bluuebunny/crossref_metadata_2025 huggingface dataset:
Small angle neutron scattering from D2O–H2O nanodroplets and binary nucleation rates in a supersonic nozzle
Authors: Heath Christopher H. , Streletzky Kiril A. , Wyslouzil Barbara E. , Wölk Judith , Strey Reinhard
Published: March 2003

Small angle neutron scattering (SANS) experiments were used to characterize binary nanodroplets composed of D2O and H2O. The droplets were formed by expanding dilute mixtures of condensible vapor in a N2 carrier gas through a supersonic nozzle, while maintaining the onset of condensation at a fixed position in the nozzle. It is remarkable, given the small coherent scattering length density of light water, that even the pure H2O aerosol gave a scattering signal above background. The scattering spectra were analyzed assuming a log-normal distribution of droplets. On average, the geometric radius of the nanodroplets rg was rg=13 (±1) nm, the polydispersity ln σr was ln σr=0.19 (±0.07), and the number density N was N=(2±0.2)⋅1011 cm−3. The aerosol volume fractions derived from the SANS measurements are consistent with those derived from the pressure trace experiments, suggesting that the composition of the droplets was close to that of the initial condensible mixture. A quantitative analysis of the scattering spectra as a function of the isotopic composition gave further evidence that the binary droplets exhibit ideal mixing behavior. Because both the stagnation temperature T0 and the location of onset were fixed, the temperature corresponding to the maximum nucleation rate was constant at TJ max=229 (±1) K. Thus, the experiments let us estimate the isothermal peak nucleation rates as a function of the isotopic composition. The nucleation rates were found to be essentially constant with Jmax equal to (3.6±0.5)⋅1016 cm−3 s−1 at a mean supersaturation of 44 (±3).
Construction of electronic diabatic states within a molecular orbital scheme
Authors: Troisi Alessandro , Orlandi Giorgio
Published: March 2003

A new procedure is proposed to construct a diabatic representation that is readily implemented in the molecular orbital-self-consistent field-configuration interaction scheme. It is based on the calculation of adiabatic wave functions at a reference geometry Q0 and of the appropriate modifications to be made to molecular orbitals for Q≠Q0 in order to force the derivative couplings for all electronic states to be exactly zero in the space around Q0. This approach is applied to the construction of the diabatic basis and to the calculation of the associated (adiabatic) vibronic coupling for a number of well-characterized systems. The properties and the limitations of this diabatic basis are discussed.
The singlet electronic excited states of the F2 molecule
Authors: Moriyama H. , Wasada-Tsutsui Y. , Sekiya M. , Tatewaki H.
Published: March 2003

By using multireference single excitation configuration interaction calculations and multireference single and double excitation CI calculations, we consider the Σu+1, Πg1, and Πu1 excited states of the F2 molecule which lie between 4.3 and 14.1 eV above the ground state. The basis set is composed of 13s, 10p, 7d, and 2 f contracted Gaussian-type functions, and covers molecular orbitals spanned by 4s, 4p, and 3d Rydberg orbitals. Of the Σu+1 states, G 1Σu+ is sometimes disregarded, presumably because it is not directly observed by optical measurements, but is inferred from perturbations in the visible and ultraviolet spectra. We find that G 1Σu+ arises from the shallow local minimum in the lowest Σu+1 potential curve, which also has a stable minimum corresponding to the state designated C 1Σu+. The experimental excitation energies (T0 values) for G 1Σu+ are 12.81–12.87 eV according to electron energy loss spectroscopy, and our theoretical value is 13.06 eV. Agreement between the experiment and the calculation is quite close. The state has a mixed ionic-Rydberg character with an interesting Rydberg portion. The experimental and calculated T0 values for C 1Σu+ are, respectively, 11.57 and 11.59 eV, suggesting that the present calculation for the state is reliable. Ambiguity found in experimental assignments of the vibrational levels for C 1Σu+ is settled here. The Πg1 and Πu1 states are also discussed.
Excited state tautomerism of the DNA base guanine: A restricted open-shell Kohn–Sham study
Authors: Langer Holger , Doltsinis Nikos L.
Published: March 2003

The relative stabilities of the six lowest energy tautomers of the DNA base guanine have been investigated in the first excited singlet state, S1, employing the restricted open-shell Kohn–Sham (ROKS) method. Comparison of the S1 optimized geometries to the respective ground-state structures reveals large distortions for the keto tautomers, whereas the enol tautomers remain essentially planar. Harmonic vibrational spectra in the S1 state have been calculated using the ROKS potential energy surfaces. Adiabatic excitation energies together with characteristic vibrational features of the individual guanine tautomers enable us to unambiguously assign recent experimental IR-UV spectra. Velocity autocorrelation functions obtained from adiabatic excited state Car–Parrinello molecular dynamics simulations demonstrate that anharmonic effects only play a minor role.
Correlation energy estimates in periodic extended systems using the localized natural bond orbital coupled cluster approach
Authors: Flocke N. , Bartlett R. J.
Published: March 2003

A new approach for the determination of correlation energies in periodic extended systems is proposed using the high transferability of amplitudes and integrals from natural bond orbital coupled cluster (NBO CC) calculations performed for small subunits. It is shown that the NBO CC calculations can in fact deliver detailed correlated wave function information for extended periodic systems. As an example we apply the ideas presented in this paper to determine an estimate for the valence correlation energy in diamond at the CCSD level.
The potential energy surface for spin-aligned Li3(1 4A′) and the potential energy curve for spin-aligned Li2(a 3Σu+)
Authors: Colavecchia F. D. , Burke J. P. , Stevens W. J. , Salazar M. R. , Parker G. A. , T Pack R.
Published: March 2003

A global potential energy surface (PES) for the 1 4A′ spin-aligned state of Li3 is presented. The surface is constructed as a many body expansion of the potential which is the sum of pairwise additive two-body potentials plus a three-body term. The two-body potential is that for the a 3Σu+ state of the lithium dimer. It combines the most recent Rydberg–Klein–Rees potential available [A. Ross (private communication)] with well-known short and long range expansions and accurately reproduces all known experimental data. To obtain the three-body contributions, an ab initio PES was computed at 1122 points using full configuration interaction for the three valence electrons with an augmented Gaussian basis and the effective core potentials of Stevens, Basch, and Krauss [W. J. Stevens et al., J. Chem. Phys. 81, 6026 (1984)] for the other electrons. The two-body interactions are also calculated using the same basis and then subtracted from the full interaction to give the three-body term. To construct the three-body potential at arbitrary configurations we use interpolation for small perimeters of the triangle formed by the triatomic system and an analytic fitting function for large perimeters. A switching function guarantees the smoothness of the potential function everywhere. The equilibrium position occurs at D3h symmetry with a bond distance of 5.861a0, nearly 2a0 smaller than the equilibrium value of 7.886a0 of the lithium dimer. The well depth at the equilibrium is 4112.64 cm−1. This is considerably deeper than the well depth of 1001.22 cm−1 for the pairwise additive potential at its equilibrium. Three-body effects are even more important for Li3 than in the recently reported Na3 case [J. Higgins et al., J. Chem. Phys. 112, 5751 (2000)], and the nonadditive three-body term cannot be neglected in any calculation on this system.
Quadratically convergent algorithm for fractional occupation numbers in density functional theory
Authors: Cancès Eric , Kudin Konstantin N. , Scuseria Gustavo E. , Turinici Gabriel
Published: March 2003

The numerical solution of the electronic structure problem in Kohn–Sham density functional theory may in certain cases yield fractional occupancy of the single-particle orbitals. In this paper, we propose a quadratically convergent approach for simultaneous optimization of orbitals and occupancies in systems with fractional occupation numbers (FONs). The starting guess for orbitals and FONs is obtained via the relaxed constraint algorithm. Numerical results are presented for benchmark cases.
Anomalous sedimentation of a small Brownian sphere in a vertical circular cylinder of periodically varying radius
Authors: Dorfman Kevin D. , Yariv Ehud , Dill Loren H. , Brenner Howard
Published: March 2003

The mean velocity of a Brownian sphere sedimenting under the influence of gravity through a viscous fluid confined within a vertical circular cylinder whose radius changes periodically along its effectively infinite length is investigated for circumstances where, owing to the small radius of the sphere relative to the minimum cylinder radius, conventional hydrodynamic wall effects are absent. Using generalized Taylor dispersion theory, explicit results are presented for the case where the periodic changes in cylinder radius occur abruptly, representing the computationally and conceptually simplest configuration. Despite the apparent absence of wall effects, the (mean) settling velocity of the sphere is always less than its Stokes law value, and depends nonlinearly upon the gravity force. Moreover, the sphere’s dispersivity fails to obey a Stokes–Einstein relation. This anomalous behavior may be exploited to enhance the force-driven separation of physicochemically disparate Brownian particles.
Resonance Raman study of the solvent dynamics for ultrafast charge transfer transition in 4-nitro-4′-dimethylamino-azobenzene
Authors: Biswas Nandita , Umapathy Siva
Published: March 2003

Contribution of solvent reorganization energy is known to be significant for ultrafast charge transfer processes, when the solvent relaxation times are slower than the rate of charge transfer. In this paper, we show that from resonance Raman intensities of a charge transfer transition in combination with Heller’s time-dependent wave packet approach and Brownian oscillator model, one can have a reasonable estimate for the different types of solvent (inertial as well as diffusive) and vibrational reorganization energies. Resonance Raman spectra have been recorded for 4-nitro-4′-dimethylamino-azobenzene (DA) that undergoes photoinduced charge transfer transition, in acetonitrile and benzonitrile. In the two solvents, the total solvent reorganization energy is partitioned into its inertial and diffusive components from the available information on their relaxation time scales. Thus, partitioning of the solvent reorganization energy reveals the importance of the extent of contribution of the two components to the charge transfer rates. The short time dynamics of DA in the two solvents is then examined from a priori knowledge of the ground state normal modes in order to convert the wave packet motion in dimensionless displacements to internal coordinates. The dynamics in DA infers that within 20 fs after photoexcitation from the ground to the charge transfer state, the excited state evolution occurs along N–O, N=N, C–N, and C–C stretching vibrations.
The dynamic Jahn–Teller effect applied to the temperature-dependent paramagnetic susceptibilities of Ti3+:Al2O3
Authors: Quade C. Richard , Mires Raymond W. , Brumage William H. , Dorman C. Franklin
Published: March 2003

The dynamic Jahn–Teller effect has been used in the analysis of the temperature-dependent paramagnetic susceptibilities of Ti3+:Al2O3. Two important changes have been made from the original analysis: (1) Parameters as determined from the dynamic Jahn–Teller effect are incorporated, and (2) All Van Vleck susceptibilities have been forced to sum to zero as required by quantum theory. The results of the analysis show that χ∥ contains a constant term of the order 1.22×10−8 emu/g that does not arise from Ti3+ singles. The data over the temperature range 18 to 100 K indicate that g∥1=2.54, a value 25% larger than that determined from the infrared Zeeman effect. It is possible to determine the perpendicular Van Vleck susceptibility for the ground state which is governed by the magnetic interaction of the lowest two energy levels with a substantial Jahn–Teller contribution. Finally, the suppression of the susceptibilities below 4.12 K can be modeled by a factor for the removal of singles from the susceptibilities by interactions with nearest neighbors, next-nearest neighbors, etc.
Density-dependent solvation dynamics in a simple Lennard-Jones fluid
Authors: Marques Martins Márcio , Stassen Hubert
Published: March 2003

The density dependence of time correlation functions for the solvation energy in a simple Lennard-Jones liquid has been investigated by molecular dynamics computer simulations. Considering argon dissolved in liquid argon, mechanical solvation dynamics has been studied treating interactions between excited solute states and the solvent by changes in the Lennard-Jones well-depth parameter, the Lennard-Jones size parameter, and a combination of the two. Densities have been varied from supercritical to triple point densities at a constant temperature of 151 K. In addition, a thermodynamic state close to the argon triple point has been considered. All the solvation energy time correlation functions have been broken down into their partial two- and three-body contributions giving an insight into the cancellation effect of solvation dynamics. It is found that the well-depth solvation process produces slowly decaying time correlation functions for the solvation dynamics at lower densities. In this case, the solvation dynamics becomes faster with increasing density due to long time cancellations between two-body time correlations with positive amplitudes and negative three-body contributions. In contrast, the size parameter solvation process is much faster. The corresponding solvation dynamics time correlations decay rapidly already at low liquid densities with two- and three-body contributions significantly stronger correlated than the total solvation energy time correlation function. Describing the excited solute by changes in the well-depth and the size parameter, the dynamical features resemble much of the solvation dynamics obtained from changes only in the size parameter.
Anomalous x-ray diffraction studies of Sr2+ hydration in aqueous solution
Authors: Ramos S. , Neilson G. W. , Barnes A. C. , Capitán M. J.
Published: March 2003

Anomalous x-ray diffraction experiments were carried out at two energies below the K edge of strontium on a 3.5 molal solution of strontium chloride in water. The first order difference method was applied to the corrected and normalized data sets, and a difference function for the Sr2+ coordination obtained. Fourier transformation of this function gave a modified total radial distribution function for Sr2+, which when analyzed showed that there are eight to nine nearest neighbor water molecules sited at 2.67 Å from the Sr2+ ion. A second coordination shell is also apparent in the range 4.5⩽r(Å)⩽5.5 which can accommodate Cl− counterions and a second shell of water molecules. The results obtained for the distance between Sr2+ and O are in excellent agreement with those obtained from standard x-ray diffraction, extended x-ray absorption fine structure, and neutron diffraction isotopic substitution. The hydration structure obtained for Sr2+ is compared with the structure of two other ions in the alkaline earth series (Mg2+ and Ca2+).
Critical fluctuations in the domain structure of lipid membranes
Authors: Halstenberg S. , Schrader W. , Das P. , Bhattacharjee J. K. , Kaatze U.
Published: March 2003

Between 100 kHz and 2 GHz ultrasonic attenuation spectra of two aqueous solutions of vesicles from 1,2-dimyristoyl-L-3-phosphatidylcholine have been measured at 13 temperatures around the main phase transition temperature of the membranes. The spectra are analyzed in terms of an asymptotic high frequency background contribution and three relaxation terms. Two of these terms can be represented by a discrete relaxation time, respectively, the other one extends over a significantly broader frequency range than a Debye-type relaxation term. It was found to nicely follow the predictions of the Bhattacharjee–Ferrell model of three-dimensional critical fluctuations. This finding has been additionally verified by measurements of the scaling function and by an analysis of the relaxation rate of order parameter fluctuations following from the fit of the experimental scaling function data to the theoretical form. Theoretical arguments are presented to indicate why the three-dimensional theory applies so well to the quasi-two-dimensional membrane system.
Non-Condon theory of nonadiabatic electron transfer reactions in V-shaped donor–bridge–acceptor complexes
Authors: Milischuk Anatoli , Matyushov Dmitry V.
Published: March 2003

The rate of nonadiabatic long-distance electron transfer (ET) is derived for the direct and superexchange electronic coupling between the donor and acceptor. The model takes into account a non-Condon thermal modulation of the electronic coupling through the interaction of the system transition dipoles with the polarization fluctuations of the solvent. Going from a linear donor–bridge–acceptor complexes to a bent, V-shaped geometry lowers the system symmetry resulting in several novel properties of the ET matrix element based on the fact that permanent and transition dipoles in the system are not polarized along the direction of ET. The effective ET matrix element HET gains two zeros as a function of the donor–acceptor vertical energy gap. The positions of zeros of HET depend on the sign relations between the donor–bridge and bridge–acceptor electronic couplings and corresponding transition dipoles. The ET matrix element becomes dependent on solvent through the solvent refractive index and the inhomogeneous width of the donor–acceptor optical transition. The former factor is mainly responsible for the temperature slope of |HET|. The derivative d|HET|/dT switches its sign at the zero points of HET. The distance decay parameter of the donor–acceptor coupling is shown to vary linearly with the inhomogeneous width of the optical transition between the donor and acceptor.
Multiaxial deformations of end-linked poly(dimethylsiloxane) networks. 4. Further assessment of the slip-link model for chain-entanglement effect on rubber elasticity
Authors: Urayama Kenji , Kawamura Takanobu , Kohjiya Shinzo
Published: March 2003

The Edwards–Vilgis slip-link model for the chain-entanglement effect on rubber elasticity is critically assessed on the basis of quasiequilibrium biaxial stress—strain data of end-linked polydimethylsiloxane (PDMS) networks with different entanglement densities. The PDMS networks with different entanglement densities were prepared by end-linking end-reactive long precursor PDMS in solutions with different solvent contents. The slip-link model, in which trapped entanglement is modeled by fictitious mobile slip-link attaching two entangled chains, satisfactorily describes the biaxial data over the entire range of deformation for all the networks examined. The model-specific parameters, i.e., slippage of slip-link (η) and inextensibility of network (α), were employed as adjustable parameters in data-fitting. The fitted values of η and α vary reasonably with the degree of dilution at network preparation, i.e., entanglement density. With an increase in dilution, i.e., decrease in entanglement density, η increases, whereas α decreases. In addition, the fitted values of η and α are in good agreement with the estimates from another molecular approach independent of mechanical testings: η=Me/Mc, where Me and Mc are the molecular masses between neighboring entanglements and between adjacent cross-links, respectively; α=nj−1/2, where nj is the number of Kuhn segments between adjacent elastically effective junctions including cross-links and trapped entanglements. The satisfactory data-fit with the model parameters of physically reasonable magnitudes supports the validity of the slip-link model for entanglement effects on rubber elasticity.
A quantitative study of the interaction of two Richtmyer–Meshkov-unstable gas cylinders
Authors: Tomkins C. , Prestridge K. , Rightley P. , Marr-Lyon M. , Vorobieff P. , Benjamin R.
Published: March 2003

We experimentally investigate the evolution and interaction of two Richtmyer–Meshkov-unstable gas cylinders using concentration field visualization and particle image velocimetry. The heavy-gas (SF6) cylinders have an initial spanwise separation of S/D (where D is the cylinder diameter) and are simultaneously impacted by a planar, Mach 1.2 shock. The resulting flow morphologies are highly reproducible and highly sensitive to the initial separation, which is varied from S/D≈1.2 to 2.0. The effects of the cylinder–cylinder interaction are quantified using both visualization and high-resolution velocimetry. Vorticity fields reveal that a principal interaction effect is the weakening of the inner vortices of the system. We observe a nonlinear, threshold-type behavior of inner vortex formation around S/D=1.5. A correlation-based ensemble-averaging procedure extracts the persistent character of the unstable flow structures, and permits decomposition of the concentration fields into mean (deterministic) and fluctuating (stochastic) components.
The effect of compatibilizer on the coalescence of two drops in flow
Authors: Ha J. W. , Yoon Yosang , Leal L. G.
Published: March 2003

This paper reports results from an experimental study of the effects of copolymer/compatibilizer on the coalescence of two equal size drops in the flow field produced by a four-roll mill. The data encompass two different fluid systems, both with PDMS as the suspending fluid and PBd as the drops, and an acid-base complex of PDMS–NH3+ −OOC–PBd adsorbed at the interface that we shall refer to as a copolymer. The two systems differ in the ratio of viscosities (λ) of the drop to the suspending fluid, one having λ=0.19 and the other λ=1.3. For the lower viscosity ratio system, as the amount of adsorbed copolymer is increased, the drainage time for coalescence in a head-on collision is increased monotonically and the critical capillary number for coalescence in a glancing collision is also reduced monotonically in a manner that appears qualitatively consistent with a slowing of the film drainage process due to Marangoni stresses. Detailed trajectory measurements for drops with copolymer show agreement with predicted theoretical results for spherical drops without copolymer, but with an increased viscosity ratio. With copolymer present, we also find that coalescence occurs for the largest capillary numbers only after the drops begin to be pulled apart by the external flow. For the higher viscosity ratio system, the effect of increasing the copolymer concentration is nonmonotonic. For very small concentrations, there is a major decrease in the critical capillary number for coalescence and a corresponding increase in the drainage time prior to coalescence, but as the copolymer concentration is further increased, the film drainage time decreases and the critical capillary number increases to a value that is intermediate between the clean interface result, and the result for the smallest copolymer concentration. This is shown to be due to a dependence of the critical coalescence angle on copolymer concentration that was not present in the lower viscosity ratio system. We conclude by speculating about mechanisms, in addition to the Marangoni effect, that might “explain” these observations.
Complex flow transitions in a homogeneous, concentrated emulsion
Authors: Shapley Nina C. , d’Avila Marcos A. , Walton Jeffrey H. , Powell Robert L. , Dungan Stephanie R. , Phillips Ronald J.
Published: March 2003

Results are presented for the flow of homogeneous, concentrated, oil-in-water emulsions subjected to a shear flow between rotating, horizontal concentric cylinders. Nuclear magnetic resonance imaging (NMRI) was used to measure velocity profiles. This technique allows velocity profiles to be measured noninvasively within a flowing, concentrated emulsion. It was observed that below a critical velocity, in a portion of the gap, the fluid moves in a direction opposite to the outer, rotating cylinder. Above this critical velocity, the emulsion corotates with the outer cylinder. Theoretical analysis suggests that the transitions are driven by buoyancy effects. The corotating and counter-rotating flow states at different rotation speeds can be characterized by a single dimensionless parameter Γ̂, which relates buoyancy and viscous effects.
Ab initio potential energy surface and rovibrational energies of H3O+ and its isotopomers
Authors: Huang Xinchuan , Carter Stuart , Bowman Joel
Published: March 2003

A new potential energy surface, based on high quality ab initio electronic structure calculations, is presented for the hydronium ion (H3O+). The new potential surface is used in rigorous calculations of vibrational energies of H3O+, D3O+, H2DO+, and HD2O+. Comparison with experiment shows significant improvement over our previous calculations using an earlier potential [X. Huang, S. C. Carter, and J. M. Bowman, J. Phys. Chem. B 106, 8182 (2002)]. Vibrational calculations are also presented with a new version of the code MULTIMODE. In this version the maximum number of coupled modes in the potential in any grouping of modes is increased from four (the previous maximum) to five. The importance of five-mode terms in the potential is demonstrated for several vibrational states in H3O+ and H2DO+. Also, in the new version of MULTIMODE the number of coupled modes in the Coriolis term can be varied independently from the number of coupled modes in the potential. Rovibrational calculations for J=1 are also presented for H3O+ and D3O+ and compared with experiment for H3O+.
Water adsorption on hydrophilic mesoporous and plane silica substrates: A grand canonical Monte Carlo simulation study
Authors: Puibasset Joël , Pellenq Roland J.-M.
Published: March 2003

The adsorption properties of water on silica surfaces at 300 K are evaluated by way of grand canonical Monte Carlo simulations, a well suited technique to investigate thermodynamical properties of interfacial fluids. The PN–TrAZ potential is used to describe atomic interactions between water and substrate, and the SPC model for water–water interactions. To measure confinement effects, we compare adsorption on different plane surfaces to a realistic disordered mesoporous material previously obtained by off lattice reconstructions known to reproduce in a realistic way the geometrical complexity of real Vycor of high specific surface, in terms of surface area, chord distribution, and correlation peak in small angle neutron scattering spectra. The adsorption isotherm, and isosteric heat of adsorption are calculated and compared to experimental data found in the literature for Vycor. They show good agreement, the PN–TrAZ potential for the adsorbate–surface interaction being able to give a satisfactory description of hydrophilic properties of plane and curved silica surfaces. For pressure less than 0.65 times the vapor pressure of SPC water at 300 K, the model shows identical adsorption properties for plane and mesoporous surfaces, validating the t-plot concept for such materials, i.e., the fact that the amount of fluid adsorbed is proportional to the surface area of the substrate irrespective of the pore size, as long as the surface chemistry (the OH density in this case) is kept constant. However, for very low relative pressure (10−3) an analysis of the isosteric heat of adsorption shows the presence of highly energetic sites on the mesoporous Vycor sample probably related to curvature, defects or roughness absent from perfectly plane surfaces. For relative pressure higher than 0.65 a sudden rise in Vycor adsorption is the signature of capillary condensation in the mesoporosity of the material, in agreement with experimental measurements in Vycor. Such a rise is absent on plane surfaces as expected for this confinement-induced capillary condensation.
Density functional theory investigation of H adsorption on the basal plane of boron-doped graphite
Authors: Ferro Y. , Marinelli F. , Allouche A. , Brosset C.
Published: March 2003

The scope of this paper is the theoretical study of hydrogen atom interaction with the boron-doped graphite surface taken as a model for the interactions that occur in controlled thermonuclear fusion devices. This work is carried out in the framework of the density functional theory. The boron-doped graphite surfaces are modeled using a small modified C16H10 cluster, in which one or two carbon atoms are substituted by boron. The efficiency of the C16H10 cluster in modeling the H-graphite interaction has already been established in a previous paper [J. Chem. Phys. 116, 8124 (2002)]. In this study, we show that the boron atom: (i) is not a stable adsorption site for H, that it induces (ii) an increase in the H binding energy, (iii) an increase in the permeability to H of the boron-doped graphite layer, and (iv) a long range electronic perturbation in its graphitic environment. A good agreement is found between our results and experimental studies dealing with erosion mechanisms of boron-doped graphite exposed to incident hydrogen ions fluxes.
Detection of phase transition of monolayers at the air–water interface by compression using Maxwell displacement current and optical second harmonic generation
Authors: Tojima Atsushi , Manaka Takaaki , Iwamoto Mitsumasa , Zhong-can Ou-Yang
Published: March 2003

The spontaneous and nonlinear dielectric polarization of Langmuir monolayers is expressed using orientational order parameters. With the theoretical calculations, the Maxwell displacement current (MDC) measurement coupled with optical second harmonic generation (SHG) measurement is shown to be helpful for the study of orientational orders and phase transitions in monolayers. Spontaneous and nonlinear polarization induced in monolayers are detected by MDC and SHG, respectively. Using the MDC-SHG measurement, monolayers of 4′-n-octyl-4-cyanobiphenyl on the water surface were examined by monolayer compression. The phase transitions such as from planar isotropic phase to polar and tilting orientational alignment phases were clearly revealed.
Growth of room-temperature “arsenic free” infrared photovoltaic detectors on GaSb substrate using metamorphic InAlSb digital alloy buffer layers
Authors: Plis E. , Rotella P. , Raghavan S. , Dawson L. R. , Krishna S. , Le D. , Morath C. P.
Published: March 2003

We report the growth of a high-quality graded InAlSb digital alloy buffer layer on GaSb substrates. The metamorphic buffer layer relaxes the lattice matching constraint and allows the growth of heterostructures without the use of a second group V element. Cross-sectional transmission electronic microscopy images reveal a very low dislocation density in the buffer layer. Using such a buffer layer, a room-temperature InGaSb photovoltaic detector with λcutoff∼3 μm has been fabricated with an external quantum efficiency >70%.
Radial source flows in porous media: Linear stability analysis of axial and helical perturbations in miscible displacements
Authors: Riaz Amir , Meiburg Eckart
Published: March 2003

Linear stability results are presented for axial and helical perturbation waves in radial porous media displacements involving miscible fluids of constant density. A numerical eigenvalue problem is formulated and solved in order to evaluate the relevant dispersion relations as functions of the Peclet number and the viscosity ratio. In contrast to the constant algebraic growth rates of purely azimuthal perturbations [C. T. Tan and G. M. Homsy, Phys. Fluids 30, 1239 (1987)], axial perturbations are seen to grow with a time-dependent growth rate. As a result, there exists a critical time up to which the most dangerous axial wavenumbers are larger, and beyond which the most dangerous azimuthal wavenumbers have higher values. This raises the possibility that early on, the smaller flow scales appear in the axial direction, whereas the later flow stages are dominated by smaller azimuthal features. By rescaling the axial wavenumber, the explicit appearance of time can be eliminated. The maximum growth rate of axial perturbations, as well as their most dangerous and cutoff wavenumbers, are seen to increase with the Peclet number and the viscosity ratio. The most dangerous wavenumber is observed to shift towards the lower end of the spectrum as the Peclet number increases. With increasing viscosity contrast, it first moves towards the lower part of the spectrum, only to shift towards the higher end later on. In the limit of large Pe, asymptotic solutions are obtained for the growth of axial disturbances. Numerical solutions of the full eigenvalue problem generally show good agreement with these asymptotic solutions for large Peclet numbers. Over the entire range of wave vector directions between the purely axial and azimuthal extrema, helical waves display an approximately constant maximum growth rate. The wavenumber of maximum growth as well the maximum growth rate of helical waves can be evaluated from the corresponding purely azimuthal and axial problems. This suggests that in three-dimensional flows the nature of the initial conditions plays an important role.
Interaction of two equal vortices on a β plane
Authors: Velasco Fuentes O. U. , Velázquez Muñoz F. A.
Published: March 2003

The interaction of two equal vortices under the influence of a gradient of background vorticity (β) is studied numerically and experimentally. If the initial shape and vorticity distribution of the vortices is fixed, two parameters determine the evolution: the normalized intercentroid distance d*=d/R, where R is the radius of the vortex; and the normalized gradient of background vorticity β*=βR/ω, where ω is the peak vorticity of the vortex. Alternate ways of identifying regimes of behavior in the parameter plane (d*,β*) are presented. These are applied to numerical simulations of interaction of vortices with steplike, steep and smooth vorticity profiles. It is found that the critical distance for merger decreases with increasing β* for all vortex types, and that vortices with smooth vorticity profile are the most merger-prone vortices. Laboratory experiments were done in a rotating water tank with a flat sloping bottom providing the β effect. The vortices produced have a smooth vorticity profile and show the same behavior observed in the simulations, except that, as a result of viscous effects, the critical merger distance is shifted towards larger values of d*.
Three-dimensional velocity field for wavy Taylor–Couette flow
Authors: Akonur Alp , Lueptow Richard M.
Published: March 2003

The stability of wavy supercritical cylindrical Couette flow has been studied extensively, but few measurements of the velocity field in flow have been made. Particle image velocimetry was used to measure the azimuthal and radial velocities in latitudinal planes perpendicular to the axis of rotation for wavy cylindrical Couette flow in the annulus between a rotating inner cylinder and a fixed outer cylinder. These measurements were matched to previous measurements of the axial and radial velocity measured in several meridional planes resulting in an experimentally measured, time-resolved, three-dimensional, three-component velocity field for wavy cylindrical Couette flow. Using this complete velocity field it is possible to evaluate details of the flow field. The vortical motion transports azimuthal momentum radially while the axial exchange of fluid between vortices in wavy flow transports azimuthal momentum axially. As the Reynolds number increases, these effects strengthen. Streams of net axial flow stretch axially along the length of the annulus and wind around the vortices from the inner cylinder to the outer cylinder and back while also winding azimuthally in the annulus. The azimuthal velocity measured at the center of a vortex is similar to the azimuthal wave speed. Measurements of the azimuthal velocity in cylindrical surfaces concentric with the axis of rotation suggest that the origin of the waviness is related to a jet-like azimuthal velocity profile rather than the radial outflow jet. Near both cylinder walls, the shear stress is quite large, decreasing to near zero at the middle of the annular gap.
An equality about the velocity derivative skewness in turbulence
Authors: Qian J.
Published: March 2003

We study velocity derivative skewness S of incompressible homogeneous isotropic turbulence. By using exact relations of isotropic turbulence and various typical models of second-order structure function DLL(r) and energy spectrum E(k), it is found that −S=C(kc/kd)2 when Taylor-microscale Reynolds number Rλ is high. Here, C is a coefficient, kc is the center wavenumber of energy dissipation spectrum, and kd is the Kolmogorov wavenumber. Therefore, the problem of Reynolds number dependence of S becomes the problem of Reynolds number dependence of kc/kd. In the inertial range, we have scaling DLL(r)∼rζ2 and E(k)∼k−(ζ2+1), ζ2 is the second-order inertial-range scaling exponent. Equality −S=C(kc/kd)2 is valid in the case of ζ2>2/3 (intermittency models of Kolmogorov’s 1962 theory) as well as in the case of ζ2=2/3 (Kolmogorov’s 1941 theory).
Orientation-dependent integral equation theory for a two-dimensional model of water
Authors: Urbič T. , Vlachy V. , Kalyuzhnyi Yu. V. , Dill K. A.
Published: March 2003

We develop an integral equation theory that applies to strongly associating orientation-dependent liquids, such as water. In an earlier treatment, we developed a Wertheim integral equation theory (IET) that we tested against NPT Monte Carlo simulations of the two-dimensional Mercedes Benz model of water. The main approximation in the earlier calculation was an orientational averaging in the multidensity Ornstein–Zernike equation. Here we improve the theory by explicit introduction of an orientation dependence in the IET, based upon expanding the two-particle angular correlation function in orthogonal basis functions. We find that the new orientation-dependent IET (ODIET) yields a considerable improvement of the predicted structure of water, when compared to the Monte Carlo simulations. In particular, ODIET predicts more long-range order than the original IET, with hexagonal symmetry, as expected for the hydrogen bonded ice in this model. The new theoretical approximation still errs in some subtle properties; for example, it does not predict liquid water’s density maximum with temperature or the negative thermal expansion coefficient.
Ultrashort, nonlinear, optical time response of Fe-doped InGaAs/InP multiple quantum wells in 1.55-μm range
Authors: Guézo M. , Loualiche S. , Even J. , Le Corre A. , Folliot H. , Labbé C. , Dehaese O. , Dousselin G.
Published: March 2003

Pump-probe experiments are used to characterize Fe-doped InGaAs/InP multiple quantum wells for ultrafast saturable absorption applications. Sample photoresponse time is well controlled by iron doping from the nanosecond range to a value as short as 0.45 ps for an iron concentration of 6×1018 cm−3. Furthermore, the switching amplitude, that is, the differential transmission ratio (DTR), of samples having increasing Fe doping (null to 6×1018 cm−3) presents two decreasing regimes at moderate optical excitation levels. The first one is explained well by the degree of quality of the material, whereas the second one is interpreted as an inhibition of exciton formation at high doping levels. This interpretation is highlighted by an analysis of the temporal evolution of the DTR. The influence of the doping localization (in the well, in the barrier, or in the whole structure) is studied as well. Finally, an attractive way to maintain a constant value of the DTR with an ultrafast response time (2.6 ps) is proposed and demonstrated.
Inverse velocity statistics in two-dimensional turbulence
Authors: Biferale Luca , Cencini Massimo , Lanotte Alesandra S. , Vergni Davide
Published: March 2003

We present a numerical study of two-dimensional turbulent flows in the enstropy cascade regime, with different large-scale energy sinks. In particular, we study the statistics of more-than-differentiable velocity fluctuations by means of two sets of statistical estimators, namely inverse statistics and second-order differences. In this way, we are able to probe statistical fluctuations that are not captured by the usual spectral analysis. We show that a new set of exponents associated to more-than-differentiable fluctuations of the velocity field exists. We also present a numerical investigation of the temporal properties of u measured in different spatial locations.
Evolution and instability of monopolar vortices in a stratified fluid
Authors: Beckers M. , Clercx H. J. H. , van Heijst G. J. F. , Verzicco R.
Published: March 2003

The evolution of initially axisymmetric shielded pancake-like vortices in a nonrotating linearly stratified fluid has been investigated experimentally and numerically. The evolution process and the shape of tripoles in laboratory experiments depend on the experimental parameter values. In order to investigate this phenomenon we have considered the influence of the Reynolds (Re) and Froude (F) numbers on the tripole formation process. Also, the role of the (absolute) ratio γ between the vorticity values of the satellite vortices and the core vortex on the tripole evolution has been investigated. Additionally, a set of numerical simulations has been performed to enable an examination of the role of Reynolds numbers (up to Re=10 000) and Froude numbers (F=0.1, 0.2, 0.4, and 0.8) outside the experimentally accessible range. The steepness parameter α was varied between 2 and 8 in order to estimate the relative importance of the different modes constituting the perturbation. From this study we conclude that tripole formation and dipole splitting in a linearly stratified fluid can be well described in terms of the parameter set (Re,F,α).
Temporally multiplexed holographic polymer-dispersed liquid crystals
Authors: Qi J. , Sousa M. E. , Fontecchio A. K. , Crawford G. P.
Published: March 2003

We report on a temporal multiplexing technique to fabricate multiple reflection gratings in holographic polymer-dispersed liquid crystals. By time-sequentially exposing the sample, two switchable reflection gratings are formed in a single film. The reflectance of the resulting gratings is well controlled by the individual exposure time. The observed transmission spectra are in excellent agreement with our model based on the reaction-diffusion equation to predict the kinetics of formation and the 2×2 matrix method to model the optical performance.
Potential for a hydrogen water-plasma laser
Authors: Mills R. L. , Ray P. C. , Mayo R. M.
Published: March 2003

A stationary, electronically excited, population inversion of atomic hydrogen, H, has been observed in a low-pressure water-vapor microwave discharge plasma. The inverted H population was evident from the relative intensities of the transitions within the Lyman series (n=2, 3, 4, and 5 to n=1) and within the Balmer series (n=3, 4, 5, 6, 7, and 8 to n=2). Back illumination with a broadband (vis/IR) lamp source showed depopulation of the n=5 state. Lines of the Balmer series of n=5, and 6 to n=2 and the Paschen series of n=5 to n=3 were of particular importance because of the potential to design blue and 1.3 μm infrared lasers, respectively, which are ideal for many communications and microelectronics applications. High-power hydrogen gas lasers are anticipated at wavelengths over a broad spectral range from far infrared to violet which may be miniaturized to micron dimensions. Such a hydrogen laser may prove to be the most versatile and useful of all.
Dynamics of the local entanglement on two vortex filaments described by the Korteweg–de Vries equation
Authors: Ohtsuka Kazumichi , Takaki Ryuji , Watanabe Shinsuke
Published: March 2003

An entanglement equation is found to reduce a Korteweg–de Vries (KdV) equation. An entanglement equation is the localized induction approximation of the Biot–Savart equation describing the dynamics of a co-rotating vortex pair. A soliton solution together with a cnoidal wave solution of the KdV equation shows that vortex filaments make a local entanglement when the distance between a pair of vortex filaments locally decreases.
Improving the mechanical properties of single-walled carbon nanotube sheets by intercalation of polymeric adhesives
Authors: Coleman Jonathan N. , Blau Werner J. , Dalton Alan B. , Muñoz Edgar , Collins Steve , Kim Bog G. , Razal Joselito , Selvidge Miles , Vieiro Guillermo , Baughman Ray H.
Published: March 2003

Organic polymers, such as poly(vinyl alcohol), poly(vinyl pyrrolidone), and poly(styrene), were intercalated into single-walled carbon nanotube sheets by soaking the sheets in polymer solutions. Even for short soak times, significant polymer intercalation into existing free volume was observed. Tensile tests on intercalated sheets showed that the Young’s modulus, strength, and toughness increased by factors of 3, 9, and 28, respectively, indicating that the intercalated polymer enhances load transmission between nanotubes.
Sub-5-nm-spatial resolution in scanning spreading resistance microscopy using full-diamond tips
Authors: Álvarez D. , Hartwich J. , Fouchier M. , Eyben P. , Vandervorst W.
Published: March 2003

Scanning spreading resistance microscopy is a two-dimensional carrier profiling technique now widely used for the characterization of silicon (Si) devices as well as other semiconductor materials. Whereas the state-of-the-art spatial resolution for this technique using commercial-diamond-coated silicon probes is limited to 10–20 nm, enhanced resolution is demonstrated through the use of full-diamond tips integrated in Si cantilevers. Sub-5-nm-spatial resolution is obtained on fully depleted silicon on isolator devices, putting the technique closer to the characterization requirements of the forthcoming semiconductor dimensions. Resistance and scanning electron microscope measurements clearly show that this enhanced resolution results from a smaller effective radius for full diamond tips as compared to the diamond-coated Si probes.
Strain-driven facet formation on self-assembled InAs islands on GaAs (311)A
Authors: Wang Z. M. , Wen H. , Yazdanpanah V. R. , Shultz J. L. , Salamo G. J.
Published: March 2003

The shape of InAs three-dimensional islands grown on GaAs(311)A substrates by molecular-beam epitaxy was investigated by in situ scanning tunneling microscopy. The island is found to be laterally surrounded by (111)A and {110} facets together with a convex curved region close to the (100) facet. The top ridge of the islands is atomically resolved to be the most recently discovered high-index surface {11,5,2}. This observation points to the importance of the study of nanostructure growth on high-index surfaces and their characterization.
Theoretical investigation of a two-dimensional photonic crystal slab with truncated cone air holes
Authors: Tanaka Yoshinori , Asano Takashi , Akahane Yoshihiro , Song Bong-Shik , Noda Susumu
Published: March 2003

The effects of truncated cone air holes on propagation losses from line defect waveguides in two-dimensional (2D) photonic crystal (PC) slabs are investigated. It is shown that coupling between TE-like waveguide modes and TM-like slab modes due to out-of-plane structural asymmetries can result in large propagation losses. It is also shown that coupling, and therefore propagation loss, does not occur in a frequency range where wave vectors of TE-like waveguide modes do not match projections of those of TM-like slab modes. The results are thought to be applicable to other structures exhibiting out-of-plane asymmetries, such as 2D PC slabs attached to silicon on insulator substrates.