bluuebunny/crossref_metadata_2025 huggingface dataset:
Reflection measurement technique of electro-optic coefficients in lithium niobate crystals and poled polymer films
Authors: Shuto Yoshito , Amano Michiyuki
Published: July 2002

A simple reflection technique proposed by Teng and Man [Appl. Phys. Lett. 56, 1734 (1990)] as well as independently by Schildkraut [Appl. Opt. 29, 2839 (1990)] for measuring the electro-optic coefficients of poled polymer films is applied to measure the r33 values of both z-cut lithium niobate crystal and diazo-dye-substituted polymer films. The measured r33 value of the lithium niobate crystal is in excellent agreement with the known value, and the observed r33 values of the poled polymer films agree well with those predicted from the second-harmonic-generation measurements.
Model of superlattice yield stress and hardness enhancements
Authors: Chu Xi , Barnett Scott A.
Published: July 2002

A model is presented that explains the yield stress and hardness enhancements that have been observed in superlattice thin films. The stress required for dislocations to glide across layers with different shear moduli was calculated using an expression that accounts for core effects and all interfaces in trapezoidal or sawtooth composition modulations. The predicted strength/hardness enhancement increased with increasing superlattice period Λ, before reaching a saturation value that depended on interface widths. A second mechanism, where dislocations glide within individual layers, was important at large Λ and gave a decrease in strength/hardness with increasing Λ. The combination of these two mechanisms gives a strength/hardness maximum versus Λ in good quantitative agreement with experimental results for nitride and metal superlattices. The results indicate that superlattice strength/hardness depends strongly on interface widths and the difference in shear moduli of the two components for Λ values below the maximum, and on the average shear modulus for larger Λ.
Transparent glass ceramics for 1300 nm amplifier applications
Authors: Tick P. A. , Borrelli N. F. , Cornelius L. K. , Newhouse M. A.
Published: July 2002

The properties of an oxyfluoride glass ceramic that possesses high transparency after ceramming are described. Approximately 25 vol % of this material is comprised of cubic, fluoride nanocrystals and the remainder is a predominantly oxide glass. When doped with Pr+3, the fluorescence lifetime at 1300 nm is longer than in a fluorozirconate glass, suggesting that a significant fraction of the rare-earth dopant is preferentially partitioned into the fluoride crystal phase. This material has the added advantage of being compatible with ambient air processing.
Optical and electrical characterization of boron impurities in silicon carbide grown by physical vapor transport
Authors: Jenny J. R. , Skowronski M. , Mitchel W. C. , Hobgood H. M. , Glass R. C. , Augustine G. , Hopkins R. H.
Published: July 2002

Undoped SiC crystals grown by physical vapor transport have been characterized by temperature dependent Hall effect and near infrared optical absorption measurements. Crystals with reduced nitrogen content were found to exhibit p-type conductivity with carrier concentrations in the 5×1014–1×1016 cm−3 range at room temperature. The Fermi level position determined from Hall effect measurements at elevated temperatures was 0.35 eV above valence band. The primary acceptor-type impurity was identified as substitutional boron with total concentration of uncompensated acceptors in the 1×1017–5×1018 range. This interpretation was confirmed by near infrared absorption spectra, which were dominated by a broad photoionization band with a threshold at 0.7 eV and a maximum at 1.75 eV. The shape of the band was fitted, and the thermal ionization energy of the defect was found to be in the 0.3–0.4 eV range. A correlation between the photoionization band intensity, and the uncompensated boron content was used to determine the value of maximum optical cross section of boron photoionization band, which was 4.17×10−17 cm2. In addition to photoionization band, boron-containing samples exhibited set of narrow absorption lines near the fundamental absorption edge. Based on correlation with boron content and line position in different SiC polytypes, these lines were identified as due to excitons bound to neutral boron acceptors.
Nonlinearity of the radio-frequency sheath
Authors: Klick Michael
Published: July 2002

The nonlinearity of the plasma sheath for frequencies well above the ion plasma frequency is analyzed. The work focuses on the relation of sheath voltage and displacement current for the low-pressure discharge. The simple model of a stepwise electron density distribution within the sheath and a generalized model are derived. Both approaches result in a fundamental and general relation of sheath voltage, displacement current, and time dependent sheath width. Experiments at asymmetrical rf discharges show the models to be realistic. The temporal dependence of the sheath width is calculated from experimental data.
Intense ultraviolet and blue upconversion fluorescence in Tm3+-doped fluoroindate glasses
Authors: Kishimoto Shoichi , Hirao Kazuyuki
Published: July 2002

Much more intense ultraviolet (UV) and blue upconversion fluorescence of Tm3+ was observed in a new stable fluoroindate glass compared with other fluoride or oxide glasses. The intense upconversion intensity and high thermal stability of these fluoroindate glasses indicate a high possibility for the realization of a UV and blue upconversion laser. It is reported that the compositional dependence of the fluorescence intensity is largely dependent on the phonon energy of the host glass. The dependence of the phonon energy of the host glasses on contributions from multiphonon decay on the fluorescence intensity is also discussed.
Strain effect on electronic and optical properties of GaN/AlGaN quantum-well lasers
Authors: Suzuki Masakatsu , Uenoyama Takeshi
Published: July 2002

In order to clarify the strain effect on the GaN-based lasers and to give the important guideline on their device design, the subband structure and the optical gains of strained wurtzite GaN/AlGaN quantum wells are theoretically investigated on the basis of k⋅p theory. First-principles band calculations are used for deriving the unknown physical parameters. It is found that neither compressive nor tensile biaxial strains in the c plane are so effective on the reduction of the threshold carrier density as conventional zinc-blende lasers and that the uniaxial strain in the c plane is very useful for reducing it. The relation between the uniaxial strain’s direction and the optical polarization is also clarified. As a result, we suggest that the uniaxial strain in the c plane is one of the preferable approaches for the efficient improvement of the GaN-based lasers performance.
Electrical and mechanical properties of ferroelectric thin films laser ablated from a Pb0.97Nd0.02(Zr0.55Ti0.45)O3 target
Authors: Lappalainen J. , Frantti J. , Lantto V.
Published: July 2002

Ferroelectric Nd-modified lead-zirconate-titanate (PZT) thin films were fabricated as capacitor structures with platinum electrodes using pulsed laser ablation deposition. Single-crystal MgO (100) and thermally oxidized Si (100) were used as substrates. The ablation processes were carried out at room temperature in a pressure of 4×10−5 mbar in a vacuum chamber. A pulsed XeCl excimer laser with the wavelength of 308 nm was used for the ablation of both platinum and Pb0.97Nd0.02(Zr0.55Ti0.45)O3 targets. For the PZT films with thicknesses between 300 and 600 nm, a laser-beam fluence of 1.0 J/cm2 was used. Amorphous PZT films were postannealed at 675 and 650 °C in the cases of MgO and silicon substrates, respectively. The dielectric constant and the loss angle were measured at room temperature as a function of the film thickness. On the MgO substrate the dielectric constant of the films increased from 400 to 600 with the increasing film thickness, while in the films on the silicon substrate the dielectric constant was typically 140. The Curie temperature of the films was about 360 °C. The remanent polarization was about 18 μC/cm2 in the films deposited on MgO, but in the films on the silicon substrate the polarization values were much lower. The conductivity of the PZT thin films was studied as a function of temperature and electric field. Low-field resistivities of the order of 1012 Ω cm were measured at room temperature. Macroscopic mechanical stresses in the PZT films were measured by the x-ray diffraction method. The films on the MgO substrate were in a compressive stress, while in the films on the silicon substrate a higher tensile stress was found. The dielectric constant was found to decrease and the coercive field to increase with the increasing mechanical stress.
Microwave-induced low-temperature crystallization of amorphous silicon thin films
Authors: Lee Jeong No , Choi Yong Woo , Lee Bum Joo , Ahn Byung Tae
Published: July 2002

Microwave heating was utilized for solid phase crystallization of amorphous silicon films. The amorphous silicon thin films were deposited in the temperature range of 100–400 °C by plasma enhanced chemical vapor deposition and annealed by microwave heating at 550 °C in nitrogen. Microwave heating lowered the annealing temperature and reduced the annealing time for complete crystallization. For example, the a-Si film deposited at 400 °C was fully crystallized in 3 h at 550 °C. On microwave heating, the hydrogen in the amorphous films diffused out very quickly, but there was no change in structural disorder following hydrogen evolution. The lower temperature crystallization of a-Si films compared to conventional furnace annealing is due to the interaction between microwave and silicon atoms. The grain size of the crystallized silicon films was in the range of 0.55–0.78 μm, depending on the deposition temperature. These grain sizes are not so small compared to those of Si films crystallized by conventional furnace heating, while the crystallization time is much shorter.
Shear response of lead zirconate titanate piezoceramics
Authors: Mueller V. , Zhang Q. M.
Published: July 2002

The piezoelectric shear strain S5 of several commercial lead zirconate titanate (PZT) piezoceramics was evaluated under the nonresonant condition in a sinusoidal ac-field E1(t) applied perpendicular to the poling direction. Results obtained on donor doped (soft PZT) and acceptor doped (hard PZT) ceramics are compared. At fields sufficiently below the limiting field Elim necessary to electrically depole the sample, we find a linear, nonhysteretic relationship between S5 and the polarization P1. In soft PZT ceramics, the effective piezoelectric shear coefficient d15=S5/E1 shows a pronounced ac-field dependence which was fitted according to d15(E1)=dlin[1+(dnlE1)α] with α≈1.2. The results indicate that irreversible motion of non-180° walls causes the nonlinearity of PZT and the contribution of 180° walls to the linear and nonlinear coefficients is negligible. The analysis of the relationship between linear and nonlinear coefficients obtained at different ceramic systems suggests that there exists another extrinsic contribution to the permittivity in PZT which may not be attributed to domain wall motion but may be responsible to the dielectric dispersion at microwave frequencies.
Hydrogen–oxygen interaction in silicon at around 50 °C
Authors: Markevich V. P. , Suezawa M.
Published: July 2002

Formation kinetics of oxygen–hydrogen (O–H) complexes which give rise to an infrared absorption line at 1075.1 cm−1 have been studied in Czochralski-grown silicon crystals in the temperature range of 30–150 °C. Hydrogen was incorporated into the crystals by high temperature (1200 °C) in diffusion from H2 gas. It was found that the observed kinetics can be explained as being due to an interaction of mobile neutral hydrogen-related species with bond-centered oxygen atoms. The binding energy of the O–H complex was determined to be 0.28±0.02 eV. An activation energy for migration of hydrogen-related species responsible for the formation of the O–H complexes was found to be 0.78±0.05 eV. It was shown that atomic hydrogen and H2*, a complex containing two hydrogen atoms, one at bond-centered site and another one at antibonding site, cannot account for the hydrogen–oxygen interaction considered. Hydrogen molecules (H2) located at tetrahedral interstitial site are suggested to be the species which interact with interstitial oxygen atoms and form the complex giving rise to the absorption line at 1075.1 cm−1.
Dielectric properties of Bi doped SrTiO3 ceramics in the temperature range 500–800 K
Authors: Yu Zhi , Ang Chen , Vilarinho P. M. , Mantas P. Q. , Baptista J. L.
Published: July 2002

The (Sr1−1.5xBix)TiO3 (0.0133⩽x⩽0.133) ceramic system reveals several sets of dielectric permittivity peaks in different temperature ranges. Dielectric permittivity and dielectric loss peaks were detected in the temperature range 500–800 K and in the present article the dielectric polarization behavior is presented and discussed. The activation energy of the dielectric relaxation is in the range of 0.99–1.12 eV. It is suggested that the permittivity peaks are related to the movement of oxygen ions or oxygen vacancies.
Improved Monte Carlo method for ion transport in ion-molecule asymmetric systems at high electric fields
Authors: Yousfi M. , Hennad A. , Eichwald O.
Published: July 2002

An improved Monte Carlo method is developed for the simulation of the ion transport in classical drift tube in the case of ion-molecule asymmetric systems such as O−/O2 or N+/N2. The aim of this new method is to overcome the problem of incident ions which vanish at relative high electric field due to asymmetric charge transfer or electron detachment. These ion removal processes are compensated by a fictitious ion creation which improves the accuracy of the ion distribution function and swarm coefficient calculations. The classical ion-molecule collision processes occurring in weakly ionized gases at room temperature (elastic collisions including energy exchange and thermal motion of background gases and also inelastic collisions) are taken into account. This new method is then validated and the transport and reaction coefficients have been given for a large range of E/N (a part of them for the first time in the literature) in O−/O2 and N+/N2 systems.
Finite-temperature full random-phase approximation model of band gap narrowing for silicon device simulation
Authors: Schenk Andreas
Published: July 2002

An analytical model of the band gap narrowing (BGN) in silicon was derived from a non-self-consistent finite-temperature full random-phase approximation (RPA) formalism. Exchange-correlation self-energy of the free carriers and correlation energy of the carrier-dopant interaction were treated on an equal basis. The dispersive quasi-particle shift (QPS) in RPA quality was numerically calculated for a broad range of densities and temperatures. The dispersion was found to be smooth enough for the relevant energies to justify the rigid shift approximation in accordance with the non-self-consistent scheme. A pronounced temperature effect of the BGN only exists in the intermediate density range. The contribution of the ionic part of the QPS to the total BGN decreases from 1/3 at low densities to about 1/4 at very high densities. Based on the numerical results, Padé approximations in terms of carrier densities, doping, and temperature with an accuracy of 1 meV were constructed using limiting cases. The analytical expression for the ionic part had to be modified for device application to account for depletion zones. The model shows a reasonable agreement with certain photoluminescence data and good agreement with recently revised electrical measurements, in particular for p-type silicon. The change of BGN profiles in a bipolar transistor under increasing carrier injection is demonstrated.
Combined electrical and mechanical properties of titanium nitride thin films as metallization materials
Authors: Patsalas P. , Charitidis C. , Logothetidis S. , Dimitriadis C. A. , Valassiades O.
Published: July 2002

Titanium nitride (TiNx) thin films, ∼100 nm thick, were deposited on Si(100) substrates by dc reactive magnetron sputtering. The effects of the substrate bias voltage and deposition temperature on their optical, electrical, and mechanical properties have been studied. It was found a strong correlation between the electrical and mechanical properties of the films which are significantly improved with increasing the substrate bias voltage and the deposition temperature. The low resistivity (43 μΩ cm), combined with the high hardness and elastic modulus values, suggest the TiNx as a promising metallization material in Si technology.
Universality and size effects in the Barkhausen noise
Authors: Durin Gianfranco , Zapperi Stefano
Published: July 2002

We show that the Barkhausen avalanches exhibit power law distributions and scaling exponents belonging to two distinct universality classes. We explain these results in terms of the critical behavior of the domain wall at the depinning transition, with exponents set by the long-range dipolar interactions arising from local magnetostatic fields, and by the elastic curvature of the wall. We are also able to predict the precise dependence of the cutoff on the demagnetizing factor k due to sample size. These predictions are experimentally confirmed on three samples (two polycrystalline 6.5 wt % Si–Fe and an amorphous Fe21Co64B15 under applied tensile stress) which are progressively cut in order to increase k. All these results allow us to link the material microstructure and the sample geometry to the macroscopic noise properties.
Direct study of energy transfer of vibrationally highly excited CS2 molecules
Authors: Dove J. E. , Hippler H. , Troe J.
Published: July 2002

Collisional energy transfer at 300 K of vibrationally highly excited CS2 molecules (excitation energies 28 660 and 32 640 cm−1) is studied directly by time resolved UV absorption spectroscopy after laser excitation at 351 and 308 nm. Average energies 〈ΔE〉 transferred per collision for 27 bath gases are derived. A marked dependence of 〈ΔE〉 on the excitation energy is noted; the steepness of this dependence is not the same for all bath gases. The results are compared with energy transfer data for vibrationally highly excited large molecules.
Rate constants of the reactions of metastable N2(A 3Σ+u) in v=0, 1, 2, and 3 with ground state O2 and O
Authors: Thomas J. M. , Kaufman F.
Published: July 2002

Overall rate constants for the removal of N2(A,v) by O2 and O were measured at 298 K in a rapidly pumped flow reactor using laser-induced fluorescence (LIF) detection of N2(A,v) by excitation in the first positive system of N2, B(3πg)←A(3Σ+u). O atoms were generated in microwave discharges of pure O2 prepared by thermal decomposition of KMnO4. Measured rate constants for N2(A,v)+O2 increased from 2.5×10−12(v=0) to 5.7×10−12 cm3 s−1(v=3). For N2(A,v)+O(3P), they were an order of magnitude larger, rising from 3.5×10−11(v=(0) to 5.2×10−11 cm3 s−1(v=3). They are compared with previous work and discussed in terms of the likely molecular interaction that they represent.
Control of selectivity of chemical reaction via control of wave packet evolution
Authors: Tannor David J. , Rice Stuart A.
Published: July 2002

A time-dependent formulation of two-photon spectroscopy is employed to show that selectivity of reactivity can be achieved via coherent two-photon processes. The problem of finding the optimum waveform (i.e., coherent pulse sequence) that will maximize the formation of a desired chemical species is formulated as a problem in the calculus of variations, and solved for two different cases.
The diagonal correction to the Born–Oppenheimer approximation: Its effect on the singlet–triplet splitting of CH2 and other molecular effects
Authors: Handy Nicholas C. , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

The prediction of the diagonal correction to the Born–Oppenheimer approximation is now possible by ab initio analytic methods, as has recently been shown by Yarkony and Lengsfield. At the general restricted Hartree–Fock (GRHF) level of approximation, the procedure is straightforward: solutions of the coupled perturbed Hartree–Fock equations (CPHF) and some overlap integrals are all that are required. This correction is evaluated for a series of small molecules with various basis sets: H2O, H2O+, CH2, HCF, H+5, and F2. It is interesting to observe that the value of this correction (0.11 kcal) for the singlet–triplet splitting of CH2 is larger than the relativistic correction, and that the theoretical value for Tnre (BO)≡9.23±0.20 kcal has come even closer to the best ab initio prediction of 9.4 kcal.
On the evaluation of nonadiabatic coupling matrix elements for MCSCF/CI wave functions using analytic derivative methods. III. Second derivative terms
Authors: Lengsfield Byron H. , Yarkony David R.
Published: July 2002

A method for the efficient evaluation of nonadiabatic coupling matrix elements of the form 〈Ψ J(r;R)‖(∂2/∂R2α) Ψ I(r;R)〉r is presented. The electronic wave functions Ψ J and Ψ I are assumed to be MCSCF/CI wave functions whose common molecular orbital basis is determined within the state averaged MCSCF (SA-MCSCF) approximation. The method derives its efficiency by exploiting analogies with analytic CI second derivative techniques and from the first and second derivative coupled perturbed SA-MCSCF equations. This method is compared with an existing finite difference procedure which is reformulated to take maximal advantage of analytic gradient methods.
Molecular dynamics computer simulation of polymer fiber microstructure
Authors: Brown D. , Clarke J. H. R.
Published: July 2002

Microscopic multichain models of amorphous and crystalline regions of an oriented polymer fiber have been examined using molecular dynamics computer simulation. A lamina model was used in which the chains were suspended between two fixed parallel planes representing the faces of adjacent crystallites. The ordered structure consisted of 16 parallel chains each containing 25 segments. The amorphous structure contained 24 chains and was characterized by a distribution of chain lengths, entanglements, end defects, and loops. The values of the geometric and interaction parameters for the polymer chains were taken from previous studies of low molecular mass liquid alkanes, so that the model approximates to polyethylene, with each CH2 group represented as a single interaction site. The structures were brought to equilibrium at 300 K and stress–strain curves obtained for elongation/compression in a direction perpendicular to the crystal faces. The influence of interchain forces on these curves and on the Young’s moduli (which differed by more than an order of magnitude for the two structures) have been analyzed and compared with theoretical predictions. The frequency dependent stress across the interfaces of the ordered structure was analyzed in terms of angle bending motions of the chains (so-called accordion modes). The relation between the associated frequency spectrum and chain length was similar to that observed in real crystalline polyethylene. Young’s moduls estimated from the velocity of sound was in good agreement with that obtained from the static stress/strain curve.
Bridge/atop site conversion of CO on Ni(111): Determination of the binding energy difference
Authors: Tang S. L. , Lee M. B. , Yang Q. Y. , Beckerle J. D. , Ceyer S. T.
Published: July 2002

A rapid site exchange process is observed in the equilibrated chemisorbed layer of CO on Ni(111). Following adsorption at 298 K, the relative populations of CO adsorbed on atop sites and twofold bridge sites are monitored by the high resolution electron energy loss intensities of the respective CO vibrational modes as a function of surface temperature. Since equilibrium is established, the binding energy difference between the terminal and bridge adsorption sites is determined. The bridge site is more stable than the atop site by 0.94±0.15 kcal/mol at a coverage of 0.13. As the coverage is increased to 0.42, the difference in binding energies decreases to 0.44±0.07 kcal/mol. At saturation coverage, 0.5, the binding energy difference effectively becomes very large, resulting in CO occupation of the twofold bridge sites exclusively.
Nonlinear UV excitation: Neutral vs ionic channels of molecular fragmentation
Authors: Craig B. B. , Faust W. L. , Chattopadhyay S. K.
Published: July 2002

Following multiphoton or nonlinear UV excitation (22 ps, 266 nm, ∼2.5 mJ) of simple molecules at low pressure, we have measured the temporal evolution of emissive product populations with subnanosecond time resolution. For several parents, our data support the importance, beneath the I. P., of fragmentation to yield neutral species competitive with further excitation to yield parent ions. The primary neutrals and ions are subject to further excitation. Channels of dissociation are recognized as prompt (unimolecular) and delayed (ion–molecule) components of fragment emission. Representative data for CH2CO are presented. A discussion of some recent REMPI results on this molecule is also given. The question has arisen whether rapid production of neutrals, energetically requiring nhν>I.P. of the parent, occurs through superexcited species. We suggest that a conclusive demonstration of such pathways has yet to be achieved.
Theoretical study of low-lying 1Σ+ and 1Π states of CO. I. Potential energy curves and dipole moments
Authors: Cooper David L. , Kirby Kate
Published: July 2002

Potential energy curves, electronic wave functions, and electric dipole moments for the X 1Σ+, A 1Π, 2 1Σ+, 3 1Σ+, and 2 1Π states of CO have been computed using multiconfiguration self-consistent field and configuration interaction methods. The calculations have been carried out at 17 internuclear distances from 1.6 to 8.0 a0. The second and third 1Σ+ states are shown to have double-minimum potentials which correlate to the B 1Σ+ and C 1Σ+ Rydberg states at small internuclear distances. The outer wells of these potentials correspond to the recently observed D 1Σ+ state and to a new state, which we designate C′ 1Σ+. The 2 1Π state, identified spectroscopically as E 1Π, dissociates adiabatically to ground state atoms and appears to be quasibound by a large hump with a maximum at ≈3.25 a0. Spectroscopic constants for all these states have been computed and are generally in excellent agreement with experimental data. The implications of this work for the interpretation of the experimentally observed predissociations, which dominate the photodestruction of CO, are discussed.
Sustained spiral waves in a continuously fed unstirred chemical reactor
Authors: Tam W. Y. , Horsthemke W. , Noszticzius Z. , Swinney H. L.
Published: July 2002

Previous experiments on the self-organization of spatial patterns in chemical reactors have generally been restricted to closed (batch) reactors. In such reactors the system relaxes irreversibly and uncontrollably towards thermodynamic equilibrium. It is difficult to make comparisons with existing theories which address asymptotic (long time) behavior because of the transient nature of the spatial patterns and the lack of well-defined control parameters in such experiments. We report a novel disk-shaped reactor that can be maintained far from thermodynamic equilibrium indefinitely by a continuous feed of reagents. Chemical patterns are formed inside a thin layer of inert gel that suppresses any convective motion. The feed to the gel is uniform and normal to the plane in which pattern can form. The reactor is used to conduct the first quantitative study of transitions between well-defined states with different patterns.
Time-independent theory of one-particle Green’s functions
Authors: Kutzelnigg Werner , Mukherjee Debashis
Published: July 2002

A new time-independent theory of Green’s functions is presented, which is based on a Fock space Liouvillean and its resolvent. Unlike current time-independent approaches in this formalism there is no need to introduce a complete operator manifold like that of Manne and Dalgaard for inner projection, nor for invoking a ‘‘killer condition.’’ A perturbative construction of the Green’s functions involves a perturbation expansion of both the resolvent of the Liouvillean and of the wave function. The approach is very general and is by no means limited to a closed-shell reference function. For functions of the latter type a very compact approach is possible in a particle–hole formalism, which automatically leads to the Feynman–Dyson series. An essential point in its derivation is that the perturbation expansion of the resolvent and of the wave function lead to diagrams with the same topology but with different energy denominators but that such diagrams can be added up to a single one, which may contain both ‘‘Rayleigh–Schrödinger’’ and ‘‘Brillouin–Wigner’’ denominators. It is shown that a consistent theory can be based on ‘‘principal-value’’ Green’s functions. The one-particle Green’s function is treated in some detail, the polarization propagator is only briefly discussed.
The rheology of Brownian suspensions
Authors: Bossis G. , Brady J. F.
Published: July 2002

The viscosity of a suspension of spherical Brownian particles is determined by Stokesian dynamics as a function of the Péclet number. Several new aspects concerning the theoretical derivation of the direct contribution of the Brownian motion to the bulk stress are given, along with the results obtained from a simulation of a monolayer. The simulations reproduce the experimental behavior generally observed in dense suspensions, and an explanation of this behavior is given by observing the evolution of the different contributions to the viscosity with shear rate. The shear thinning at low Péclet numbers is due to the disappearance of the direct Brownian contribution to the viscosity; the deformation of the equilibrium microstructure is, however, small. By contrast, at very high Péclet numbers the suspension shear thickens due to the formation of large clusters.
Computer simulations of the dielectric properties of water: Studies of the simple point charge and transferrable intermolecular potential models
Authors: Alper Howard E. , Levy Ronald M.
Published: July 2002

A series of very long molecular dynamics simulations has been completed for the rigid simple point charge (SPC) and transferrable intermolecular potential 4P (TIP4P) water models with reaction field boundary conditions. The dielectric constant corresponding to these models was evaluated in two ways: (1) by calculating the fluctuations in the mean square dipole moment of the system in the absence of an applied field and (2) by evaluating the polarization response of the system to an applied field. Consistent values for the dielectric constant are obtained by the two methods. For the TIP4P water model, the dielectric constant ε0 is calculated to be ∼50 at 293 K, in agreement with previously published results. For the rigid SPC model, ε0 is calculated to be ∼68 at 300 K and ∼59 at 350 K. The calculated dielectric constant is shown to be very sensitive to the way in which the truncation of long-range forces is treated, although the short-range liquid properties are insensitive to the truncation procedure. The effects on the dielectric response of system size, cutoff radius, and the smoothing of long range interactions at the cutoff boundary are examined. It is found that the introduction of a smoothing function causes perturbations of the water–water angular orientation correlation functions which can result in large errors in the calculated dielectric constant.
Trajectory surface-hopping study of electronically inelastic collisions of CN(A 2Π) with He: Comparison with exact quantum calculations
Authors: Parlant Gérard , Alexander M. H.
Published: July 2002

The trajectory surface-hopping procedure recently introduced by Parlant and Gislason [J. Chem. Phys. 91, 4416 (1989)] is used to calculate rotationally resolved state-to-state cross sections for the energy transfer between the 2Π3/2 and 2Π1/2 fine-structure rotational manifolds of the A electronic state of CN induced by collision with He at thermal collision energy. Quantum scattering calculations for the same process are also reported. The results obtained by both techniques are compared and found to be in encouraging agreement.
A time-dependent wave packet approach to atom–diatom reactive collision probabilities: Theory and application to the H+H2 (J=0) system
Authors: Neuhauser Daniel , Baer Michael , Judson Richard S. , Kouri Donald J.
Published: July 2002

This paper describes a new approach to the study of atom–diatom reactive collisions in three dimensions employing wave packets and the time-dependent Schrödinger equation. The method uses a projection operator approach to couple the inelastic and reactive portions of the total wave function and optical potentials to circumvent the necessity of using product arrangement coordinates. Reactive transition probabilities are calculated from the state resolved flux of the wave packet as it leaves the interaction region in the direction of the reactive arrangement channel. The wave packet does not need to be propagated into the asymptotic reactive region in order to determine accurate vibrationally resolved, but rotationally summed reaction probabilities. The present approach is used to obtain such vibrationally resolved reaction probabilities for the three-dimensional H+H2 (J=0) hydrogen exchange reaction, using a body-fixed system of coordinates.
Jump transition due to a time-dependent bifurcation parameter: An experimental, numerical, and analytical study of the bistable iodate–arsenous acid reaction
Authors: Laplante J. P. , Erneux T. , Georgiou M.
Published: July 2002

We study the response diagram of the bistable iodate–arsenous acid reaction and investigate analytically and experimentally the jump transitions due to a time-dependent parameter. This parameter is changed linearly in time and its rate of change is characterized by a dimensionless parameter ε. We observe that the jump transition does not occur at a limit point of the steady states but is delayed. Asymptotic studies predict that this delay is proportional to an O(ε2/3) quantity as ε→0. The ε2/3 law is in agreement with the experimental observations of the transition between the lower and upper branches of steady states even if ε is not a small quantity [T. Erneux and J. P. Laplante, J. Chem. Phys. 90, 6129 (1989)]. In this paper, we investigate experimentally the second transition of the iodate–arsenous acid reaction, namely, the transition between the upper and lower branches of the steady states. We have found that the delay does not satisfy the ε2/3 law but rather is following an ε law. By comparing the experimental results and the results obtained from an analytical and numerical study of the bistable reaction, we show that the values of ε used in the experiments are too large to follow the ε2/3 law.
State resolved photodissociation of vibrationally excited water: Rotations, stretching vibrations, and relative cross sections
Authors: Vander Wal R. L. , Scott J. L. , Crim F. F.
Published: July 2002

The state resolved photodissociation of highly vibrationally excited water molecules using laser induced fluorescence detection of the OH product demonstrates the control that the initially selected state exerts over product state populations. These vibrationally mediated photodissociation experiments, in which one photon prepares a highly vibrationally excited molecule and a second photon dissociates it, determine the role of overall rotations and of O–H stretching vibrations as well as measure the relative cross section for the photodissociation of water. The rotational state of the vibrationally excited water molecule governs the rotational state of the OH product of the dissociation, in agreement with ab initio calculations and previous measurements on single rotational states excited in the fundamental asymmetric stretching vibration band. The initially selected vibrational state of the water molecule determines the vibrational energy disposal in the products, which agrees with a simple qualitative model based on the pattern of the initially selected vibrational wave function. Dissociating vibrational states with similar energies but very different nuclear motions produces dramatically different product vibrational state populations. The vibrational energy initially present in the surviving bond primarily appears as vibrational excitation of the product. Dissociation of the ‖04〉− state produces no vibrationally excited OH, but dissociation of the ‖13〉− state produces mostly vibrationally excited products.

These qualitative notions agree well with recently detailed ab initio calculations. The relative photodissociatiuib cross section of the highly vibrationally excited molecule shows structure over the wavelength range of 218.5 to 266 nm that reflects the nodal pattern of the intermediate vibrational state in the dissociation and confirms the predictions of theoretical calculations.
A quantum-mechanical heat engine operating in finite time. A model consisting of spin-1/2 systems as the working fluid
Authors: Geva Eitan , Kosloff Ronnie
Published: July 2002

The finite-time operation of a quantum-mechanical heat engine with a working fluid consisting of many noninteracting spin-1/2 systems is considered. The engine is driven by an external, time-dependent and nonrotating magnetic field. The cycle of operation consists of two adiabats and two isotherms. The analysis is based on the time derivatives of the first and second laws of thermodynamics. Explicit relations linking quantum observables to thermodynamic quantities are developed. The irreversible operation of this engine is studied in three cases: (1) The sudden limit, where the performance is found to be the same as that of the spin analog of the Otto cycle. This case provides the lower bound of efficiency. (2) The step-cycle operation scheme. Here, the optimization of power is carried out in the high-temperature limit (the ‘‘classical’’ limit). The results obtained are similar to those of Andresen et al. [Phys. Rev. A 15, 2086 (1977)]. (3) The Curzon–Ahlborn operation scheme. The semigroup approach is used to model the dynamics. Then the power production is optimized. All the results obtained for Newtonian engines operating by the same scheme, such as the Curzon–Ahlborn efficiency, apply in the high-temperature limit. These results are obtained without the additional assumption of proximity to thermal equilibrium, implicitly implied by the use of Newtonian heat conduction in the original derivation. It seems that the results of the Curzon–Ahlborn analysis are always obtained in the high-temperature limit, irrespective of the details of the model. The performance beyond the classical limit is optimized numerically. The classical approximation is found to be valid for most of the spin-polarization range. The deviations from the classical limit depend heavily upon the specific nature of both the working fluid and the heat baths and exhibit great diversity and complexity.
A quantitative analysis of guest periodicity in one-dimensional inclusion compounds
Authors: Rennie Andrew J. O. , Harris Kenneth D. M.
Published: July 2002

We consider those crystalline inclusion compounds that can be modeled as one-dimensional systems, and develop a mathematical model in which the one-dimensional inclusion compound comprises a linear, infinite host channel (with periodic repeat distance ch along the channel axis) containing a regular, periodic arrangement of n guest molecules (with periodic repeat distance cg ). The principal aim of this paper is to develop a method for the quantitative prediction of cg, and a theoretically justified procedure is described for using known potential energy functions for host–guest and guest–guest interactions as a route towards deriving this information. Fundamental to the analysis is the definition of an appropriate energy expression—denoted the ‘‘characteristic energy’’ of an inclusion compound Ê(α,n)—that directly indicates the relative energetic favorability of inclusion compounds with different guest periodicities α(α=cg/ch). Then, via a graphical method—the characteristic energy diagram, which is essentially a means of constructing a bounded region that will contain the points (α,Ê(α,n))—a restricted range containing the optimal value of α can be determined. A procedure is provided for constructing the characteristic energy diagram using computed potential energy functions for ‘‘real’’ inclusion compounds, and the method for interpretation of the diagram is discussed in full.

Furthermore, our method of analysis not only permits an assessment of the optimum guest periodicity for the inclusion compound, but also addresses, from energetic considerations, the question of whether ‘‘lock-in’’ of the host and guest substructures will occur. The applicability of our method of analysis is illustrated using potential energy functions computed for the hexadecane/urea inclusion compound. The characteristic energy diagram is constructed for this system and used to deduce that the optimal guest periodicity is in the range cg=22.6 Å±0.1 Å, in excellent agreement with recent experimental results. At this optimal periodicity, the interaction between adjacent guest molecules is repulsive; any naive approach to predict cg by finding the minimum of the guest–guest interaction energy curve is blatantly incorrect.
Hyperfine structure of the NaK c 3Σ+ state and the effects of perturbation
Authors: Ishikawa Kiyoshi , Kumauchi Takahiro , Baba Masaaki , Katô Hajime
Published: July 2002

Scanning the frequency of a single-mode dye laser crossed with a molecular beam of NaK, we have measured the excitation spectrum by monitoring selectively the fluorescence intensity of transition to the a 3Σ+ state. The B 1Π(v=8) and c 3Σ+(v=22) levels are perturbed around J=5 and the c 3Σ+(v=22) and b 3Π(v≊62) levels are perturbed around J=24. The transition lines to the perturbed levels are fully resolved. The hyperfine splitting, which is induced by the coupling with the nuclear spins of the Na and K atoms, is observed for levels of the c 3Σ+ state and the perturbed states. The magnitude of the hyperfine splitting, the line intensity, and the energy shift are analyzed, and their relation with the perturbation is studied. The ratio of electron spin densities at the sodium and potassium atoms in the c 3Σ+ state is estimated to be 0.71:0.15 from the magnitudes of hyperfine splitting.
Free energy model for inhomogeneous fluid mixtures: Yukawa-charged hard spheres, general interactions, and plasmas
Authors: Rosenfeld Yaakov
Published: July 2002

A free energy model for the inhomogeneous hard-sphere fluid mixture was derived recently [Phys. Rev. Lett. 63, 980 (1989)], which is based on the fundamental geometric measures of the particles. Along with an updated assessment of its accuracy, this model is first generalized for charged hard-sphere fluid mixtures, in which every particle carries a central Yukawa charge, and it is then extended to general fluid mixtures in external fields. The Yukawa-charged hard-sphere mixture provides a quite general reference system for many interesting physical systems including plasmas, molten salts, and colloidal dispersions, the screening parameter enabling to interpolate between the long range Coulomb forces and the short range hard cores. A special renormalization property of the Yukawa potential provides the means to derive the exact Onsager-type lower bound for the potential energy of the mixture, and its related asymptotic strong-coupling limit of the liquid pair correlation functions. These results are obtained analytically for the general homogeneous mixture with Yukawa interactions. They enable to extend the fundamental measure free energy model to inhomogeneous charged Yukawa mixtures, with the charge contributions given by a truncated second order expansion from the uniform (bulk) fluid limit.

The resulting free energy model, which interpolates between the ideal-gas and ‘‘ideal-liquid’’ limits, then leads to a self-consistent method for calculating the density profiles for general fluid mixtures in external fields. This method is equivalent to an ansatz of ‘‘universality of the bridge functional.’’ The ‘‘bridge functional’’ consists of all the terms beyond the second order, in the expansion of the excess free energy functional around a reference uniform fluid. The self-consistency is imposed by applying the general method in the special case when the external potential is generated by a ‘‘test particle’’ at the origin of coordinates. In this limit, our general method for nonuniform fluids corresponds to an established and successful theory for the bulk uniform fluid pair structure, namely the thermodynamically consistent modified-hypernetted-chain theory, with the bridge functions now generated by an explicit and demonstratively accurate, ‘‘universal,’’ hard-sphere bridge functional. As a stringent test for the general model, the strongly coupled one-component plasma, in the bulk and near a hard wall, is considered in some detail.
Fluctuations in electrochemical systems. II. Application to a diffusion limited redox process
Authors: Gabrielli C. , Huet F. , Keddam M.
Published: July 2002

A model of the stochastic behavior of the electrochemical interface is presented when a redox reaction limited by the diffusion of the reacting species occurs on the electrode surface. As shown in the first paper [J. Chem. Phys. 99, 7232 (1993)] it leads to the current–voltage curve, the electrochemical impedance and the power spectral density (PSD) of the electrochemical noise (current fluctuations in potentiostatic control or potential fluctuations in galvanostatic control). A comparison with experiment is performed for the potassium ferri-ferrocyanide reaction on platinum. The noise generated by reaction-diffusion is measured after eliminating the parasitic noise sources. At equilibrium the Nyquist theorem is experimentally checked, whereas far from equilibrium the measured excess noise is found to be in good agreement with the theory.
Structure factors for regular polymer gels and networks
Authors: Hammouda Boualem
Published: July 2002

Structure factors, P(q), for regular dendrimeric polymer gels and two-dimensional tetrafunctional networks are calculated. Since the low-q limit (q being the scattering wave number) region is uninteresting for these systems (the Guinier region yields gel and network overall sizes), this paper focuses on the intermediate q region, whereby the scattering radiation is probing sizes larger than single blocks but smaller than the whole macromolecule. Kratky plots [q2P(q)vs q2] are presented for starburst dendrimer gels with varying functionality and correlation range r (with respect to a reference block) and for ‘‘crumpled sheet’’ tetrafunctional networks with varying r. For regular networks, the structure factor has contributions from the various ‘‘lattice animals’’ that correspond to a chosen pair of correlated blocks. A simple method based on multivariate Gaussian distributions is used to express the mean square intermonomer distance 〈r2ij〉 for not too complicated correlation diagrams (r<4).
The stabilization of phase III and phase I in sodium sulfate by aliovalent cation substitution
Authors: Dharmasena Gamini , Frech Roger
Published: July 2002

Both phase I and III in Na2SO4 have been stabilized at room temperature by substitution with the sulfate salts of the aliovalent cations Y3+, La3+, Dy3+, Ce4+, and Ca2+. Phase stabilization has been studied using laser Raman spectroscopy to measure the internal optic modes of the sulfate anion. Phase III is stabilized by a few tenths of a mole percent of the substituent, while phase I is stabilized by roughly ten times that concentration. Stabilization of both phase I and phase III does not appear to depend significantly on the charge of the guest ion. The enthalpy and entropy of the phase III to phase I transition was found to decrease markedly with increasing substituent cation concentration.