bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical modeling of the OH stretch infrared spectrum of carboxylic acid dimers based on first-principles anharmonic couplings
Authors: Florio Gina M. , Zwier Timothy S. , Myshakin Evgeniy M. , Jordan Kenneth D. , Sibert Edwin L.
Published: January 2003

Carboxylic acid dimers serve as prototypical systems for modeling the unusual spectral behavior of the hydride stretch fundamental. Large anharmonic effects associated with the pair of cooperatively strengthened OH⋯O=C hydrogen bonds produces complicated infrared spectra in which the OH stretch oscillator strength is spread over hundreds of wave numbers, resulting in a complicated band sub-structure. In this work cubic anharmonic constants are computed along internal coordinates associated with the intramolecular OH stretch, intermolecular stretch, and OH bend internal coordinates for the formic acid and benzoic acid dimers. These are then projected onto the normal coordinates to produce mixed states that are used in computing the OH stretch infrared spectrum. For the benzoic acid dimer the calculations accurately reproduce for three deuterated isotopomers the overall breadth and much of the vibrational sub-structure in the observed spectra. For the formic acid dimer, the spectrum is calculated using a model employing a subset of the cubic force constants as well as using the full cubic force field. The spectra calculated for the formic acid dimer are sparser and somewhat more sensitive to the exact positions of the anharmonically coupled states than that of the benzoic acid dimer. Again semiquantitative agreement with experiment is obtained.
Efficient generation of low-energy folded states of a model protein
Authors: Gordon Heather L. , Kwan Wai Kei , Gong Chunhang , Larrass Stefan , Rothstein Stuart M.
Published: January 2003

A number of short simulated annealing runs are performed on a highly-frustrated 46-“residue” off-lattice model protein. We perform, in an iterative fashion, a principal component analysis of the 946 nonbonded interbead distances, followed by two varieties of cluster analyses: hierarchical and k-means clustering. We identify several distinct sets of conformations with reasonably consistent cluster membership. Nonbonded distance constraints are derived for each cluster and are employed within a distance geometry approach to generate many new conformations, previously unidentified by the simulated annealing experiments. Subsequent analyses suggest that these new conformations are members of the parent clusters from which they were generated. Furthermore, several novel, previously unobserved structures with low energy were uncovered, augmenting the ensemble of simulated annealing results, and providing a complete distribution of low-energy states. The computational cost of this approach to generating low-energy conformations is small when compared to the expense of further Monte Carlo simulated annealing runs.
Temperature dependence of the collisional energy transfer of OH(v=10) between 220 and 310 K
Authors: Lacoursière Jean , Dyer Mark J. , Copeland Richard A.
Published: January 2003

The temperature dependence of the thermally averaged collisional removal cross section of OH (X 2∏, v=10) by O2, N2O, and CO2 is measured between 220 and 310 K using a two-laser pump–probe technique and a specially designed vacuum-isolated flow cell. OH molecules are generated in v=6–9 by the reaction of hydrogen atoms and ozone. The (10,7) vibrational transition is excited with pulsed near-infrared laser light to create a population of OH (v=10) molecules. The temporal evolution of the v=10 population is monitored as a function of collider gas pressure by a time-delayed ultraviolet laser pulse. The probe step uses laser-induced fluorescence by exciting the B 2∑+–X 2∏ (0,10) transition and detecting the fluorescence from the B 2∑+–A 2∑+ (0,6–8) transitions. From 310 to 223 K, the OH (v=10) removal cross section increases by 35±21, 33±14, and 58±48 percent for the colliders O2, N2O, and CO2, respectively. This inverse temperature dependence is typical of a loss mechanism governed by long-range attractive forces.
Velocity map imaging of the photodissociation of CF3I: Vibrational energy dependence of the recoil anisotropy
Authors: Aguirre F. , Pratt S. T.
Published: January 2003

The photodissociation of jet-cooled CF3I into CF3+I(2P3/2) and CF3+I*(2P1/2) has been investigated between 304 and 277 nm by using velocity map ion imaging. The two-dimensional images provide detailed information on the partition of available energy into kinetic and internal energy of the photofragments. Vibrational structure with spacing of 695±100 cm−1 is resolved in both I and I* images, indicating excitation of the umbrella mode ν2 of the CF3 photofragment. The fragment recoil anisotropies β(I) and β(I*) are determined as a function of the excitation wavelength and their variations are interpreted in terms of the crossing between the Q03 and Q11 dissociative electronic states. The high-resolution images allow the determination of the variation of the anisotropy parameter β as a function of the vibrational level of CF3 fragment, and provide a complementary method for the determination of the C–I bond energy. The vibrational dependence of the anisotropy values is discussed in terms of final-state interactions between the CF3 umbrella motion and the C–I dissociation coordinate, as discussed previously by Hennig et al. [J. Chem. Phys. 84, 544 (1986)].
Ultrafast excitation of out-of-plane vibrations and vibrational energy redistribution after internal conversion of 4-nitroaniline
Authors: Kozich V. , Werncke W. , Vodchits A. I. , Dreyer J.
Published: January 2003

Ultrafast vibrational excitation and energy redistribution in the electronic ground state after internal conversion of 4-nitroaniline (PNA) and isotope labeled analogs is investigated by picosecond anti-Stokes resonance Raman spectroscopy. In PNA, PNA-di-15N and PNA-D4, anti-Stokes Raman lines of overtones and/or combination bands of out-of-plane vibrations display risetimes close to the decay time of the electronically excited state of about 0.5 ps and pronounced excess populations. Compared to such fast dynamics, the strongly Raman active totally-symmetric modes show a considerably slower picosecond rise time. Our results indicate primary excitation of out-of-plane vibrations by internal conversion and secondary excitation of strongly Raman active vibrations by redistribution of the vibrational energy.
A theoretical study of cis–trans isomerization in HONO using an empirical valence bond potential
Authors: Guo Yin , Thompson Donald L.
Published: January 2003

The empirical valence bond (EVB) approach proposed by Chang and Miller [J. Phys. Chem. 94, 5884 (1990)] for constructing global potential energy surfaces has the potential for treating large molecular systems. In this work, we slightly modify the expression of Chang and Miller by adding a damping function along the reaction coordinate to ensure the proper asymptotic behavior of the potential. We apply the EVB method to describe the cis–trans isomerization in HONO, and compute the isomerization rates in the low-energy tunneling regime by employing the semiclassical method that treats tunneling within classical trajectory calculations.
On the structure of Lennard-Jones fluids confined in crystalline slitlike pores
Authors: Patrykiejew A. , Sałamacha L. , Sokołowski S.
Published: January 2003

The structure of Lennard-Jones fluid in slitlike pores with crystalline walls is studied by canonical ensemble Monte Carlo simulation method. It is assumed that each pore wall is a perfect (100) plane of the face centered cubic crystal. The effects of the wall–wall separation on the ordering and freezing of a confined fluid are investigated for a series of systems with the density corresponding to the two completely filled layers of commensurate c(2×2) phase. It is demonstrated that the structure of a solid phase very strongly depends on the pore diameter. Thus, the formation of partially filled film that spans the space between the both pore walls occurs only when the pore width is smaller than a certain value. For wider pores only two monolayers at each pore wall occur. Then, the formation of commensurate structures of square symmetry in each layer occurs only over certain ranges of the pore width, while for other pore widths, the incommensurate phases of hexagonal symmetry in each layer are formed. The melting of solid phases has been found to be either the first order or continuous transition. In particular, the first order melting transitions have been found to be accompanied by a change (decrease) in the number of adsorbate layers within a pore. We have been able to single out seven regions of the pore width, which exhibit different structure and properties of adsorbed phases.
Melting of nanostructured Sn probed by in-situ x-ray diffraction
Authors: Depero L. E. , Bontempi E. , Sangaletti L. , Pagliara S.
Published: January 2003

In-situ x-ray diffraction allowed us to track the melting process of nanosized Sn droplets obtained by Sn evaporation on a substrate heated above the Sn melting temperature. We provide further evidence that the melting temperature of nanosized Sn particles is dramatically decreased with respect to the bulk value. The results, obtained by the analysis of in-situ x-ray diffraction patterns, indicate that thermal expansion of lattice parameters is anisotropic, the differences being related to the tetragonal crystal structure of β-Sn. Moreover, the behavior of Sn–Sn distance against the temperature suggests that the melting temperature is related to a critical Sn–Sn distance.
Ab initio potential energy surface for vibrational state calculations of H2CO
Authors: Yagi Kiyoshi , Oyanagi Chikako , Taketsugu Tetsuya , Hirao Kimihiko
Published: January 2003

A highly accurate potential energy function for H2CO has been developed at the CCSD(T)/cc-pVTZ level. The potential energy function is generated by the modified Shepard interpolation of the local fourth-order Taylor expansions defined at three reference points, i.e., the equilibrium structure plus two symmetrically equivalent structures located in the strongly coupled region of CH symmetric (Q1) and antisymmetric (Q5) stretching vibrational coordinates. The vibrational self-consistent field and the following vibrational configuration interaction methods have been applied to determine the fundamentals, overtones, and combination bands of H2CO. It is shown that our proposed potential energy function and a conventional quartic force field provide the different result related to the assignment of the 1151 and 113161 bands. The calculated vibrational energies are in good agreement with the corresponding experimental values, showing the mean absolute deviation of 7.7 cm−1.
On multiple states in single-layer flows
Authors: Baines Peter G. , Whitehead J. A.
Published: January 2003

For free surface flows over obstacles in a channel of constant width, there is a range of parameter values where two steady flow states are possible, with the state that is actually obtained being determined by the past history. Specifically, one of these flow states is wholly supercritical (i.e., no waves can propagate upstream against the flow) over the obstacle. The other has a hydraulic jump that travels to upstream infinity, and the flow undergoes a subcritical (i.e., waves can propagate in both directions) to supercritical transition at the obstacle crest. A new, third steady solution is described here, in which a hydraulic jump is stationary over the upstream face of a long obstacle. This new solution is contiguous with the other two, and in a sense, lies between them. It is shown that this new solution is unstable, in that if the stationary jump is displaced to a location with a slightly different bottom height, it will move further in the same direction. By this criterion, jumps are unstable on upslope flow, and stable on downslope flow. These properties, and the general character of hysteresis implied by these multiple hydraulic equilibria, have been tested with two series of experiments. The new solution is ordinarily not found because of its instability, but it can be viewed by manually balancing the unstable jump. Comparisons were also made between the observed abrupt transitions between flow states, and the predictions of hydraulic theory. Qualitatively the agreement is quite good, with differences attributable to experimental factors that are not contained in two-dimensional long wave hydraulics.
Dynamics of glass-forming liquids. VII. Dielectric relaxation of supercooled tris-naphthylbenzene, squalane, and decahydroisoquinoline
Authors: Richert Ranko , Duvvuri Kalyan , Duong Lien-Thi
Published: January 2003

We have measured the dielectric relaxation of several molecular organic supercooled liquids, ααβ-tris-naphthylbenzene (C36H24 and C36H10D14), squalane, and decahydroisoquinoline. The dynamics is studied in the frequency range 10−2–107 Hz, equivalent to temperatures between Tg and approximately 1.2×Tg. For the very low dielectric loss materials, a resolution of tan δ≈3×10−5 is required in order to observe the details of the relaxation behavior. Characteristic quantities like fragility, relaxation time dispersion, time–temperature superposition, and the slow Johari–Goldstein type β relaxation are among the properties discussed.
Phase diagrams for sonoluminescing bubbles: A comparison between experiment and theory
Authors: Toegel Ruediger , Lohse Detlef
Published: January 2003

Phase diagrams for single bubble sonoluminescence (SBSL) are calculated. The employed model is based on a set of ordinary differential equations and accounts for the bubble hydrodynamics, heat exchange, phase change of water vapor, chemical reactions of the various gaseous species in the bubble (N2, O2, and H2O being the most important among these), and diffusion/dissolution of the reaction products in the liquid. The results of the model are compared in detail to various phase diagram data from recent experimental work, among which are air-water systems as well as systems with a xenon-nitrogen mixture as the saturated gas. Excellent quantitative agreement is found for all considered cases. Moreover, we find that the onset of SBSL is hysteretic. When starting with air typical temperatures before onset are 5500 and 15 000 K thereafter. In the light emitting regime the bubbles are found to nearly entirely consist of argon.
Hydration structure of met-enkephalin: A molecular dynamics study
Authors: Dudowicz Jacek , Freed Karl F. , Shen Min-yi
Published: January 2003

All-atom molecular dynamics (MD) simulations for a single molecule of met-enkephalin in aqueous solution have been used to study the solvent structure surrounding the hydrated penta-peptide. The hydrophobicity (or hydrophicility) of the individual met-enkephalin atoms is analyzed in terms of the solute–solvent, site–site radial distribution functions gAB(r) that are computed for the all possible AB atom pairs. The MD simulated gAB(r) are also used for testing the reference interaction site model theory calculations by Hirata and co-workers for the same system.
On the actinic wavelength dependence of OClO photochemistry in solution
Authors: Wallace Paul M. , Bolinger Josh C. , Hayes Sophia C. , Reid Philip J.
Published: January 2003

The actinic (i.e., photolysis) wavelength dependence of chlorine dioxide (OClO) photochemistry in solution is investigated. Femtosecond pump–probe studies performed with 355-nm photoexcitation are presented for OClO dissolved in water and acetonitrile. Relative to earlier studies employing 400-nm photoexcitation, a decrease in primary-photoproduct geminate-recombination efficiency is observed in both solvents. This decrease in recombination efficiency is especially dramatic in acetonitrile where the recombination quantum yield is found to be only 0.08±0.04. The solvent and actinic-wavelength dependence of geminate recombination is discussed with respect to the recent theoretical work of Benjamin and co-workers [J. Chem. Phys. 116, 8930 (2002)]. Substantial increases in the optical density at 267 nm are also observed that cannot be explained by considering the increase in ClO concentration accompanying a reduction in geminate-recombination efficiency exclusively. Instead, these increases are consistent with an increase in the quantum yield for ClOO and/or Cl production suggesting that the partitioning between OClO photoproduct channels is actinic-wavelength dependent.
Electron-impact ionization of the simple alcohols
Authors: Rejoub R. , Morton C. D. , Lindsay B. G. , Stebbings R. F.
Published: January 2003

Absolute partial and total cross sections for electron-impact ionization of methanol, ethanol, and 1-propanol are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ion species are collected with equal efficiency irrespective of their initial kinetic energies. The total cross section for each target is obtained as the sum of the partial cross sections. The overall uncertainty in most of the absolute cross sections is ±6%–8%. Significant discrepancies are seen between the only prior methanol partial cross section determination and this study but the majority of published total cross section measurements and calculations are in good agreement with this work.
High pressure effects on the structure and spectroscopy of V3+ substitutional defects in Cs2NaYCl6. An ab initio embedded cluster study
Authors: Seijo Luis , Barandiarán Zoila
Published: January 2003

In this paper we present the results of ab initio model potential embedded cluster average coupled pair functional calculations on (VCl6)3− embedded in a reliable representation of the Cs2NaYCl6 host. They are aimed at complementing recent high pressure, room temperature, broad band luminescence studies with detailed theoretical data. In particular, we study the effects of high hidrostatic pressure (i) on the structure of V3+ substitutional defects in four low lying electronic states (a3T1g, T2g3, T2g1, and b3T1g), (ii) on the individual a1g and eg force constants and equilibrium displacements, and (iii) on the zero-phonon and Franck–Condon energy differences between them. It is shown that only the eg equilibrum displacements between different electronic sates are large and pressure dependent, while the a1g ones are small and essentially pressure independent. A value of κ=4×10−3 kbar−1 is suggested for the isothermal bulk modulus of the Cs2NaYCl6 elpasolite, because it allows for an excellent match between the theoretical dependence on the unit cell volume and the experimental dependence on applied pressure of several properties: equilibrium displacements, vibrational frequencies, and minimum-to-minimun and vertical energy differences.
C–C bond fission pathways of chloroalkenyl alkoxy radicals
Authors: Zhang Dan , Zhang Renyi , Allen David T.
Published: January 2003

Density-functional theory and ab initio molecular orbital calculations have been employed to determine the structures and energetics of the chloroalkenyl alkoxy radicals arising from Cl-initiated reactions of isoprene as well as the transition states and products of their decomposition reactions. Geometry optimizations of the various species were performed at the Becke three parameter Lee–Yang–Parr (B3LYP)/6-31G(d,p) level, and single-point energies were computed using second-order Møller–Plesset and coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations. The activation and reaction energies of C–C bond scission of the alkoxy radicals are in the ranges of 12–25 and −3–22 kcal mol−1, respectively. Using the obtained activation barriers and transition state structures, we have calculated the high-pressure limit decomposition rates of the chloroalkenyl alkoxy radicals using transition state theory, ranging from 1×10−5 to 2×104 s−1. The results indicate that C–C bond decomposition of the chloroalkenyl alkoxy radicals is rather slow and likely plays a minor role in the Cl-isoprene reactions. Implications of the present results on the formation yields of methyl vinyl ketone, methacrolein, and 1-chloro-3-methyl-3-buten-2-one are discussed.
Oscillatory and chaotic thermocapillary convection in a half-zone liquid bridge
Authors: Ueno Ichiro , Tanaka Shiho , Kawamura Hiroshi
Published: January 2003

Thermocapillary-driven convection in a half-zone liquid bridge was investigated experimentally. The induced flows were categorized into several regimes mainly through the pattern of the suspended particle motion in the bridge and the surface temperature variation. Special attention was paid to the flow structures far beyond the critical condition. Chaotic and turbulent flows were realized in this configuration. They were distinguished from the periodic oscillatory flow by applying the pseudo-phase-space reconstruction from the time series of the surface temperature variation.
Passive scalar mixing in a spatially developing shear layer: Comparison of one-dimensional turbulence simulations with experimental results
Authors: Ashurst Wm. T. , Kerstein A. R. , Pickett L. M. , Ghandhi J. B.
Published: January 2003

One-dimensional turbulence (ODT) simulations of a spatially developing planar shear layer are used to obtain the probability density function (PDF) of the concentration of a passive scalar. An asymmetry of the PDF with respect to transverse location in this self-similar flow appears in the ODT results and in experimental results obtained with planar laser-induced fluorescence of acetone. Using the ODT simulations, estimates are presented of the effect of measurement volume upon the resulting PDFs. Variable density effects on the concentration of the denser fluid within incompressible flow are also examined.
Visualization of a central bleed jet in an axisymmetric, compressible base flow
Authors: Bourdon C. J. , Dutton J. C.
Published: January 2003

The central bleed jet in an axisymmetric, compressible base flow with a Mach 2.46 free stream was visualized using an acetone planar laser-induced fluorescence (PLIF) technique. Three bleed flow rates, a low-bleed case (I=0.0038) and two others that bracket the optimal bleed case (I=0.0113 and I=0.0226), were examined. The injection parameter, I, is defined here as the bleed mass flow rate normalized by the product of the base area and freestream mass flux. This study shows that the bleed jet in the low-bleed case is almost instantly deflected outward toward the outer shear layer after it exits from the bleed orifice. When I=0.0113, the bleed fluid carries enough momentum that the bleed jet remains coherent and generally aligned along the symmetry axis for approximately one base radius before it is deflected outward by the primary recirculation region. For the highest bleed rate examined (I=0.0226), the bleed jet remains coherent and aligned along the symmetry axis for approximately 2.5 base radii downstream, where the impingement of freestream fluid causes the bleed jet to symmetrically eject mass downstream. Because the behavior of the bleed jet is much different for pre- and post-optimal bleed rates, some general conclusions can be made about the effect of bleed rate on base drag. For the two lower-bleed cases, there is a base pressure increase because the bleed fluid partially isolates the primary recirculation region from the outer flow, and provides part of the mass necessary for entrainment in the outer shear layer. The bleed jet in the high-bleed case is not as effective at increasing the base pressure, because the jet is shielded from the outer shear layer by the secondary recirculation region, which exists in the annulus between the bleed orifice and the base corner.
Infrared and Raman features of a series of α,ω-bis(arylthio)oligothiophenes as molecular wires. A π-electron delocalization efficiency study
Authors: Casado J. , Hicks R. G. , Hernández V. , Myles D. J. T. , Ruiz Delgado M. C. , López Navarrete J. T.
Published: January 2003

Fourier transform infrared and Raman spectra of a novel series of neutral α,ω-bis(mesitylthio)oligothiophenes and some α,ω-bis(mesitylthio)oligothiophenes containing ethylenedioxy groups at the β positions, with various chain lengths have been investigated. Because these compounds contain carbon-sulfur linkages they can be viewed as models for prototypical surface-bound molecular wires in which the oligomer is linked to the surface by a thiol-metal bond. Thus, interactions between the conjugated moiety and the metallic surface or cluster can be simulated. These studies are very important in order to apply molecular systems in micro and optoelectronic devices. The statements of effective conjugation coordinate theory and density functional theory calculations let us describe the specific electronic interactions between the mesitylthio groups and the oligothiophene backbone. An increased π-electron delocalization or conjugation length with respect to the α,ω-bis(alkyl)oligothiophenes of the same length has been noticed. The inclusion of C–S bridges between the end groups and the central π-conjugated backbone increases the π-electron delocalization within the oligomeric chain and precludes the oligothiophene moiety from conjugative interactions with the mesityl rings.
The valence-excited states T1–T4 and S1–S2 of acetylene: A high-level MR-CISD and MR-AQCC investigation of stationary points, potential energy surfaces, and surface crossings
Authors: Ventura Elizete , Dallos Michal , Lischka Hans
Published: January 2003

Valence-excited singlet (S1,S2) and triplet (T1–T4) states of acetylene have been studied by means of extended multireference electron correlation techniques (MR-CISD, MR-CISD+Q, and MR-AQCC). Extrapolations to the basis set limit have been performed. Minima and saddle points have been calculated using a recently developed analytic gradient method for excited states. Planar as well as nonplanar structures have been considered. In particular, the existence of an asymmetric, planar cis-type minimum on the S2 surface has been confirmed conclusively. Moreover, an intersection S1/S2 has been located close to this minimum. This situation will most probably affect the interpretation of the absorption bands attributed to the trans 1 1Bu state. In-plane and out-of-plane saddle points for cis–trans isomerization have been determined and characterized by harmonic vibrational analysis. Several interesting surface crossings for different electronic states (S1/S2, T2/T3, and S1/T3) have been characterized. Implications of the flatness of the T3 surface around linear structures and the location of the S1/T3 crossing seam on the anomalities observed in the ZAC spectrum of the Ã 1Au state are discussed.
A comparison of polarized double-zeta basis sets and natural orbitals for full configuration interaction benchmarks
Authors: Abrams Micah L. , Sherrill C. David
Published: January 2003

We compare several standard polarized double-zeta basis sets for use in full configuration interaction benchmark computations. The 6-31G**, DZP, cc-pVDZ, and Widmark–Malmqvist–Roos atomic natural orbital (ANO) basis sets are assessed on the basis of their ability to provide accurate full configuration interaction spectroscopic constants for several small molecules. Even though highly correlated methods work best with larger basis sets, predicted spectroscopic constants are in good agreement with experiment; bond lengths and harmonic vibrational frequencies have average absolute errors no larger than 0.017 Å and 1.6%, respectively, for all but the ANO basis. For the molecules considered, 6-31G** gives the smallest average errors, while the ANO basis set gives the largest. The use of variationally optimized basis sets and natural orbitals are also explored for improved benchmarking. Although optimized basis sets do not always improve predictions of molecular properties, taking a DZP-sized subset of the natural orbitals from a singles and doubles configuration interaction computation in a larger basis significantly improves results.
Determination of interface energy band diagram between (100)Si and mixed Al–Hf oxides using internal electron photoemission
Authors: Afanas’ev V. V. , Stesmans A. , Tsai W.
Published: January 2003

Internal electron photoemission is used to investigate the electronic structure of the Si/Hf–Al oxide interfaces as a function of oxide composition. In the mixed oxides, the energy position and the density of states in the conduction band show little sensitivity to the Hf content. At the same time, the energy band diagram of the Si/oxide interface appears to be very close to that of the Si/Al2O3 interface, suggesting that the conduction band is derived mostly from the states of Al ions.
The joint cascade of energy and helicity in three-dimensional turbulence
Authors: Chen Qiaoning , Chen Shiyi , Eyink Gregory L.
Published: January 2003

Three-dimensional (3D) turbulence has both energy and helicity as inviscid constants of motion. In contrast to two-dimensional (2D) turbulence, where a second inviscid invariant—the enstrophy—blocks the energy cascade to small scales, in 3D there is a joint cascade of both energy and helicity simultaneously to small scales. It has long been recognized that the crucial difference between 2D and 3D is that enstrophy is a nonnegative quantity whereas the helicity can have either sign. The basic cancellation mechanism which permits a joint cascade of energy and helicity is illuminated by means of the helical decomposition of the velocity into positively and negatively polarized waves. This decomposition is employed in the present study both theoretically and also in a numerical simulation of homogeneous and isotropic 3D turbulence. It is shown that the transfer of energy to small scales produces a tremendous growth of helicity separately in the + and − helical modes at high wave numbers, diverging in the limit of infinite Reynolds number. However, because of a tendency to restore reflection invariance at small scales, the net helicity from both modes remains finite in that limit. Since energy and helicity are not separately conserved in the + and − modes, there are four “fluxlike” quantities for both invariants, which correspond to transfer either out of large scales or into small scales and either to + helical or to − helical modes. The helicity fluxes out of large scales in the separate + and − channels are not constant in wave number up to the Kolmogorov dissipation wave number kE but only up to a smaller wave number kH, recently identified by Ditlevsen and Giuliani [Phys. Fluids 13, 3508 (2001); Phys. Res. E 63, 036304 (2001)]. However, contrary to their argument, the net helicity flux is shown to be constant all the way up to the Kolmogorov wave number: there is no shorter inertial range for helicity cascade than for energy cascade. The transfer of energy and helicity between + and − modes, which permits the joint cascade, is shown to be due to two distinct physical processes, advection and vortex stretching.
Vortices in film flow over strongly undulated bottom profiles at low Reynolds numbers
Authors: Wierschem A. , Scholle M. , Aksel N.
Published: January 2003

We present an experimental study of gravity driven films flowing down sinusoidal bottom profiles of high waviness. We find vortices in the valleys of the undulated bottom profile. They are observed at low Reynolds numbers down to the order of 10−5. The vortices are visualized employing a particle image velocimeter with fluorescent tracers. It turns out that the vortices are generated beyond a critical film thickness. Their size tends to a finite value for thick films. The critical film thickness depends on the waviness of the bottom undulation, the inclination angle, and on the surface tension but not on the Reynolds number. Increasing the waviness, a second vortex can be generated.
Thermocapillary convection with moving contact points
Authors: Jiang Y. , Badr H. , Floryan J. M.
Published: January 2003

Marangoni convection in a cavity subject to two types of heating, i.e., heating through the sidewalls and heating by a point source from above, has been investigated. The assumed wetting conditions permit motion of the interface along the sidewalls subject to a constant contact angle constraint. The analysis considers complete interface deformation effects. The results determined for large Biot and zero Marangoni numbers show the existence of limit points beyond which steady, continuous interface cannot exist. The limit points define the maximum capillary number Ca and the maximum cavity length L permitted. The permitted values of the Reynolds number may be bounded from below and from above depending on the values of other transport parameters and on the form of the external heating. Interface approaching bottom of the cavity leading, most likely, to the formation of a dry spot, represents the main factor limiting the existence of steady convection. The topology of the flow field is similar to the case when the wetting conditions result in the fixed location of the contact points while the topology of the interface is qualitatively different. The change of the contact conditions from the fixed contact points to the fixed contact angles results in a significant reduction of the range of parameters that guarantee the existence of a continuous interface.
Thermal stability of Ti-catalyzed Si nanowires
Authors: Kamins T. I. , Li X. , Williams R. Stanley
Published: January 2003

The stability of long, narrow Si nanowires grown by catalytic decomposition of a Si-containing gas during annealing was investigated because their use in devices will depend on their stability during subsequent processing. The nanowires can be annealed without visible change well above 900 °C in N2 after air exposure. However, annealing in a H2 ambient more readily degrades their structure. Nanowires without a native surface oxide are stable up to 850 °C, but change significantly at 900 °C. Surface diffusion appears to be sufficient to allow constant-diameter nanowire regions to form.
Magneto-optical imaging with diluted magnetic semiconductor quantum wells
Authors: Gourdon C. , Jeudy V. , Menant M. , Roditchev D. , Tu Le Anh , Ivchenko E. L. , Karczewski G.
Published: January 2003

Heterostructures containing CdMnTe-diluted magnetic semiconductor quantum wells are proposed as magneto-optic layers (MOL) for high-resolution Faraday microscopy at low temperature. The Faraday rotation is enhanced up to 54.4 deg T−1 by embedding the quantum wells in a semiconductor–metal optical cavity. These MOL allowed the observation of the intermediate state pattern in a superconductor layer with spatial and magnetic resolutions of 1 μm and 10 mT, respectively.
Bulk-grain resistivity and positive temperature coefficient of ZnO-based varistors
Authors: Fernández-Hevia D. , de Frutos J. , Caballero A. C. , Fernández J. F.
Published: January 2003

We analyze the conditions that allow the bulk-grain regions of a polycrystalline semiconductor to be explored through electrical measurements. The temperature dependence of grain resistivity in ZnO varistors (300–430 K) is presented, and a positive temperature coefficient is found. This is consistent with a free-carrier density approaching exhaustion, and an electron mobility controlled mainly by lattice (both optical and acoustical) scattering. No grain conductivity activation energy is to be found above room temperature and, therefore, ac-impedance techniques can be inadequate for the evaluation of grain conductivity and shallow donor activation energy.
Shaped electrode and lens for a uniform radio-frequency capacitive plasma
Authors: Sansonnens L. , Schmitt J.
Published: January 2003

Plasma inhomogeneity caused by standing-wave effects in rf parallel plate reactors can be removed if one of the electrodes is replaced by a shaped electrode. The proposed shape is calculated using Maxwell’s equations in vacuum. In this solution, the electric field has a radially uniform vertical component, and a radial component which remains negligible for a plasma gap small compared to the quarter wavelength. Perturbations to this vacuum solution in presence of a plasma should remain small if the damping length due to dissipation in the plasma is large compared to the reactor radius and if the rf skin depth remains large compared to the plasma thickness.
Terahertz pulse shaping via optical rectification in poled lithium niobate
Authors: Lee Y.-S. , Amer N. , Hurlbut W. C.
Published: January 2003

We demonstrate a technique for terahertz pulse shaping via optical rectification in the pre-engineered domain structure of poled lithium niobate crystals. The terahertz wave forms coincide with the crystal domain structures. The one-dimensional nonlinear wave equation simulates the experimental results with a good qualitative agreement.
Cavitation inception by almost spherical solid particles in water
Authors: Marschall H. B. , Mørch K. A. , Keller A. P. , Kjeldsen M.
Published: January 2003

The tensile strength of water increases when solid particles are filtered out, and it becomes greater the smaller the remaining particles are. Natural particles are of random shape, making parametric studies on the relationship between tensile strength and particle characteristics difficult. In this investigation, using degassed tap water from which natural particles larger than about 1 μm had been filtered out, the tensile strength was measured before and after seeding with almost spherical solid balls of diameters from 3 up to 76 μm. The smallest balls, one type being hydrophobic, the other hydrophilic, had no measurable influence on the tensile strength, though they were notably larger than the remaining natural nuclei. Seeding with the larger balls, hydrophilic as well as hydrophobic ones, reduced the tensile strength compared with that measured for unseeded, filtered water, but at most down to 1/3. On this basis it is concluded that a greater tensile strength is connected to the almost spherical solid balls than that due to natural particles of the same size. The critical cavities developed from the larger balls had radii much smaller than those of the balls themselves. This supports the hypothesis that cavitation nuclei are related to the fine scale surface structures observed on the balls, and in dependence of their global radii of curvature, critical cavities are developed. A model of this development is presented.
Synthesis and electronic structure of epitaxially stabilized Sr2−xLaxVO4 (0⩽x⩽1) thin films
Authors: Matsuno J. , Okimoto Y. , Kawasaki M. , Tokura Y.
Published: January 2003

We have synthesized a-axis-oriented epitaxial thin films of layered vanadate Sr2−xLaxVO4 on LaSrAlO4 (100) substrates using a pulsed-laser deposition technique. Epitaxial interface energy stabilizes compounds from V4+ (x=0) to V3+ (x=1), for which none of bulk single crystal has been synthesized. The transport property including its anisotropy has been studied to reveal metallic conduction for x=0.15 and 0.2. The two-dimensional nature of the crystals gives insulating behavior along the c axis for all the compositions. Optical measurements indicate that the doped electrons are responsible for the insulator-to-metal transition as well as for the systematic variation of electronic structure from V4+ to V3+-based Mott insulators.
Bright single-active layer small-molecular organic light-emitting diodes with a polytetrafluoroethylene barrier
Authors: Gao Yudi , Wang Liduo , Zhang Deqiang , Duan Lian , Dong Guifang , Qiu Yong
Published: January 2003

Single-layer organic light-emitting diodes (OLEDs) with a small molecule, tris(8-hydroxyquinoline) aluminum (Alq3) as the only active material, have been prepared. In order to achieve an efficient hole injection, a thin layer of polytetrafluoroethylene (Teflon) was inserted between the anode and the Alq3 layer. The effect of the Teflon layer thickness upon the device performance has also been investigated. A brightness of 16 000 cd/m2 with 6 nm thick Teflon layer was achieved, whereas the conventional double-layer OLEDs with N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4,4′-diamine and Alq3 showed only 9500 cd/m2 in our experiments. The single-layer structure technology is of great importance to the OLED’s commercialization due to its possible lower cost and higher production efficiency. And it is reasonable to infer that, based on this work, higher device performance could be realized by screening both the active material and the barrier layer material.
Two-dimensional viscous flow in a granular material with a void of arbitrary shape
Authors: Raja Sekhar G. P. , Sano O.
Published: January 2003

The effect of the presence of a void of characteristic size R0 in a granular material with permeability k on an otherwise undisturbed uniform flow U∞ is investigated analytically. The cavity region is assumed to be two-dimensional, and its cross section is slightly deformed from a circle. The entire flow field is determined by matching the solution of the Stokes equation inside the hole with that of the generalized Darcy’s equation (or Brinkman’s equation) in the granular material. The effect of the hole increases with the value of ζ0=R0/k. When ζ0 is much larger than unity, which corresponds to the case for typical groundwater flow, the velocity at the center of the circular cylindrical hole U0 becomes 3U∞. In addition this velocity depends on the shape and orientation of the cavity, and attains still larger value. The volume flux Q0 flowing into the hole from the upstream side also depends on ζ0 and its configuration, which is typically two times for a circular cavity in comparison with Q∞ that would otherwise flow into the same region. These results have fundamental importance on the formation of the waterway network, as well as the accurate estimation of the underground water velocity.
Carbon-nanotube-based nonvolatile memory with oxide–nitride–oxide film and nanoscale channel
Authors: Choi Won Bong , Chae Soodoo , Bae Eunju , Lee Jo-Won , Cheong Byoung-Ho , Kim Jae-Ryoung , Kim Ju-Jin
Published: January 2003

We have fabricated a single-wall carbon-nanotube (CNT)-based nonvolatile memory device using SiO2–Si3N4–SiO2 (ONO) layers as a storage node. The memory device is composed of a top gate structure with a channel width of a few nanometers and the ONO layer embedded between CNT and gate electrode. When the bias voltage between the CNT and gate electrode increases to 4 V, charges are tunneled out from CNT surfaces and captured to the traps in the ONO layers. Stored charges on the trap sites make the threshold voltage shift of 60 mV and is independent of charging time, suggesting that the ONO has traps with a quasiquantized energy state. The quantized state is related to the localized high electric field associated with CNT channel. The CNT-field-effect transistor with an ONO storage node could be used for an ultrahigh-density nonvolatile memory.
Capacitively coupled hot-electron nanobolometer as far-infrared photon counter
Authors: Anghel Dragoş-Victor , Kuzmin Leonid
Published: January 2003

We show theoretically that hot-electron nanobolometers consisting of a small piece of normal metal, capacitively coupled to a superconducting antenna through a pair of normal metal–insulator–superconductor (NIS) tunnel junctions may be used as far-infrared photon counters. To make the device most effective at high counting rates, we suggest the use of the bolometer in the simplest configuration, when the NIS tunnel junctions are used as both an electron cooler and thermometer. The absorbtion of the photon in the normal metal produces a pulse in the electron temperature, which is measured by the NIS junctions. The counter may resolve photons up to 0.3–0.4 mm wavelength and has a typical re-equilibration time constant of about 20 ns.
A simple model for self-sustained oscillations in homogeneous shear flow
Authors: Yakhot Victor
Published: January 2003

Generation of the large-scale coherent vortical structures in homogeneous shear flow couples dynamical processes of energy and enstrophy production. In the large rate of strain limit, the simple estimates of the contributions to the energy and enstrophy equations result in a dynamical system, describing experimentally and numerically observed self-sustained nonlinear oscillations of energy and enstrophy. It is shown that the period of these oscillations is independent of the box size and the energy and enstrophy fluctuations are strongly correlated.
Ferromagnetism in Mn-implanted ZnO:Sn single crystals
Authors: Norton D. P. , Pearton S. J. , Hebard A. F. , Theodoropoulou N. , Boatner L. A. , Wilson R. G.
Published: January 2003

We have investigated the magnetic properties of Mn-implanted n-type ZnO single crystals that are codoped with Sn. Theory predicts that room-temperature carrier-mediated ferromagnetism should be possible in manganese-doped p-type ZnO, although Mn-doped n-type ZnO should not be ferromagnetic. While previous efforts report only low-temperature ferromagnetism in Mn-doped ZnO that is n type via shallow donors, we find evidence for ferromagnetism with a Curie temperature of ∼250 K in ZnO that is codoped with Mn and Sn. As a 4+ valence cation, Sn should behave as a doubly ionized donor, thus introducing states deep in the gap. Hysteresis is clearly observed in magnetization versus field curves. Differences in zero-field-cooled and field-cooled magnetization persists up to ∼250 K for Sn-doped ZnO crystals implanted with 3 at. % Mn. Increasing the Mn concentration to 5 at. % significantly reduces the magnetic hysteresis. This latter observation is inconsistent with the origin for ferromagnetism being due to segregated secondary phases, and strongly suggests that a near-room-temperature dilute magnetic semiconducting oxide has been realized. Based on these results, ZnO doped with Mn and Sn may prove promising as a ferromagnetic semiconductor for spintronics.
Influence of polarization charges in Al0.4Ga0.6N/GaN barrier varactors
Authors: Saglam M. , Mutamba K. , Megej A. , Sydlo C. , Hartnagel H. L. , Daumiller I.
Published: January 2003

In this letter, we investigate the influence of polarization charges on the characteristics of metal–organic chemical-vapor-deposition-grown Al0.4Ga0.6N/GaN heterostructure barrier varactors (HBVs). The current–voltage and capacitance–voltage characteristics of the AlGaN/GaN HBVs shift asymmetrically due to the combination of piezoelectric (stress-induced) and spontaneous polarization effects in the barriers. Depending on the polarization of the bias voltage, different values of capacitance modulation, 1.4 for negative bias and 1.1 for positive bias, are observed. The obtained measurement results are supported by a theoretical analysis involving polarization charges at layer interfaces as well as their effect on the barrier height and the width of the depletion region.
Ion beam synthesis of superconducting MgB2 thin films
Authors: Peng Nianhua , Shao G. , Jeynes C. , Webb R. P. , Gwilliam R. M. , Boudreault G. , Astill D. M. , Liang W. Y.
Published: January 2003

Superconducting MgB2 thin films have been fabricated by 80 keV B11 ion implantation into commercial Mg ribbon with B11 doses up to 1018 ions/cm2, followed by thermal annealing at 500 °C. Temperature dependent dc magnetization measurements confirmed superconducting phase transitions between 11 and 18 K for samples containing nanocrystalline MgB2 grains embedded in Mg substrate with a small amount of MgO inclusion.
Generation of ultrawide-band chirped sources in the infrared through parametric interactions in periodically poled crystals
Authors: Marcus G. , Zigler A. , Englander A. , Katz M. , Ehrlich Y.
Published: January 2003

A method to generate chirped ultrawide-band sources with a chirp bandwidth of about 50% in the infrared is described and experimentally verified. It is based on optical parametric generation in periodically poled crystals with a chirped Ti:sapphire as a pump source. We have demonstrated a 27% wide bandwidth in the signal branch and 45% bandwidth in the idler branch when a periodically poled KTP crystal was pumped by a chirped Ti:sapphire laser with 12 nm full width at half maximum bandwidth.