bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetoresistive stripe temperature calculation
Authors: Tobin Vincent M. , Atesmen Kemal
Published: July 2002

A method of calculating the steady-state operating temperature of a shielded magnetoresistive element in three dimensions is described. The method of images is used to account for any number of layers of different thermal conductivities, which may include, for example, gaps, shields, undercoat, overcoat, and substrate. The temperature may be calculated at any point, allowing the average temperature of the stripe to be calculated. The effect of the thermal coefficient of resistance is included. Results of calculations using the model compare favorably to measurements.
Low temperature magnetic and transport anisotropy in Pr0.7Sr0.3MnO3: The magnetostriction scenario
Authors: Wolfman J. , Prellier W. , Mercey B. , Simon Ch.
Published: July 2002

Angle-resolved magnetic and transport measurements of a Pr0.7Sr0.3MnO3 thin film as a function of the in-plane magnetic field direction are discussed. The origins of the anisotropic magnetotransport evidenced near Curie temperature and at T=30 K are shown to strongly differ. Quantitative magnetic simulations of the angular dependence of the magnetization at T=30 K were used to evaluate the anisotropy energy in low field. A rotation of the in-plane easy axis induced by high fields at T=30 K is attributed to the magnetostriction caused by the praseodymium ions. Furthermore, a model taking into account this plausible magnetostrictive effect is shown to qualitatively reproduce the observed low temperature transport anisotropy.
Magnetic properties and crystal structure of HoNiAl and UNiAl hydrides
Authors: Kolomiets A. V. , Havela L. , Rafaja D. , Bordallo H. N. , Nakotte H. , Yartys V. A. , Hauback B. C. , Drulis H. , Iwasieczko W. , DeLong L. E.
Published: July 2002

The effect of hydrogenation on the magnetic properties and crystal structure of 4f and 5f intermetallics was studied on HoNiAlH2.0, UNiAlH2.3 and UNiAlD2.1. The incorporation of hydrogen into HoNiAl leads to a decrease of the magnetic ordering temperature from 13 to 6 K, while it increases from 19 K to values in the range 90–100 K in the UNiAl–H system. The crystal lattices of all compounds are anisotropically expanded. Moreover, the crystal symmetry is lowered to orthorhombic in HoNiAlH2.0, but remains hexagonal for UNiAlH(D)x. Three different deuterium positions were found for both compounds.
Preisach analysis of reversible magnetization in SmCo/Fe films
Authors: Cornejo Daniel R. , Missell Frank P.
Published: July 2002

SmCo(50 nm)/Fe(y nm) bilayers (y=0, 2.5, 5, 20) were prepared by magnetron sputtering on Si substrates. The films showed coercivities in the range 1.5–14 kOe as well as remanence enhancement, characteristic of exchange-coupled systems. Henkel plots for these bilayers showed that the demagnetizing interactions increase as the thickness of the Fe layer increases. For each film, a set of minor loops was measured in order to determine the Preisach distributions. With suitable analytic forms for these distributions, we obtained a description of the experimental data for the major loops, the minor loops and recoil curves. The comparison with experiment is excellent. From the moving Preisach model we have determined the evolution of the reversible and irreversible magnetization components during the magnetization and demagnetization processes. The most important source of Mrev is the reversible rotation of the magnetization in the grains of the SmCo layer. The soft Fe layer, when well coupled to the SmCo layer, contributes mainly to Mrev, but is also an important source of Mirr.
Effects of reactive ion beam treatment of a sapphire surface to optimite the deposition of GaN films
Authors: Kim Hyun-Jeong , Byun Dongjin , Kim Gyeungho , Kum Dong-Wha
Published: July 2002

We examined the possibility of employing the surface treatment of sapphire using reactive ion beam (RIB) pretreatment as an alternative process for GaN growth in metalorganic chemical vapor deposition (MOCVD). As a result of the RIB treatment, etching of the sapphire surface and the formation of a very thin, disordered AlON layer was observed which was partially crystallized during the main growth of GaN. Reduction in both dislocation density and lattice strain of GaN on RIB treated sapphire was obtained. Partial crystallization in the RIB layer promoted two-dimensional growth on the crystallized regions and relieved the misfit strain through relaxation of the disordered RIB layer. The optical properties of the GaN films could be optimized by the proper choice of the ion beam energy. The present results clearly show that RIB pretreatment of the sapphire surface can be used to improve the properties of GaN films grown by MOCVD.
Solid containing rotationally free nanocrystalline γ-Fe2O3: Material for a nanoscale magnetic compass?
Authors: Tejada Javier , Zhang XiXiang , Kroll Elizabeth , Bohigas Xavier , Ziolo Ronald F.
Published: July 2002

A nanocomposite material has been characterized that contains nanometer size magnets that are free to rotate in response to an applied magnetic field. The composite consists of 5–10 nm crystals of γ-Fe2O3 dispersed in a solid methanol polymer matrix. The material was prepared by freezing a methanol-based ferrofluid of γ-Fe2O3 and subjecting it to a magnetic field applied in alternate directions to anneal the matrix. Before the field treatment, the solid displays magnetic behavior characteristic of an ordinary nanoscopic magnetic material. It is superparamagnetic above the blocking temperature (160 K) and hysteretic below, showing magnetic remanence and coercivity. After the field treatment to anneal the matrix, the same solid shows only Curie–Weiss behavior above and below the blocking temperature over the temperature range from 4.2 to 200 K and in response to applied magnetic fields as low as 1.59 kA/m. The data are consistent with a solid containing rotationally free, nanoscopic magnets encased in cavities of very small dimensions. The free rotation of the particles precludes the observation of magnetic relaxation phenomena that are characteristic of magnetic solids and ferrofluids. The present solid portends a class of magnetic materials with very little or no electrical and magnetic loss.
Response to “Comment on ‘Analysis of high-resolution x-ray diffraction in semiconductor strained layers’ ” [J. Appl. Phys. 87, 8212 (2000)]
Authors: Dunstan D. J. , Kimber A. C.
Published: July 2002

The author of the comment is confused about a paradox and a philosophical error he identifies in our multivariate linear regression analysis of data. In this response, we clarify the issue, showing that the alleged paradox arises equally in a single-variate regression analysis and even in the use of repeated measurements to give a mean and standard deviation. We clarify also the question of best choices of reflections.
Investigation of threading dislocation blocking in strained-layer InGaAs/GaAs heterostructures using scanning cathodoluminescence microscopy
Authors: Russell J. J. , Zou J. , Moon A. R. , Cockayne D. J. H.
Published: July 2002

Threading dislocation glide relieves strain in strained-layer heterostructures by increasing the total length of interface misfit dislocations. The blocking theory proposed by Freund [J. Appl. Phys. 68, 2073 (1990)] predicts the thickness above which gliding threading dislocations are able to overcome the resistance force produced by existing orthogonal misfit dislocations. A set of wedge-shaped samples of InxGa1−xAs/GaAs (x=0.04) strained-layer heterostructures was grown using molecular-beam epitaxy in order to test the theory of dislocation blocking over a range of thicknesses within one sample. Scanning cathodoluminescence microscopy techniques were used to image the misfit dislocations. The cathodoluminescence results confirm the model proposed by Freund.
Diffusion of Ge in Si1-xGex/Si single quantum wells in inert and oxidizing ambients
Authors: Griglione Michelle , Anderson Timothy J. , Haddara Yaser M. , Law Mark E. , Jones Kevin S. , van den Bogaard Alex
Published: July 2002

The interdiffusion of Si/Si0.85Ge0.15/Si single quantum well (SQW) structures subjected to inert- and oxidizing-ambient annealing was investigated as a function of temperature (900–1200 °C) and time. Point defect injection allowed modification of the vacancy and interstitial mediated components of interdiffusion, DV and DI. Diffusion profiles of samples processed in inert and oxidizing ambients were similar, which indicates a vacancy-dominated mechanism. Activation energies of diffusion in inert and oxidizing ambients were found to be 5.8 and 5.0 eV, respectively. A fractional interstitial component fI of ∼0.10 was estimated for the lower temperatures, while a significantly smaller fI of ∼0.02 was estimated for the higher temperatures. Experiments using SQWs with buried boron marker layers showed that dislocations in the Si1-xGex trap point defects and affect interdiffusion behavior.
Effects of modulated electric field form and frequency on the electro-optical properties of CdS0.1Se0.9 nanocrystals
Authors: Wang Y. S. , Wang R. Z. , Sun P. , Tu Q. Y. , Yan Q. L. , Huang P.
Published: July 2002

The electroabsorption (EA) of CdS0.1Se0.9 nanocrystals was measured under pulse-like modulated fields and recovered both with similar frequency (1f ) and with double frequency (2 f ) of the applied field. The EA properties and the effects of applied field form and recovered frequency on the EA properties were studied. Electric field induced strong change of absorption (10−3) was observed in resonant and nonresonant regions. The EA signal in the resonant region was due to the quantum confined Stark effects and the signal in the nonresonant region was due to the change of local field that resulted in the change of refractive index, the change of absorption. The quadratic dependence of EA signal intensity on the electric fields indicated that the nanocrystals had third nonlinear optical susceptibility. The EA signal intensity measured with 1f mode was about ten times of that measured with 2 f mode. Perhaps the lower response measured with 2 f mode was due to the measured mode and the different response coefficient in different frequency regions.
Electronic structure of FeO and RuO
Authors: Krauss M. , Stevens W. J.
Published: July 2002

The electronic structure of FeO and RuO is examined using multiconfiguration self-consistent-field (MC-SCF) wave functions that go asymptotically to minimally correlated fragment atoms. Core electrons are eliminated by the use of relativistic effective potentials (REP) which also are used to calculate the spin-orbit coupling constant for the 5Δ, 5Π, and 5Φ states. The electronic structure of the FeO and RuO 5Δ states is described in terms of interacting singly charged ions, M+ and O−, which are bound primarily by a strong sigma bond. The natural orbitals are determined and evaluated in detail. The characteristics of these orbitals are used to hypothesize an aufbau for the ground states of most of the first and second row transition metal oxides. In addition to the 5Δ states, the 5Σ+, 5Π, 5Φ, 5Σ−, 5Γ, 7Σ+, 7Π, 7Φ, 3Δ, 3Σ+, 3Π, and 3Φ states were also studied. FeH and RuH 6Δ, 4Δ, and 4Φ states were also examined to evaluate the basis set.
Calculation of ESR spectra and related Fokker–Planck forms by the use of the Lanczos algorithm. II. Criteria for truncation of basis sets and recursive steps utilizing conjugate gradients
Authors: Vasavada Kashyap V. , Schneider David J. , Freed Jack H.
Published: July 2002

The complex symmetric Lanczos algorithm (LA) has proven to be a very efficient means of calculating magnetic resonance line shapes and spectral densities associated with Fokker–Planck forms. However, the relative importance of the various components of the basis set in an accurate representation of the spectrum and the proper number of recursive steps are not easily assessed in practice using the Lanczos algorithm. A systematic and objective procedure for the determination of optimal basis sets and number of recursive steps is developed using a generalization of the conjugate gradient method (CGM) appropriate for the type of complex symmetric matrices occuring in these problems. The relative importance of the individual basis vectors is determined by using the CGM to obtain the ‘‘solution vector’’ from the set of algebraic equations defining the spectrum. This is done at several values of the sweep variable (e.g., the frequency or the magnetic field). The maximum (over these values of sweep variable) for each component of the solution vector is taken to be a measure of the overall importance of the corresponding basis

vector in the complete spectrum. Using this method signficant basis set truncation is conveniently possible. The number of recursive steps needed for an accurate representation of the spectrum is easily obtained by monitoring the residual in the approximate solution vector at the center of the spectrum and by recognizing the close relationship between the LA and the CGM. It is this relationship that enables construction of the Lanczos tridiagonal matrix with the CGM which can either be used to calculate the cw ESR spectrum directly or else the eigenvalues. The information obtained from the CGM can be used to ‘‘turbocharge’’ the LA by taking advantage of the nearly optimal basis set and number of recursive steps. Significant savings in computation time are possible, and relative savings are greatest for the most difficult problems. This is illustrated with a variety of examples of slow-motional cw ESR spectra and of the new two-dimensional electron-spin-echo technique. In keeping with the greater sensitivity of the latter technique to motional dynamics, it is consistently found to require significantly larger optimal basis sets and number of recursive steps for an accurate representation. One of the most challenging problems for both types of spectroscopy is the case of macroscopically oriented samples where the macroscopic director is tilted at an angle relative to the applied static magnetic field, since this removes much of the symmetry in the problem. This case is found to yield to very significant truncation of basis sets, and a new symmetry-based decoupling of certain basis vectors was found in this study for the particular example of a 90° tilt angle.
A trapped polymer chain in random porous media
Authors: Baumgärtner A. , Muthukumar M.
Published: July 2002

Dynamic and static properties of a polymer chain without self-excluded volume, which performs Brownian motion between randomly distributed impenetrable fixed obstacles, have been investigated by Monte Carlo simulations and analyzed by scaling considerations. The mean square radius of gyration 〈S2〉, the center-of-mass diffusion coefficient D, and the longest relaxation time τ are functions of x=(1−p)(N)1/2 for all chain lengths N and porosities p above the percolation threshold. Simulations have been performed for x≲10. With increasing x the radius of gyration exhibits a crossover from Gaussian statistics 〈S2〉∼N to a collapsed state where 〈S2〉 is independent of N. This phenomenon is attributed to the effects of both the lack of self-excluded volume and the presence of an effective self-attractive potential arising from random repulsion between polymer and the solid particles of the medium. The strong dependency upon chain length of D∼N−2.9±0.3 and τ∼N4.0±0.4 is conjectured to result from randomly distributed bottlenecks and traps in the porous solid. If these local constraints are released by arranging the obstacles in a periodic array, familiar reptation dynamics and 〈S2〉∼N are observed.
A comparison between theoretical and experimental state-to-state charge transfer cross sections for H++H2 at 20 eV: Evidence for quantum effects
Authors: Baer Michael , Niedner Gereon , Toennies J. Peter
Published: July 2002

A three−dimensional close coupling quantum mechanics calculation is performed for hydrogen ion−molecule collisions. The results are compared to those obtained from the trajectory surface hopping method. (AIP)
Electrical conductivity and equation of state of shock-compressed liquid oxygen
Authors: Hamilton D. C. , Nellis W. J. , Mitchell A. C. , Ree F. H. , van Thiel M.
Published: July 2002

The electrical conductivity of shock-compressed liquid oxygen has been measured in the dynamic pressure range 18–43 GPa(180–430 Kbar). A double-shock equation-of-state point was also measured. The data and Hugoniot calculation, based on a chemical equilibrium model, indicate that liquid oxygen partially dissociates and forms a two-component conductive fluid. Details of the experimental design are given and the data are discussed in terms of electronic transport in disordered systems.
Effective medium potentials for molecule–surface interactions: H2 on Cu and Ni surfaces
Authors: No/rskov J. K.
Published: July 2002

A new approximate method is developed for the calculation of the adiabatic potential energy surface for a molecule outside a metal surface. It is computationally fast enough to be useful in simulations of the dynamics of adsorbing and desorbing molecules. The method is characterized by the fact that the functional form of the total energy expression is derived from density functional theory, that each of the terms entering can be given a precise physical interpretation, and that most of the parameters entering can be calculated, within the local density approximation. The method is explicitly derived for H2 outside metal surfaces and the applicability is illustrated for H2 adsorbing on various Cu and Ni surfaces. Although very approximate, the calculated potentials seem to include a number of features observed experimentally: Ni is more active in dissociating H2 than Cu, and open surfaces are more active than close-packed ones. Moreover, the method is simple enough that one can contemplate studying variations in dissociation pathways over the surface unit cell. For the Cu surfaces these variations are substantial accounting for at least part of the variation of the sticking coefficient with the kinetic energy of the incoming molecule. Because of the transparent nature of the energy expression, all these trends can be given a simple physical interpretation.
Pulse shaping and coherent Raman spectroscopy in condensed phases
Authors: Yan Y. J. , Mukamel S.
Published: July 2002

A microscopic theory for ultrafast coherent Raman spectroscopy of polyatomic molecules in condensed phases is developed. For off resonant excitation, an effective Hamiltonian that controls the molecular dynamics in a Raman process is derived. The limitations of the driven oscillator model are clarified, and generalized equations of motion, which hold for resonant and off resonant excitation, are derived. Spectral selectivity using pulse shaping is discussed.
Cluster relations for multireference coupled-cluster theories: A model study
Authors: Jankowski K. , Paldus J. , Wasilewski J.
Published: July 2002

The validity of various coupled-cluster conditions, that were recently formulated for both the valence-universal open-shell coupled-cluster theory based on Lindgren’s exponential ansatz and the state-universal theory employing the Jeziorski–Monkhorst ansatz, are examined for two model systems consisting of two and four slightly stretched, interacting hydrogen molecules. These cluster conditions ascertain the possibility to represent four-electron terms in the wave operator through appropriate products of two-electron terms and thus justify the truncation of the open-shell cluster operators employed to at most two-electron terms. A single parameter that determines the geometry of the studied model systems makes it possible to continuously vary the extent of quasidegeneracy between the two lowest energy states over a wide range. It is shown that the cluster conditions are relatively well satisfied only in the strongly quasidegenerate region, except for a few very small amplitudes whose effect on the resulting energies should be insignificant. To assess the validity of the often made assumption of negligibility of one-electron amplitudes, the generalized cluster conditions involving these terms were also examined. The role played by these additional terms was shown to be small. Finally, in the valence-universal case, the role of orbital choice was also investigated by performing cluster analysis with both system and core spin orbitals. The effect of using different orbital alternatives was again found to be small.
The electronic and molecular structure of ferrocene
Authors: Park Changyok , Almlöf Jan
Published: July 2002

The metal–ligand distance in ferrocene has been calculated with several different electronic structure methods. The only correlation treatment able to reproduce the experimental value is the MCPF (modified coupled-pair functional) approach with all 66 valence electrons correlated. Large basis sets are necessary to account for the dispersion interaction between the rings.
On the Born–Green–Yvon equation and triplet distributions for hard spheres
Authors: Taylor Mark P. , Lipson J. E. G.
Published: July 2002

The Born–Green–Yvon integral equation for hard spheres is studied using two closures which provide improvements to the traditional Kirkwood superposition approximation (KSA). These rigorous corrections to the KSA arise from a diagrammatic expansion of the triplet potential of mean force which can be carried out in terms of either the Mayer f-function or the total correlation function h. While the short-ranged f-bond corrections improve the calculated pair distribution function at contact, they otherwise distort this function and thus give very poor compressibility results. The long-ranged h-bond corrections are found to give overall improvement to the pair distribution function and, in particular, give nearly the correct phase of this function. Furthermore, the triplet distribution function computed with the second-order h-bond correction is found to be reasonably close to Monte Carlo results.
Angular distributions in electronically adiabatic hyperthermal collisions. An eikonal approach
Authors: Cohen Joel M. , Micha David A.
Published: July 2002

A general method is described to calculate transition integrals, including phase interference effects, from eikonal wave functions. It introduces Jacobians to evaluate the divergence of trajectory bundles, and to transform integration variables from space coordinates to trajectory coordinates. Calculations are performed for angular distributions in Li+Hg and H2+Hg collisions using Lennard-Jones potentials that are compared with accurate partial-wave sums. Very good agreement is found at hyperthermal collision energies.
Multipolar electrolyte solution models. III. Free energy of the charged and dipolar hard sphere mixture
Authors: Peters Günther H. , Eggebrecht John
Published: July 2002

Helmholtz free energies, internal energies, and pressures of mixtures of charged and dipolar hard spheres are obtained from Monte Carlo computer simulation. Ionic and solvent contributions to these properties of this electrolyte solution model are reported for a large number of states, with variations in total density, ionic concentration, and the ratio of ionic charge to solvent polarity. From this data we show that, in contrast to solution theories based on replacement of the solvent by a continuum with dielectric constant ε0, there is a correspondence in properties of states at fixed κ02=κ2ε0, where κ is the inverse Debye length.
Relationship between dissociation energies, force constants, and bond lengths for some N–F and O–F bonds
Authors: Politzer Peter , Habibollahzadeh Dariush
Published: July 2002

A nonlocal density functional procedure is used to compute the dissociation energies of some N–F and O–F bonds that show an anomalous relationship between force constants and bond lengths. The dissociation energies are found to increase in the same direction as the force constants. The trend in the bond lengths is interpreted in terms of the quantities of charge in the N–F and O–F bond regions.
Structure, dynamics, and thermodynamics of model (H2O)8 and (H2O)20 clusters
Authors: Wales David J. , Ohmine Iwao
Published: July 2002

We present molecular dynamics simulations of (H2O)8 and (H2O)20, paying particular attention to the possibility of solid-like/liquid-like coexistence. Four differently parametrized rigid molecule potentials are examined for (H2O)8; only the most promising is applied to (H2O)20. In every case, we find evidence for time-scale coexistence in the statistics of the short-time averaged temperature. In several cases, we also observe loops in the microcanonical caloric curve [T(E)], indicating the formal existence of two stable states over a finite range of energy. Further evidence is provided by systematic quenching, both by comparison with the dynamics and in terms of model density of states calculations of the microcanonical T(E), energy distribution function f(E), Helmholtz free energy A(T), and heat capacity Cv(T). We discuss two possible approaches to these thermodynamic functions from the distribution of local energy minima and compare the results with those for atomic clusters bound by the Lennard-Jones potential.
Nonlinear response functions for birefringence and dichroism measurements in condensed phases
Authors: Cho Minhaeng , Fleming Graham R. , Mukamel Shaul
Published: July 2002

The fourth-rank tensorial nonlinear response function which controls the response of a chromophore in the condensed phase to three arbitrarily polarized external fields is calculated. Internal (vibrational and rotational) degrees of freedom are incorporated using wave packets in Liouville space (the doorway/window picture). The rotational contribution of the chromophore is expressed in terms of a conditional probability for the rotational diffusion model. We apply this formalism to transient dichroism and birefringence spectroscopies.
Nascent rotational and vibrational distributions in both products of the reaction Zn(4 1P1)+H2O→ZnH(X 2Σ+)+OH(X 2Π)
Authors: Kuwahara Kazuya , Ikeda Hiroyuki , Umemoto Hironobu , Sato Tohru , Takano Kazuto , Tsunashima Shigeru , Misaizu Fuminori , Fuke Kiyokazu
Published: July 2002

The reaction Zn(4 1P1)+H2O→ZnH(X 2Σ+)+OH(X 2Π) was studied under thermal equilibrium conditions at 700 K. The nascent internal state distributions of both products ZnH and OH were determined by using a pump-and-probe technique. The rotational distributions of ZnH and OH were both Boltzmann-like for their v″=0 vibrational levels. However, the rotational temperatures were significantly different—12 000 K for ZnH and 900 K for OH. ZnH was also vibrationally excited. The nascent vibrational distribution of ZnH was determined to be 10(v″=0):13(v″=1):7(v″=2):2(v″=3). In contrast, no excitation in the OH vibration was observed. Such a nonstatistical energy partitioning is explained by considering a short-lived Zn–H–OH intermediate in a nonlinear geometry.
Maximal entropy spectral fluctuations and the sampling of phase space
Authors: Remacle F. , Levine R. D.
Published: July 2002

An analytical functional form for the distribution of intensities in an absorption spectrum is derived. Deviations from the purely statistical ‘‘Porter–Thomas’’ distribution are shown to be directly related to finite time information on the dynamics in phase space. The predicted distribution is wider than the purely statistical one with a higher proportion of very low intensity transitions. The derivation is based on a maximum entropy form of the spectrum. The constraints used are the values of the survival amplitude at finite number of times. The amplitude is obtainable as the Fourier transform of an observed spectrum or as the result of a dynamical computation. The optimal choice of the time points which characterize the spectrum, is discussed and a numerical algorithm is provided. Extensive spectral fluctuations occur when more than one time scale is needed to characterize the dynamics. This separation of time scales is also manifested as a clump structure in the spectrum of maximal entropy. The formalism also provides the distribution of line spacings and the ‘‘correlation hole’’ in the time autocorrelation function is discussed as an illustration.
The concept of reaction route
Authors: Masuda Makihiko
Published: July 2002

The stoichiometry of a complex reaction network of the Belousov–Zhabotinsky reaction is discussed by using the concept of reaction route. The consistency of stoichiometric relations between the reaction network and its reduced mechanism is shown to be analyzed by a criterion proposed previously.
Thermal energy reactions of CO+2 with chloromethanes
Authors: Tsuji Masaharu , Funatsu Tsuyoshi , Matsumura Ken-ichi , Nishimura Yukio
Published: July 2002

Rate constants and product ions have been determined for thermal energy reactions of CO2+ with CHnCl4−n(n=0–3) by using an ion-beam apparatus. Total rate constants are (8.7±3.7), (6.7±3.1), (9.1±4.1), and (4.9±1.6)×10−10 cm3 s−1 for CH3Cl, CH2Cl2, CHCl3, and CCl4, respectively. These values amount to 38%–61% of the collision rate constants estimated from either the Langevin or averaged dipole oriented theory. Although charge transfer followed by the successive loss of a Cl atom is the major product channel, parent ions are formed from CH3Cl and CH2Cl2 with branching ratios of 33%±5% and 25%±3%, respectively. The reaction mechanisms are interpreted in terms of the electronic states of the parent molecular ion accessible in the charge-transfer processes. The lack of formation of parent ions from CHCl3 and CCl4 is explained as due to complete (pre)dissociation of ionic states below 13.78 eV.
Influence of bond length variation on correlated static exchange potential: A case study in e−–N2 scattering
Authors: Ghose Keya Basu , Pal Sourav
Published: July 2002

We discuss in this note how the correlated static exchange potential changes with bond length for N2 molecule where the earlier extensive results at equilibrium exist. We have used many-body coupled cluster technique for this study. Its relevance to e−–N2 scattering is also discussed.
Dynamic sedimentation of linear micellar aggregates in a centrifugal field
Authors: Duyndam Alexander , Odijk Theo
Published: July 2002

We formulate an equation for the dynamic sedimentation of linear micelles (both rodlike and flexible) in the limit of local thermodynamic equilibrium when the recombination and scission of micelles are fast processes. A self-similar solution of the first kind is proposed for rodlike micelles in a free-draining approximation. The analytical structure of the associated nonlinear ordinary differential equation is very different from that pertaining to solutions of nonaggregating molecules. The theory may be of use in an experimental determination of the growth parameters by ultracentrifugation.
Response to ‘‘Comment on ‘14N spin relaxation studies of N2 in buffer gases. Cross sections for molecular reorientation and rotational energy transfer’ ’’ [J. Chem. Phys. 100, 1775 (1994)]
Authors: Jameson Cynthia J. , Jameson A. Keith , ter Horst Marc A.
Published: July 2002

The origin of the factor 1/4 in the equation describing the quadrupolar relaxation times for linear molecules in the dilute gas phase is clarified. The quadrupolar relaxation cross-sections for nitrogen molecule are around 30 Å2 as reported by us and not around 120 as claimed in the comment.
Delocalization length and structure of oligopyrroles and of polypyrrole from their vibrational spectra
Authors: Zerbi G. , Veronelli M. , Martina S. , Schlüter A. D. , Wegner G.
Published: July 2002

Oligopyrroles with increasing chain length and polypyrroles were recently synthesized with new chemical methods. In this paper, we analyze their infrared and Raman spectra. The spectra can be nicely interpreted in terms of recent theoretical concepts of vibrational dynamics of polyconjugated systems. The spectra thus become a tool for deriving information on the structure of these materials.
Connectivity in a binary mixture of randomly centered spheres with selective particle clustering
Authors: Bresme F. , Abascal J. L. F.
Published: July 2002

We report simulation results for the pair-connectedness functions and the percolation threshold of a binary mixture of randomly centered spheres with selective particle clustering. The study is intended to check integral equation results and to provide a reference to assess association phenomena in ionic systems.
Coupled chaotic states and apparent noise in experiment and model
Authors: Hauser Marcus J. B. , Schneider Friedemann W.
Published: July 2002

We present an experimental and model study of the effect of mass coupling of two similar chaotic states in the Belousov–Zhabotinskii (BZ) reaction. At high coupling strengths the coupled chaotic states become synchronized as shown by a high correlation coefficient. When the coupling strength is decreased, the coupled system passes through a symmetry–breaking transition from synchronized to asynchroneous chaos. At the transition point the direct experimental evaluation of the maximum Lyapunov exponent of the single chaotic system is possible from the coupling strength. At very low coupling strengths the correlation approaches zero. The differences and sums of the two chaotic time series in each reactor are also investigated. At high and moderate coupling strengths, the differences and the sums are verified to be deterministically chaotic on the basis of their fractal dimensionalities, for example. However, for weakly coupled (and uncoupled) chaotic states our analysis with state-of-the-art methods shows that the sum and differences behave as if they are stochastic, although the time series of individual reactions always remain deterministically chaotic. The reason for this apparent ‘‘statistical’’ behavior at low (and zero) coupling strengths is the limited number of the experimental (∼16 000 data points) and numerical data points available for analysis for large Hausdorff dimensions (DH≳3.6). Hyperchaos could not be detected in the coupled chaotic BZ systems.
Study of random copolymers in dilute solution
Authors: Swift B. W. , Olvera de la Cruz M.
Published: July 2002

Single chain AB random copolymers in a solvent which is good for B and bad for A are studied by Monte Carlo simulation. The chain statistics of the polymer coil are analyzed as a function of the number of chain beads, N, and A monomer fraction, fe, in the regime where the chain obeys self-avoiding walk (SAW) statistics. The results are compared to a modified Flory argument [S. P. Obukhov, J. Phys. A 19, 3655 (1986)]. The argument predicts a correction to the mean radius of gyration of the coil, Rg, which scales as N−0.5 in the SAW regime. We fit the model equation to the simulations data and find good agreement. We also calculate the dispersion of Rg as a function of fe. We find that as fe is increased from fe=0 to fe=0.3, the dispersion of Rg increases. Finally, we generate contours of A and B monomer density as a function of the distance from the center of mass of the chain. The contours show the emergence of an A rich region approximately the size of Rg at the center of the coil suggesting the structure of collapsed heterogeneous macromolecules [H. S. Chan and K. A. Dill, Physics Today 2, 24 (1993)].
A self-consistent determination of the atomic and electronic structure of a model liquid metal
Authors: Winn Martyn , Kahl Gerhard
Published: July 2002

Based on a Lennard-Jones two-body interatomic potential supplemented by a tight-binding description of the valence electron band structure, we propose an iterative scheme for the self-consistent determination of the atomic and electronic structure of a model liquid metal. The procedure makes use of recent liquid-state theories of the electronic density of states, modern thermodynamically self-consistent integral equation theories of atomic structure, and ideas from tight-binding bond theory. Results are presented and discussed for several phase points of the model system.
Diffusion-controlled, heterogeneous reaction in a material with a bimodal poresize distribution
Authors: Loewenberg Michael
Published: July 2002

A theoretical analysis is developed for diffusion-controlled, heterogeneous reaction in a material with a bimodal poresize distribution. A configurational averaging procedure is used to describe a sparse, random network of macropore capillaries contained in a reactive, otherwise homogeneous matrix. The analysis is valid for a wide range of reactivities, and is particularly suited for conditions where reactant concentration varies on a length scale comparable to the diameter of, or the spacing between the macropores. The results depend on the reactivity on the matrix, the void volume of the macropore network, and the macropore to matrix diffusivity ratio. A local effectiveness parameter, which depends on the reactivity and macropore void volume, characterizes the reaction behavior in the material. An exact numerical solution is obtained that depends analytically on the parameters of the problem, and an accurate analytical representation is derived that depends very simply on the local effectiveness, the macropore void volume, and the diffusivity ratio; typical results are presented. The solution reduces to simplified models of the heterogeneous reaction in the low- and high-reactivity limits, where reactant concentration varies on a length scale that is large compared to the macropore spacing or small compared to the macropore diameter.
The mutual diffusion coefficient for the van der Waals binary mixtures of types II, III, IV, and V
Authors: Castillo R. , Garza C. , Dominguez H.
Published: July 2002

In the framework of the mean-field kinetic variational theory, a numerical study is presented to understand the concentration dependence of the mutual diffusion coefficient in terms of molecular sizes and interaction parameters for the van der Waals binary mixtures of types II, III, IV, and V, in the scheme of Scott and van Konynenburg. This work is an extension to the study for systems of type I presented by us quite recently. In addition, the behavior of the mutual diffusion coefficient of the van der Waals mixture is compared with that of the hard-sphere mixture and for the case of systems of type II, with experimental data of actual systems: water/n-propanol, n-hexane/acetone, and n-heptane/acetone. The mutual diffusion coefficients for the last two systems were determined by us with the Taylor dispersion technique. The mutual diffusion coefficients for the systems n-hexane/acetone and n-heptane/acetone are reported here at 298.15 and 303.15 K, respectively, along all the concentration range. The explicit model used here allows us to obtain an explanation about the role played by the variables that determine the concentration dependence of the mutual diffusion coefficient for actual systems.
Polydiacetylene chains diluted in their single-crystal monomer matrix
Authors: Spagnoli S. , Berréhar J. , Lapersonne-Meyer C. , Schott M.
Published: July 2002

The spectroscopic properties of the chains of the polydiacetylene (PDA) poly-4BCMU, diluted in their single-crystal monomer matrix are studied and discussed in terms of the single polymer chain electronic properties, the monomer structural changes, and their mutual influence. The polymer exciton transition energy shifts considerably (by 0.3 eV) as temperature decreases, down to a value that may be the lowest transition energy yet observed in a PDA. Two first-order structural transitions of the monomer crystal are observed near 320 and 220 K. They show up in the polymer spectroscopic properties—absorption and resonance Raman scattering—as discontinuities of both wave numbers and linewidths. So, the polymer chains behave as probes of the monomer crystal structural changes. A new electronic transition is observed below the main exciton one, possibly corresponding to a state of g symmetry. A weak fluorescence is observed at low temperature. Its origin coincides with the exciton absorption. Therefore, there is no configurational relaxation in the exciton state during the fluorescence lifetime.
New measurement method of the autocorrelation function in the quasielastic light scattering with the sinusoidal electric field
Authors: Ito Kohzo , Ooi Shuichi , Nishi Noriaki , Kimura Yasuyuki , Hayakawa Reinosuke
Published: July 2002

A new method for measuring the autocorrelation function in the quasielastic light scattering with the sinusoidal electric field specialized for the periodically stationary process proper to QELS-SEF. It has been found from the experimental results for a polystyrene latex solution with the new correlator that we can obtain the complex electrophoretic mobility μ* and the apparent diffusion constant Dapp in the frequency range much wider than the conventional QELS-SEF method.
Rotational resonance with multiple-pulse scaling in solid-state nuclear magnetic resonance
Authors: Spencer Richard G. S. , Fishbein Kenneth W. , Levitt Malcolm H. , Griffin Robert G.
Published: July 2002

Multiple-pulse techniques are applied to rotational resonance experiments in solid-state nuclear magnetic resonance. The usual rotational resonance condition is satisfied when an integral multiple of the magic-angle spinning speed equals the difference in isotropic chemical shifts of the two members of a homonuclear spin-1/2 pair. We show that sequences of rapid periodic radio-frequency pulses scale and rotate both the Zeeman and dipole–dipole Hamiltonians, leading to a modification of the resonance condition and to the introduction of new, single- and double-quantum, rotational resonances. Experimental results are presented which demonstrate these effects in the spectra of doubly 13C-labeled solids.
Direct determination of J coupling between two crystallographically equivalent spins from two-dimensional J-resolved nuclear magnetic resonance spectra under variable-angle-spinning conditions
Authors: Wu Gang , Wasylishen Roderick E.
Published: July 2002

We describe a new solid-state nuclear magnetic resonance (NMR) method for directly measuring the indirect spin–spin coupling J between two crystallographically equivalent, but magnetically nonequivalent spins. We show that under conditions of fast variable-angle-spinning (VAS), two-dimensional (2D) J-resolved NMR spectra can yield an accurate value of the J coupling between a pair of crystallographically equivalent spins. This information is not available from fast magic-angle-spinning (MAS) NMR spectra, or from NMR spectra of isotropic fluids unless a third spin is introduced into the spin system. Furthermore, the observation of J in VAS 2D J-resolved experiments does not rely on the presence of a nonzero dipolar coupling between the equivalent spins. This contrasts with slow-spinning MAS experiments involving crystallographically equivalent spin pairs, where recoupling of the J interaction requires that the two nuclei are also dipolar coupled.
Electrokinetic properties of a charged tracer in a time-dependent external field
Authors: Alarcón-Waess O. , Velasco R. M.
Published: July 2002

In a previous paper [Cruz de León et al., Chem. Phys. Lett. 207, 294 (1993)], we have derived a formalism to calculate the dynamic friction function of a charged tracer macroparticle in a multicomponent ionic solution. Here we apply this theoretical scheme to describe the electrokinetic properties of a charged tracer immersed in a solution in the presence of a time-dependent external driving force. We derive analytic expressions for the self-diffusion coefficient and the electrophoretic mobility. We compute the Van Hove self-correlation function and compare it with previous results. As an example we compute the electrophoretic mobility in the presence of a time-independent external driving force within well-defined limits and approximations. We obtain complete results in the absence of hydrodynamic interactions. Our results derived here provide a framework to describe the dynamics of a charged tracer.
Is ‘‘cross-talk’’ between the lanthanide charge reservoir layers and superconducting planes in cuprates possible?
Authors: Smentek-Mielczarek Lidia
Published: July 2002

The powerful tool of the perturbed function approach has been used to create a theoretical model which explains the observed features and spectroscopic properties of rare earth ions in conducting and superconducting cuprates.
Herzberg–Teller coupling and configuration interaction in a metalloporphyrin model: 1,3,5,7-tetramethylcyclo-octatetraene dianion
Authors: Sztainbuch Isaac W. , Soos Zoltan G. , Spiro Thomas G.
Published: July 2002

1,3,5,7-tetramethylcyclo-octatetraene dianion (TMCOT−2) is employed as a metalloporphyrin model. Its UV-VIS absorption spectrum shows two electronic bands, an intense band (Soret or B) at ∼215 nm and a weaker state (Q band) at 382 nm, while cyclo-octatetraene dianion (COT−2) only has a B band at 236 nm. An exact Pariser–Parr–Pople (PPP) calculation of COT−2 predicts a strong Soret band at 231 nm and a vanishing Q band intensity at 366 nm. Resonance Raman spectrum with excitation near the Q and Soret bands of TMCOT−2 shows enhancement patterns remarkably similar to those of metalloporphyrins. Nontotally symmetric modes are enhanced in resonance with the Q band, while the totally symmetric modes are enhanced in resonance with the B state. The cyclic polyene TMCOT−2 spectrum thus provides direct evidence for Herzberg–Teller coupling of the Q band and B state.
Intramolecular energy flow and bond dissociation in iodoacetylene and iododiacetylene
Authors: Kim Y. H. , Ree J. , Lee C. S. , Shin H. K.
Published: July 2002

Intermolecular and intramolecular energy flow and subsequent bond dissociation in collinear collisions I–C≡C–H+Ar and I–C≡C–C≡C–H+Ar have been studied by classical trajectory techniques over the collision energy range of 0 to 10 eV. When the molecule is initially in the ground state, the overall energy transfer in I–C≡C–H+Ar is very small, but in I–C≡C–C≡C–H+Ar it is large. The collisionally perturbed C–H bond stores a large amount of energy from translation for a brief period during the early stage of collision and transfers most of it to the inner region of the molecule, specifically to the low frequency C–I vibration. Thus the high-frequency vibration of the perturbed C–H bond during the collision plays a crucial role in determining the extent of intramolecular energy transfer and, in turn, C–I dissociation. But in nondissociative collisions, there is another series of the C–H vibration at the latter stage of collision, transferring energy back to translation. This study also considers collision-induced intramolecular energy flow in the molecule with an initially excited C–H bond. The relaxation of the low-lying C–H excitation is very slow on a nanosecond time scale. However, when the excitation is high, the vibrational frequency of the C–H bond is significantly weakened, thus becoming comparable to that of the triple bond, in which case the isolating effect of the adjacent C≡C bond is no longer important and intramolecular energy flow becomes efficient.
He-atom scattering study of Xe coadsorbed with alkali metal atoms on graphite
Authors: White J. D. , Lakin J. V. , Strauss M. A. , Diehl R. D.
Published: July 2002

The coadsorption of alkali atoms with Xe on a graphite substrate has been studied using He-atom scattering and low-energy electron diffraction (LEED). When Xe is adsorbed on a graphite surface which has a low density of alkali metal adatoms, ordered Xe monolayers and bilayers are formed. Differences in adsorption rate and desorption temperatures of the Xe indicate that the presence of alkali metal atoms on the surface increase the sticking coefficient and the desorption temperatures of Xe adatoms. As deduced from specular He-atom scattering desorption curves, this stronger adsorption behavior appears to be a local effect, causing the largest changes for Xe atoms next to the alkali metal adatoms. An increase in the Xe–graphite stretching frequency is also observed with inelastic He-atom scattering, consistent with the stronger adsorption of Xe. We observed no evidence for Xe adsorption onto full alkali metal overlayers down to 40 K, consistent with theoretical studies which indicate small physisorption well depths for rare gases on alkali metal surfaces.