bluuebunny/crossref_metadata_2025 huggingface dataset:
Observation of large vibration-to-vibration energy transfer collisions (ΔE≳3500 cm−1) in quenching of highly excited NO2 by CO2 and N2O
Authors: Hartland Gregory V. , Qin Dong , Dai Hai-Lung
Published: July 2002

Time-resolved Fourier transform infrared emission spectra, recorded after 475 nm excitation of NO2 in a CO2 or N2O bath, show IR emission from collisionally populated vibrational levels of the bath gas. The frequency of the observed bands proves that the emission arises from either the (1,00,1), (0,2l,1), and/or (0,00,2) levels of CO2 or N2O. From the pressure dependence of the emission intensity it was determined that these levels are populated by single collisions with excited NO2. Under typical conditions (1:10 ratio of NO2 to bath gas and 1–2 Torr total pressure) a steady state concentration is reached in our experiments where 0.016±0.006 multiply excited CO2 molecules, or 0.03±0.01 multiply excited N2O molecules were generated per laser excited NO2. A transition dipole coupling model is applied to explain these results, where the resonance conditions for vibration-to-vibration energy transfer are relaxed by extensive vibronic and vibrational couplings in highly excited NO2. In this model the energy-dependent transition dipole of excited NO2 is derived from the time-resolved IR emission spectra. The probability of Δv=1 energy transfer collisions for excited NO2 with CO2 or N2O can be accurately calculated. However, the number of multiply excited species produced (Δv≳1) is grossly underestimated. Analysis of the time-resolved data shows that the probability for Δv≳1 V–V energy transfer is ca. two orders of magnitude larger than the probability predicted by the dipole coupling model, and that NO2 molecules with energies as low as 5000 cm−1 have a non-negligible probability for exciting the overtone levels of CO2 and N2O. Finally, it was found that the dipole coupling model also underestimates the probability for the ΔE≳10 000 cm−1 supercollisions deduced in previous experiments (see Refs. ).
Emission cross sections and line shapes for photodissociating triiodide in ethanol: Experimental and computational studies
Authors: Johnson Alan E. , Myers Anne B.
Published: July 2002

Resonance Raman spectra and emission cross sections have been measured for triiodide in ethanol. The spectra show a long progression in the symmetric stretch and a broad background, which we attribute to a combination of solvent-dephasing-induced fluorescence and unresolved Raman combination bands of the symmetric and antisymmetric stretches. We also observe the fundamental of the antisymmetric stretch, indicating that the symmetry of the triiodide ion is broken by the environment on the resonance Raman time scale. Using two skewed LEPS potentials to describe the excited states, we are able to calculate emission band shapes and cross sections that agree very well with experiment. However, the time scale for solvent-induced electronic dephasing is unclear due to the difficulty in partitioning the broad background into separate Raman and fluorescence contributions.
Energetics and kinetics of CO and NO adsorption on Pt{100}: Restructuring and lateral interactions
Authors: Yeo Y. Y. , Vattuone L. , King D. A.
Published: July 2002

Calorimetric heats of adsorption and sticking probabilities are reported for NO and CO on both the reconstructed hex and the unreconstructed (1×1) surfaces of Pt{100} by single crystal adsorption calorimetry (SCAC), at room temperature. The hex surface reverts to the (1×1) structure during adsorption of both gases, as previously reported. The initial heat of adsorption on the (1×1) surface is 215 kJ/mol for CO and 200 kJ/mol for NO. Adsorbate–adsorbate interactions determine not only the dependence of the heat of adsorption on coverage but also the formation of different ordered structures. A model is suggested to explain the observed dependence of the differential heat on coverage and the LEED patterns, and a Monte Carlo simulation is performed to derive the corresponding differential heat, thus allowing estimates to be made of the magnitude of adsorbate–adsorbate interactions. For CO adsorption, the critical contribution is the pairwise interaction energy εd between molecules in nnn sites while for NO triplet formation is suggested with significant repulsive interaction between molecules in the same triplet (εt) and an even stronger repulsion between triplet pairs (εtt).

NO–NO repulsive interactions (εt=20 kJ/mol, εtt=80 kJ/mol) are considerably stronger than CO–CO interactions (εd=5 kJ/mol); thus, at half monolayer coverage CO gives rise to a c(2×2) pattern while NO gives a c(2×4) pattern. Moreover, with CO the coverage can be increased to 0.75 ML, with the formation of compressed structures, while for NO the saturation coverage is just 0.5 ML. The differential heat on the hex surface is also discussed showing the possible role of adsorption at defect sites in the energetics of the system. The surface energy difference between the clean (1×1) and hex surfaces is obtained as 20 kJ(mol Pts)−1 by comparing the integral heats of adsorption of CO on both surfaces at θ=0.5, when the final states of the two systems are identical.
The quantum heat engine and heat pump: An irreversible thermodynamic analysis of the three-level amplifier
Authors: Geva Eitan , Kosloff Ronnie
Published: July 2002

The manifestations of the three laws of thermodynamics are explored in a model of an irreversible quantum heat engine. The engine is composed of a three-level system simultaneously coupled to hot and cold heat baths, and driven by an oscillating external field. General quantum heat baths are considered, which are weakly coupled to the three-level system. The work reservoir is modeled by a classical electro-magnetic field of arbitrary intensity, which is driving the three-level system. The first law of thermodynamics is related to the rate of change of energy obtained from the quantum master equation in the Heisenberg picture. The fluxes of the thermodynamic heat and work are then directly related to the expectation values of quantum observables. An analysis of the standard quantum master equation for the amplifier, first introduced by Lamb, is shown to be thermodynamically inconsistent when strong driving fields are used. A generalized master equation is rigorously derived, starting from the underlying quantum dynamics, which includes relaxation terms that explicitly depend upon the field. For weak fields the generalized master equation reduces to the standard equation. In very intense fields the amplifier splits into two heat engines. One engine accelerates as the field intensifies, while the other slows down and eventually switches direction to become a heat pump. The relative weight of the slower engine increases with the field intensity, leading to a maximum in power as a function of the field intensity. The amplifier is shown to go through four ‘‘phases’’ as the driving field is intensified, throughout all of which the second law of thermodynamics is generally satisfied. One phase corresponds to a ‘‘refrigeration window’’ which allows for the extraction of heat out of a cold bath of temperatures down to the absolute zero. This window disappears at absolute zero, which is conjectured to be a dynamical manifestation of the third law of thermodynamics.
Quantum control of dissipative systems: Exact solutions
Authors: Cao Jianshu , Messina Michael , Wilson Kent R.
Published: July 2002

Optimal quantum control theory, which predicts the tailored light fields that best drive a system to a desired target, is applied to the quantum dissipative dynamics of systems linearly coupled to a Gaussian bath. To calculate the material response function required for optimizing the light field, the analytical solution is derived for the two-level Brownian harmonic oscillator model and the recently developed method for directly simulating the Gaussian force is implemented for anharmonic Brownian oscillators. This study confirms the feasibility of quantum control in favorable condensed phase environments and explores new quantum control features in the presence of dissipation, including memory effects and temperature dependence.
Computer simulations of liquid HF by a newly developed polarizable potential model
Authors: Jedlovszky Pál , Vallauri Renzo
Published: July 2002

A new polarizable potential model of liquid HF is developed. The model is tested by comparing simulated thermodynamic and structural properties of liquid HF with experimental and ab initio molecular dynamics results. Properties of the isolated HF dimer obtained with this model are also compared with both ab initio and experimental data. It turns out that this model can describe the thermodynamic properties of liquid HF with a good accuracy in the entire temperature range of the liquid state at atmospheric pressure, and also give the energy and geometry of the isolated HF dimer in a good agreement with experimental and ab initio results. The obtained pair correlation functions of liquid HF agree also reasonably well with experimental and ab initio molecular dynamics findings, although the position of the peaks describing hydrogen bonding appears to be at 0.1–0.3 Å higher distances. Such a deviation is opposite to what has been observed with our previous nonpolarizable model, which resulted in peak positions generally at lower r values than the experimental and ab initio molecular dynamics data. This fact indicates that in spite of the still existing small deviations, the inclusion of the polarizability improves the potential model also from a structural point of view.
Grand canonical ensemble molecular dynamics simulations: Reformulation of extended system dynamics approaches
Authors: Lynch Gillian C. , Pettitt B. Montgomery
Published: July 2002

The extended system Hamiltonian for carrying out grand canonical ensemble molecular dynamics simulations is reformulated. This new Hamiltonian includes a generalized treatment of the reference state partition function of the total chemical potential that reproduces the ideal gas behavior and various previous partitionings of ideal and excess terms. Initial calculations are performed on a system of Lennard–Jones particles near the triple point and on liquid water at room temperature.
High resolution Raman spectroscopy from vibrationally excited states populated by a stimulated Raman process: 2ν1−ν1 of CH412
Authors: Martı́nez R. Z. , Bermejo D. , Santos J. , Champion J. P. , Hilico J. C.
Published: July 2002

A new pump and probe technique for investigating vibrationally excited states via high resolution Raman spectroscopy is proposed. In a first step, a vibrational state, typically not accessible by a dipole moment transition from the ground state (IR forbidden, Raman allowed transition), is populated in a doubly pulsed stimulated Raman process. This state is long lived as the radiative decay to the ground state is highly inefficient. After a delay of several nanoseconds, the high resolution spectrum corresponding to a transition from this excited level is recorded. The second process follows the quasi-cw stimulated Raman spectroscopy scheme reported previously. Experimental aspects are discussed and the spectrum of 2ν1−ν1 of CH412 is presented as an illustration of the technique. The analysis of this spectrum has been done in two ways: first, by fitting simultaneously the ν1 and 2ν1 bands using an isolated band model; second, using a model based on the tetradecad-pentad system. The standard deviations of both fits are of the order of the experimental precision. The interpretation of the spectroscopic constants of the corresponding models is discussed.
Relaxation dynamics of C60
Authors: Walsh Tiffany R. , Wales David J.
Published: July 2002

The relaxation dynamics of C60 from high-energy isomers to Buckminsterfullerene is examined using a master equation approach. An exhaustive catalog of the C60 fullerene isomers containing only five- and six-membered rings is combined with knowledge of the Stone-Wales rearrangements that connect all such isomers. Full geometry optimizations have been performed for all the minima and the transition states which connect them up to six Stone-Wales steps away from the global minimum. A density-functional tight-binding potential was employed to provide a quantum mechanical description of the bonding. The resulting picture of the potential energy landscape reveals a “weeping willow” structure which offers a clear explanation for the relatively long relaxation times observed experimentally. We also predict the most important transient local minima on the annealing pathway.
Dynamics of fragment recoil in the femtosecond photodissociation of triiodide ions in liquid solution
Authors: Hess Stephan , Bürsing Helge , Vöhringer Peter
Published: July 2002

Novel femtosecond, multiple pulse experiments including polarization control are performed to elucidate the dynamics of fragment recoil in the 400-nm photolysis of triiodide ions in liquid ethanol solution. The instantaneous resonance Raman response of the dynamic system, induced at well defined delays after impulsive bond fission of the parent ion, displays a time-dependent vibrational frequency of the diatomic fragment. This time dependence is interpreted through interactions between the fragments which decay with increasing interfragment separation. Simultaneously, the instantaneous anisotropic response of the reactive system exhibits electronically coherent contributions at very early times after bond breakage and provides direct evidence for the existence of electronic degeneracies in the vicinity of the Franck-Condon region. The decay of this coherent anisotropic component reflects a dynamic lifting of these electronic degeneracies upon recoil of the product species and decay of residual interactions between the fragments. From both experiments it can be concluded that in liquid solution it takes about 2 ps until the fragments arrive in the asymptotic limit of the reaction with negligible product interactions.
Enhanced orientation of polar molecules by combined electrostatic and nonresonant induced dipole forces
Authors: Friedrich Bretislav , Herschbach Dudley
Published: July 2002

Recent experiments have demonstrated the efficacy of orienting low rotational states of a linear polar molecule in a static electric field, εS, or aligning a molecule (polar or not) in an intense nonresonant laser field, εL. We present theoretical results showing that the combined action of εS and εL can markedly sharpen orientation, particularly by introducing a pseudo-first-order Stark effect for tunneling doublets created by the polarizability interaction. Also, if εS and εL are not collinear, the molecular axis can be localized with respect to φ as well as θ, since M states as well as J states undergo hybridization. Another benefit is a means to eliminate “wrong way orientation” which otherwise occurs for “low-field seeking” states.
Low-energy electron scattering by H2O, H2S, H2Se, and H2Te
Authors: Varella Márcio T. do N. , Bettega Márcio H. F. , Lima Marco A. P. , Ferreira Luiz G.
Published: July 2002

We report elastic differential, integral, and momentum transfer cross-sections for H2X molecules (X: O, S, Se, and Te) obtained at the static exchange level of approximation. The energy range considered was from 2 up to 30 eV for H2O and from 5 up to 30 eV for the other molecules. Our calculations were performed with the Schwinger multichannel method with pseudopotentials [M. H. F. Bettega, L. G. Ferreira, and M. A. P. Lima, Phys. Rev. A 47, 1111 (1993)], combined with a Born closure procedure in order to account for the long-range potential due to the permanent dipole moment of the targets. Our calculated cross-sections for H2O and H2S are in good agreement with other theoretical results. Agreement with available experimental data is also encouraging. It was found that molecular size plays a crucial role in the scattering process. The influence of heavy and H atoms in the collisions is also discussed. For the integral cross-sections of the heavier molecules we also investigated incident energies below 5 eV, looking for possible shape resonances. Through the symmetry decomposition of the integral cross-sections and the eigenphase sum analysis, we found shape resonances for H2S, H2Se, and H2Te at the B2 symmetry. For H2Te, we have also found a shape resonance at the A2 symmetry. For all molecules a very broad structure was found at the A1 symmetry. This is the first work to report such resonances for H2Se and H2Te.
Dynamic reaction paths and rates through importance-sampled stochastic dynamics
Authors: Zuckerman Daniel M. , Woolf Thomas B.
Published: July 2002

We extend a previously developed method, based on Wagner’s stochastic formulation of importance sampling, to the calculation of reaction rates and to a simple quantitative description of finite-temperature, average dynamic paths. Only the initial and final states are required as input—no information on transition state(s) is necessary. We demonstrate the method for a single particle moving on the two-dimensional Müller–Brown potential surface. Beyond computing the forward and reverse rates for this surface, we determine the average path, which exhibits “saddle point avoidance.” The method may be generalized to arbitrary numbers of degrees of freedom and to arbitrary types of stochastic dynamics.
Tensor analysis of the second-order nonlinear optical susceptibility of chiral anisotropic thin films
Authors: Kauranen Martti , Van Elshocht Sven , Verbiest Thierry , Persoons André
Published: July 2002

We present a new measurement technique based on second-harmonic generation to investigate thin molecular films with in-plane anisotropy. The technique does not rely on continuous azimuthal patterns of the second-harmonic intensities. Instead, the second-harmonic intensities are recorded at a number of selected distinct azimuthal orientations of the sample. The signals are recorded as functions of the state of polarization of the fundamental laser beam. Only normalized polarization line shapes are required. As the line shapes need not be mutually calibrated, the technique is limited only by the accuracy of the individual polarization measurements and is applicable also to samples with inhomogeneous surface coverage of molecules. The technique is applied to anisotropic chiral thin films with C2 symmetry made of helicenebisquinone. Full analysis of the nonlinear susceptibility tensor of the samples is performed and the effects of anisotropy and chirality separated. The part of the nonlinearity arising from chirality is shown to dominate the nonlinear response.
A link between the two-body and three-body interaction energies of fluids from molecular simulation
Authors: Marcelli Gianluca , Sadus Richard J.
Published: July 2002

Molecular simulation data are reported that indicate that there is a simple empirical relationship between two-body and three-body interaction energies. The significance of this relationship is that three-body interactions can be estimated accurately from two-body interactions without incurring the computational penalty of three-body calculations. The relationship is tested by performing Gibbs ensemble simulations for the vapor–liquid equilibria of argon. The results are in good agreement with calculations that explicitly evaluate all three-body interactions.
The relationship between monopole harmonics and spin-weighted spherical harmonics
Authors: Dray Tevian
Published: July 2002

We compare two independent generalizations of the usual spherical harmonics, namely monopole harmonics and spin-weighted spherical harmonics, and make precise the sense in which they can be considered to be the same. By analogy with the spin-gauge language, raising and lowering operators for the monopole index of the monopole harmonics can immediately be written down.
A q deformation of the Gauss distribution
Authors: Leeuwen Hans van , Maassen Hans
Published: July 2002

The q deformed commutation relation aa*−qa*a=1 for the harmonic oscillator is considered with q∈[−1,1]. An explicit representation generalizing the Bargmann representation of analytic functions on the complex plane is constructed. In this representation the distribution of a+a* in the vacuum state is explicitly calculated. This distribution is to be regarded as the natural q deformation of the Gaussian.
Algebraic structures and invariant manifolds of differential systems
Authors: Figueiredo A. , Rocha Filho T. M. , Brenig L.
Published: July 2002

Algebraic tools are applied to find integrability properties of ODEs. Bilinear nonassociative algebras are associated to a large class of polynomial and nonpolynomial systems of differential equations, since all equations in this class are related to a canonical quadratic differential system: the Lotka–Volterra system. These algebras are classified up to dimension 3 and examples for dimension 4 and 5 are given. Their subalgebras are associated to nonlinear invariant manifolds in the phase space. These manifolds are calculated explicitly. More general algebraic invariant surfaces are also obtained by combining a theorem of Walcher and the Lotka–Volterra canonical form. Applications are given for Lorenz model, Lotka, May–Leonard, and Rikitake systems.
Arnol’d’s transformation—An example of a generalized canonical transformation
Authors: Stump Daniel R.
Published: July 2002

The duality transformation described by Arnol’d, which relates the orbits for different central potentials through a change of variables, is shown to be an example of a generalized canonical transformation, in an extended phase space that includes time as a dynamical variable.
The Potts model and the Tutte polynomial
Authors: Welsh D. J. A. , Merino C.
Published: July 2002

This is an invited survey on the relation between the partition function of the Potts model and the Tutte polynomial. On the assumption that the Potts model is more familiar we have concentrated on the latter and its interpretations. In particular we highlight the connections with Abelian sandpiles, counting problems on random graphs, error correcting codes, and the Ehrhart polynomial of a zonotope. Where possible we use the mean field and square lattice as illustrations. We also discuss in some detail the complexity issues involved.
Collision of a vortex pair with a contaminated free surface
Authors: Tryggvason Grétar , Abdollahi-Alibeik Javad , Willmarth William W. , Hirsa Amir
Published: July 2002

Collision of a viscous, two-dimensional vortex pair with a contaminated, free surface is studied numerically. The Froude number is assumed to be small, so the surface remains flat. The full Navier–Stokes equations and a conservation equation for the surface contaminant are solved numerically by a finite difference method. The shear stress at the free surface is proportional to the contamination gradient, and simulations for several values of the proportionality constant (W), as well as Reynolds numbers, have been performed. The evolution is also compared with full-slip and no-slip boundaries. As the vortices approach the surface, the upwelling between them pushes the contaminant outward, reducing the amount directly above the vortices, and leading to a clean region for low W. As W is increased the clean region becomes smaller, and eventually no clean region is formed. Except for very low W, the contaminant layer leads to the creation of secondary vortices, causing the original vortices to rebound in a similar way as vortices colliding with a no-slip boundary. For one case, the numerical results are compared with experimental measurements with satisfactory results. Computations of a vortex pair colliding obliquely with a contaminated surface and head-on collision of axisymmetric vortex rings are also presented.
Bifurcation of current sheets in plasmas
Authors: Hesse M. , Schindler K.
Published: July 2002

Bifurcation theory techniques are used for the quantitative investigation of a plane current sheet in plasmas with electric current as the control parameter. The existence of a bifurcation point is shown and the bifurcating solution is calculated. Stability results are achieved for the plane sheet and the bifurcating branches. The results allow conclusions to be drawn about the dynamic evolution of the system.
Experiments on nonlinear cross waves
Authors: Lichter S. , Shemer L.
Published: July 2002

Surface water waves are generated by a paddle-type wavemaker operating at one end of a long tank. In addition to a progressing wave field at the forcing frequency, a subharmonic cross wave is generated in the neighborhood of the wavemaker. At lower forcing amplitudes there is a Benjamin–Feir instability of the progressing wave. At large forcing amplitudes, the fundamental decays rapidly along the channel. The cross wave dominates the near field and is strongly modulated on a slow time scale. During each modulation period a soliton propagates away from the wavemaker. The near-field standing cross wave undergoes a transformation into a progressing wave in the far field.
Propagation of discontinuities in the Hasegawa–Mima equation
Authors: Nycander J.
Published: July 2002

The propagation of curves with discontinuous third derivatives of the potential in a fluid described by the Hasegawa–Mima equation is studied. It is shown that the area inside a closed curve of this kind is conserved, and a law governing the evolution of the magnitude of the discontinuity is derived. The hypothesis that such curves carry soliton properties is put forward.
Experimental evidence of nonlocal hydrodynamic influence on the dynamic contact angle
Authors: Blake T. D. , Bracke M. , Shikhmurzaev Y. D.
Published: July 2002

The dynamic contact angle formed when a liquid curtain impinges onto a moving solid is measured for aqueous glycerol solutions in different flow regimes. It is usually assumed that the dynamic contact angle is simply a function of the contact-line speed and the material properties of the contacting media. The new results show that this is not the case. For a given gas/liquid/solid combination and a given contact-line speed, the dynamic contact angle can be varied by varying the flow rate of the liquid and/or the curtain height, that is by varying the flow field near the contact line. The possibility of attributing this effect merely to free-surface bending and interpreting the results in terms of the so-called “apparent” contact angle is discussed and ruled out on the basis of some general qualitative arguments and analysis of the characteristic length scales involved. A probable connection between the observed effect and the physical mechanism of interface disappearance and formation incorporated in a recently developed theory of wetting is discussed.
Microwave generation from an axially extracted virtual cathode oscillator with a guide magnetic field
Authors: Kostov K. G. , Nikolov N. A.
Published: July 2002

The operation of a virtual cathode oscillator (vircator) with strong axial magnetic field has been experimentally studied. Depending on the cathode–anode gap and cathode diameter, the operating voltage varies from 200 kV up to 480 kV with 2–7 kA diode current. Microwave emission is produced by the oscillating virtual cathode. The central microwave frequency follows the beam plasma frequency. It varies by 11.5 GHz up to 22 GHz, depending on the current density. The oscillation frequency does not depend on the guide magnetic field magnitude. A maximal output power of 15±5 MW in asymmetric transverse magnetic (TM) modes is achieved by the axially extracted vircator. Variation of the magnetic field intensity in a range of 0–40 kG has an insignificant effect upon the emitted microwave power. An electron beam power to microwave power conversion efficiency of approximately 1% is obtained.
A Lagrangian study of dynamics and singularity formation at magnetic null points in ideal three-dimensional magnetohydrodynamics
Authors: Klapper I. , Rado A. , Tabor M.
Published: July 2002

The ideal three-dimensional incompressible magnetohydrodynamics equations are analyzed at magnetic null points using a generalization of a method from fluid dynamics. A closed system of ordinary differential equations governing the evolution of traces of matrices associated with the fluid velocity and magnetic field gradients are derived using a model for the pressure Hessian. It is shown rigorously that the eigenvalues of the magnetic field gradient matrix are constant in time and that, in the model, a finite time singularity occurs with characteristics similar to the magnetic field-free case.
Growth of epitaxial films of CdTe and (Cd,Mn)Te on GaAs substrates
Authors: Siegrist T. , Segmüller Armin , Mariette H. , Holtzberg F.
Published: July 2002

Epitaxial films of CdTe and (Cd,Mn)Te were grown on (001) GaAs substrates in a vacuum evaporator. Conventional Bragg diffraction indicated that the films had a single orientation, either (a) CdTe(001) ∥ GaAs(001), or (b) CdTe(111) ∥ GaAs(001). Grazing-incidence x-ray diffraction showed that most of the films had both orientations with (a) CdTe[110] ∥ GaAs[110] and (b) CdTe[112̄] ∥ GaAs[110]. Only one of the two possible domains was found in case (b). A mosaic of well-crystallized islands appears to accommodate the large film strain of −12.8% due to the lattice mismatch between CdTe and GaAs.
Polydimethylsiloxane flow defined for experiments in fluid dynamics
Authors: Weijermars Ruud
Published: July 2002

Linear polydimethylsiloxanes (PDMS’s) are liquid polymers between 200 and 550 K and are well suited for experimental studies in fluid mechanics because (1) they are commercially available in a wide range of molecular lengths, (2) they are transparent and colorless, and (3) their flow behavior can be predicted from their molecular length. A particular PDMS can flow either as a viscous or strain rate softening fluid, depending upon its molecular length and the applied strain rate. Six different flow regimes are defined here to aid the selection of the appropriate PDMS for any particular experiment in fluid dynamics. Diffusion is sufficiently slow in PDMS’s of long molecular length so that finite strains can be visualized by the deformation of strain markers within it. This allows laboratory modeling of three-dimensional strain histories in fluids which previously could only be approached by two-dimensional numerical modeling.
Solid neon moderator for producing slow positrons
Authors: Mills A. P. , Gullikson E. M.
Published: July 2002

Slow positrons can be obtained by moderating the energetic β+ particles from a radioactive source. We find that solid Ne makes a more efficient moderator than any other material known to date. The efficiency ε, defined as the number of slow positrons per β+ emitted by the source, is (0.30±0.02)% for a flat layer of Ne covering a 22Na deposit. In a cylindrical geometry, ε is (0.70±0.02)%, more than twice the previous best efficiency obtained with single-crystal tungsten. The energy spectrum for Ne has a full width at half-maximum of 0.58 eV, somewhat broader than the spectrum of positrons from a single-crystal metal. Moderators made from the other solid rare gases have a much lower efficiency and a larger energy spread.
Crystallography and microstructure of Y1Ba2Cu3O9−x, a perovskite-based superconducting oxide
Authors: Beyers R. , Lim G. , Engler E. M. , Savoy R. J. , Shaw T. M. , Dinger T. R. , Gallagher W. J. , Sandstrom R. L.
Published: July 2002

We have investigated the crystallography and microstructure of Y1Ba2Cu3O9−x with transmission electron microscopy and x-ray diffraction. Y1Ba2Cu3O9−x is a distorted, oxygen-defect perovskite with ordering of the yttrium and barium ions. Its unit cell is orthorhombic with space group Pmm2 and lattice parameters a=3.893 Å, b=11.688 Å, and c=3.820 Å. The structure is heavily twinned on {101} type planes, possibly due to a tetragonal-to-orthorhombic transition above room temperature.
Properties of La1.8Sr0.2CuO4 superconductors
Authors: Panson A. J. , Wagner G. R. , Braginski A. I. , Gavaler J. R. , Janocko M. A. , Pohl H. C. , Talvacchio J.
Published: July 2002

Critical current, critical field, and carrier density measurements have been made on bulk samples of La1.8Sr0.2CuO4 to assess the potential of such oxide superconductors for practical applications. The importance of preparing samples in a high oxygen pressure was documented. The upper critical field at T=0 was estimated to be 530 kOe. From magnetization hysteresis loops, critical current densities were determined between 0 and 60 kOe. At 60 kOe, the values were 2×103 A/cm2 at 4.2 K and 4×102 A/cm2 at 18 K in samples that exhibited characteristics of weak flux pinning. The effective carrier density at 48 K was 1×1021 cm−3, approximately half of the expected upper limit. A set of microscopic superconducting parameters has been derived from transition temperature, resistivity, and upper and lower critical field measurements made on a single specimen.
Redistribution of arsenic in silicon during high pressure thermal oxidation
Authors: Choi Seong S. , Numan M. Z. , Chu W. K. , Srivastava J. K. , Irene E. A.
Published: July 2002

The redistribution of arsenic in Si has been studied in terms of the relative rates of oxidation and diffusion in silicon. Redistribution of arsenic in silicon during oxidation is dependent on the ratio of oxidation rate to the diffusivity in silicon, B/D1/2, as well as on the thermodynamic equilibrium segregation coefficient. It was found that for the value of B/D1/2 larger than 50, most of the arsenic impurity becomes trapped in SiO2. For the smaller value of B/D1/2, snowplowing of arsenic results.
Study of interface states in the metal-semiconductor junction using deep level transient spectroscopy
Authors: Zhang Han , Aoyagi Yoshinobu , Iwai Sohachi , Namba Susumu
Published: July 2002

A new technique is reported for determining the interface states in the metal-semiconductor junction by deep level transient spectroscopy. The continuous interface state distribution in the Al-GaAs junction has been studied for the energy range from 0.19 to 0.57 eV below conduction band by the technique and it is found to be of the order of 4.3×1010–7.9×1010 eV−1 cm−2. The results have shown that the technique is very effective and credible.
Nucleation and characterization of pseudomorphic ZnSe grown on molecular beam epitaxially grown GaAs epilayers
Authors: Gunshor R. L. , Kolodziejski L. A. , Melloch M. R. , Vaziri M. , Choi C. , Otsuka N.
Published: July 2002

The heteroepitaxial growth of ZnSe on GaAs epilayers grown by molecular beam epitaxy is found to occur via a two-dimensional growth mechanism. Alternatively, nucleation on a GaAs substrate exhibits three-dimensional growth characteristics. The differentiation of the type of nucleation is evidenced by reflection high-energy electron diffraction intensity oscillations, as well as the dynamic behavior of the diffraction patterns. Photoluminescence measurements of pseudomorphic ZnSe epilayers grown on GaAs epilayers provide a direct measurement of ZnSe deformation potentials.
InGaAs/InAlAs multiquantum well intersubband absorption at a wavelength of λ=4.4 μm
Authors: Levine B. F. , Cho A. Y. , Walker J. , Malik R. J. , Kleinman D. A. , Sivco D. L.
Published: July 2002

We report, for the first time, intersubband absorption experiments in doped InGaAs/InAlAs multiquantum well superlattices and observe a resonance peak at a wavelength of λ=4.4 μm which is in good agreement with theory. This material system may be useful for detectors in the λ=3–5 μm spectral region.
Self-electro-optic effect device and modulation convertor with InGaAs/InP multiple quantum wells
Authors: Bar-Joseph I. , Sucha G. , Miller D. A. B. , Chemla D. S. , Miller B. I. , Koren U.
Published: July 2002

We report the first observation of the self-electro-optic effect in InGaAs/InP multiple quantum wells, grown by organometallic vapor phase epitaxy. Clear bistability and switching are observed over a range of 40 nm around 1.61 μm with 20–30 V bias. We demonstrate the operation of a modulation convertor, which converts a modulation from a carrier at 1.6 μm onto a carrier at 0.85 μm and vice versa.
The tetranucleotide UCAY directs the specific recognition of RNA by the Nova K-homology 3 domain
Authors: Jensen Kirk B. , Musunuru Kiran , Lewis Hal A. , Burley Stephen K. , Darnell Robert B.
Published: July 2002

The Nova family of proteins are target antigens in the autoimmune disorder paraneoplastic opsoclonus-myoclonus ataxia and contain K-homology (KH)-type RNA binding domains. The Nova-1 protein has recently been shown to regulate alternative splicing of the α2 glycine receptor subunit pre-mRNA by binding to an intronic element containing repeats of the tetranucleotide UCAU. Here, we have used selection-amplification to demonstrate that the KH3 domain of Nova recognizes a single UCAY element in the context of a 20-base hairpin RNA; the UCAY tetranucleotide is optimally presented as a loop element of the hairpin scaffold and requires protein residues C-terminal to the previously defined KH domain. These results suggest that KH domains in general recognize tetranucleotide motifs and that biological RNA targets of KH domains may use either RNA secondary structure or repeated sequence elements to achieve high affinity and specificity of protein binding.
Rad54 protein stimulates the postsynaptic phase of Rad51 protein-mediated DNA strand exchange
Authors: Solinger Jachen Armon , Heyer Wolf-Dietrich
Published: July 2002

Rad54 and Rad51 are important proteins for the repair of
double-stranded DNA breaks by homologous recombination in eukaryotes.
As previously shown, Rad51 protein forms nucleoprotein filaments on
single-stranded DNA, and Rad54 protein directly interacts with such
filaments to enhance synapsis, the homologous pairing with a
double-stranded DNA partner. Here we demonstrate that
Saccharomyces cerevisiae
Rad54 protein has an additional
role in the postsynaptic phase of DNA strand exchange by stimulating
heteroduplex DNA extension of established joint molecules in
Rad51/Rpa-mediated DNA strand exchange. This function depended on the
ATPase activity of Rad54 protein and on specific protein:protein
interactions between the yeast Rad54 and Rad51 proteins.
Allogeneic β-islet cells correct diabetes and resist immune rejection
Authors: Pericin Marcus , Althage Alana , Freigang Stefan , Hengartner Hans , Rolland Eric , Dupraz Philippe , Thorens Bernard , Aebischer Patrick , Zinkernagel Rolf M.
Published: July 2002

Allogeneic MHC-incompatible organ or cell grafts are usually promptly rejected by immunocompetent hosts. Here we tested allogeneic β-islet cell graft acceptance by immune or naïve C57BL/6 mice rendered diabetic with streptozotocin (STZ). Fully MHC-mismatched insulin-producing growth-regulated β-islet cells were transplanted under the kidney capsule or s.c. Although previously or simultaneously primed mice rejected grafts, STZ-treated diabetic mice accepted islet cell grafts, and hyperglycemia was corrected within 2–4 weeks in absence of conventional immunosuppression. Allogeneic grafts that controlled hyperglycemia expressed MHC antigens, were not rejected for >100 days, and resisted a challenge by allogeneic skin grafts or multiple injections of allogeneic cells. Importantly, the skin grafts were rejected in a primary fashion by the grafted and corrected host, indicating neither tolerization nor priming. Such strictly extralymphatic cell grafts that are immunologically largely ignored should be applicable clinically.
A low threshold level of expression of mutant-template telomerase RNA inhibits human tumor cell proliferation
Authors: Kim Moses M. , Rivera Melissa A. , Botchkina Inna L. , Shalaby Refaat , Thor Ann D. , Blackburn Elizabeth H.
Published: July 2002

The ribonucleoprotein telomerase synthesizes telomeric DNA by copying an intrinsic RNA template. In most cancer cells, telomerase is highly activated. Here we report a telomerase-based antitumor strategy: expression of mutant-template telomerase RNAs in human cancer cells. We expressed mutant-template human telomerase RNAs in prostate (LNCaP) and breast (MCF-7) cancer cell lines. Even a low threshold level of expression of telomerase RNA gene constructs containing various mutant templates, but not the control wild-type template, decreased cellular viability and increased apoptosis. This occurred despite the retention of normal levels of the endogenous wild-type telomerase RNA and endogenous wild-type telomerase activity and unaltered stable telomere lengths.
In vivo
tumor xenografts of a breast cancer cell line expressing a mutant-template telomerase RNA also had decreased growth rates. Therefore, mutant-template telomerase RNAs exert a strongly dominant-negative effect on cell proliferation and tumor growth. These results support the potential use of mutant-template telomerase RNA expression as an antineoplastic strategy.
Transgenic potato (
Solanum tuberosum
) tubers synthesize the full spectrum of inulin molecules naturally occurring in globe artichoke (
Cynara scolymus
) roots
Authors: Hellwege Elke M. , Czapla Sylvia , Jahnke Anuschka , Willmitzer Lothar , Heyer Arnd G.
Published: July 2002

The ability to synthesize high molecular weight inulin was transferred to potato plants via constitutive expression of the
1-SST
(sucrose:sucrose 1-fructosyltransferase) and the
1-FFT
(fructan: fructan 1-fructosyltransferase) genes of globe artichoke (
Cynara scolymus
). The fructan pattern of tubers from transgenic potato plants represents the full spectrum of inulin molecules present in artichoke roots as shown by high-performance anion exchange chromatography, as well as size exclusion chromatography. These results demonstrate
in planta
that the enzymes sucrose:sucrose 1-fructosyltransferase and fructan:fructan 1-fructosyltransferase are sufficient to synthesize inulin molecules of all chain lengths naturally occurring in a given plant species. Inulin made up 5% of the dry weight of transgenic tubers, and a low level of fructan production also was observed in fully expanded leaves. Although inulin accumulation did not influence the sucrose concentration in leaves or tubers, a reduction in starch content occurred in transgenic tubers, indicating that inulin synthesis did not increase the storage capacity of the tubers.
The human
GRAF
gene is fused to
MLL
in a unique t(5;11)(q31;q23) and both alleles are disrupted in three cases of myelodysplastic syndrome/acute myeloid leukemia with a deletion 5q
Authors: Borkhardt Arndt , Bojesen Stig , Haas Oskar A. , Fuchs Uta , Bartelheimer Dominique , Loncarevic Ivan F. , Bohle Rainer M. , Harbott Jochen , Repp Reinald , Jaeger Ulrich , Viehmann Susanne , Henn Traudl , Korth Petra , Scharr Dirk , Lampert Fritz
Published: July 2002

We have isolated the human
GRAF
gene (for GTPase regulator associated with the focal adhesion kinase pp125
FAK
). This gene was fused with
MLL
in a unique t(5;11)(q31;q23) that occurred in an infant with juvenile myelomonocytic leukemia.
GRAF
encodes a member of the Rho family of the GTPase-activating protein (GAP) family. On the protein level, it is 90% homologous to the recently described chicken
GRAF
gene that functions as a GAP of RhoA
in vivo
and is thus a critical component of the integrin signaling transduction pathway. The particular position of the human
GRAF
gene at 5q31 and the proposed antiproliferative and tumor suppressor properties of its avian homologue suggest that it also might be pathogenetically relevant for hematologic malignancies with deletions of 5q. To investigate this possibility, we sequenced 4–5 individual cDNA clones from 13 cases in which one allele of GRAF was deleted. We found point mutations within the GAP domain of the second
GRAF
allele in one patient. In two additional patients we found an insertion of 52 or 74 bp within the
GRAF
cDNA that generates a reading frame shift followed by a premature stop codon.
GRAF
maps outside the previously defined commonly deleted 5q31 region. Nevertheless, inactivation of both alleles in at least some cases suggests that deletions and mutations of the
GRAF
gene may be instrumental in the development and progression of hematopoeitic disorders with a del(5q).
Analysis of the chromosome sequence of the legume symbiont
Sinorhizobium meliloti
strain 1021
Authors: Capela Delphine , Barloy-Hubler Frédérique , Gouzy Jérôme , Bothe Gordana , Ampe Frédéric , Batut Jacques , Boistard Pierre , Becker Anke , Boutry Marc , Cadieu Edouard , Dréano Stéphane , Gloux Stéphanie , Godrie Thérèse , Goffeau André , Kahn Daniel , Kiss Ernö , Lelaure Valérie , Masuy David , Pohl Thomas , Portetelle Daniel , Pühler Alfred , Purnelle Bénédicte , Ramsperger Ulf , Renard Clotilde , Thébault Patricia , Vandenbol Micheline , Weidner Stefan , Galibert Francis
Published: July 2002

Sinorhizobium meliloti
is an
α-proteobacterium that forms agronomically important
N
2
-fixing root nodules in legumes. We report here the
complete sequence of the largest constituent of its genome, a 62.7%
GC-rich 3,654,135-bp circular chromosome. Annotation allowed assignment
of a function to 59% of the 3,341 predicted protein-coding ORFs, the
rest exhibiting partial, weak, or no similarity with any known
sequence. Unexpectedly, the level of reiteration within this replicon
is low, with only two genes duplicated with more than 90% nucleotide
sequence identity, transposon elements accounting for 2.2% of the
sequence, and a few hundred short repeated palindromic motifs (RIME1,
RIME2, and C) widespread over the chromosome. Three regions with a
significantly lower GC content are most likely of external origin.
Detailed annotation revealed that this replicon contains all
housekeeping genes except two essential genes that are located on
pSymB. Amino acid/peptide transport and degradation and sugar
metabolism appear as two major features of the
S.
meliloti
chromosome. The presence in this replicon of a large
number of nucleotide cyclases with a peculiar structure, as well as of
genes homologous to virulence determinants of animal and plant
pathogens, opens perspectives in the study of this bacterium both as a
free-living soil microorganism and as a plant symbiont.
Microinjection and growth of bacteria in the cytosol of mammalian host cells
Authors: Goetz Monika , Bubert Andreas , Wang Gefu , Chico-Calero Isabel , Vazquez-Boland Jose-Antonio , Beck Markus , Slaghuis Joerg , Szalay Aladar A. , Goebel Werner
Published: July 2002

Most facultative intracellular bacteria replicate in specialized phagosomes after being taken up by mammalian cells. Relatively few intracellular bacteria escape the phagosomal compartment with the help of cytolytic (pore-forming) proteins and replicate in the host cell cytosol. Without such toxins, intracellular bacteria cannot reach this cellular compartment. To circumvent the requirement of an “escape” step, we developed a procedure allowing the efficient direct injection of bacteria into the cytosol of mammalian cells. With this technique, we show that most bacteria, including extracellular bacteria and intracellular pathogens that normally reside in a vacuole, are unable to replicate in the cytosol of the mammalian cells. In contrast, microorganisms that replicate in the cytosol, such as
Listeria monocytogenes
,
Shigella flexneri
, and, to some extent, enteroinvasive
Escherichia coli
, are able to multiply in this cellular compartment after microinjection. Further
L. monocytogenes
with deletion in its PrfA-regulated
hpt
gene was found to be impaired in replication when injected into the cytosol. Complementation of the
hpt
mutation with a plasmid carrying the wild-type
hpt
gene restored the replication ability in the cytosol. These data indicate that cytosolic intracellular pathogens have evolved specific mechanisms to grow in this compartment of mammalian cells.