bluuebunny/crossref_metadata_2025 huggingface dataset:
Probe diffusion in an aqueous polyelectrolyte solution
Authors: Gorti Sridhar , Ware Bennie R.
Published: July 2002

Using the technique of fluorescence photobleaching recovery, we have measured the tracer diffusion coefficients of probe particles in aqueous solutions of polystyrene sulfonate at various ionic strengths. Data were fit well by the expression D/D0=exp[−acν′], where D is the probe diffusion coefficient at polymer concentration c, D0 is the probe diffusion coefficient in the absence of polymer, and a and ν′ are familiar scaling parameters. The parameter ν′ ranged from a maximum of 1.0 for the conditions with maximum intersegment interaction (low salt, high-molecular weight, high-charge density) to a minimum of about 0.5 for the high-salt, low-charge density cases. The parameter a showed little sensitivity to polymer charge density or ionic strength but was a strong function of probe size. No discontinuities in the probe diffusion coefficients or analytical parameters were observed under any solution conditions, even though experimental variations included conditions that are expected to correspond to the transition to the semidilute concentration regime and to the ordinary–extraordinary phase transition.
Importance of membrane fluidity on bilayer interactions
Authors: Sornette Didier , Ostrowsky Nicole
Published: July 2002

A new model is proposed to account for the observed enhancement with bilayer fluidity of the short-range membrane interaction. It involves spontaneous, although hindered, membranes undulations, whose role is essentially to renormalize the membrane interaction. This effect is shown to enhance the short-range repulsion and partially account for experimental results. Relevance of this formalism to other problems of surface science is also pointed out.
Order–disorder transitions in Langmuir–Blodgett films. III. Polarized Raman studies of cadmium arachidate using integrated optical techniques
Authors: Rabe J. P. , Swalen J. D. , Rabolt J. F.
Published: July 2002

Polarized Raman measurements of Langmuir–Blodgett (LB) multilayers of cadmium arachidate (CdA) have been obtained through the application of integrated optical techniques. Excitation of either the transverse electric (TE) or transverse magnetic (TM) mode of an asymmetric slab waveguide was alternately used to produce Raman scattering in the CdA multilayers deposited on its surface. Analysis of the 90° scattered light with a polarizer placed at two mutually perpendicular positions for both the TE and TM modes of the waveguide produced a set of four measurements which were used to determine the orientation and lateral order of the CdA monolayer components. The extent of order in the LB film was then investigated as a function of temperature and compared with that of bulk CdA. The temperature of the order–disorder transition was found to be similar in both cases and consistent with that found previously from infrared studies.
1H shielding anisotropy in Mg(OH)2: The isolated OH− group
Authors: Sears R. E. J. , Kaliaperumal R. , Manogaran S.
Published: July 2002

The proton NMR shielding tensor was measured at room temperature in the hexagonal layered compound magnesium hydroxide. The isotropic shielding is 29.9±2 and the anisotropy is 12.7±2 ppm, on the 1H absolute scale. From a Gierke and Flygare (GF) analysis of the data the OH− spin–rotation constant was found to be 51±14 kHz. The GF contributions from the neighbors of a given OH− group were subtracted from the experimental shielding and corrections were made for the effects of vibrational motion to give an isotropic shielding of 29.9±2, with an anisotropy of 13.7±2 ppm, for an isolated OH−. The OH− shielding anisotropy is in good agreement with the ab initio calculations of Ratcliffe et al., but the isotropic shielding is not. Proton spin–lattice relaxation measurements at 300 and 77 K gave evidence of proton conductivity in Mg(OH)2.
Monte Carlo calculation of the quantum partition function via path integral formulations
Authors: Kono H. , Takasaka A. , Lin S. H.
Published: July 2002

Using Bennett’s Monte Carlo (MC) method, we calculate the quantum partition functions of path integral formulations. First, from numerically exact results for a harmonic oscillator and a double-well potential, we discuss how fast each approximate partition function converges to the exact value as the number of integral variables involved in the formulation is increased. It turns out that most effective and most suitable for the MC simulation is Takahashi and Imada’s path integral fomulation based on a modified Trotter formula in which the original potential is replaced with an effective one. This formulation is well balanced between the following two factors: the effect of zero potential energy is underestimated, resulting in an improper increase in the partition function; and, on the other hand, effective potential restricts the motion of fictitious particles born in the formulation so that the partition function value tends to be smaller. Fictitious particles can be treated as classical ones. We therefore can apply Bennett’s MC method to calculating the ratio of two quantum partition functions (of a system under consideration and a reference system). As the number of fictitious particles N is increased, choice of reference system becomes less and less important and multistage sampling becomes dispensable. This, to some extent, compensates for the expense that N is larger than the real particle number. The tunneling mechanism of fictitious particles in the simulation is discussed.
Electric-field-induced Raman spectroscopy
Authors: Andrews David L. , Blake Nick P.
Published: July 2002

In this final paper of a series on electric-field-induced spectroscopy, the general theory of electro-optical effects in vibrational Raman spectroscopy is developed. It is demonstrated that the electrical polarization of a dipolar fluid can lead to appreciable intensity enhancement of certain lines in its Raman spectrum, and a modification of the polarization rules normally applicable to an isotropic medium. Exact intensity expressions are presented, and it is shown how the complete symmetry analysis of all Raman transitions can be accomplished with five intensity measurements, introducing new possibilities for species characterization close to electrode surfaces. It is also demonstrated that in intense electric fields a novel nonlinear electro-optical channel exists, whereby the electric field perturbs the stationary states of the system and gives rise to selection rules completely different to those which normally apply in Raman scattering. This can result in the appearance of entirely new lines in the spectrum. Detailed examination of the intensity expressions for the electro-optical route reveal that in this case five intensity measurements are required for a complete elucidation of the Raman spectrum.
Aperiodicity resulting from two-cycle coupling in the Belousov–Zhabotinskii reaction. III. Analysis of a model of the effect of spatial inhomogeneities at the input ports of a continuous-flow, stirred tank reactor
Authors: Györgyi László , Field Richard J.
Published: July 2002

Deterministic chaos is a well-established phenomenon in continuous-flow, stirred tank reactor (CSTR) experiments with the oscillatory Belousov–Zhabotinskii (BZ) reaction. However, it has not yet been possible to reproduce the experimentally observed, robust chaos in simulations using realistic models of the homogeneous chemical kinetics of the BZ reaction. That it may be necessary to consider spatial inhomogeneities in modeling the BZ chaos is suggested by the existence of strong stirring effects on the aperiodic behavior and by the difficulty of reproducing chaos under the same conditions in reactors of different physical configuration. Such inhomogeneity might result from a lack of perfect mixing in the CSTR, especially near the inlets, or from diffusion of species at low flow rates from the CSTR reaction mixture into the tips of the inlets. The presence of spatial inhomogeneities allows coupling between essentially independent oscillators, a well-known source of chaos. Such a model using a realistic representation of the BZ kinetics leads to an eight-variable set of ordinary differential equations. Numerical analysis of these equations by continuation methods and by numerical integration shows the existence of broad regions of chaos and various hysteresis effects involving limit cycles, a steady state and/or a strange attractor. Tristability was found in calculations in a narrow flow rate range. The computed behavior appears for parameter values closely related to the values used experimentally to obtain similar phenomena, and the visual similarity of the computed and experimental strange attractors is striking. The experimental routes to chaos, period doubling, intermittency, and secondary Hopf bifurcations are all reproduced in the simulations. The computed and experimental structures of periodic windows observed within chaotic regions also are very similar.
Intermolecular site–site correlation functions of athermal hard-sphere chains: Analytic integral equation theory
Authors: Chiew Yee C.
Published: July 2002

The intermolecular site–site correlation function gij(r) of freely jointed athermal tangent hard-sphere chains (HSC) are determined based on a particle–particle description of the chain system. We developed a mathematical framework through which the functions gij(r) can be obtained within the context of the Percus–Yevick (PY) integral equation theory. The computational scheme involves the analytic solution to the structure of a multicomponent particle mixture that is subject to some specified connectivity constraints. In particular, we determined the site–site correlation functions gij(r) of homonuclear 4-mer chains, and homonuclear 4-mer chains in a hard-sphere solvent. Analytic expressions for the site–site and site–solvent correlation functions at contact, i.e., gij(σ+), are obtained. Site–site correlation functions determined from the PY theory show the ‘‘correlation hole’’ behavior at low density. Comparison of the PY results with computer simulation data shows that the theory yields accurate values for an averaged ‘‘total’’ correlation function, and captures the essential features of the site–site correlation functions gij(r). In addition the effect of hard-sphere solvent particles on the structure of HSC molecules is also investigated.
Molecular dynamics studies of polar/nonpolar fluid mixtures. I. Mixtures of Lennard-Jones and Stockmayer fluids
Authors: de Leeuw S. W. , Smit B. , Williams C. P.
Published: July 2002

The results of canonical ensemble molecular dynamics calculations of mixtures of Lennard–Jones and Stockmayer fluids are reported. To study solely the influence of the polarity, the Lennard-Jones parameters were identical for both components. The excess mixing properties show a strong asymmetry with respect to composition for large dipolar strength. The free energy of mixing is obtained through a thermodynamic integration procedure. The results strongly suggest that, for reduced dipolar strengths μ2>3.15, demixing occurs into a phase rich in polar component and an almost pure Lennard-Jones fluid. It is shown that perturbation theory yields fairly accurate results for the dipolar energy and free energy of the mixture. For the free energy of mixing, qualitatively correct results are obtained. The structure and orientational correlation functions of the mixture are discussed. The radial distribution function for pairs of polar molecules show a marked increase in local ordering with dipolar strength for low concentrations of the polar component, indicating that strong clustering of polar molecules occurs at these concentrations. The orientational order is also seen to increase very strongly with dipole moment at these concentrations. The pair correlation function for pairs of Lennard-Jones atoms shows little dependence on dipolar strength of Stockmayer molecules at these concentrations. The distribution function for pairs of unlike molecules reflects the increasingly dissimilar character of these molecules as the dipolar strength increases. For large concentrations of Stockmayer molecules, the opposite effect is observed, albeit less pronounced, in that the pair-correlation function for Lennard-Jones atoms shows an increase in local ordering as μ increases, whereas the radial distribution function for Stockmayer pairs remains relatively unaffected with increasing μ. These results are interpreted in terms of a frustation model. Results are given for the variation of the dielectric constant of the mixture with composition and dipolar strength.
Collective lipid motions in bilayer membranes studied by transverse deuteron spin relaxation
Authors: Stohrer Jürgen , Gröbner Gerhard , Reimer Dirk , Weisz Klaus , Mayer Christian , Kothe Gerd
Published: July 2002

The anisotropy and pulse frequency dispersion of the spin–spin relaxation time TCP2E from Carr–Purcell–Meiboom–Gill pulse sequences is employed to evaluate the major contribution to transverse 2H spin relaxation in bilayer membranes. Analysis of the experiments is achieved in terms of a density operator formalism, employing the stochastic Liouville approach. From a comparison of the observed angular and frequency dependences of TCP2E with those predicted for order director fluctuations, we conclude that collective lipid motions constitute the dominant transverse relaxation process. Computer simulations provide the viscoelastic parameters of the lipid membranes. For 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayers at T=318 K an average elastic constant of K=2×10−11 N and an effective viscosity of η=0.1 P have been determined. Using the experimentally accessible value for the long wavelength cutoff of the elastic modes, one obtains the mean square amplitude of the director fluctuations 〈θ20〉=0.04. This corresponds to an order parameter of SOF=0.94. Apparently, the contributions of the collective motions to the measured order parameters are marginal.
Jet emission spectroscopy of OH and OD near 1850 Å: First observation of a 2Π→2Π electronic transition of OD
Authors: Huber K. P. , Holland F. , Coxon J. A.
Published: July 2002

The complex many-line systems observed at around 1850 Å from discharges containing H2O or D2O [A. Michel, Z. Naturforsch. 12a, 887 (1957); P. Felenbok and J. Czarny, Annu. Astrophys. 27, 244 (1964)] have been reinvestigated at high resolution in the emission from supersonic jet expansions. Mixtures of He with D2O produce a conspicuous progression of red shaded bands that overlap the many-line spectra but have not been reported before. The new transitions originate at large internuclear distance in a single 2Π vibronic level of OD at 88 294 cm−1 above the minimum of the ground-state potential, and they end on high vibrational levels, v″=13,...,18, of X 2Π. Term values and spectroscopic parameters for the initial and final states have been determined; the upper state, labeled C′2Π, is inverted with a spin-orbit interaction that is close to the A value of X 2Πi. Analogous bands of OH have not been detected, and in contrast with the many-line emissions of both isotopes which persist in the presence of Ar as well as of He and in discharges through pure water vapor, the new C′2Π→X 2Π bands of OD appear only in mixtures with He. Guided by ab initio calculations for the 2Π excited states of OH [E. F. van Dishoeck and A. Dalgarno, J. Chem. Phys. 79, 873 (1983)], the interpretation of the experimental results concludes that all of the observed emission at around 1850 Å originates in a group of more or less strongly mixed 2Π levels which reflect the radiative properties and excitation characteristics of two interacting stable diabatic states and which escape predissociation by a repulsive third 2Π state.
Three-dimensional velocity analysis combining ion imaging with Doppler spectroscopy: Application to photodissociation of HBr at 243 nm
Authors: Kinugawa Tohru , Arikawa Tatsuo
Published: July 2002

A simple and sensitive method combining ion imaging with Doppler spectroscopy has been developed to measure full three-dimensional velocity distributions of low energy scattering particles. This technique was applied to the photolysis study of an HBr molecular beam at 243 nm.
Photochemistry of adsorbed molecules. XI. Charge-transfer photodissociation and photoreaction in chloromethanes on Ag(111)
Authors: Dixon-Warren St. J. , Jensen E. T. , Polanyi J. C.
Published: July 2002

Photoinduced charge-transfer (CT) dissociation of adsorbates is reported in this paper for a series of chloromethanes (RCl) adsorbed on Ag(111). The chloromethanes were CCl4, CHCl3, CH2Cl2, CH3Cl, and CCl3Br. The observation that Cl− ions were emitted following UV laser irradiation of the chloromethane covered metal surface gave direct evidence for CT photodissociation RCl/Ag(111)+hν→(RCl−)‡/Ag(111)→Cl−+R/Ag, in which a photoexcited substrate electron caused dissociation of RCl. The yield of Cl− emission for varying R decreased in a manner similar to the gas-phase dissociative attachment cross sections at low electron energy (≤1 eV) for RCl (σgDA). The cross section for Cl− emission as a function of RCl coverage showed a sharp maximum at the completion of the first molecular layer of CCl4, CHCl3, CH2Cl2, and CCl3Br. This enhanced cross section was ascribed to exothermicity of a photoreaction of R with the metal surface which imparted translational energy to Cl− permitting it to escape from its image potential. The cross section, σS/PRXNCT for photoinduced surface reaction yielding Cl–Ag(111) at the RCl covered surface was measured for CCl4, CHCl3, CH2Cl2, and CH3Cl. The yield of Cl–Ag(111) for varying R was found to change in a manner similar to the total low energy electron capture cross section in the gas phase σgtot. The yield decreased only slightly with increasing laser wavelength in striking contrast to the direct photolysis cross section. A CT mechanism was proposed in which a temporary negative ion formed by photoinduced CT reacted with the metal surface RCl/Ag(111)+hν→(RCl−)‡→R/Cl–Ag(111). This CT photoreaction was shown to occur with a photon energy ∼1 eV below the work function. Hot electrons, rather than free electrons, were the major agent for inducing photoreaction at all wavelengths.
Polymer-induced forces between colloidal particles. A Monte Carlo simulation
Authors: Dickman Ronald , Yethiraj Arun
Published: July 2002

Monte Carlo simulations of the fluctuating bond lattice model are used to determine the force between colloidal particles immersed in a nonadsorbing polymeric fluid. Monodisperse systems with chain lengths of 20 to 100 segments are studied at occupation fractions ranging from 0.1 to 0.6, covering the semidilute and dense regimes. The variation of the force with concentration, particle diameter, and interparticle separation is in qualitative agreement with predictions of scaling theory and of integral equations for the colloid–polymer system. In semidilute solutions the force is purely attractive and displays an approximately linear dependence upon separation for small colloid separations. At higher concentrations the force is repulsive, for certain separations.
Evaporation model of cluster scattering from surfaces
Authors: Marković Nikola , Pettersson Jan B. C.
Published: July 2002

We present classical trajectory calculations of ArnNem (n+m=111, 859) clusters scattering from a rigid surface. The dynamics of energy transfer and cluster decomposition during surface scattering is investigated for incident velocities of 100–700 m/s. The initial translational energy is at impact effectively transferred into internal degrees of freedom of the cluster. The overall energy transfer efficiency is very high but not complete, leaving too much energy in translation. No fragmentation takes place below 200 m/s. At incident velocities below 450 m/s, evaporation of small fragments from the heated cluster takes place in thermal equilibrium with the vibrational degrees of the cluster. This thermal evaporation is also the dominating ejection channel up to 700 m/s. Above 450 m/s, the formation of a compressed zone at impact opens up a new channel with ejection of fast fragments parallel to the surface plane. This effect becomes increasingly important at higher velocities. An evaporation model where fragmentation of the heated cluster takes place as isotropic and thermal ejection of small fragments is concluded to account for the major fragmentation processes observed.
Stochastic dynamics of a subsystem interacting with a solid body with application to diffusive processes in solids
Authors: Tsekov Roumen , Ruckenstein Eli
Published: July 2002

In this paper the dynamics of a mechanical subsystem interacting with a solid body is studied. The Newton equations are transformed to a set of stochastic generalized Langevin equations describing the evolution of the coordinates of the subsystem particles. The solid is modeled as a bath of interacting harmonic oscillators, and the effect of their spatial correlations on the statistical properties of the Langevin forces is accounted for. The most important result is the relation established between the static interaction of the subsystem with the solid body and the dissipative and fluctuation forces. In the particular case of a subsystem consisting of a single particle, an expression is derived for the friction tensor in terms of the static interaction potential and Debye cutoff frequency of the solid. The analysis is applied in the latter case to some simple processes occurring in solids, such as adsorption, desorption, and diffusion.
Direct observation of stacking disorder in a colloidal crystal
Authors: Verhaegh Nynke A. M. , van Duijneveldt Jeroen S. , van Blaaderen Alfons , Lekkerkerker Henk N. W.
Published: July 2002

Colloidal crystals of rhodamine labeled silica spheres dispersed in chloroform have been studied with fluorescence confocal scanning laser microscopy. We report the first determination of the three-dimensional crystalline stacking in the bulk of a concentrated dispersion. The structure was found to consist of a random stacking of close-packed planes. The results are compared with light scattering experiments.
Far-infrared permanent and induced dipole absorption of diatomic molecules in rare-gas fluids. I. Spectral theory
Authors: Roco J. M. M. , Hernández A. Calvo , Velasco S.
Published: July 2002

We present a spectral theory for the far-infrared absorption spectrum of a very diluted solution of diatomic molecules in a rare-gas fluid, that includes permanent and induced contributions. The absorption coefficient is given as the convolution of a translational spectrum and a rotational spectrum. The former is described in terms of time correlation functions associated to the induced dipole moment. The latter is discussed on the basis of a model consisting of a quantum rigid rotor interacting with a thermal bath, making use of time correlation functions associated to the different anisotropic orders of the solute–solvent intermolecular potential. Non-Markovian and line mixing effects are taken into account. Explicit expressions for the five leading contributions of the induced dipole moment are given.
Photodissociation spectroscopy and dynamics of the N2O−2 anion
Authors: Osborn David L. , Leahy David J. , Cyr Douglas R. , Neumark Daniel M.
Published: July 2002

The spectroscopy and dissociation dynamics of the N2O−2(C2v) anion have been investigated using the technique of fast ion beam translational spectroscopy. A newly developed pulsed supersonic discharge source is described for the production of internally cold N2O−2. A structured absorption band beginning near 580 nm is observed, and is assigned to the B̃(2A2)←X̃(2B2) transition with the aid of ab initio calculations. Two dissociation channels from the upper state are observed: (1) O−+N2O and (2) NO−+NO. Translational energy and angular distributions are measured for both channels at several excitation energies. The translational energy distribution for channel (1) at 570 nm shows resolved structure corresponding to N2O vibrational excitation. The translational energy distributions for channel (1) are reasonably well described by prior distributions, indicating this channel results from dissociation from the N2O−2 ground electronic state. In contrast, channel (2) appears to result from dissociation on a repulsive excited electronic state. From the translational energy distributions for channel (1), we obtain the bond dissociation energy and heat of formation (at 0 K) for N2O−2: D0(O–N2O)=1.40±0.03 eV and ΔH0f,0(N2O−2)=0.58±0.04 eV.
Underpotential deposition of Cu on Au(111) in sulfate-containing electrolytes: A theoretical and experimental study
Authors: Zhang Jun , Sung Yung-Eun , Rikvold Per Arne , Wieckowski Andrzej
Published: July 2002

We study the underpotential deposition of Cu on single-crystal Au(111) electrodes in sulfate-containing electrolytes by a combination of computational statistical-mechanics based lattice-gas modeling and experiments. The experimental methods are in situ cyclic voltammetry and coulometry and ex situ Auger electron spectroscopy and low-energy electron diffraction. The experimentally obtained voltammetric current and charge densities and adsorbate coverages are compared with the predictions of a two-component lattice-gas model for the coadsorption of Cu and sulfate. This model includes effective, lateral interactions out to fourth-nearest neighbors. Using group-theoretical ground-state calculations and Monte Carlo simulations, we estimate effective electrovalences and lateral adsorbate–adsorbate interactions so as to obtain overall agreement with experiments, including both our own and those of other groups. In agreement with earlier work, we find a mixed (√3×√3) phase consisting of 2/3 monolayer Cu and 1/3 monolayer sulfate at intermediate electrode potentials, delimited by phase transitions at both higher and lower potentials. Our approach provides estimates of the effective electrovalences and lateral interaction energies, which cannot yet be calculated by first-principles methods.
The role of energy denominators in self-consistent field (SCF) calculations for open shell systems
Authors: Kollmar Christian
Published: July 2002

The transformation of the molecular orbitals (MO’s) of open shell systems during a self-consistent field iteration step is compared for two different approaches: the usual procedure of iterative diagonalization of a Fock matrix and a simplified direct approach using the gradient and the one-electron Hessian matrix in a space of orbital rotation parameters. It is shown that the frequent failure of the former is due to inappropriate energy denominators given by the difference of MO energies. In contrast to closed shell systems, the elements of the one-electron Hessian matrix cannot be represented consistently by such differences in the case of open shell systems. The results of numerical calculations support this finding.
Thermal rate constant calculation using flux–flux autocorrelation functions: Application to Cl+H2→HCl+H reaction
Authors: Wang Haobin , Thompson Ward H. , Miller William H.
Published: July 2002

An efficient method was recently introduced by Thompson and Miller [J. Chem. Phys. 106, 142 (1997)] for calculating thermal rate constants using the flux–flux autocorrelation function with absorbing boundary conditions. The method uses an iterative method to exploit the low rank feature of the Boltzmannized flux operator and subsequently only propagates the eigenvectors that have significant contributions to the rate constant. In the present article, this method is used to calculate the thermal rate constants of the Cl+H2→HCl+H reaction in the temperature range of 200–1500 °K. Total angular momentum is treated by employing the body-fixed axis frame, both exactly and also via various approximations. Comparisons with previous exact and approximate theoretical results are made.
The ion pairing and hydration structure of Ni2+ in supercritical water at 425 °C determined by x-ray absorption fine structure and molecular dynamics studies
Authors: Wallen Scott L. , Palmer Bruce J. , Fulton John L.
Published: July 2002

The ion pairing structure of Ni(Br)2 solutions (0.2 and 0.4 molal) under supercritical conditions was determined using x-ray absorption fine structure (XAFS) spectroscopy. These first measurements of the average bulk structure show that approximately one Br− counterion is associated with each Ni2+. The Ni2+-to-Br− distance of 2.40 Å is very accurately determined and the strength of this interaction, as indicated by the Debye–Waller factor (σ2=0.009 Å2), shows that the bromine anion is very tightly bound to the nickel cation under these supercritical conditions. In addition to the onset of ion pairing interactions, there is also a dramatic transition in the hydration structure. Results show a loss of about 50% of the waters in the first shell upon going from ambient to a hydrothermal condition of 425 °C and 690 bar. Finally, we use molecular dynamics simulations with refined intermolecular potentials to directly calculate XAFS spectra that are shown to quantitatively reproduce the experimental results for the ion pair and hydration structure.
On the ground state of NiH2
Authors: Barysz Maria , Papadopoulos Manthos G.
Published: July 2002

The uncertainty over the symmetry of the ground state of NiH2 is resolved, showing that it is a bent  1A1. The computations have been performed using a complete active space self-consistent field wave function (CASSCF), a second order perturbation method (CASPT2), and quasirelativistic corrections to the energy and geometry.
Fragmentation of ionized liquid helium droplets: A new interpretation
Authors: Callicoatt Berton E. , Förde Kirk , Jung Lilian F. , Ruchti Thomas , Janda Kenneth C.
Published: July 2002

Pure liquid helium droplets of mean size 〈N〉=100–15 000 atoms ionized by electron impact show surprisingly similar ion fragment distributions. For all cluster sizes He2+ is the most probable cluster ion fragment, accounting for 30%–70% of the total ion yield. The high relative intensity of He2+ for the larger clusters shows that the droplets dissipate the ionization energy through an impulsive process, which ejects He2+ from the cluster, rather than by thermal evaporation. The other helium ion fragments that have been the focus of previous studies are most likely formed by a similar mechanism.
Evidence for phase separation during the crystallization of hyperquenched glassy clathrate hydrate forming solutions
Authors: Tulk C. A. , Ba Y. , Klug D. D. , McLaurin G. , Ripmeester J. A.
Published: July 2002

Tetrahydrofuran, a water-soluble cyclic ether molecule, is well known for its ability to form clathrate hydrates (cubic structure II, melting point 277.5 K). An aqueous solution of THF (17:1 molecular ratio) was hyperquenched (cooled at a rate of 106 Ks−1) to 77 K to give recoverable samples of ∼1 g of metastable amorphous material. The crystallization process was studied on three complementary length scales by x-ray powder diffraction, quadrupole echo double resonance (QEDOR) NMR spectroscopy, and Raman spectroscopy as the amorphous material was annealed at temperatures between 80 and 190 K. Results show that phase separation of the two components occurred predominantly during the annealing process, resulting in clusters of crystalline THF and ice Ic before clathrate hydrate crystallization was initiated. During the hyperquenching process, the decreasing molecular mobility of water molecules between room temperature and the fictive temperature (the temperature below which the solution becomes structurally immobilized upon hyperquenching) inhibits growth of the crystal nuclei that normally would result in crystalline clathrate hydrate formation. Annealing the samples between 110–140 K showed that the local hydrogen bonded O–H--O structure of the water molecules changed toward the arrangement characteristic of crystalline ice Ic. We propose that this process forces the THF out of the glassy solution, thus increasing the THF cluster size in the deposit so that these become visible to diffraction. Further annealing, above 150–160 K caused large-scale growth of crystalline clathrate hydrate material, which then could be associated with a reduction in the THF cluster size, and ice Ic.
Self-consistent-field modeling of complex molecules with united atom detail in inhomogeneous systems. Cyclic and branched foreign molecules in dimyristoylphosphatidylcholine membranes
Authors: Meijer L. A. , Leermakers F. A. M. , Lyklema J.
Published: July 2002

We have developed a detailed self-consistent-field model for studying complex molecules in inhomogeneous systems, in which all the molecules are represented in a detailed united atom description. The theory is in the spirit of the approach developed by Scheutjens and co-workers for polymers at interfaces and self-assembly of surfactants and lipids into association colloids. It is applied to lipid membranes composed of dimyristoylphosphatidylcholine (DMPC). In particular, we looked at the incorporation of linear, branched, and cyclic molecules into the lipid bilayers being in the liquid phase. Detailed information on the properties of both the lipids and the additives is presented. For the classes of linear and branched alcohols and phenol derivatives we find good correspondence between calculated partition coefficients for DMPC membranes and experimental data on egg-yolk PC. The calculated partitioning of molecules of isomers, containing a benzene ring, two charged groups (one positive and one negative) and 16 hydrocarbon segments, into DMPC membranes showed variations of the partition coefficient by a factor of 10 depending on the molecular architecture. For zwitterionic additives we find that it is much more difficult to bring the positive charge into the membrane core than the negative one. This result can be rationalized from information on the electrostatic potential profile of the bare membrane, being positive in both the core and on the membrane surface but negative near the position of the phosphate groups. For several tetrahydroxy naftalenes we found that, although the partition coefficient is barely influenced, the average orientation and position of the molecule inside the membrane is strongly dependent on the distribution of the hydroxyl groups on the naphthalene rings. The orientation changes from one where the additive spans the membrane when the hydroxyls are positioned on (2,3,6,7) positions, to an orientation with the rings parallel to the membrane surface and located near the head group–hydrophobic core interface for the hydroxyls at the (1,3,5,7) positions. We propose that, when our model is used in combination with octanol/water partitioning data, a very accurate prediction is possible of the affinity of complex molecules for lipid membranes.
A new, fully ab initio investigation of the NO(X 2Π)Ar system. I. Potential energy surfaces and inelastic scattering
Authors: Alexander Millard H.
Published: July 2002

We report new coupled-cluster [CCSD(T)] ab initio calculations of the two potential energy surfaces (PES’s) of the Ar–NO complex. Successively larger basis sets are used to extrapolate to the complete basis set limit. Although qualitatively very similar to our earlier PES’s [M. H. Alexander, J. Chem. Phys. 99, 7725 (1993)], the new PES’s have substantially deeper wells (De=116 cm−1). Full close-coupled integral inelastic cross sections were determined at collision energies of 442 and 1774 cm−1 for transitions out of the lowest NO rotational level (j=ω=1/2). For transitions into higher rotational levels of the same spin–orbit manifold the cross sections are little changed from previously calculated values, and still in some disagreement with recent experiments. For transitions in which the spin–orbit manifold changes (ω=1/2→3/2), the calculated integral cross sections are larger than the previously calculated values at E=442 cm−1 but agree quite closely at E=1774 cm−1. At both energies, however, the calculated cross sections for spin–orbit changing transitions are noticeably smaller than experimentally derived values.
Effective potentials for ion–DNA interactions
Authors: Lyubartsev A. P. , Laaksonen A.
Published: July 2002

Effective, solvent-mediated interaction potentials between alkali metal ions (Li+, Na+, K+, and Cs+) and selected sites on the DNA surface are derived from molecular dynamics simulations data. The effective potentials reproduce the corresponding ion–ion and ion–DNA radial distribution functions, obtained in the detailed full-atomistic molecular dynamics simulations. In this work, we have first calculated the ion–DNA and ion–ion radial distribution functions from molecular dynamics simulations of a periodic fragment of double-helix DNA in ionic aqueous solution. Thereafter we applied the inverse Monte Carlo method, previously suggested by us [Phys. Rev. E 52, 3730 (1995)], to derive the effective interaction potentials. Finally, a considerably larger DNA fragment was constructed together with its surrounding solvent environment and Monte Carlo simulations were performed to calculate the ion distributions and the relative binding affinities of different alkali ions to DNA. The results agree well with available experimental data.
Diffusion-controlled kinetics of the helix–coil transition with square barrier hydrogen bonds
Authors: Jun Bokkyoo , Weaver David L.
Published: July 2002

The coil α-helix (and reverse) transition in peptides is modeled as a sequential diffusive kinetic process in which the fundamental event is the diffusion back and forth over a square barrier to propagate or dissolve a single hydrogen bond. The model is solved exactly numerically in one-dimension (the reaction coordinate), for helix and coil probabilities as a function of (1) time, (2) the number of hydrogen bonds, and (3) temperature. In addition, a modified first-passage time is calculated as the time scale of the coil to helix transition. The results of the diffusion model calculations are compared with recent experiments and we show how the model may give insight into protein folding kinetics. The mechanistic diffusion model complements the Master equation model applied previously to the coil–helix folding problem and provides insight into the choice of a useful reaction coordinate for the process.
Impulsive stimulated thermal scattering study of structural relaxation in supercooled glycerol
Authors: Paolucci Dora M. , Nelson Keith A.
Published: July 2002

Impulsive stimulated thermal scattering (ISTS) studies are conducted on glycerol in the supercooled liquid range. ISTS is used to measure the Debye–Waller factor, and to characterize the structural relaxation dynamics in the range from a few nanoseconds to hundreds of microseconds. The results are compared to predictions of mode-coupling theory (MCT), and suggest that inclusion of important microscopic degrees of freedom may be needed for MCT to successfully describe hydrogen-bonded liquids like glycerol.
A variable coefficient Korteweg–de Vries equation: Similarity analysis and exact solution. II
Authors: Nirmala N. , Vedan M. J. , Baby B. V.
Published: July 2002

A Korteweg–de Vries (KdV) equation with time-dependent coefficients is studied in this paper. The similarity transformation for this system is investigated and an exact solution in a particular case is obtained. The Ablowitz–Ramani–Segur (ARS) conjecture is used to identify the integrability of the system. It is found that in some special cases the system may be integrable.
Anisotropic fluid with SU(2)-type structure in general relativity: A model of localized matter
Authors: Letelier Patricio S. , Verdaguer Enric
Published: July 2002

A model of anisotropic fluid with three perfect fluid components in interaction is studied. Each fluid component obeys the stiff matter equation of state and is irrotational. The interaction is chosen to reproduce an integrable system of equations similar to the one associated to self-dual SU(2) gauge fields. An extension of the Belinsky–Zakharov version of the inverse scattering transform is presented and used to find soliton solutions to the coupled Einstein equations. A particular class of solutions that can be interpreted as lumps of matter propagating in empty space-time is examined.
The zero curvature formulation of the sKdV equations
Authors: Das Ashok , Roy Shibaji
Published: July 2002

The fermionic extensions of the KdV equation are derived from the zero curvature condition associated with the superalgebra OSp(2‖1). This derivation clarifies why there are only two such extensions possible and why only one of them is supersymmetric. A Lenard type of derivation of these equations is also presented.
Characteristic functions and invariants of supermatrices
Authors: Kobayashi Yuji , Nagamachi Shigeaki
Published: July 2002

The characteristic functions and the invariants of supermatrices are studied. It is shown that the Euclidean algorithm is useful in obtaining a system of invariants.
Phase-space representations of general statistical physical theories
Authors: Singer Matthias , Stulpe Werner
Published: July 2002

It is shown that Hilbert-space quantum mechanics and many other statistical theories can be represented on some phase space, in the sense that states can be identified with probability measures and observables can be described by functions. In the general context of statistical dualities, informationally complete observables are introduced and a theorem on their existence is proven. The correspondence between these observables and the injective affine mappings from the states into the probability measures on phase space, i.e., the phase-space representations, is pointed out. In particular, a description of all observables by functions is presented, such that all expectation values can, in arbitrarily good physical approximation, be calculated as integrals. Moreover, some new aspects of the particular case of those phase-space representations of quantum mechanics that are related to certain joint position-momentum observables are discussed.
Spectral properties of a simple Hamiltonian model
Authors: de Oliveira César R.
Published: July 2002

The spectral properties of a Hamiltonian dependent on a parameter α are investigated. Due to the linearity in momentum and periodicity it was possible to strengthen some known results for the analogous kicked model. It is rigorously shown that, for any potential in a ‘‘large’’ set, the spectrum is pure for each fixed value of α. Depending on the arithmetic character of α the spectrum is pure absolutely continuous, pure singular continuous, or pure point. It is also shown that in some sense the classical dynamics is identical to the quantum one.
Time-dependent Kaluza–Klein soliton solutions
Authors: Liu Hongya , Wesson Paul , Ponce de Leon J.
Published: July 2002

Time-dependent generalizations of the standard static, spherically symmetric solutions of the empty Kaluza–Klein or (4+1) Einstein equations are considered herein. Such time-dependent solutions exist because Birkhoff’s theorem does not hold in (4+1) gravity. The solutions depend on the time and radius but not the fifth or internal coordinate, and fall into four classes with distinct mathematical properties. Physically, some of our solutions represent a static 4D space–time with an oscillating or contracting extra dimension, which suggests their relevance to particle physics. Others represent an expanding 3D space with a contracting extra dimension, which suggests their relevance to cosmology.
Localization of energy in general relativity
Authors: Maluf José W.
Published: July 2002

In the framework of the teleparallel equivalent of general relativity the energy density εg of asymptotically flat gravitational fields can be naturally and unambiguously defined. Upon integration of εg over the whole three dimensional space the ADM energy is obtained. εg is used to calculate the energy inside a Schwarzschild black hole.
Quantization of the electromagnetic field on static space–times
Authors: Furlani E. P.
Published: July 2002

A quantization of the electromagnetic four-potential in the Lorentz gauge on static space–times with compact Cauchy surfaces is developed herein. For the classical problem, the field solution is obtained by use of a Hilbert space approach in which time evolution reduces to a unitary mapping of Cauchy data. For the quantum problem, a representation of the CCRs on a Fock space is constructed. The gauge condition is imposed as a constraint on state vectors and determines a physical subspace. The field equations are recovered on the physical Fock space (Gupta–Bleuler method).
Self-gravitating nonlinear scalar fields
Authors: Malec Edward
Published: July 2002

We investigate the Cauchy problem for the Einstein - scalar field equations in asymptotically flat spherically symmetric spacetimes, in the standard 1+3 formulation. We prove the local existence and uniqueness of solutions for initial data given on a space-like hypersurface in the Sobolev H1∩H1,4 space. Solutions exist globally if a central (integral) singularity does not form and/or outside an outgoing null hypersurface. An explicit example demonstrates that there exists a local evolution with a naked initial curvature singularity at the symmetry centre.
Einstein’s equations with asymptotically stable constraint propagation
Authors: Brodbeck Othmar , Frittelli Simonetta , Hübner Peter , Reula Oscar A.
Published: July 2002

We introduce a proposal to modify Einstein’s equations by embedding them in a larger symmetric hyperbolic system. The additional dynamical variables of the modified system are essentially first integrals of the original constraints. The extended system of equations reproduces the usual dynamics on the constraint surface of general relativity, and therefore naturally includes the solutions to Einstein gravity. The main feature of this extended system is that, at least for a linearized version of it, the constraint surface is an attractor of the time evolution. This feature suggests that this system may be a useful alternative to Einstein’s equations when obtaining numerical solutions to full, nonlinear gravity.
Laminar–turbulent transition of wall-jet flows subjected to blowing and suction
Authors: Cohen Jacob , Amitay Michael , Bayly Bruce J.
Published: July 2002

A new family of solutions describing the mean flow of an incompressible two-dimensional wall jet subjected to boundary suction or blowing was found, in which the vertical velocity at the wall varies in a power law with downstream distance. Linear stability calculations indicate that two unstable modes, an inviscid inflectional point instability and a viscous instability, may coexist in the flow. It is demonstrated theoretically that the relative importance of each mode can be controlled by subjecting the wall jet to small amounts of blowing or suction. Experimental results, in the absence of suction and blowing, support theoretical findings about the coexistence of the two instability modes.
Vortical nature of thermal plumes in turbulent convection
Authors: Cortese T. , Balachandar S.
Published: July 2002

Thermal plumes in Rayleigh–Bénard convection have been observed to occur with strong vertical component of vorticity, resulting in spiraling hot updrafts and cold downdrafts. Results from two different simulations at Rayleigh numbers 9800 and 33 000 times the critical Rayleigh number close to the boundaries show rapid increase in the conditional averages of vertical velocity and temperature perturbation at large values of vertical vorticity. This result along with the spatial distribution of vertical vorticity indicates a strong correlation between narrow regions of upmoving hot fluid (or downmoving cold fluid) and local vertical vorticity. The horizontal dimension of these vortical plumes is an order of magnitude smaller than the layer depth, but they extend up to half the convective layer in the vertical direction.