bluuebunny/crossref_metadata_2025 huggingface dataset:
Efficient frequency upconversion of Tm3+ ions in Yb3+ doped barium-thorium fluoride glass
Authors: Yeh D. C. , Sibley W. A. , Suscavage M. J.
Published: July 2002

If infrared light can be efficiently changed to visible light it would have important implications for optical detection and display devices. In this paper the efficiency of the two-photon upconversion process between Yb3+ and Tm3+ ions in BaF2/ThF4 heavy metal fluoride glass is investigated as a function of Tm3+ concentration and temperature. It is found that for the concentration of 0.05-mol % TmF3 and a projected incident infrared intensity of 1 W/cm2, the projected efficiency at 300 K would be 1.2%.
Spectroscopic and fast photographic studies of excimer laser polymer ablation
Authors: Dyer P. E. , Sidhu J.
Published: July 2002

Spectroscopy and fast photography have been used to study the luminous plume produced by ablative etching of polyimide and polyethylene-terephthalate polymer films with the XeCl laser. The time dependence of the expansion for various environmental gas pressures has been studied in this way. The results show that at low pressures (<0.5 mb) the expansion approaches a free expansion into a vacuum, while at 15 mb the time dependence is close to that predicted by an ideal blast wave model. It is suggested that a significant fraction of the luminous species detected spectroscopically may be created in the shock front at the plume/environmental gas interface.
Theoretical determination of net radiation force and torque for a spherical particle illuminated by a focused laser beam
Authors: Barton J. P. , Alexander D. R. , Schaub S. A.
Published: July 2002

Series expressions for the net radiation force and torque for a spherical particle illuminated by an arbitrarily defined monochromatic beam are derived utilizing the spherical-particle/arbitrary-beam interaction theory developed in an earlier paper. Calculations of net force and torque are presented for a 5-μm-diam water droplet in air optically levitated by a tightly focused (2 μm beam waist diameter) TEM00-mode argon-ion (λ=0.5145 μm) laser beam for on and off propagation axis, and on and off structural resonance conditions. Several features of these theoretical results are related to corresponding experimental observations.
Domain-wall dynamics and Barkhausen effect in metallic ferromagnetic materials. I. Theory
Authors: Alessandro Bruno , Beatrice Cinzia , Bertotti Giorgio , Montorsi Arianna
Published: July 2002

The Barkhausen effect (BE) in metallic ferromagnetic systems is theoretically investigated by a Langevin description of the stochastic motion of a domain wall in a randomly perturbed medium. BE statistical properties are calculated from approximate analytical solutions of the Fokker–Planck equation associated with the Langevin model, and from computer simulations of domain-wall motion. It is predicted that the amplitude probability distribution P0(Φ̇) of the B flux rate Φ̇ should obey the equation P0(Φ̇)∝Φ̇c̃−1 exp(−c̃Φ̇/〈Φ̇〉), with c̃>0. This result implies scaling properties in the intermittent behavior of BE at low magnetization rates, which are described in terms of a fractal structure of fractal dimension D<1. Analytical expressions for the B power spectrum are also derived. Finally, the extension of the theory to the case where many domain walls participate in the magnetization process is discussed.
Photoemission studies of chemical bonding and electronic states at the Fe/Si interface
Authors: Li Baoqi , Ji Mingron , Wu Jianxin , Hsu Chenchia
Published: July 2002

Chemical bonding and reactions at the Fe/Si interface have been studied as a function of Fe overlayer thickness and annealing temperature. The chemical properties (composition, electronic structure, and reactivity) were observed under ultrahigh vacuum conditions using x-ray photoemission, ultraviolet photoemission (UPS), and Auger electronic spectroscopy. Both core line and valence-band states have been measured. At room temperature, intermixing of atoms across the clean Fe/Si interface occurs. In the initial stage the Fe2p3/2 core line shifts 0.4 eV to a higher binding energy relative to pure Fe metal. This is nearly the same as the chemical shift of the Fe2p3/2 core line in FeSi2. With increasing coverage, the Fe2p3/2 core line shifts decrease and subsequently approach that of pure Fe metal. In the meanwhile, the Fe2p3/2 core line shapes changed gradually with greater width and asymmetry upon metal deposition. At the lower coverage, the UPS spectra are dominated by a broadband, but at high coverage, two peaks corresponding to Fe3d and Fe3d-Si3p bonding are observed. Upon deposition,the nonbonding d-state peak shifts to the Fermi level and the spectrum becomes that of pure Fe metal. All these results are discussed in the context of an interstitial diffusion model between Si and Fe at the Fe/Si interface.
Reversible and irreversible magnetization in soft iron-based polycrystalline materials
Authors: Bertotti G. , Fiorillo F. , Pasquale M.
Published: July 2002

The magnetization process has been investigated in nonoriented SiFe alloys over a wide induction range, from about 10−4 T to technical saturation. Special attention has been devoted to the role of grain size 〈s〉. The low-field behavior of the initial magnetization curve is characterized by a number of remarkable features: (1) the permeability μr tends, in the limit of vanishing applied field H, to a constant value, independent of 〈s〉; (2) the increase of μr with H exhibits a nonlinear behavior, in contrast with the Rayleigh law; and (3) the larger the grain size, the steeper the rise of μr with H. This effect is attributed to an increased ease of propagation of locally induced magnetization reversals at larger 〈s〉 values. Experiments put in evidence a correlation between μr and the length of propagation λ, which is found to vary as μ1/2r. However, at high inductions (above about 1 T), larger grains imply a reduction of permeability, due to the growth of unfavorable crystallographic textures. Grain size also affects power losses to a large extent. Measurements performed at 1 and 1.5 T show that the hysteresis component Ph and the excess eddy component Pe, respectively, vary as 1/√〈s〉 and √〈s〉 in the investigated range 45 μm≤〈s〉≤360 μm.
Oxygen and fluorine atom kinetics in electron cyclotron resonance plasmas by time-resolved actinometry
Authors: Booth J. P. , Sadeghi N.
Published: July 2002

The kinetics of O and F atoms in O2-based plasmas has been studied by time-resolved optical emission spectroscopy (actinometry) in modulated plasmas. The sticking coefficient αO of O atoms on the stainless-steel reactor walls was 0.09±0.01 in O2 plasmas containing fluorine (added as either SF6 or F2), but was about 0.5 in a pure O2 plasma. This explains the significant increase in steady-state O density as a few percent of fluorinated gas is added. The corresponding value for F atoms, αF, was 0.06±0.01, almost independent of conditions. The method also indicates the relative importance of the different electron-impact-induced mechanisms (direct excitation of ground-state atoms and dissociative excitation of feedstock molecules) for the production of emitting atoms [O 3p3P (844 nm) and F 3s2P (703 nm)] in plasmas. These results show that the widely used (steady-state) actinometry technique using 844-nm emission from O 3p3P atoms is an unreliable measure of ground-state [O] variations.
Atomic-level stress in an inhomogeneous system
Authors: Cheung Kin S. , Yip Sidney
Published: July 2002

The distribution of local stress in an atomic system can be calculated from its mechanical definition. While this leads, for a homogeneous system, to an expression equivalent to that derived from the virial theorem (thermodynamic definition), when local inhomogeneities are present the equivalence can be demonstrated only in the sense of a volume average which effectively eliminates the inhomogeneities. Atomistic simulation results are presented to show that for a relaxed crystal with a planar free surface, the normal stress on each atomic plane parallel to the surface, when calculated according to the mechanical definition, vanishes uniformly as expected, whereas if the virial expression were applied the resulting stress distribution shows unphysical oscillations in the surface region.
Micro-Raman study of stress distribution in local isolation structures and correlation with transmission electron microscopy
Authors: De Wolf I. , Vanhellemont J. , Romano-Rodríguez A. , Norström H. , Maes H. E.
Published: July 2002

Stress in local isolation structures is studied by micro-Raman spectroscopy. The results are correlated with predictions of an analytical model for the stress distribution and with cross-sectional transmission electron microscopy observations. The measurements are performed on structures on which the Si3N4 oxidation mask is still present. The influence of the pitch of the periodic local isolation pattern, consisting of parallel lines, the thickness of the mask, and the length of the bird’s beak on the stress distribution are studied. It is found that compressive stress is present in the Si substrate under the center of the oxidation mask lines, with a magnitude dependent on the width of the lines. Large tensile stress is concentrated under the bird’s beak and is found to increase with decreasing length of the bird’s beak and with increasing thickness of the Si3N4 film.
Effects of low work function metals on the barrier height of sulfide-treated n-type GaAs(100)
Authors: Samaras J. E. , Darling Robert B.
Published: July 2002

A comparative study of the Schottky barrier height variation on sulfide-treated GaAs(100) surfaces with low work function metal contacts was made using current-voltage and capacitance-voltage measurements. Five different wet chemical sulfide treatments were found to cause little variation in the Sm (0.72 eV) and Mg (0.59 eV) Schottky barrier heights, but caused significant variation in the Al (0.58–0.75 eV) barrier heights when compared to the untreated control diodes. A low temperature (160 °C) anneal was found to cause variation in all of these, uniformly raising the barrier heights of the Sm (+0.07 eV) and Al (+0.04 eV) contacts, and degrading the Mg contacts. These results demonstrate the critical importance of both the reaction specifics and the stability of the interface on the Schottky barrier height.
Effects of several kinds of anisotropy on the coercivity behaviors of iron oxides
Authors: Yang Zhijun , Yao Kaiyi
Published: July 2002

Coercivity behaviors of iron oxides during phase changes between Fe3O4 and γ-Fe2O3 for granular powders and continuous thin films, which can be interpreted in a unified framework by an intermediate product hypothesis, are investigated. There exists a critical value ccr of Fe3O4 concentration for different samples. The intermediate product Fe3−zO4 in oxidation and reduction processes is a mixture of two phases when Fe3O4 concentration c is smaller than ccr; but it may exist in the form of a homogeneous solid solution (single-phase) (Fe3O4)1−x(γ-Fe2O3)x when c is larger than ccr. The difference in lattice constants will produce a tensive stress (σ≳0) on γ-Fe2O3 and compressive stress (σ<0) on Fe3O4 at interfaces. Superposition of a positive stress anisotropy on the easy axes 〈111〉 of magnetocrystalline anisotropy for Fe3O4 and on the easy axes 〈110〉 for γ-Fe2O3 will cause an increase in the coercivity Hc for spherical particle and thin film samples. The coercivity behavior of our acicular particle sample is contrary to that of thin film and spherical particle samples. Also, the switching field distribution curve showed a peak near c=0.4. There always exist some two-phase intermediate compounds in Fe3−zO4 due to the incomplete reaction of single domain particles. The decrease in coercivity of acicular particles is affected by the shape anisotropy related to changes in effective particle sizes.
Loss measurements on amorphous alloys under sinusoidal and distorted induction waveform using a digital feedback technique
Authors: Bertotti G. , Ferrara E. , Fiorillo F. , Pasquale M.
Published: July 2002

A newly developed digital feedback wattmeter, allowing power loss measurements in soft laminations under generic induction waveform is presented. The system is intrinsically free of auto-oscillations, typical drawback of analog feedback circuits, and can therefore be operated in a wide frequency range (0.5 Hz–100 kHz). Loss measurements performed by this setup under sinusoidal and distorted induction waveforms on Co based amorphous ribbons are reported. It is shown that the classical approximation to the prediction of power losses under distorted induction largely fails to account for the experimental results. A novel theoretical approach, based on the statistical theory of losses, is discussed and successfully applied to the experiments. In particular, it is shown that knowledge of the loss components under sinusoidal induction at a given magnetizing frequency permits one to make an accurate prediction of the effect of distortion at that frequency as well as other ones. Illustrative applications at 1 kHz and 100 kHz are reported.
Finite-time Carnot refrigerators with wall gain and product loads
Authors: Agrawal D. C. , Menon V. J.
Published: July 2002

Finite-time thermodynamics is used to investigate theoretically the wall gain and product loads on a refrigerator based on an endoreversible Carnot cycle. Explicit formulas are obtained for the operating time, coefficient of performance, and the cooling rate and the procedure is illustrated numerically for typical refrigerators.
Cation distribution of Li-Ti mixed ferrites (abstract)
Authors: Reddy M. Bhagavantha , Reddy P. Venugopal
Published: July 2002

Lithium-titanium mixed ferrites of different compositions have been prepared by the well known ceramic technique. Thermo power studies of these samples, after usual characterization by x-ray diffraction (XRD), bulk density, porosity, etc. have been measured by the differential method. Room temperature values of the Seebeck coefficient are found to vary from 2.3 to 114 μV/K. It is an established fact that the thermo power measurement would also be used to study the cation distributions on octahedral and tetrahedral sublattices of ferro spinel. This technique was successfully used in the study of cation distribution of Fe3O4, Fe3O4-TiFe2O4, etc. As such the cation distribution of Li-Ti mixed ferrites of P.I. has been arrived at using thermo-power data by assuming that small polorans rather than electrons/holes are responsible for the process of conduction. A knowledge of cation distribution among octahedral sites of a spinel ferrite is essential to understand the mechanism responsible for electrical properties like electrical conductivity, thermo power, dielectric properties, etc. Saturation magnetization of all the samples has also been obtained by the well known oscillation method. From the cation distribution it has been observed that the concentration of Ti4+ on octahedral sites is found to increase while those of Fe3+ and Fe2+ are found to decrease rapidly.
X-ray photoelectron spectroscopy analysis of polyimide films modified by ultraviolet pulsed laser radiation at 193 nm
Authors: Brezini A. , Zekri N.
Published: July 2002

Polyimide films were treated in air with an argon-fluorine ultraviolet excimer laser (λ=193 nm). X-ray photoelectron spectroscopy analysis was performed on the modified surfaces after laser exposure at various fluences around the expected threshold fluence Fth. From the present data the photochemical process (photoablation) leads to produce a carbon-rich surface by ejecting mainly oxides of carbon and nitrogen compounds above a particular fluence Fe.
Infrared to visible upconversion in Er3+-doped-lead-germanate glass: Effects of Er3+ ion concentration
Authors: Pan Z. , Morgan S. H. , Loper A. , King V. , Long B. H. , Collins W. E.
Published: July 2002

The upconversion mechanism of Er3+ ions has been studied for lead-germanate glasses containing Er2O3 concentrations from 0.1 mol % to 2 mol %. Intense green emission was observed at room temperature due to 4S3/2→4I15/2 transition excited by a cw near-infrared laser beam at 797 nm. This green emission shows a similar intensity for samples with different Er3+ ion concentrations. A weak blue emission of 410 nm originating from the 2H9/2→4I15/2 transition was also observed. This blue emission and a red emission from the 4F9/2 level increase with the increase in Er3+ ion concentration. The bright green emission is attributed to the excited level absorption while the blue emission is due to a third step excitation where energy transfer between excited ions owing to their Coulomb interaction plays a key role.
Increased exchange anisotropy due to disorder at permalloy/CoO interfaces
Authors: Moran T. J. , Gallego J. M. , Schuller Ivan K.
Published: July 2002

The exchange anisotropy of Ni80Fe20 (permalloy) films deposited onto bulk single crystal CoO substrates using molecular-beam epitaxy is strongly affected by the interface structure. Contrary to expectations for uncompensated interfaces, the loop shift increases for samples with greater interface disorder. The results are discussed in the light of proposed exchange anisotropy models. The results suggest that antiferromagnet domain dynamics are an important part of the exchange anisotropy mechanism.
Growth of {100} textured diamond films by the addition of nitrogen
Authors: Cao G. Z. , Schermer J. J. , van Enckevort W. J. P. , Elst W. A. L. M. , Giling L. J.
Published: July 2002

Localized {100} fiber textured diamond films were grown by addition of 20–200 ppm nitrogen into the gas phase during hot-filament chemical-vapor deposition (CVD). Cathodoluminescence indicates the presence of the nitrogen-vacancy system in the {100} textured diamond, whereas a blue ‘‘band A’’ luminescence is normally observed in diamond films grown without nitrogen addition. The results demonstrate that the nature of the substrates used for growth has no appreciable influence on the {100} texture, which implies that this fiber texture is obtained by competitive growth and selection of facets. The interaction of nitrogen with the {100} surface is a highly important factor in this process. Homoepitaxial growth shows that the addition of a small amount of nitrogen greatly enhances the growth rate of the {100} faces, making 〈100〉 the fastest growth direction in comparison with the 〈110〉 and 〈111〉 directions. This is attributed to breaking of a part of the dimers on the (2×1) reconstructed {100} surface by nitrogen compounds. The {100} texture in narrow, ring-shaped areas on diamond layers grown by the flame technique can also be attributed to the occurrence of a certain amount of nitrogen in the gas phase. It is demonstrated that the flame grown polycrystalline diamond layers have morphologies and cathodoluminescence features that are consistent with those observed in the hot-filament CVD diamond films grown with the addition of nitrogen.
Effects of surface topology and texture on exchange anisotropy in NiFe/Cu/NiFe/FeMn spin valves
Authors: Park C.-M. , Min K.-I. , Shin K. H.
Published: July 2002

We have investigated the effects of texture and interfacial roughness on the exchange anisotropy in NiFe/Cu/NiFe/FeMn spin valves with different buffer layers (no buffer, Cu, or FeMn) on Si(100) or Si(111) substrates by magnetron sputtering. The crystalline structure, surface topology, and exchange anisotropy field (Hex) were characterized. The exchange anisotropy was established all in (111), (200), and (220) textured samples and there was no systematic relationship between the type of texture and Hex. However, it was found that Hex increased as the surface roughness decreased. The results lead us to believe that interfacial roughness rather than crystallographic texture controls the development of the exchange anisotropy.
Simulations of multipactor-assisted breakdown in radio frequency plasmas
Authors: Vender D. , Smith H. B. , Boswell R. W.
Published: July 2002

Breakdown in low pressure radio frequency (rf) plasmas is investigated with particle-in-cell simulations in a model gas based on argon. A one dimensional model with a realistic rf wave form is used in a range of pressures between 1 and 50 mTorr. Dynamic scaling and electron generated secondaries are introduced in order to simulate breakdown realistically, and it is possible to follow the discharge development from a few initial electrons to the steady state plasma with ne ∼109 cm−3. The results confirm that breakdown is controlled by the resonant multiplication of secondary electrons known as the multipactor. For the pressures investigated, however, ionization is the main source of new charges and a model based on a typical electron trajectory is introduced to account for the growth in the number of electrons in the discharge gap. The model is in good agreement with the simulation results.
Iteration scheme for the solution of the two-dimensional Schrödinger-Poisson equations in quantum structures
Authors: Trellakis A. , Galick A. T. , Pacelli A. , Ravaioli U.
Published: July 2002

A fast and robust iterative method for obtaining self-consistent solutions to the coupled system of Schrödinger’s and Poisson’s equations is presented. Using quantum mechanical perturbation theory, a simple expression describing the dependence of the quantum electron density on the electrostatic potential is derived. This expression is then used to implement an iteration scheme, based on a predictor-corrector type approach, for the solution of the coupled system of differential equations. We find that this iteration approach simplifies the software implementation of the nonlinear problem, and provides excellent convergence speed and stability. We demonstrate the approach by presenting an example for the calculation of the two-dimensional bound electron states within the cross section of a GaAs-AlGaAs based quantum wire. For this example, the convergence is six times faster by applying our predictor-corrector approach compared to a corresponding underrelaxation algorithm.
Size effects in surface segregation
Authors: Cserháti Cs. , Szabó I. A. , Beke D. L.
Published: July 2002

The effect of the slab thickness (d) on the segregation behavior has been investigated for a phase separating system in a slab consisting of atomic planes parallel to the free surfaces. The calculations were carried out using the Fowler–Guggenheim approximation in the monolayer limit. It is shown, that by varying the surface fraction ξ, a surface phase transition can occur, similar to a semi-infinite system where a change in temperature can lead to a phase transition (S-shaped segregation isotherm). Furthermore, the existence of a minimum on the free energy curve versus d was examined. It was demonstrated that such a minimum can exist in the nanocrystalline region.
Minimization of ion micromotion in a Paul trap
Authors: Berkeland D. J. , Miller J. D. , Bergquist J. C. , Itano W. M. , Wineland D. J.
Published: July 2002

Micromotion of ions in Paul traps has several adverse effects, including alterations of atomic transition line shapes, significant second-order Doppler shifts in high-accuracy studies, and limited confinement time in the absence of cooling. The ac electric field that causes the micromotion may also induce significant Stark shifts in atomic transitions. We describe three methods of detecting micromotion. The first relies on the change of the average ion position as the trap potentials are changed. The second monitors the amplitude of the sidebands of a narrow atomic transition, caused by the first-order Doppler shift due to the micromotion. The last technique detects the Doppler shift induced modulation of the fluorescence rate of a broad atomic transition. We discuss the detection sensitivity of each method to Doppler and Stark shifts, and show experimental results using the last technique.
Temperature dependence of the enhanced excited state absorption in erbium-doped garnets
Authors: Maeda Yoshinobu , Yamada Toshikazu
Published: July 2002

Temperature dependence of an enhanced excited state absorption was investigated in erbium-doped garnet crystals using 787.3 and 787.8 nm laser diodes. The enhanced absorption was obtained at temperature ranges of 15–570 K, 15–420 K, and 15–1200 K in a yttrium aluminum garnet, a yttrium scandium gallium garnet, and a lutetium aluminum garnet, respectively. Absorption peaks corresponding to both the I1615/2−I49/2 and the I1613/2−I211/2 transitions of Er3+ in garnets were shifted from those for 300 K at high temperature. The enhanced absorption weakened at high temperature because an absorption cross section from the I413/2 level to the H211/2 level decreased due to the shift of the absorption peaks. It has been confirmed that a mechanism of the enhanced absorption can be explained by calculating the rate equations based on an analytical model for a five-level system of the Er3+ ion. It is suggested that the enhanced absorption weakened at temperatures lower than 12 K because the number of phonons decreased due to low temperature.
Scaling approach to conical indentation in elastic-plastic solids with work hardening
Authors: Cheng Yang-Tse , Cheng Che-Min
Published: July 2002

We derive, using dimensional analysis and finite element calculations, several scaling relationships for conical indentation in elastic-plastic solids with work hardening. Using these scaling relationships, we examine the relationships between hardness, contact area, initial unloading slope, and mechanical properties of solids. The scaling relationships also provide new insights into the shape of indentation curves and form the basis for understanding indentation measurements, including nano- and micro-indentation techniques. They may also be helpful as a guide to numerical and finite element calculations of indentation problems.
Fabrication of narrow-band-gap hydrogenated amorphous silicon by chemical annealing
Authors: Futako Wataru , Takeoka Shinya , Fortmann Charles M. , Shimizu Isamu
Published: July 2002

Hydrogenated amorphous silicon films with optical band gaps narrower than 1.7 eV have been prepared by a chemical annealing process involving the sequential deposition of 10–30 Å of amorphous silicon followed by a plasma treatment, hydrogen and hydrogen–argon plasma treatments were investigated. Thick homogeneous films were built up by repeating the sequence many times. The formation of microcrystalline structure could be completely suppressed by the proper choice of the substrate temperature and hydrogen–argon mixture. Argon radical impingement results in hydrogen abstraction from the growth surface resulting in films with hydrogen contents as low as 3 at. %. These low hydrogen content films had a correspondingly low optical band gap of ∼1.6 eV. Raman spectra analysis indicates that the silicon–silicon bonding environment is independent of the optical band gap. However, the optical band gap is very sensitive to the content and type of hydrogenated structure present in the material. Analysis of the electronic transport indicates that these films have uniformly good transport properties.
Photoacoustic frequency transmission technique: Electronic deformation mechanism in semiconductors
Authors: Todorović D. M. , Nikolić P. M. , Bojičić A. I.
Published: July 2002

A quantitative analysis of the electronic deformation (electronic strain) contribution to the photoacoustic signal in semiconductor samples was done. The photoacoustic (PA) effect was investigated as a function of the modulation frequency in a transmission detection configuration. The theoretical model for this configuration was given for the thermal and elastic processes besides the carrier transport characteristic and the relation for electronic elastic deformation was derived by the theory of the elastic thin plate vibrations. Analysis showed that the electronic deformation mechanisms can have an important infleucne on the PA signal in the frequency range typical for gas-microphone detection. The experimental photoacoustic data for Si samples were measured and they exhibited a satisfactory agreement with the theoretical model.
Calorimetric, thermomechanical, and rheological characterizations of bulk glass-forming Pd40Ni40P20
Authors: Wilde G. , Görler G. P. , Willnecker R. , Fecht H. J.
Published: July 2002

Calorimetric, thermomechanical, and rheological properties of undercooled liquid Pd40Ni40P20 were determined within a wide temperature range above the glass transition. The concept of the limiting fictive temperature was applied to the entire set of measurements to compare the different properties adequately. It was found that an equilibrium state of a sample that is defined by its calorimetric glass temperature corresponds to a similar equilibrium state for the specific volume and for the shear viscosity as well. The Kauzmann temperature as one of the most important material characteristics concerning the glass transition could be determined with high accuracy leading to the evaluation of the free volume persistent in the samples. Viscosity values of the liquid extending over a range of about nine orders in magnitude could be described best by the free volume theory evaluated by Cohen and Grest, provided that experimentally obtained parameters were used for the calculations. The comparison between nonequilibrium measurements at isochronous heating and model calculations in the framework of bimolecular reaction kinetics shows that good agreement can be achieved using thermodynamic parameters that have been obtained from equilibrium measurements. However, systematic deviations indicate also the limitations of the model that are related to the intrinsic dynamic heterogeneity of the vitreous state.
The OH+HBr reaction revisited
Authors: Ravishankara A. R. , Wine P. H. , Wells J. R.
Published: July 2002

Rate coefficients for the reaction OH+HBr → Br+H2O have been measured at 298 K using two different experimental techniques: (1) 266 nm pulsed laser photolysis of O3/H2O/HBr/He mixtures followed by time resolved resonance fluorescence detection of OH (LFP-RF) and (2) 248 nm pulsed laser photolysis of H2O2/HBr/Ar mixtures followed by pulsed laser induced fluorescence detection of OH (LFP-LIF). In both sets of experiments, the HBr concentration was monitored in situ in the slow flow system by UV photometry. Measured rate coefficients in units of 10−12 cm3 molecule−1 s−1 are 11.4±0.3 (LFP-RF) and 10.7±0.3 (LFP-LIF) where the uncertainties are 2σ and represent precision only. The absolute accuracy of the results is estimated to be ±10%. The experimental k1 values are in good agreement with a previous LFP-RF study in our laboratory but significantly faster than values reported by several other groups.
Microphase separation in multiblock copolymer melts
Authors: Mayes Anne M. , Olvera de la Cruz Monica
Published: July 2002

The microphase separation transition for asymmetric triblock and (A–B)m star copolymer melts is investigated following a mean-field approach proposed by Leibler for diblock copolymer melts. Continuous transitions are found in all triblock architectures and in stars for any m at some composition fc. The phase diagram for the equilibrium morphologies is notably altered by varying the architecture of triblock copolymers.
Molecular dynamics simulations of aqueous ionic clusters using polarizable water
Authors: Dang Liem X. , Smith David E.
Published: July 2002

The solvation properties of a chlorine ion in small water clusters are investigated using state-of-the-art statistical mechanics. The simulations employ the polarizable water model developed recently by Dang [J. Chem. Phys. 97, 2659 (1992)]. The ion–water interaction potentials are defined such that the successive binding energies for the ionic clusters, and the solvation enthalpy, bulk vertical binding energy, and structural properties of the aqueous solution agree with the best available results obtained from experiments. Simulated vertical electron binding energies of the ionic clusters Cl−(H2O)n, (n=1–6) are found to be in modest agreement with data from recent photoelectron spectroscopy experiments. Minimum energy configurations for the clusters as a function of ion polarizability are compared with the recent quantum chemical calculations of Combariza, Kestner, and Jortner [Chem. Phys. Lett. 203, 423 (1993)]. Equilibrium cluster configurations at 200 K are described in terms of surface and interior solvation states for the ion, and are found to be dependent on the magnitude of the Cl− polarizability assumed in the simulations.
Ion-induced nucleation: A density functional approach
Authors: Kusaka I. , Wang Z.-G. , Seinfeld J. H.
Published: July 2002

Density functional theory is applied to ion-induced nucleation of dipolar molecules. The predicted reversible work shows a sign preference, resulting in a difference in the nucleation rate by a factor of 10–102, for realistic values of model parameters. The sign effect is found to decrease systematically as the supersaturation is increased. The asymmetry of a molecule is shown to be directly responsible for the sign preference in ion-induced nucleation.
Solvent structure and hydrodynamic effects in photoinduced electron transfer
Authors: Swallen S. F. , Weidemaier Kristin , Fayer M. D.
Published: July 2002

A previously developed statistical mechanical theory describing photo-induced electron transfer and geminate recombination in liquid solutions has been modified to account for realistic finite-volume solvent effects. This work introduces physically important effects caused by the solvent which fundamentally affect the rates and spatial distribution of charge transfer events. The finite volume of solvent molecules gives rise to a nonuniform distribution of particles around an electron donor, which is incorporated into the theory by a two-particle radial distribution function (rdf). The Percus–Yevick solutions for the rdf can give numerically useful values for the solvent structure, g(R) although any form of g(R) can be used with the method. The nonuniform particle distribution significantly affects the electron transfer rates and the distribution of ion pairs formed by forward electron transfer, particularly at short times. In addition, finite solvent size affects the rate of relative diffusion between any donor–acceptor pair. These ‘‘hydrodynamic effects’’ slow down the interparticle diffusion rates when near contact, resulting in a major change in the long time behavior of photoexcited electron transfer systems. This work formally introduces the mathematical modifications to charge transfer theory necessary to account for the solvent structure and hydrodynamic effect and illustrates the results with model calculations. These calculations show that analysis of experiments with theories that do not include the rdf and hydrodynamic effects can result in significant errors in the interpretation of data.
Calculation of vibrational (J=0) excitation energies and band intensities of formaldehyde using the recursive residue generation method
Authors: Poulin Nicolas M. , Bramley Matthew J. , Carrington Tucker , Kjaergaard Henrik G. , Henry Bryan R.
Published: July 2002

We use the recursive residue generation method (RRGM) with an exact kinetic energy operator to calculate vibrational excitation energies and band intensities for formaldehyde. The basis is a product of one-dimensional potential optimized discrete variable representation (PO-DVR) functions for each coordinate. We exploit the symmetry by using symmetry adapted basis functions obtained by taking linear combinations of PO-DVR functions. Our largest basis set consists of 798 600 functions (per symmetry block). The Lanczos tridiagonal representation of the Hamiltonian is generated iteratively (without constructing matrix elements explicitly) by sequential transformations. We determine a six-dimensional dipole moment function from the ab initio dipole moment values computed at the QCISD level with a 6-311++G(d,p) basis set. We converged all A1, B2 and B1 vibrational states up to the combination band with two quanta in the C–O stretch and one quantum in a C–H stretch at about 6 350 cm−1 above zero point energy. We present a simulated (J=0) infrared spectrum of CH2O for transitions from the ground state.
Ionization energies of anthracene, phenanthrene, and naphthacene
Authors: Zakrzewski V. G. , Dolgounitcheva O. , Ortiz J. V.
Published: July 2002

Ab initio electron propagator calculations on the lowest vertical ionization energies of anthracene, phenanthrene, and naphthacene have been performed with the partial third order and outer valence Green’s function approximations. Agreement with photoelectron spectra is very close, enabling the clarification of previous assignments, some of which were contradictory. With the present approximations, Feynman–Dyson amplitudes are equivalent to canonical molecular orbitals. Plots of these one-electron functions aid in the interpretation of the spectra by revealing patterns of delocalization and locations of nodes.
A molecular-thermodynamic model for polyelectrolyte solutions
Authors: Jiang Jianwen , Liu Honglai , Hu Ying , Prausnitz J. M.
Published: July 2002

Polyelectrolyte solutions are modeled as freely tangent-jointed, charged hard-sphere chains and corresponding counterions in a continuum medium with permitivity ε. By adopting the sticky-point model, the Helmholtz function for polyelectrolyte solutions is derived through the r-particle cavity-correlation function (CCF) for chains of sticky, charged hard spheres. The r-CCF is approximated by a product of effective nearest-neighbor two-particle CCFs; these are determined from the hypernetted-chain and mean-spherical closures (HNC/MSA) inside and outside the hard core, respectively, for the integral equation theory for electrolytes. The colligative properties are given as explicit functions of a scaling parameter Γ that can be estimated by a simple iteration procedure. Osmotic pressures, osmotic coefficients, and activity coefficients are calculated for model solutions with various chain lengths. They are in good agreement with molecular simulation and experimental results.
Self-consistent field, ab initio molecular orbital and three-dimensional reference interaction site model study for solvation effect on carbon monoxide in aqueous solution
Authors: Sato Hirofumi , Kovalenko Andriy , Hirata Fumio
Published: July 2002

We have developed a three-dimensional (3D) extension of the reference interaction site model-self-consistent field (RISM-SCF) method to treat the electronic structure of a solvated molecule. The site–site treatment of the solute–solvent correlations involving the approximation of radial averaging constitutes a bottleneck of the RISM-SCF method, and thus lacks a 3D picture of the solvation structure for complex solutes. To resolve this problem, we devised out a 3D generalization of the RISM integral equations which yields the 3D correlation functions of interaction sites of solvent molecules around a solute of arbitrary shape. In the present article, we propose a SCF combination of the ab initio molecular orbital (MO) methods and 3D-RISM approach. A benchmark result for carbon monoxide in ambient water is also presented.
Perfect fluid spheres admitting a one-parameter group of conformal motions
Authors: Herrera L. , Ponce de León J.
Published: July 2002

Some exact analytical solutions of the Einstein equations for perfect fluids were found under the assumption of spherical symmetry and the existence of a one-parameter group of conformal motions. The first solution exhibited represents a nonstatic homogeneous spherically symmetric distribution of matter which is singular at t=0. Two other solutions represent contracting and expanding fluids, respectively, whose evolution tends asymptotically to a static sphere with a surface gravitational potential equal to (1)/(3) . These two solutions possess vanishing pressure surfaces which are not the boundary of matter except in the static limit. Finally an oscillating distribution of matter is presented.
Invariant supersymmetric multilinear forms and the Casimir elements of P-type Lie superalgebras
Authors: Scheunert Manfred
Published: July 2002

The Casimir elements of the P-type Lie superalgebras are investigated. Depending on the class of algebras under consideration either there do not exist any nontrivial Casimir elements at all or else the Casimir elements are highly degenerate. Basic to the investigation is a lemma about invariant supersymmetric multilinear forms on a finite-dimensional module over a Lie superalgebra. Some comments on the Cartan subalgebras of a Lie superalgebra are also included. An Appendix provides some information on multilinear algebra with ε-commuting scalars.
Stability of axial flow in an annulus with a rotating inner cylinder
Authors: Lueptow Richard M. , Docter Andreas , Min Kyungyoon
Published: July 2002

Flow between concentric cylinders with the inner cylinder rotating and an axial pressure gradient imposed in the annulus reveals a rich variety of flow regimes depending on the flow conditions. The occurrence of these flow regimes was studied experimentally by both visually and optically detecting the transition from one flow regime to another over a wide range of Taylor numbers for moderate axial Reynolds numbers. Seven flow regimes of toroidal vortices were identified, including Taylor vortices, wavy vortices, random wavy vortices, modulated wavy vortices, turbulent modulated wavy vortices, turbulent wavy vortices, and turbulent vortices. The toroidal vortices in these flow regimes look similar to the corresponding vortices when there is no axial flow, except that they translate with the axial flow at a speed slightly greater than the bulk axial velocity. Three flow regimes of helical vortices were observed at low Taylor numbers, including laminar helical vortices, stationary helical vortices, and wavy helical vortices. Depending on the flow parameters, the helical vortices had both positive and negative helix angles with respect to the bulk flow and appeared either stationary or moving downstream. Another flow regime consisting of the repeating sequential appearance of turbulent wavy vortices, turbulent helical vortices, and turbulent vortices was also observed.
The interaction of resonant magnetic perturbations with rotating plasmas
Authors: Fitzpatrick R. , Hender T. C.
Published: July 2002

The penetration of a helical magnetic perturbation into a rotating tokamak plasma is investigated. In the linear regime, it is found that unless the frequency of the imposed perturbation matches closely to one of the natural mode frequencies, reconnection at the rational surface is suppressed by a large factor. In order to deal with the problem in the nonlinear regime a theory of propagating, constant-ψ magnetic islands is developed. This theory is valid provided the island width greatly exceeds any microscopic scale length (but still remains small compared with the minor radius), and the magnetic Reynolds number of the plasma is sufficiently large. An island width evolution equation is obtained which, in addition to the usual Rutherford term, contains a stabilizing term due ultimately to the inertia of the plasma flow pattern set up around the propagating island. A complete solution is presented for the case where the island and its associated flow pattern are steady. In the nonlinear regime, a fairly sharp threshold is predicted for the magnitude of the applied perturbation. Below this threshold, the induced islands are rotationally suppressed and partially dragged along by the rotating plasma, and above it the islands are virtually fully reconnected and ‘‘locked’’ at the applied frequency of the perturbation. Numerical results from an initial value code are presented, which show good agreement with the analytic predictions. Finally, it is demonstrated that these theories can be used to interpret data recently obtained from the COMPASS-C device [Controlled Fusion and Plasma Heating 1990 (EPS, Geneva, 1990), Vol. 1, p. 379]. In particular, a positive explanation is given of why in some cases an applied quasistatic resonant magnetic perturbation can stabilize magnetohydrodynamic modes, but in others leads to a disruption.
Forced magnetic reconnection
Authors: Hahm T. S. , Kulsrud R. M.
Published: July 2002

By studying a simple model problem, the time evolution of magnetic field islands which are induced by perturbing the boundary surrounding an incompressible plasma with a resonant surface inside is examined. The reconnection and island formation process for sufficiently small boundary perturbations occurs on the tearing mode time scale defined by Furth, Killeen, and Rosenbluth [Phys. Fluids 6, 459 (1963)]. For larger perturbations the time scale is that defined by Rutherford [Phys. Fluids 16, 1903 (1973)]. The resulting asymptotic equilibrium is such that surface currents in the resonant region vanish. A detailed analytical picture of this reconnection process is presented.
Nonlinear reduced Braginskii equations with ion thermal dynamics in toroidal plasma
Authors: Zeiler A. , Drake J. F. , Rogers B.
Published: July 2002

Starting from the Braginskii fluid equations, a set of nonlinear reduced equations are derived which describe the low frequency dynamics of electron and ion energy and density in a toroidal plasma. The equations have an energy integral. The equations are appropriate for studying the relation between electron and ion thermal transport and particle transport in low temperature plasma near the edge of plasma confinement devices.
Drifts, boundary conditions and convection on open field lines
Authors: Cohen Ronald H. , Ryutov Dmitri
Published: July 2002

In a number of plasmas of practical interest, including the scrape-off layer of a tokamak with a divertor or toroidal limiter, some gas discharge devices, and in the vicinity of spacecraft, magnetic field lines intersect bounding surfaces at shallow angles. Under these circumstances a number of interesting and important effects arise. Drifts can compete with parallel flows in establishing the boundary conditions for plasma mass-flow and current (sheath current–voltage characteristics). This paper derives the mass-flow constraints including both poloidal and radial drifts, reviews the current boundary conditions, and surveys the consequences, including along-field density and heat-flux asymmetries, convection created by a wavy surface, generation of electric fields, and surface currents associated with shadows from surface structures, and modification of instability growth rates.
Blocking site-to-site translocation of a misactivated amino acid by mutation of a class I tRNA synthetase
Authors: Bishop Anthony C. , Nomanbhoy Tyzoon K. , Schimmel Paul
Published: July 2002

The genetic code is established by the aminoacylation reactions of tRNA synthetases. Its accuracy depends on editing reactions that prevent amino acids from being assigned to incorrect codons. A group of class I synthetases share a common insertion that encodes a distinct site for editing that is about 30 Å from the active site. Both misactivated aminoacyl adenylates and mischarged amino acids attached to tRNA are translocated to this site, which, in turn, is divided into subsites—one for the adenylate and one for the aminoacyl moiety attached to tRNA. Here we report that a specific mutation in isoleucyl-tRNA synthetase prevents editing by blocking translocation. The mutation alters a widely conserved residue that is believed to tether the amino group of mischarged tRNA to its subsite for editing. These and other data support a model where editing is initiated by translocation of the misacylated amino acid attached to tRNA to create an “editing complex” that facilitates subsequent rounds of editing by translocation of the misactivated adenylate.