bluuebunny/crossref_metadata_2025 huggingface dataset:
Screening and the quantitative π-model description of the optical spectra and polarizations of phenyl based oligomers
Authors: Castleton C. W. M. , Barford W.
Published: September 2002

The long standing problem of the inability of many semiempirical models to correctly predict the polarization of the higher dipole allowed optical transitions of phenyl based π-conjugated polymers and molecules is examined and related to the issue of internal and external screening of π–π electron Coulomb interactions within the molecules. Following a review of previous theoretical and experimental work, π electron only the Complete Neglect of Differential Overlap (CNDO) model is presented which, for the first time, is able to predict accurately the energies and symmetries of all the observed optical transitions of benzene, biphenyl and trans-stilbene, up to ∼8–10 eV. In so doing, it is demonstrated that the problem with previous calculations was the noninclusion of screening from outside the π electron system itself. By fitting separately the spectra in hydrocarbon based condensed phases, in the gas phase and in solid rare gas matrices, and comparing the resulting model parameters, we show that, while the effects of screening from the environment are certainly noticeable, the most important spectral features—in particular the ordering of dipole allowed transitions—come from effective screening by the σ electrons. We find that both of these effects can be adequately accounted for within a π electron only model by using a dielectric constant and appropriate parameter renormalization.
Functionals of the square kinetic energy density
Authors: Maximoff Sergey N. , Ernzerhof Matthias , Scuseria Gustavo E.
Published: September 2002

We construct exchange energy functionals that depend on the non-interacting square kinetic energy density τ2(r)=14∑k|Δφk(r)|2 rather than the electron density. Since τ2 is a non-local functional of the electron density, it may describe non-local effects beyond standard generalized gradient approximation models. These effects may be essential in dealing with such a non-local quantity as the exchange energy. The problem of v-representability of τ2 for a slightly perturbed electron gas and Coulomb systems is discussed. A gradient expansion technique that permits construction of gradient-corrected functionals of τ2 is developed, and the performance of the new exchange functionals is analyzed.
Nuclear motion and Breit–Pauli corrections to the diamagnetism of atomic helium
Authors: Bruch L. W. , Weinhold F.
Published: September 2002

The nuclear motion, Born–Oppenheimer and Breit–Pauli corrections to the diamagnetic susceptibility χd of an isolated ground-state helium atom are evaluated and found to be less than 0.1% of the static nonrelativistic result χd(0).
Sensitivity of the liquid bridge hydrodynamics to local capillary contributions
Authors: Chénier E. , Delcarte C. , Kasperski G. , Labrosse G.
Published: September 2002

In the usual models of thermocapillary flows, a vorticity singularity occurs at the contact free surface–solid boundaries. The steady axisymmetric hydrodynamics of the side-heated liquid bridge of molten metal is addressed here for its sensitivity to the size δ of a length scale explicitly introduced to regularize the problem. By linear stability analysis of the flows, various stable steady states are predicted: The already known steady states which are reflection-symmetric about the mid-plane, but also others which do not possess this property. The thresholds in Ma of the associated bifurcations are strongly dependent on δ, and converge with δ→0 towards low values. Published data give these results some physical relevance.
Modeling the hydrodynamic fluctuations of self-associating fluids: An application to the Brillouin scattering of 1-octanol
Authors: Raudino Antonio , Sassi Paola , Morresi Assuntina , Cataliotti Rosario Sergio
Published: September 2002

A theoretical model, encompassing the concentration fluctuations of associated and unassociated species, has been developed in order to explain the non-Landau behavior observed in self-associating fluids in the hydrodynamic regime. The complex dynamics for the formation of different species is depicted as a pseudo two-components system formed by unbound molecules coexisting with a unique associated specie, the mean aggregation number of which is dictated by thermodynamic requirements. The model has been tested in the case of a hydrogen-bonding system, 1-octanol, over a wide range of temperatures and wave vectors. The model explains, albeit qualitatively, most of the observed deviations from the Landau behavior observed in the Rayleigh–Brillouin spectra at different values of the k-vector.
Like-charge colloid–polyelectrolyte complexation
Authors: Messina René , Holm Christian , Kremer Kurt
Published: September 2002

We investigate the complexation of a highly charged sphere with a long flexible polyelectrolyte, both negatively charged in a salt-free environment. Electroneutrality is insured by the presence of divalent counterions. Using molecular dynamics within the framework of the primitive model, we consider different Coulomb coupling regimes. At strong Coulomb coupling we find that the adsorbed chain is always confined to the colloidal surface but forms different conformations that depend on the linear charge density of the chain. A mechanism involving the polyelectrolyte overcharging is proposed to explain these structures. At intermediate Coulomb coupling, the chain conformation starts to become three-dimensional, and we observe multilayering of the highly charged chain while for lower charge density the chain wraps around the colloid. At weak Coulomb coupling, corresponding to an aqueous solvent, we still find like-charge complexation. In this latter case the chain conformation exhibits loops.
Carrier recombination at single dislocations in GaN measured by cathodoluminescence in a transmission electron microscope
Authors: Albrecht M. , Strunk H. P. , Weyher J. L. , Grzegory I. , Porowski S. , Wosinski T.
Published: September 2002

We study radiative and nonradiative recombination at individual dislocations in GaN by cathodoluminescence performed in a transmission electron microscope. The dislocations are produced by indentation of dislocation free single crystals and have a-type Burgers vectors (b=1/3〈112̄0〉). They are aligned along 〈112̄0〉 directions in the basal plane. Our direct correlation between structural and optical properties on a microscopic scale yields two main results: (i) 60°-basal plane dislocations show radiative recombination at 2.9 eV; (ii) screw-type basal plane dislocations act as nonradiative recombination centers. We explain the nonradiative recombination by splitting this dislocation into 30° partials that have dangling bonds in the core. The dissociation width of these dislocations is <2 nm.
Reply to “Comment on ‘The effect of the charge density of microemulsion droplets on the bending elasticity of their amphiphilic film’ ” [J. Chem. Phys. 117, 4077 (2002)]
Authors: Farago B. , Gradzielski M.
Published: September 2002

We show that the criticism as expressed by Lisy is irrelevant. The formalism he developed is identical to the one we used and the only term in which we differ is a matter of choice of reference system. In any case he completely misses the main point of the paper with respect to the physics of the system, namely the apparently lacking influence of the charge density on the bending elasticity.
Secondary instabilities of rotating flows in the presence of a magnetic field: The case of circular streamlines
Authors: Fabijonas Bruce R.
Published: September 2002

The stability of standing waves in a rotating column of fluid and constant magnetic field is examined. These standing waves are exact solutions to the nonlinear equations, and it is shown that these waves are unstable with respect to high-frequency, short wavelength perturbations. The overall growth rates of these instabilities are unaffected by the perturbations in the magnetic field in almost all but the most extreme parameter regimes. This perturbation analysis can be viewed as a secondary perturbation analysis of the rotating flow.
Spectral analysis of electron transfer kinetics. II
Authors: Jung YounJoon , Cao Jianshu
Published: September 2002

Electron transfer processes in Debye solvents are studied using a spectral analysis method recently proposed. Spectral structure of a nonadiabatic two-state diffusion equation is investigated to reveal various kinetic regimes characterized by a broad range of physical parameters; electronic coupling, energy bias, reorganization energy, and solvent relaxation rate. Within this unified framework, several kinetic behaviors of the electron transfer kinetics, including adiabatic Rabi oscillation, crossover from the nonadiabatic to adiabatic limits, transition from the incoherent to coherent kinetic limits, and dynamic bath effect, are demonstrated and compared with results from previous theoretical models. Dynamics of the electron transfer system is also calculated with the spectral analysis method. It is pointed out that in the large reorganization energy case the nonadiabatic diffusion equation exhibits a nonphysical behavior, yielding a negative eigenvalue.
Turbulence inside a vortex: Take two
Authors: Cotel Aline J.
Published: September 2002

In this paper, a modified theory based on earlier work on stratified entrainment is proposed to explain the persistence of trailing vortices in the far field. These vortices grow at a laminar rate even though the Reynolds number is high and in the turbulent regime.
A quasi-equilibrium beam compression in a recirculator for heavy ion inertial fusion
Authors: Kikuchi Takashi , Nakajima Mitsuo , Horioka Kazuhiko
Published: September 2002

A recirculating and quasi-equilibrium compression process has been studied as a feasible scheme for the beam buncher for heavy ion inertial fusion. The recirculating beam buncher consists of induction linacs and bending sections. Induction modulators apply the velocity tilt for the beam compression. The applied voltage is controlled so as to moderate the velocity tilt before the bending sections. As a result, the longitudinal repulsion force of the beam is balanced with the bunching force for the most part of the compression process. While the beam is gradually compressed under the quasi-equilibrium condition, the transverse mismatch from the velocity tilt of the bunching beam is suppressed to an allowable level. In this paper, the basic concept of the quasi-equilibrium compression and an estimation of the potentiality as an option of accelerator systems for heavy ion inertial fusion are represented.
Phenomenology of squeezing and sliding of molecularly thin Xe, CH4 and C16H34 lubrication films between smooth and rough curved solid surfaces with long-range elasticity
Authors: Persson B. N. J. , Samoilov V. N. , Zilberman S. , Nitzan A.
Published: September 2002

The properties of Xe, CH4 and C16H34 lubricant confined between two approaching solids are investigated by a model that accounts for the curvature and elastic properties of the solid surfaces. We consider both smooth surfaces, and surfaces with short-scale roughness. In most cases we observe well defined molecular layers develop in the lubricant film when the width of the film is of the order of a few atomic diameters, but in some cases atomic scale roughness inhibit the formation of these layers, and the lubricant exhibit liquid-like properties. An external squeezing-pressure induces discontinuous, thermally activated changes in the number n of lubricant layers. We observe that the layering transition tends to nucleate in disordered or imperfect regions in the lubrication film. We also present and discuss results of sliding dynamics for Xe and C16H34 lubrication films.
Nonlinear stability and instability in collisionless trapped electron mode turbulence
Authors: Baver D. A. , Terry P. W. , Gatto R. , Fernandez Eduardo
Published: September 2002

A two-field model for collisionless trapped electron mode turbulence has both finite amplitude-induced stability and instability, depending on wave number. Effects usually identified with nonlinear plasma instability (self-trapping, kinetics, 3D mode structure, magnetic shear) are absent. Nonlinear stability and instability reside in E×B advection of density. It drives modes of a purely damped branch of the dispersion relation to finite amplitude and changes the rate at which free energy is released into the turbulence by shifting the density-potential cross phase. Analysis shows that modes of the purely damped branch cannot be ignored in saturation, and that the linear growth rate is a poor indicator of driving at finite amplitude, invalidating mixing length and quasilinear approximations. Using statistical closure theory, the nonlinear eigenmode and growth rate are determined from the saturation level of modes on all branches, stable and unstable, and the nonlinear cross phase that governs finite-amplitude instability.
On the reactivity of NH formed from photoinduced decomposition of HN3 in an Ar matrix at 12 K toward N2 and CO: A combined matrix isolation and quantum chemical study
Authors: Himmel Hans-Jörg , Junker Markus , Schnöckel Hansgeorg
Published: September 2002

Herein we report on the reactivity of NH produced from photolytically induced decomposition of HN3 in Ar matrices at 12 K. This reactivity was experimentally probed in matrix-isolation experiments and theoretically by detailed quantum chemical calculations. The mechanisms for reactions of HN with N2 and CO were examined. The results of the experiments show that triplet NH(3Σ) is formed as a detectable product of the photoinduced HN3 decomposition, indicating that the reaction of HN in its triplet electronic state with N2 to give HN3 (1A′) is opposed to a significant barrier. Using quantum chemical methods [coupled-cluster single double triple, complete active space self-consistent field, second-order Møller–Plesset] the barrier of this spin-forbidden reaction was estimated to be about 104–130 kJ mol−1 (depending on the level of theory), in good agreement with the results of earlier experimental and theoretical studies. On the other hand, our experiments show that HN(3Σ) reacts under matrix conditions with CO to give HNCO (1A′). The calculations show that the barrier for this reaction indeed is significantly lower (∼35 kJ mol−1) than the one for the reaction of HN(3Σ) with N2. Triplet–singlet conversion in both systems is achieved either through spin–orbit coupling or interaction with the “phonon-bath” of the Ar matrix environment.
Constrained statistical thermodynamic treatment of friction
Authors: Diestler D. J.
Published: September 2002

The molecular description of friction at a single, ideal microscopic contact of the sort realizable in scanning surface probe devices is greatly complicated by wide variations in the temporal regime t*≡texpt/tr of the measurement, where texpt is the time taken to measure the frictional force Fs and tr is the time required for the system to attain a state of thermodynamic equilibrium. At one extreme (t*≫1) the system remains in equilibrium for the duration of the measurement and one can employ statistical thermodynamics (in practice, Monte Carlo simulation) to compute Fs, which depends only on the thermodynamic state. At the other extreme (t*≪1) the system remains out of equilibrium. One must then account for the dynamic history of the system, typically by means of nonequilibrium molecular dynamics. The range of t* between these extremes can be handled within a single theoretical framework based on the concept of “equivalent equilibrated states.” Through addition of auxiliary potential fields to the Hamiltonian specific degrees of freedom of the system can be constrained. The properties of the constrained system are computed from the free energy of the system trapped in the equivalent equilibrated state by the constraints. The constraints are chosen to correspond to t*. The results of the theory applied to a one-dimensional model demonstrate dramatically the impact of history on Fs.
Monte Carlo simulation on separation and co-crystallization of a mixture of short polyethylene chains in a thin film
Authors: Xu Guoqiang , Mattice Wayne L.
Published: September 2002

Thin films of a mixture of two kinds of short polyethylene chains, n-tricontane and n-tetracontane, are simulated both at high and low temperatures by a dynamic Monte Carlo method on a high coordination lattice, which bridges the coarse-grained and the fully atomistic simulations. Films are obtained from an equilibrated model for the melt by increasing one of the three periodic boundaries to a very large value. The melting temperatures (Tm) of the two pure components in the simulation are obtained from a sharp transition of various parameters, such as the probability of trans conformation of C–C bonds, orientation order parameters, energies, etc. At high temperature, 420 K (above Tm of the two pure components), the chains have a trend to separate with each other. The shorter chains are enriched on the free surface due to their larger fraction of chain ends. At low temperature (below Tm of the two pure components), two situations exist, which depend on the prior history of the mixture. If the quench takes place from a homogeneous mixture the chains crystallize together with almost the same density profile except for a large fluctuation in the bulk region of the thin film. On the other hand, if the quench takes place from the equilibrated thin film at 420 K, the shorter chains remain enriched on the surface, and the segregation of the shorter chain is enhanced.
Three-dimensional flow in a cylinder with sidewall mass addition
Authors: Staab P. L. , Kassoy D. R.
Published: September 2002

Three-dimensional, time-dependent, nonlinear flow dynamics within a cylinder with sidewall mass injection are investigated. A nonaxisymmetric transient injection velocity, prescribed along the sidewall boundary of a long, narrow, half-open cylinder, induces a low Mach number, high Reynolds number flow. The injection drives nearly planar axial acoustic disturbances, which interact with the injected fluid to produce the azimuthal component of vorticity on the sidewall in an inviscid manner. A smaller, but important azimuthally dependent transient disturbance, driven by the nonaxisymmetric injection disturbance leads to the axial component of vorticity on the cylinder sidewall. Both components of vorticity are shown to convect toward the center of the cylinder, diffuse radially, and convect downstream. Other results show that the axial component of vorticity produced along the sidewall is largest near the maximum of a mass source distribution in the azimuthal dimension. The amplitude of the axial vorticity component decreases significantly away from the injection surface and at other azimuthal locations. The analysis of these flow processes is based on the full three-dimensional Navier–Stokes equations. A formal multiple-scale asymptotic analysis, based on the behavior of the axial Mach number M→0, is used to derive reduced equations. It is shown that the primary rotational flow response is described by a nonlinear, convection-diffusion equation.
Multistream instabilities in three-species plasmas formed around strong double layers
Authors: Lapuerta V. , Ahedo E.
Published: September 2002

An electron–electron–ion (e-e-i) and an ion–ion–electron (i-i-e) plasma are formed on the sides of a strong double layer. A detailed parametric investigation of the linear dispersion relation of these three-species plasmas is carried out in order to determine the different current-driven instabilities that can form in each of them. Both the electron–electron (e-e) and the ion–electron (i-e) instabilities develop in the e-e-i plasma. The first one dominates over practically its whole domain of existence and presents up to five different types. The second one involves the three plasma species and differs from the classical two-species version: it is resistive for most parametric ranges, with the reactive type occupying only a small parametric region within the high-drift range. Either two i-e instabilities or one i-e and one ion–ion instability can develop simultaneously in the i-i-e plasma, depending on the ratio of the velocities of the two beams involved. For both types of plasmas, different forms of branch mixing between the two unstable modes are identified.
Fine theoretical spectroscopy using symmetry adapted cluster-configuration interaction general-R method: Outer- and inner-valence ionization spectra of CS2 and OCS
Authors: Ehara Masahiro , Ishida Mayumi , Nakatsuji Hiroshi
Published: September 2002

Fine theoretical spectroscopy has been presented by the SAC-CI (symmetry adapted cluster-configuration interaction) general-R method for the outer- and inner-valence ionization spectra of CS2 and OCS. The SAC-CI general-R method simulated the experimental spectra quite accurately and the detailed assignments of the satellite peaks were given. For CS2, four outer-valence satellites Πu2 states were calculated, one of which was attributed to the recently observed peak (1′). Numerous Σu+2 and Σg+2 satellite peaks were obtained in the inner-valence region and some of them were dominantly described by triple electron processes; the quadruple R-operators were found to be important for describing these states in the general-R method. For OCS, the relative position of the main peaks was correctly reproduced and the higher R-operators were found to be important for the ordering of A and B states. In the energy region of 24–36 eV, continuous spectra of numerous Σ+2 satellites were obtained, which reproduced the feature of the photoelectron spectrum.
The nuclear quadrupole moment of 115In from molecular data
Authors: van Stralen Joost N. P. , Visscher Lucas
Published: September 2002

The nuclear quadrupole moment of 115In has been determined by combining the experimental nuclear quadrupole coupling constants and electric field gradients, calculated at the four-component CCSD(T) level of theory, of four indium halides. Our recommended value for the nuclear quadrupole moment of 115In is 770(8) millibarn. A basis set study at the Dirac–Hartree–Fock level shows a slow convergence of the electric field gradient with respect to higher angular momentum functions.
Damping of perturbations in weakly collisional plasmas
Authors: Short R. W. , Simon A.
Published: September 2002

New analytic results are presented for the description of waves in weakly collisional plasmas. The temporal damping of a freely propagating wave and the spatial damping of a wave driven by a localized antenna are treated. Analytic forms are given for the temporal and spatial dispersion relations of the collective modes, as well as expansions in the collisionality parameter, which facilitate analysis of the approach to the collisionless limit. The damping of the distribution function perturbations associated with plasma wave echoes is also analyzed. In the limit of weak collisionality our results are essentially in agreement with those of Su and Oberman [C. H. Su and C. Oberman, Phys. Rev. Lett. 20, 427 (1968)], which have recently been disputed by Ng et al. [C. S. Ng, A. Bhattacharjee, and F. Skiff, Phys. Rev. Lett. 83, 1974 (1999)].
Characterization of copper clusters through the use of density functional theory reactivity descriptors
Authors: Jaque Pablo , Toro-Labbé Alejandro
Published: September 2002

In this paper we study nine neutral copper clusters through the theoretical characterization of their molecular structures, binding energy, electronic properties, and reactivity descriptors. Geometry optimization and vibrational analysis were performed using density functional theory calculations with a hybrid functional combined with effective core potentials. It is shown that reactivity descriptors combined with reactivity principles like the minimum polarizability and maximum hardness are operative for characterizing and rationalizing the electronic properties of copper clusters.
Vibrational structure in the optical response of small Li-cluster ions
Authors: Ellert Christoph , Schmidt Martin , Haberland Hellmut , Veyret Vincent , Bonačić-Koutecký Vlasta
Published: September 2002

The optical response of Li4+ and Li7+ exhibits three large maxima at photon energies between 1.9 and 3.6 eV, on which a fine structures could be resolved that has a peak to peak distance of about 30–40 meV. A high level ab initio calculation of Li4+ shows that this feature is due to vibrational structure of the excited electronic state. It is pointed out that the lifetime of the excited states exhibits a dramatic shortening with increasing particle size.
How much can we learn from nearest neighbor distributions?
Authors: Schlier Christoph
Published: September 2002

Nearest neighbor distributions of molecular spectra can, in principle, be used to learn from quantum spectra about the classical dynamics of a system, i.e., whether it is regular or irregular (chaotic). However, the predictive power of this method is limited due to the generally small number of spectral lines available for analysis, and the ambiguities of the procedures used. This is demonstrated here for the determination of the shape of nearest neighbor distributions in terms of a Brody parameter, which was determined from fits to samples from a Brody distribution and fits to simulated molecular spectra. The procedures are also applied to computed spectra of NO2 and SO2.
Harmonic emission with cyclotron satellite structure due to strong magnetic fields produced by ultra-intense laser–plasma interaction
Authors: Zheng Jian , Tanaka K. A. , Sentoku Y. , Offenberger A. A. , Kitagawa Y. , Kodama R. , Kurahashi T. , Mima K. , Yamanaka T.
Published: September 2002

Under the action of an intense laser and strong magnetic field, an electron undergoes oscillation at a cyclotron frequency, as well as the frequency of the incident wave. The coupling between the drift cyclotron and figure-8 motions can generate sidebands in the vicinity of all harmonics. The frequency separation between the sidebands could be used to infer the magnitude of the self-generated magnetic field.
Design of flyer-plate-driven compressible turbulent mix experiments using Z
Authors: Drake R. Paul
Published: September 2002

The use of flyer plates to drive compressible turbulent mix experiments is discussed. The experimental parameters can be optimized, in order to maximize the degree of nonlinear development of either the Rayleigh–Taylor or the Richtmyer–Meshkov instability. Analytic formulas are presented for this optimization. Results of this analysis and of simulations are shown for experiments that might be accomplished on the Z machine at Sandia National Laboratories [M. K. Matzen, Phys. Plasmas 4, 1519 (1997)]. One finds that unique experiments could be accomplished with this device.
Indium–tin–oxide-coated glass as dichroic mirror for far-infrared electromagnetic radiation
Authors: Bauer T. , Kolb J. S. , Löffler T. , Mohler E. , Roskos H. G. , Pernisz U. C.
Published: September 2002

We propose to utilize glass plates coated with indium tin oxide (ITO) as far-infrared dichroic mirrors, e.g., in optoelectronic terahertz spectroscopy for purposes such as electro-optic detection of terahertz radiation or in time-resolved experiments with pump pulses in the visible or near-infrared spectral range and probe pulses at terahertz frequencies. Measurements of the complex reflection and transmission coefficients in the spectral range from 0.1 to 2.8 THz indicate that commercially available ITO glass is suitable for this purpose.
Structural characterization of Ni–Al (111) interface by surface x-ray absorption spectroscopy
Authors: Damoc L. , Fonda E. , Le Fevre P. , Traverse A.
Published: September 2002

The evolution of the Ni/Al(111) interface has been studied in situ by x-ray absorption spectroscopy at the Ni–K edge. Ni films were deposited on bulk Al(111) with thickness ranging from 2 monolayers (ML) up to 30 ML. The aim was to determine the diffusion length of Ni and the phases that have formed. Ni diffused spontaneously at room temperature to a depth that we estimated to be of the order of 11 ML. The structure of Ni/Al(111) mixed interface has been characterized by x-ray absorption spectroscopy. With respect to previous studies on Al(110) the first phase formed on Al(111) is Al3Ni2-like instead of AlNi-like. Accordingly to previous observations, an AlNi3 phase forms on top of Al3Ni2 after the deposition of the first few monolayers. We propose that the pure Ni growth observed after deposition of 11 ML is due to the presence of the AlNi3 aluminide that acts as a diffusion barrier preventing deeper Ni penetration into Al at room temperature.
Theoretical electronic structure of the lowest-lying states of the LaF molecule
Authors: Fahs H. , Allouche A. R. , Korek M. , Aubert-Frécon M.
Published: September 2002

CAS-SCF/MRCI calculations have been performed for nine molecular states in the representation Λ(+)2S+1 (neglecting spin–orbit effects) for the molecule LaF. The corresponding 20 molecular states in the representation Ω(+/−) (including spin–orbit effects) have been calculated using a semiempirical spin–orbit pseudopotential built up for lanthanum. Calculated potential energy curves and spectroscopic constants are reported. To the best of our knowledge they are the first ones from ab initio methods for the excited states of LaF. Comparison to recent experimental data display a good agreement with averaged discrepancies ΔTe=95 cm−1, ΔRe=0.033 Å, and Δωe=5 cm−1. Predictions are provided for the new triplet transitions under investigation in our laboratory involving the excited state (2)3Π as well as for the yet unobserved state (1)3Σ+.
Natural doubly diffusive convection in three-dimensional enclosures
Authors: Bergeon A. , Knobloch E.
Published: September 2002

Numerical continuation is used to study bifurcations in doubly diffusive convection in three-dimensional enclosures driven by opposing horizontal temperature and concentration gradients, and the results are compared with the two-dimensional case. Direct numerical simulation is used to show that in certain regimes the first stable nontrivial state of the three-dimensional system is a finite amplitude nonlinear oscillation. This state may be either periodic or chaotic. The mechanism responsible for these oscillations is identified, and the oscillations shown to be an indirect consequence of the presence of a steady-state bifurcation to fundamentally three-dimensional longitudinal structures that are absent from a two-dimensional formulation. The role of global bifurcations in generating the chaotic oscillations is elucidated.
Radiation-condensation instability in a highly ionized dusty plasma
Authors: Ibáñez S. Miguel H. , Shchekinov Yuri A.
Published: September 2002

The dynamics of linear perturbations in a radiatively cooling dusty plasma is considered, with the charge of both dust (Zd) and plasma (Zp) components being allowed to vary with their densities. It is shown that in the long-wavelength limit corresponding to the characteristic cooling length, when the plasma can be treated as quasineutral, the presence of dust particles changes the criteria for radiation instability, regardless of the charging process of the dust particles. In particular, the condensation (isobaric) mode is shown to be stabilized (destabilized) if in the equilibrium, the relation between densities of the dust nd and plasma n under the quasineutrality condition, (d ln nd/d ln n)q<1 (>1) is satisfied, while the isentropic mode is stabilized (destabilized) when the opposite inequalities take place; the isochoric mode is unaltered. Numerical estimates show that these effects can be important in hot phases (T∼106 K) of the interstellar plasma, and in tokamak plasma near the walls.
The symmetric driven cavity
Authors: Farias Jason E. , McHugh John P.
Published: September 2002

The driven cavity problem is considered. The driving force is not constant, but may be a complicated pattern across the top of the cavity. The problem is motivated by certain geophysical flows, such as circulation within lakes and how they are affected by atmospheric forcing at the surface. The results are obtained computationally using the lattice Boltzmann method employed on a two dimensional hexagonal lattice. Reynolds numbers range to 3000. An antisymmetric sinusoidal velocity pattern is used to simulate the nonconstant forcing. The simulations include an initial disturbance, developed with unidirectional forcing. Results exhibit a steady symmetric, steady asymmetric or unsteady asymmetric pattern, depending on the size of initial disturbance and Reynolds number.
Nonlinear optical properties of oligothiophene self-assembled monolayers on gold substrate
Authors: Mishina E. , Miyakita Y. , Yu Q.-K. , Nakabayashi S. , Sakaguchi H.
Published: September 2002

Liner and nonlinear-optical properties of oligothiophenes self-assembled monolayers (SAM) are studied by optical second harmonic interference technique. Linear polarizability and first hyperpolarizability as functions of the number of thiophene rings are calculated using a self-consistent approach for linear and nonlinear optical properties. Both of these dependencies can be described by a power law; a nonzero first hyperpolarizability points to the asymmetry of delocalized electron system that might be due to an adsorption of thiophene molecules on a metal substrate. The influence of a dipole–dipole interaction of molecules arranged in the SAM is discussed.
Site-specific fragmentation caused by core-level photoionization: Effect of chemisorption
Authors: Nagaoka Shin-ichi , Mase Kazuhiko , Nakamura Arinobu , Nagao Masashi , Yoshinobu Jun , Tanaka Shin-ichiro
Published: September 2002

We used the energy-selected-photoelectron photoion coincidence (ESPEPICO) method to study site-specific fragmentation caused by C:1s photoionization of 1,1,1-trifluoro-2-propanol-d1 [CF3CD(OH)CH3, TFIP-d1] on a Si(100) surface. High-resolution electron energy loss spectroscopy showed that TFIP-d1 is dissociatively chemisorbed like (CF3)(CH3)CDO–Si(100), and different chemical shifts at the three carbon sites were observed by photoelectron spectroscopy. The site-specific fragmentation evident in the ESPEPICO spectra of the sub-monolayer at room temperature indicates that the TFIP-d1 there has an O–Si bond oriented in the trans position with respect to the C–CF3 bond. Here we discuss the fragmentation processes in light of the results obtained with the ESPEPICO method and the Auger-electron photoion coincidence method.
Structural origins of circular dichroism in surface second harmonic generation
Authors: Simpson Garth J.
Published: September 2002

Circular dichroic (CD) ratios often exceeding 100% have been reported in previous surface second harmonic generation (SHG) measurements of chiral surface films, offering promise for the development of unique characterization methods to study biologically interesting surface systems. In this work, the molecular and surface structural origins of these large dichroic differences were explored in theoretical treatments and modeling calculations. Several new conclusions were formed regarding chirality in electric dipole allowed SHG measurements; (1) SHG-CD is allowed even in uniaxial films of achiral chromophores, provided they assemble with asymmetry in the twist angle, (2) for systems dominated by one or more chiral tensor element, this same asymmetry in surface packing is required for SHG-CD to be observed, and (3) large SHG-CD ratios are predicted in standard reflection and transmission measurements of ultrathin films in systems with spectral overlap between multiple excited states. The methodology described provides a formalism to aid in extracting structural information from nonlinear optical measurements of chiral films.
Further remarks on the heat capacity of the restricted primitive model
Authors: Valleau John , Torrie Glenn
Published: September 2002

This concerns the critical behavior of the restricted primitive model. Luijten, Fisher, and Panagiotopoulos [J. Chem. Phys. 114, 5468 (2001)] recently suggested that canonical heat capacity data previously published by us might be consistent with nonclassical behavior, which had appeared unlikely to us. Their argument turns on a supposed similarity of behavior of our results with results they have obtained for a lattice version of the model. We point out that the behaviors are in fact very dissimilar, however, and we explain why we do not find their suggestion plausible.
Coating and polymerization of C60 with carbon: A gas phase photodissociation study
Authors: Pellarin M. , Cottancin E. , Lermé J. , Vialle J. L. , Broyer M. , Tournus F. , Masenelli B. , Mélinon P.
Published: September 2002

( C 60 ) n C q + cationic clusters are produced in a laser vaporization source by quenching the vapors from C60 and graphite targets. They are analyzed in the gas phase by abundance and photofragmentation time-of-flight mass spectroscopy. Among the large number of expected isomers, the present experiments give evidence for the stability of special arrangements that may result from the attachment of preformed carbon rings to the fullerene cage. The particular case of C10 will be discussed, considering the results obtained on complexes with one, two or even three C60 molecules. This study reveals that the incorporation of large molecules into carbon cages could be an essential mechanism during the growth process of larger fullerenes and that the polymerization of C60 molecules is promoted by carbon links involving a single atom or a dimer.
Wavelets in curvilinear coordinate quantum calculations: H2+ electronic states
Authors: Maloney A. , Kinsey James L. , Johnson Bruce R.
Published: September 2002

Multiscale wavelets are used to solve the quantum eigenvalue equations for the hydrogen molecular ion H2+ in the Born–Oppenheimer approximation. Normally restricted to Cartesian systems, “wavelets on the interval” (a normal wavelet family augmented by special edge functions) have recently been applied to such boundary value problems as the hydrogen atom in spherical polar coordinates [J. Mackey, J. L. Kinsey, and B. R. Johnson, J. Comp. Phys. 168, 356 (2001)]. These methods are extended here to ground and excited electronic states of the simplest molecule, for which the electronic Hamiltonian is separable in confocal elliptic coordinates. The set of curvilinear coordinate quantum systems for which wavelet bases have been applied is thus enlarged.
Tensile elongation (110%) observed in ultrafine-grained Zn at room temperature
Authors: Zhang X. , Wang H. , Scattergood R. O. , Narayan J. , Koch C. C. , Sergueeva A. V. , Mukherjee A. K.
Published: September 2002

Tensile tests were performed for Zn at room temperature, which show elongations of 110%–20% for average grain sizes of 240–23 nm, respectively. The ductility of ultrafine-grained and nanocrystalline Zn was found to decrease with grain size refinement. The deformation mechanisms in ultrafine-grained Zn are believed to be a mixture of grain boundary sliding of small nanograins and intra-grain dislocation creep within the large grains.
Density distribution for a polymer adsorbed at an oil–water interface
Authors: Cai Jun , Prausnitz John M.
Published: September 2002

The interaction between a polymer segment and an oil–water interface is represented by an asymmetric square-well potential where the well-depth on one side reflects water–polymer and the well depth on the other side reflects oil–polymer interactions. The polymer is represented by a Gaussian chain. The polymer’s density distribution is calculated along a coordinate perpendicular to the interface. Results are obtained as a function of the well width, the well depth and its asymmetry and, most important, the polymer’s length. For a symmetric well, the distribution shows a strong maximum at the interface provided that the polymer is sufficiently long. For an asymmetric well, the polymer is also strongly adsorbed at the interface provided that the polymer is sufficiently long and provided that the larger well-depth does not exceed a critical value that depends on the smaller well-depth. The calculations are in substantial agreement with experimental results that indicate nearly irreversible adsorption of long-chain molecules at an oil–water interface.
Thermal-dynamic reversal of fine magnetic grains with arbitrary anisotropy axes orientation
Authors: Wang Xiaobin , Bertram H. Neal , Safonov Vladimir L.
Published: September 2002

A method is developed to analyze thermally agitated magnetization reversal in a single-domain ferromagnetic grain with a uniaxial anisotropy axis oriented at an arbitrary angle to a magnetic field. Random forces and phenomenological damping consistent with thermodynamics are introduced into the dynamic equations of the normal modes. This general approach reduces to the Landau–Lifshitz–Gilbert formalism only when the magnetic energy is isotropic about the equilibrium position. A stochastic averaging method based upon a small damping parameter is used to transform the inherent two-dimensional problem into a one-dimensional form. The mean first exit time is calculated and the results for dynamic coercivity at different angles to the magnetic field are obtained. The method is ultilized to calculate the switching field versus pulse time for planar random media with noninteracting grains. The results fit well with direct Langevin simulation over short times and with Néel–Arrhenius models at long times. A general field and temperature dependent form of the attempt frequency, f0, is derived.
Rotational Brownian motion of a pair of linear molecules or dipoles with anisotropic interaction
Authors: Felderhof B. U.
Published: September 2002

The rotational Smoluchowski equation for the orientational distribution function of two identical linear molecules or dipoles with anisotropic interaction is solved by expansion in terms of a limited set of eigenfunctions of the operator for free rotational diffusion. The equilibrium self-correlation and pair time-correlation functions of the two axial unit vectors or dipole moments are evaluated for the special case of electrostatic or magnetostatic dipole interaction. The interactions appreciably influence the relaxation behavior.
The structure of a concentrated aqueous solution of tertiary butanol: Water pockets and resulting perturbations
Authors: Bowron D. T. , Moreno S. Dı́az
Published: September 2002

The structure of a 0.86 mole fraction solution of tertiary butanol in water at room temperature has been investigated by neutron diffraction with hydrogen/deuterium isotope substitution. The intermolecular correlations between the alcohol and water molecules have been extracted using the empirical potential structure refinement technique. The results highlight the creation of water pockets within the solution structure. These pockets mediate the interactions that occur between the alcohol hydroxyl groups, reducing the level of direct alcohol–alcohol hydrogen bonding. The results show that only a small amount of water is required to drive this solution in the direction of hydrophobic behavior more commonly associated with the water-rich solution compositions.
Study of the triplet and pair structure of strong electrolytes modeled via truncated Coulomb interactions
Authors: Jorge S. , Lomba E. , Abascal J. L. F.
Published: September 2002

The structure of 3:3 and 1:3 electrolyte solutions at various concentrations and several cation/anion size ratios has been analyzed in terms of triplet and pair correlation functions, by means of simulation and a triplet integral equation theory derived from the inhomogeneous Ornstein–Zernike equation. The interaction model consists of a truncated and shifted Coulomb plus the Ramanathan–Friedman repulsive core. Concentration and size and charge asymmetry are found to induce changes in the triplet structure beyond those predicted by the simple superposition approximation, which are, however, correctly reproduced by the triplet integral equation.
Electron attachment time-of-flight mass spectrometry reveals geometrical shell closings in van der Waals aggregates
Authors: Ingolfsson Oddur , Wodtke Alec M.
Published: September 2002

Using electron attachment time-of-flight mass spectrometry, we show how high-precision structural constants of van der Waals aggregates may be obtained for two kinds of homogeneous clusters, (SF6)N and (CO2)N. Furthermore, we obtain size-specific structural information over a wide range of aggregate sizes. Mass spectrometric data are presented regarding the size needed to facilitate the transition from “cluster packing,” dominated by nearest-neighbor interactions, to bulk-like packing. For both examples, it appears that the cluster-to-bulk packing transition may occur even for aggregates where the majority of the molecules resides at the surface. The critical size for the cluster-to-bulk transition may be related to the size at which molecules packed as bulk crystals can begin forming nearly spherical shapes. A discussion of the mechanism by which geometrical shell closings are visualized in electron attachment time-of-flight mass spectrometry is also presented. We postulate that these observations reflect the dynamics of electron localization in ordered crystallites with and without defects.
Measurements of microwave spectra and structural parameters for methylferrocene
Authors: Margolis David S. , Tanjaroon Chakree , Kukolich Stephen G.
Published: September 2002

Rotational transitions for methylferrocene were measured, in the 4–12 GHz range, using a Flygare–Balle type, pulsed-beam Fourier transform spectrometer. Mono-substituted ferrocenes are nearprolate asymmetric tops with a and b dipole moment components, providing numerous possible transitions in this frequency range. Eighteen rotational constants were calculated from the data for six isotopomers. 59 transitions were measured for the normal isotopomer, and much smaller data sets were obtained for the Fe54, and four C13 isotopomers. Despite the small data sets for C13 and Fe54 transitions, good fits were obtained with small standard deviations ranging from 2 to 5 kHz. The eighteen A, B, and C rotational constants were used to determine the structural parameters for methylferrocene. Measured rotational constants for the normal isotopomer are: A=1592.6050(6), B=957.2565(4), and C=825.9892(4) MHz. The values for the averaged structural parameters are: r1(Fe–C)=2.050(7) Å, r2(C–C)(cyclopentadienyl ligands)=1.433(7) Å and r3(C–CH3)=1.52(2) Å (for the methyl cyclopentadienyl ligand). Small splittings (20–30 kHz) were observed for some transitions, and these could be due to proton hyperfine structure or small internal rotation splittings. The structural parameters for this complex were calculated using density functional theory (DFT) methods, and the results are in very good agreement with the measured values. Results are compared with earlier measurements on substituted ferrocenes. Distortions of the substituted cyclopentadienyl ligand are significantly smaller than those observed for chloroferrocene.
Morphological study of aluminum tris(8-hydroxyquinoline) thin films using infrared and Raman spectroscopy
Authors: Curry R. J. , Gillin W. P. , Clarkson J. , Batchelder D. N.
Published: September 2002

We present comprehensive Raman spectra for thin films of Alq3, annealed at various temperatures up to 300 °C, over the range of 70–1800 cm−1. These spectra give strong evidence for structural rearrangement of thin films of Alq3 upon annealing at temperatures above 200 °C. Needle like crystals are observed to grow in the films and confirmed to be comprised of the α-Alq3 polymorph using the low energy Raman spectra. Furthermore, no evidence of the fac isomer or thermal interconversion between the mer and fac isomers of Alq3 was observed in either the infrared or Raman spectra of the thin films or powder. These results may have implications for the long-term efficiencies of organic light emitting diodes incorporating thin films of Alq3.