary reactive gaseous mixture
Authors: Nowakowski B.
Published: July 2002

The Boltzmann equation for reacting and diffusing components of binary mixture is solved by means of the perturbative Chapman–Enskog method. It is assumed that all molecules have identical mechanical properties and the system is in mechanical equilibrium. Typical reaction schemes consisting of bimolecular processes only are considered. Corrections of two orders to the distribution functions perturbed by the chemical reactions are calculated. The first-order correction gives a modification of the reaction rate in the reaction-diffusion equation for concentrations. The correction to the diffusion coefficient is obtained in the second-order approximation. Moreover, the solution at this order yields a term proportional to the square of the concentration gradient. These corrections are calculated for the molecular model of reactive hard spheres. For low activation energies the magnitude of the relative correction to the diffusion coefficient can be larger than that to the reaction rate.
Numerical calculation of the rate of homogeneous gas–liquid nucleation in a Lennard-Jones system
Authors: ten Wolde Pieter Rein , Ruiz-Montero Maria J. , Frenkel Daan
Published: July 2002

We report a computer-simulation study of the absolute rate of homogeneous gas–liquid nucleation in a Lennard-Jones system. The height of the barrier has been computed using umbrella sampling, whereas the kinetic prefactor is calculated using molecular dynamics simulations. The simulations show that the nucleation process is highly diffusive. We find that the kinetic prefactor is a factor of 10 larger than predicted by classical nucleation theory.
Coherent enhancement and suppression of reactive scattering and tunneling
Authors: Frishman Einat , Shapiro Moshe , Brumer Paul
Published: July 2002

An optimization approach to coherent control of scattering is introduced and used to show that one can totally suppress reaction at energy E if the number of populated initial states exceeds the number of open product states (as in endoergic reactions). Numerical results on a multichannel system are presented.
Optical spectra and thermal Schottky levels in dysprosium sesquisulfide
Authors: Gruber John B. , Zandi Bahram , Justice Bruce , Westrum Edgar F.
Published: July 2002

We report a detailed crystal-field splitting analysis of the energy levels of Dy3+(4f9) in single crystals of Dy2S3 that have the Th3P4 cubic defect structure. From an analysis of the temperature-dependent absorption spectra, we have identified seven of the eight crystal-field split energy (Stark) levels of the ground-state multiplet manifold, H15/26. Sixty-two experimental Stark levels from various multiplet manifolds of Dy3+ are compared with a calculated crystal-field splitting, whose initial crystal-field parameters, Bnm, were determined from lattice-sum calculations. The rms deviation between experimental and calculated levels is 7 cm−1. Both the experimental and calculated crystal-field splitting of the H15/26 manifold are compared with an assignment of Schottky levels obtained from a reassessment of heat capacity data reported earlier. Based on entropy considerations and verification of the Schottky level assignments by analyses of the optical and magnetic susceptibility data, we conclude that the anomaly observed in the heat capacity data near 3.4 K is due to antiferromagnetic ordering.
Phase diagram of the four-dimensional Lennard-Jones fluid
Authors: Hloucha M. , Sandler S. I.
Published: July 2002

We have used the Gibbs ensemble Monte Carlo method to investigate fluids in the N dimensional space. We consider the practical way to perform these simulations and give expressions for the long-range corrections. Results for the four-dimensional Lennard-Jones fluid are presented. The calculated phase diagram and several thermodynamic properties for this fluid are given. Estimates of the critical point and the critical exponent are obtained from the phase diagram. The simulations were performed with two different simulation techniques: random choice and a fixed sequence of Monte Carlo steps.
Temperature programmed desorption studies of OD coadsorbed with H2 on Pt(111)
Authors: Backstrand Kyle M. , Weibel Michael A. , Moision Robert M. , Curtiss Thomas J.
Published: July 2002

A molecular beam source of pure hydroxyl radicals has been developed and used to explore the water reaction catalyzed over Pt(111). An electrostatic hexapole selectively focused OD radicals from a supersonic corona discharge source onto a Pt target at a surface temperature of TS=143 K. Subsequent D2O temperature programmed desorption (TPD) spectra revealed two major features, one near TS∼170 K from desorption of molecular water overlayer and a second near TS∼210 K from the decomposition of an adsorbed OD intermediate. The latter feature was isolated and analysis of TPD spectra revealed that the D2O production reaction was approximately half-order in total oxygen coverage with a pre-exponential factor ranging from vd=4±1×1016 to 5±2×1018 molecules1/2 cm−1 s−1 and activation energy Ea=9.7±0.1 to 11.5±0.1 kcal mol−1 for initial coverage ranging from θ0=0.04 to 0.25 ML. Coadsorption studies of OD and H2 revealed that H atoms drive reactions with adsorbed OD at TS∼180 K to form all three water isotopes: D2O, HDO, and H2O. Oxygen (O2) TPD spectra contained three desorption features (TS=700 K, 735 K, and 790 K). The relative abundance of O2 from these three features was virtually the same in all low temperature (TS=143 K) TPD experiments. At elevated dosing temperatures (TS=223 K) the two features at TS=700 K and 790 K could be selectively titrated from the surface by hydrogen. The presence of hydrogen prior to OD exposure at this elevated temperature prevented the accumulation of oxygen on the surface. The implications of these observations on our mechanistic understanding of the low temperature (TS<210 K) water reaction are discussed.
Numerical procedures for sample structures on stochastic differential equations
Authors: Nakazawa Hiroshi
Published: July 2002

Procedures are discussed for the numerical construction of sample processes on stochastic differential equations. Structures of a relevant class of Gaussian white-noise functionals are analyzed to some closed forms, and new numerical algorithms for single noise problems are presented up to the accuracy of O(h3), or within errors of O(h7/2), in the sense of the mean-square norm.
Periodic solutions in spontaneously broken theories
Authors: Brihaye Yves , Kunz Jutta
Published: July 2002

A class of spontaneously broken field theories is proposed, and the occurrence of their periodic, classical solutions is investigated in detail. The emergence of multiple solutions is observed, their normal modes of oscillation are studied, and the bifurcations of the classical energy functional are related to the occurrence of zero modes. These features, also present in the Weinberg–Salam model, are illustrated here in a simple way.
Existence and blow-up of solutions to degenerate Davey–Stewartson equations
Authors: Li Yongsheng , Guo Boling , Jiang Murong
Published: July 2002

In the present paper we consider a system of degenerate Davey–Stewartson equations. We prove the global existence of weak solutions and blow-up of solutions for appropriate initial data.
Temperature dependence of the dislocation component of the modulus defect in deformed bcc metals
Authors: Pal-Val P. P.
Published: July 2002

Experimental data on temperature dependences of the dislocation component of the modulus defect are obtained for some bcc metals (Mo, Fe, and Nb) of different purity and orientation. Measurements are made at frequencies ω/2π≃90 kHz in the temperature range 6–300 K. It is found that the modulus defect in Mo and Fe single crystals increases monotonically with temperature starting from 6 K. The increase in the modulus defect becomes stronger in temperature intervals in which relaxation peaks were observed earlier on the temperature dependences of absorption. It is shown that such a behavior of the modulus defect is due to some thermally activated dislocation processes with a broad distribution of activation parameters, which occur in the given temperature range. In contrast to Mo and Fe, Nb shows no temperature dependence of the modulus defect up to 170–200 K, but at higher temperatures it increases rapidly. The existence of a broad temperature region in which the modulus defect is constant indicates that thermally activated processes with a relaxation time close to the reciprocal cyclic frequency 1/ω are either absent or suppressed considerably in this temperature range.
Adsorption of gaseous helium near Tλ at low pressures
Authors: Kachalin G. V. , Kryukov A. P. , Nesterov S. B.
Published: July 2002

Cryosorption of helium isotopes (3He and He4) on thin layers of solid argon at low gas pressures in the bulk is studied experimentally upon a change in the layer temperature from 4.2 to 2 K. It is shown that the sorption isostere for He4 displays anomalous behavior near the temperature Tλ of transition of liquid He4 into superfluid state. An abrupt change in pressure is observed for a He4 film thickness corresponding approximately to two monolayers. In experiments on cryosorption of gaseous He3 on an argon layer for a He3 film thickness corresponding approximately to one monolayer, the pressure in the bulk varies monotonically over the entire temperature interval.
Magnetostructural topological defects in two-dimensional antiferromagnets
Authors: Dudko O. K. , Kovalev A. S.
Published: July 2002

A two-dimensional model generalizing the Peierls model to the case of coupled fields of magnetization and elastic displacement distribution is proposed for describing the structure of a complex magnetoelastic topological defect which has the form of the bound state of a dislocation and a magnetic disclination in an antiferromagnet. The obtained system of nonlinear one-dimensional integro-differential equations has solutions for magnetic vortices and for magnetic disclinations connected with dislocations and having regular asymptotic forms at large distances and no singularities at the core of the topological defect.
Resonance properties of domain walls in ferromagnets with a weak exchange interaction
Authors: Gvozdikova M. V. , Kovalev A. S. , Kivshar Yuri S.
Published: July 2002

The spin dynamics of a domain wall is studied in an infinite ferromagnetic chain with an easy-axis anisotropy as well as in a chain of finite size. The dependence of the intrinsic mode frequency of the domain wall on the exchange interaction is studied for all admissible values of the latter. It is shown that this dependence is considerably modified in the region of transition of the domain wall from collinear structure to canted form.
Direct observation of a manifestation of magnetoelastic coupling in a low-dimensional virtual ferroelastic
Authors: Zvyagina G. A. , Zvyagin A. A.
Published: July 2002

The propagation velocities of acoustical vibrations are measured for different directions of propagation and polarizations in layered alkali molybdates at low temperatures. A comparison is made of the experimentally obtained absolute sound velocities in virtual ferroelastics and in systems with the cooperative Jahn–Teller effect. The temperature dependence of the sound velocities are investigated experimentally in these systems. The change in the sound velocities in an external magnetic field gives the first direct proof of the presence of an interaction between the magnetic and elastic subsystems of a virtual ferroelastic. A comparison with the theoretical calculations suggests the presence of a nonlinear coupling of the electronic subsystem with optical phonons.
A singularity method for unsteady linearized flow
Authors: Pozrikidis C.
Published: July 2002

Oscillatory viscous flow in the presence of particles is considered. The flow is represented in terms of fundamental solutions to the governing unsteady Stokes equation including the oscillating Stokeslet and dipole. It is shown that the force on a particle in arbitrary oscillatory flow is proportional to the total strength of the distributed oscillating Stokeslets necessary to represent the flow. Faxen’s laws for solid and fluid particles are derived in terms of singularity representations and slender-body theory is discussed. The theoretical results are applied in studies of the translational and rotational oscillations of a solid sphere, the translational oscillation of a spherical drop, and the axial and transverse oscillation of a prolate spheroid. It is demonstrated that a spherical drop convected in a uniform streaming flow or moving under the influence of a body force, maintains its shape, independently of surface tension. The force F on a prolate spheroid executing longitudinal or transverse oscillations is computed in an extended range of frequencies ω. The results verify the accuracy of the correlation F(ω) proposed by Lawrence and Weinbaum [J. Fluid Mech. 189, 463 (1988)].