bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature quenching of exciton luminescence intensity in ZnO/(Mg,Zn)O multiple quantum wells
Authors: Makino T. , Tamura K. , Chia C. H. , Segawa Y. , Kawasaki M. , Ohtomo A. , Koinuma H.
Published: May 2003

The temperature-dependent behavior of excitonic photoluminescence observed in ZnO/MgZnO multiple quantum wells (MQWs) in the temperature range of 5–300 K is described. In a ZnO/Mg0.27Zn0.73O MQW grown by laser molecular-beam epitaxy, the luminescence was dominated by localized exciton (LE) emission throughout the whole temperature range studied. Luminescence of free excitons (FEs) was not observed. A simple rate equation is used to describe the quenching of LE emission. The activation energy for LE luminescence quenching is of the order of the localization energy of excitons, suggesting that the thermionic emission of the LEs out of the localization potentials leads to nonradiative recombination. In a ZnO/Mg0.12Zn0.88O MQW having lower barriers, the luminescence was dominated by LE emissions at low temperatures, while the FE transition was dominating emissions at temperatures above 175 K. A rate equation assuming one nonradiative recombination channel is used to describe the quenching of the transitions observed. The activation energy for LE luminescence quenching deduced in this sample is also of the order of the localization energy of excitons. The temperature dependences of FE emission intensities are also discussed by using a simple rate equation in which a thermal release effect of LEs toward FEs is taken into account.
Evidence for shallow implantation during the growth of bismuth nanocrystals by pulsed laser deposition
Authors: Barnes J-P. , Petford-Long A. K. , Suárez-Garcı́a A. , Serna R.
Published: May 2003

The implantation of bismuth during pulsed laser deposition (PLD) has been directly observed and investigated. Bi was deposited on amorphous aluminum oxide (Al2O3) and the laser energy density on the Bi target was varied by one order of magnitude (0.4 to 5 J cm−2). Cross-sectional transmission electron micrographs reveal that, for laser energy densities above 2 J cm−2, in addition to the formation of Bi nanocrystals, there is a dark and apparently continuous layer in the Al2O3 underneath them. From previous velocity measurements, the kinetic energy of the Bi species in the plume generated at laser energy densities above 2 J cm−2 has been estimated to be around 200 eV, which gives a calculated implantation range of 1.8 nm in Al2O3. This is in good agreement with the position of the Bi-rich layer.
Kinetic extensions of the nucleation theorem
Authors: McGraw Robert , Wu David T.
Published: May 2003

Kinetic extensions of the nucleation theorem (KNT) are derived using the law of mass action and detailed balance. Results are obtained for the first- and higher-order derivatives of the nucleation rate, J, with change in supersaturation, S, in terms of the cumulants, κn, of a molecular distribution of reciprocal equilibrium cluster growth rates. At constant temperature we find d ln J/d ln S=κ1+1, an exact formulation of the nucleation theorem in terms of nucleation rate, and the extension dn ln J/d(ln S)n=(−1)n+1κn for the higher-order derivatives (n⩾2). The case n=2 is related to the Kelvin relation. Analysis of recent water vapor nucleation rates [Wölk and Strey, J. Phys. Chem. B 105, 11683 (2001)] provides molecular-based estimates for κ1 and κ2 suitable for comparison with the predictions of classical nucleation theory. The KNT is applied to ion-induced nucleation from the gas phase, by a sequence of reversible chemical reactions, and extensions to multistep kinetics and multicomponent nucleation are presented. Nucleation theorems enable one to deduce molecular-level properties directly from macroscopic rate measurements. Here we show these properties are not those of a single cluster, the critical nucleus, as approximate forms of the theorems would suggest, but instead are averages over a weighted distribution of clusters near critical size.
Structural study of the Al/Ni interface in ultrathin polycrystalline multilayers
Authors: Fonda E. , Petroff F. , Traverse A.
Published: May 2003

Several Al–Ni multilayers were prepared where the Ni thickness was varied in order to (i) detect Ni diffusion in Al and Al diffusion in Ni and (ii) determine the compounds that form at the interface during deposition at room temperature. X-ray absorption spectroscopy was carried out at the K edge of Ni to identify the local atomic arrangement around this atom. Magnetic measurements allowed the detection of pure Ni among nonmagnetic AlxNi1−x compounds. At the Al/Ni interface, Ni diffuses in Al whereas no Al diffusion in Ni was observed at the Ni/Al interface. Ni diffusion in Al results in the formation of a disordered aluminide over a thickness of about 1.7 nm, then pure Ni growth takes place in the form of islands.
Grain boundary transport in x-ray irradiated polycrystalline diamond
Authors: Conte G. , Rossi M. C. , Salvatori S. , Fabbri F. , Loreti S. , Ascarelli P. , Cappelli E. , Trucchi D.
Published: May 2003

The transport properties of a “thin” polycrystalline diamond film are analyzed after the sample exposure to 8.06-keV x-ray radiation. Structure and morphology of the as-grown film have been evaluated by Raman, x-ray diffraction, and scanning electron microscopy techniques. The transport properties have been investigated by measuring dark current–voltage characteristics in the temperature range of 60 to 360 K. Ohmic transport has been evidenced on the as-grown film up to 1.16×105 V/cm. After irradiation, nonlinear contributions to the dark current have been evidenced and related to field-assisted thermal ionization of traps. Below 200 K, hopping mechanisms have been observed. Correlations have been found among x-ray irradiation, density of traps involved in the transport processes, and the nonhomogeneous nature of the sample. A simple model of the grain boundary structure is proposed to explain the observations.
Direct numerical simulation of stagnation region flow and heat transfer with free-stream turbulence
Authors: Bae Sungwon , Lele Sanjiva K. , Sung Hyung Jin
Published: May 2003

A direct numerical simulation is performed for stagnation-region flow with free-stream turbulence. A fully implicit second-order time-advancement scheme with fourth-order finite differences and an optimized scheme are employed. The optimized scheme is developed to save computational cost. The free-stream turbulence is a precomputed field of isotropic turbulence. The present DNS results in the “damping” and “attached amplifying” regimes are found to be similar to those of the organized inflow disturbances. Emphasis is placed on the flow and temperature fields in the “detached amplifying” regime. The contours of instantaneous flow field illustrate that streamwise vortices are stretched in the streamwise direction by mean strain rate. The temperature field is also stretched in the streamwise direction near the wall. The surface contours reveal that the temperature field is influenced significantly by streamwise vorticity. Due to the dominance of the mean strain, the log-law region is not observed for ū and T̃, the inner scaling fails, but the outer scaling works. The single-point turbulence statistics and the turbulent statistics budgets are obtained. The flow statistics reflect the typical characteristics of stagnation-region flow which are generically different from those of other canonical shear flows. One of the typical features of the budgets is that the velocity pressure correlation and the turbulent transport play significant roles in the stagnation-region flow. Finally, the present simulation data are compared with experimental results. It is found that the effect of large-scale eddies on the enhancement of wall heat transfer is substantial in the turbulent stagnation-region heat transfer.
A model for the mixing time scale of a turbulent reacting scalar
Authors: Cha Chong M. , Trouillet Philippe
Published: May 2003

Current micromixing models in transported PDF (probability density function equation) modeling of initially nonpremixed, turbulent reacting flows ignore the direct influence of chemistry on the dissipation rate of reacting scalar fluctuations. Following mapping closure, a model for the time scale of reactive scalar mixing in the flamelet regime is developed and comparisons made with direct numerical simulations of a turbulent reacting jet. The modeling results show a significant improvement over the generally used estimate of substituting the time scale of the reacting scalar by that of a conserved scalar in traditional transported PDF approaches. The model can readily be applied to many existing mixing submodels used in transported PDF modeling when the time scales of the reacting scalars appear explicitly in the formulation. The submodels allow the transported PDF approach to now encompass the fast as well as slow chemistry regimes.
Power laws for rough wall turbulent boundary layers
Authors: Kotey N. A. , Bergstrom D. J. , Tachie M. F.
Published: May 2003

An assessment of the ability of power laws to describe the mean velocity profile in the overlap region of a zero pressure gradient turbulent boundary layer is reported. The experiments were performed in a wind tunnel on smooth and four different types of rough surfaces at moderate Reynolds numbers. A novel modification to the power law velocity profile is proposed to account for the effect of surface roughness in the overlap region. This modification is analogous to the use of a roughness function to produce a downward shift in the logarithmic velocity profile. The roughness parameters in the proposed equation more accurately follow the effect of roughness on skin friction than does the roughness shift ΔU+. The present study shows that power laws can be used to effectively describe the mean velocity profile over a wider range than a log law for both smooth and rough surfaces.
Optical observation of preflashover phenomena from polytetrafluoroethylene in a planar concentric structure
Authors: Zhang Guan-Jun , Yan Z. , Liu Y. S. , Yasuoka K. , Ishii S.
Published: May 2003

The optical emission from polytetrafluoroethylene (PTFE) surfaces in a planar concentric metal-polymer-metal structure was investigated under ac electric field in vacuum. Two kinds of optical pattern were observed before the flashover; they were strongly dependent on the magnitude of applied voltage. At a lower applied voltage, the light emission was weak and uniform and was observed close to the edges of both electrodes. It may be considered as electroluminescence phenomena due to the radiative recombination of electrons and holes injected from the electrode into the polymer surface layer. With increase of the applied voltage, the optical emission became significantly strong and irregular, and its activity area gradually expanded from the inner electrode to the outer electrode. The accumulation of electrons injected is supposed to form a long-term negative space charge region away from the inner electrode. A strong modification of the local electric field near its border would result in the detrapping of trapped electrons at a critical applied voltage and hence cause forward/backward secondary electron emission on the surface of PTFE, accompanied by intense light emission and a wide luminescent area, and probably lead to the final surface flashover. It is concluded that the negative charge region is likely located around ∼2.6 mm from the inner electrode edge.
Intersubband absorption in strain-compensated InAlAs/AlAs/InxGa(1−x)As (x∼0.8) quantum wells grown on InP
Authors: Lai K. T. , Missous M. , Gupta R. , Haywood S. K.
Published: May 2003

We report the observation of strong room temperature absorption peaks between 4 and 7 μm in strain-compensated In0.84Ga0.16As/AlAs/In0.52Al0.48As double-barrier multiple-quantum-well structures. The observed peaks at 4.4, 5.0, and 7.2 μm are attributed to E3→E4, E2→E3, and E1→E2 Γ-Γ electron intersubband transitions, respectively, the transition energies are in good agreement with our theoretical model. The large conduction band offset and low effective mass in this material system, as well as the possibility for strain compensation between wells and barriers, make this a promising route to efficient room temperature quantum well infrared photodetectors.
Effects of magnetic layer thickness and of head-to-medium spacing on noise in advanced particulate recording media
Authors: Mercer Tim , Bissell Philip R. , Ardeleanu Paul , Stoleriu Laurentiu , Stancu Alexandru
Published: May 2003

The effects of magnetic layer thickness on the noise characteristics of advanced metal particle tape have been observed using a dc demagnetization process generated with a uniform in-plane magnetizer. The three-dimensional (3D) surface maps of spectral noise power plotted as a function of remanent magnetization state showed a change in behavior as the recording layer became thinner. The thickest magnetic coating of 310 nm showed “trough-like” characteristics associated with conventional thick particulate media. The thinnest sample with a 165 nm magnetic coating showed a “peak-like” response that is observed in continuous thin-film media, with a graduated trend from one to the other for the two intermediate samples. A simulation of these results has been made with an 8000 spherical particle micromagnetic model of the media remanent states and a simulated magnetoresistive read head. Noise maps produced from the simulation compared well with experiment. Spatial isolation of the noise contributions was achieved by dividing the modeled magnetic layer into sublayers. This revealed that particles at the surface were the source of the “peak-like” response, and their contributions became more dominant as the magnetic coating got thinner. Further model investigations showed an association of the different sublayer characteristics with a change in the mean magnetostatic interaction field. The interaction field of particles at the surface had characteristics similar to those of 2D granular thin films and hence generated “thin film-like” noise. A further effect was associated with the spacing between the head and various regions of particles. As this was increased, not only was the amplitude of the noise reduced, but the increased region of integration meant that longer range correlations were incorporated into the observed characteristics, resulting in noise maps more like those of conventional thick particulate media.
Capillary rise in nesting cylinders
Authors: Brady Victor , Concus Paul , Finn Robert
Published: May 2003

We investigate computationally recent results concerning the question of whether liquid necessarily rises higher in a capillary tube of smaller section, when tubes are placed vertically in an infinite reservoir. The numerical results corroborate for a particular example a striking discontinuous behavior that was predicted mathematically.
Positioning of self-assembled Ge islands on stripe-patterned Si(001) substrates
Authors: Zhong Zhenyang , Halilovic A. , Mühlberger M. , Schäffler F. , Bauer G.
Published: May 2003

Self-assembled Ge islands were grown on stripe-patterned Si(001) substrates by solid source molecular beam epitaxy. The surface morphology obtained by atomic force microscopy and cross-sectional transmission electron microscopy images shows that the Ge islands are preferentially grown at the sidewalls of pure Si stripes along the [−110] direction at 650 °C or along the trenches, whereas most of the Ge islands are formed on the top terrace when the patterned stripes are covered by a strained GeSi buffer layer. Reducing the growth temperature to 600 °C results in a nucleation of Ge islands both on the top terrace and at the sidewall of pure Si stripes. A qualitative analysis, based on the growth kinetics, demonstrates that the step structure of the stripes, the external strain field, and the local critical wetting layer thickness for the islands formation contribute to the preferential positioning of Ge islands on the stripes.
Optical and electrical properties of Pr0.8Sr0.2MnO3 thin films
Authors: Suzuki Toshio , Jasinski Piotr , Petrovsky Vladimir , Zhou Xiao-Dong , Anderson Harlan U.
Published: May 2003

The results of studies on the preparation, structure, optical, and electrical properties of Pr0.8Sr0.2MnO3 thin films were presented. Dense films with a thickness of 50–70 nm were produced on monocrystalline sapphire substrates using a polymeric precursor spin coating technique. The results of the optical measurements were correlated with the annealing temperatures of the films and showed that the shape of the optical spectra changed as the structure changed from an amorphous to crystalline structure. The optical spectra was also used to determine the energy dependence of the absorption coefficient. The electrical conductivity was observed to be about three orders of magnitude higher for specimens which were crystalline as compared to amorphous.
Discrete dislocation analysis of size effects in thin films
Authors: Nicola Lucia , Van der Giessen Erik , Needleman Alan
Published: May 2003

A discrete dislocation plasticity analysis of plastic deformation in metal thin films caused by thermal stress is carried out. The calculations use a two-dimensional plane-strain formulation with only edge dislocations. Single crystal films with a specified set of slip systems are considered. The film-substrate system is subjected to a prescribed temperature history and a boundary value problem is formulated and solved for the evolution of the stress field and for the evolution of the dislocations structure in the film. A hard boundary layer forms at the interface between the film and the substrate, which does not scale with the film thickness and thus gives rise to a size effect. It is found that a reduction in the rate of dislocation nucleation can occur abruptly, which gives rise to a two-stage hardening behavior.
A molecular dynamics study of drop spreading on a solid surface
Authors: Wu Xuhong , Phan-Thien Nhan , Fan Xi-Jun , Ng Teng Yong
Published: May 2003

In this paper, the effects of size of the chains composed of the drop and initial velocity of the drop on the drop terraced spreading are studied. The chain size effects on the base radius of drop are studied at zero initial velocity. A logarithm dependence of the drop base radius on the time is found for 2-atom to 16-atom flexible chain systems with large lattice dimension of solid and a power law t1/2 for small lattice dimension of solid. The longer the chain, the slower is the spreading. With a nonzero initial velocity, the drop base radius increases with increasing initial velocity before the drop splits into smaller separated drops. As the initial velocity increases, the transition from a logarithm to a power law t1/2 relation for the time dependence of the drop base radius is first noted here.
Homotopy of exact coherent structures in plane shear flows
Authors: Waleffe Fabian
Published: May 2003

Three-dimensional steady states and traveling wave solutions of the Navier–Stokes equations are computed in plane Couette and Poiseuille flows with both free-slip and no-slip boundary conditions. They are calculated using Newton’s method by continuation of solutions that bifurcate from a two-dimensional streaky flow then by smooth transformation (homotopy) from Couette to Poiseuille flow and from free-slip to no-slip boundary conditions. The structural and statistical connections between these solutions and turbulent flows are illustrated. Parametric studies are performed and the parameters leading to the lowest onset Reynolds numbers are determined. In all cases, the lowest onset Reynolds number corresponds to spanwise periods of about 100 wall units. In particular, the rigid-free plane Poiseuille flow traveling wave arises at Reτ=44.2 for Lx+=273.7 and Lz+=105.5, in excellent agreement with observations of the streak spacing. A simple one-dimensional map is proposed to illustrate the possible nature of the “hard” transition to shear turbulence and connections with the unstable exact coherent structures.
Solid immersion lens-enhanced nano-photoluminescence: Principle and applications
Authors: Moehl S. , Zhao Hui , Don B. Dal , Wachter S. , Kalt H.
Published: May 2003

We demonstrate a far-field nano-photoluminescence setup based on the combination of a hemispherical solid immersion lens (SIL) with a confocal microscope. The spatial resolution is confirmed to be 0.4 times the wavelength in vacuum in terms of half width at half maximum. The collection efficiency is found to be about five times higher than the same microscope without SIL, which is consistent with our theoretical analysis. We investigate in detail the influence of an air gap between the SIL and the sample surface on the system performance, and prove both experimentally and theoretically the tolerance of this far-field system to an air gap of several micrometers. These features make the present setup an ideal system for spatially resolved spectroscopy of semiconductor nanostructures. In particular, we show two examples of such applications in which the present setup is clearly suitable: Studies of excitonic transport in quantum wells and spectroscopy of single quantum dots with emphasis on polarization dependence and weak-signal detection.
Low-field transverse magnetovoltage in manganite films
Authors: Moshnyaga V. , Damaschke B. , Tidecks R. , Samwer K.
Published: May 2003

The temperature and in plane magnetic field dependences of the transverse voltage, VY, were studied in patterned epitaxial films of La0.7Ca0.3MnO3(LCMO)/MgO and La0.7(Ca0.5Sr0.5)0.3MnO3(LSCMO)/MgO. Above the Curie temperature, TC, VY(T) shows the same insulating behavior as the longitudinal voltage, VX(T). With the onset of long range ferromagnetic order near TC, an additional contribution to VY appears, exhibiting a sin(2α) dependence on the angle α between the current and the magnetic field, H. An hysteretic VY(H) behavior with two minimums at the coercive field, ±HC, due to the magnetization reversal, was found in the ferromagnetic regime. A magneto voltage, ΔVY(0.5 kOe)/VY(0)=400%, and a low-field sensitivity of 20%/Oe (50 μV/Oe) for Hext=10–20 Oe in a LSCMO film were observed at room temperature. The results are explained as an anisotropic magnetoresistance (AMR) effect, which becomes maximal slightly below TC. The maximal AMR ratios, 1.7% and 1.5–2.3%, were obtained for LCMO and LSCMO films, respectively.
Hypersonic shock waves and hybridization of a-C:H thin films
Authors: Golanski A. , Stoquert J. P. , Piazza F. , Kern P. , Laemmer E. , Deruyver A. , Schulze F. S. , McDonnell L.
Published: May 2003

A distributed electron cyclotron resonance plasma reactor powered by a microwave generator operating at 2.45 GHz was used to deposit a-C:H films at room temperature on rf biased 〈100〉 Si substrates. Modifying substrate bias, substrate current density and composition of the precursor gas enabled the average deposited energy density to be varied. The physical properties of a-C:H were investigated using atomic force microscopy (AFM), x-ray photoelectron spectroscopy, and electron energy loss spectroscopy (EELS). The experimental results were correlated with the predictions of the binary collision theory. The influence of the deposited energy density on the nucleation and growth processes was investigated using both pure C2H2 and C2H2 mixed with Ar. The sp3 nucleation process is shown to be stimulated by high energy density cascades generated by (C2H2)+ and Ar+ ions. For the pure acetylene plasma, the AFM topography displays a random network of circular, crater-like objects close to 1 μm in diameter. These objects are associated with plastic flow of a-C:H and are attributed to the transversal hypersonic shock waves generated by overlapping binary collision cascades. EELS analysis shows that an increasing ion current density applied under constant substrate bias leads to an increased sp3 hybridized carbon fraction. The effect is attributed to interference between the shock waves triggered by individual ions and the corresponding high pressure transients. The probability Pi of a dynamic overlap of order i between shock waves is estimated under the assumption that in order to modify the quantum state and bonding type, the overlap must occur during the wave propagation time τ. The observed evolution of the sp3 hybridized fraction is consistent with theoretical predictions for i=2 and a propagation time τ≈1 ps, indicating that shock waves are generated during the cascade’s lifetime. Analysis of the AFM images shows that once the shock wave comes to rest, the subsequent nucleation of the sp3 hybridized component is controlled by the tensile stress-mediated nucleation process.
A novel method for the solution of the Schrödinger equation in the presence of exchange terms
Authors: Rawitscher George H. , Kang S.-Y. , Koltracht I.
Published: May 2003

In the Hartree–Fock approximation the Pauli exclusion principle leads to a Schrödinger equation of an integro-differential form. We describe the extension of a new spectral noniterative method (S-IEM), previously developed for solving the Lippmann–Schwinger integral equation with local potentials, so as to include the exchange nonlocality. We apply it to the restricted case of electron-hydrogen scattering in which the bound electron remains in the ground state and the incident electron has zero angular momentum, and we compare the acuracy and economy of the new method to two other methods. One is a noniterative solution of the integral equation as described by Sams and Kouri in 1969. Another is an iterative method introduced by Kim and Udagawa in 1990 for nuclear physics applications, which makes an expansion of the solution into an especially favorable basis obtained by a method of moments. The S-IEM method turns out to be more accurate than the two comparison methods by many orders of magnitude for the same number of mesh points.
Dispersed fluorescence spectroscopy of AlNi, NiAu, and PtCu
Authors: Fabbi Jacqueline C. , Karlsson Lars , Langenberg Jon D. , Costello Quinton D. , Morse Michael D.
Published: May 2003

Dispersed fluorescence studies of AlNi, NiAu, and PtCu have been performed, providing spectroscopic information about the ground and low-lying excited electronic states. Vibrational frequencies are reported for the ground X 2Δ5/2 state of all three molecules. In the case of AlNi, fluorescence to all five of the states originating from the 3dNi93sAl2σ2 manifold has been observed. For both NiAu and PtCu, fluorescence to two low-lying excited states in addition to the ground state was observed. Relative energies, vibrational constants, and, when possible, Ω values of these states are reported. Comparisons of the measured electronic states to the predictions of a ligand-field plus spin–orbit model are also provided, along with a comparison of the electronic structure of PtCu to that of PtH.
The microwave spectrum, ab initio analysis, and structure of the fluorobenzene–hydrogen chloride complex
Authors: Sanz M. Eugenia , Antolı́nez Sonia , Alonso José L. , López Juan C. , Kuczkowski Robert L. , Peebles Sean A. , Peebles Rebecca A. , Boman Faith C. , Kraka Elfi , Cremer Dieter
Published: May 2003

The fluorobenzene–hydrogen chloride π-hydrogen-bonded complex has been studied by high resolution microwave spectroscopy and ab initio calculations. Rotational spectra of the C6H5F–H35Cl, C6H5F–H37Cl, and C6D5F–H35Cl isotopomers were assigned using pulsed molecular beam techniques in a Fourier-transform microwave spectrometer. The spectra are consistent with a structure of the complex in which the HCl is above the fluorobenzene ring near the ring center, similar to the benzene–HCl prototype dimer. An analysis of the inertial data and the chlorine quadrupole coupling tensor results in two mathematically possible locations for the HCl subunit with respect to the fluorobenzene arising from sign ambiguities in interpreting the spectral constants. One structure has the HCl nearly perpendicular to the aromatic ring; the other has the HCl pointing toward the fluorine end of the ring. Spectral intensities for the μa and μb transitions favor the former configuration. Ab initio calculations (MP2/6-311++G(2df,2pd)+BSSE corrections) indicate that the position of the HCl is driven by electrostatic interactions with the π electrons of the benzene ring. HCl is shifted by 0.16 Å from the center of the ring toward the para-C atom, where the π density is significantly higher. In the equilibrium form, HCl is tilted by δ=14° from perpendicular to the ring with the hydrogen end toward the para-C atom. The H atom can perform an internal rotation or at least a half-circular libration (barriers smaller than 100 cm−1). An average δ value of 0.7° is estimated in reasonable agreement with the derived vibrationally averaged value of 3.8°. The complex binding energy ΔE calculated at the CCSD(T)/6-311++G(2df,2pd)+CP(BSSE) level of theory is 2.8 kcal/mol, suggesting a lower ΔE value for benzene–HCl than previously reported. Fluorobenzene–HCl possesses some charge transfer character; however, just 5.5 melectron are transferred from the benzene ring to HCl. In view of this, π–H bonding in fluorobenzene–HCl is predominantly electrostatic rather than covalent in character contrary to claims made in connection with benzene–HCl.
Bifurcation of and ghost effect on the temperature field in the Bénard problem of a gas in the continuum limit
Authors: Sone Yoshio , Doi Toshiyuki
Published: May 2003

A gas in a time-independent state under a uniform weak gravity in a general domain is considered. The asymptotic behavior of the gas in the limit that the Knudsen number of the system tends to zero (or in the continuum limit) is investigated on the basis of the Boltzmann system for the case where the flow velocity vanishes in this limit, and the fluid-dynamic-type equations and their associated boundary conditions describing the behavior of the gas in the continuum limit are derived. The equations, different from the Navier–Stokes ones, contain thermal stress and infinitesimal velocity amplified by the inverse of the Knudsen number. The system is applied to analysis of the behavior of a gas between two parallel plane walls heated from below (Bénard problem), and a bifurcated strongly distorted temperature field is found in infinitesimal velocity and gravity. This is an example showing that the Navier–Stokes system fails to describe the correct behavior of a gas in the continuum limit.
Exact self-similarity solution of the Navier–Stokes equations for a porous channel with orthogonally moving walls
Authors: Dauenhauer Eric C. , Majdalani Joseph
Published: May 2003

This article describes a self-similarity solution of the Navier–Stokes equations for a laminar, incompressible, and time-dependent flow that develops within a channel possessing permeable, moving walls. The case considered here pertains to a channel that exhibits either injection or suction across two opposing porous walls while undergoing uniform expansion or contraction. Instances of direct application include the modeling of pulsating diaphragms, sweat cooling or heating, isotope separation, filtration, paper manufacturing, irrigation, and the grain regression during solid propellant combustion. To start, the stream function and the vorticity equation are used in concert to yield a partial differential equation that lends itself to a similarity transformation. Following this similarity transformation, the original problem is reduced to solving a fourth-order differential equation in one similarity variable η that combines both space and time dimensions. Since two of the four auxiliary conditions are of the boundary value type, a numerical solution becomes dependent upon two initial guesses. In order to achieve convergence, the governing equation is first transformed into a function of three variables: The two guesses and η. At the outset, a suitable numerical algorithm is applied by solving the resulting set of twelve first-order ordinary differential equations with two unspecified start-up conditions. In seeking the two unknown initial guesses, the rapidly converging inverse Jacobian method is applied in an iterative fashion. Numerical results are later used to ascertain a deeper understanding of the flow character. The numerical scheme enables us to extend the solution range to physical settings not considered in previous studies. Moreover, the numerical approach broadens the scope to cover both suction and injection cases occurring with simultaneous wall motion.
Dynamics of ozone and OH radicals generated by pulsed corona discharge in humid-air flow reactor measured by laser spectroscopy
Authors: Ono Ryo , Oda Tetsuji
Published: May 2003

The dynamics of ozone and OH radicals are studied in pulsed corona discharge plasma in a humid-air environment. Ozone density is measured by the laser absorption method, and OH density is measured by the laser-induced fluorescence (LIF) method. A 100-ns pulsed corona discharge occurs between a series of 25 needle electrodes and a plate electrode. After the pulsed discharge, the time evolutions of ozone and OH densities are measured in humid air or a humid nitrogen-oxygen mixture. Results show that the addition of 2.4% water vapor to dry air reduces ozone production by a factor of about 6, and shortens the ozone formation time constant from 30 to 6 μs. Water vapor may reduce atomic oxygen levels leading to the decreased production of ozone by O+O2 reaction. The LIF measurement for OH radicals shows that OH density is approximately constant for 10 μs after the pulsed discharge, then decays by recombination reaction and reactions with the discharge products of oxygen, such as ozone or atomic oxygen. Absolute OH density is estimated; it is about 3×1015 cm−3 in streamers at 10 μs after discharge in the H2O(2.4%)/N2 mixture.
Mechanism of enhanced light output efficiency in InGaN-based microlight emitting diodes
Authors: Choi H. W. , Jeon C. W. , Dawson M. D. , Edwards P. R. , Martin R. W. , Tripathy S.
Published: May 2003

Micro-light emitting diode (LED) arrays with diameters of 4 to 20 μm have been fabricated and were found to be much more efficient light emitters compared to their broad-area counterparts, with up to five times enhancement in optical power densities. The possible mechanisms responsible for the improvement in performance were investigated. Strain relaxation in the microstructures as measured by Raman spectroscopy was not observed, arguing against theories of an increase in internal quantum efficiency due to a reduction of the piezoelectric field put forward by other groups. Optical microscope images show intense light emission at the periphery of the devices, as a result of light scattering off the etched sidewalls. This increases the extraction efficiency relative to broad area devices and boosts the forward optical output. In addition, spectra of the forward emitted light reveal the presence of resonant cavity modes [whispering gallery (WG) modes in particular] which appear to play a role in enhancing the optical output.
Trajectory-dependent cellularized frozen Gaussians, a new approach for semiclassical dynamics: Theory and application to He–naphtalene eigenvalues
Authors: Anderson Sybil M. , Neuhauser Daniel , Baer Roi
Published: May 2003

A semiclassical cellular method is proposed. Signals generated by semiclassical techniques generally deteriorate over time as trajectories become chaotic. One approach to remedy this problem has been to have each trajectory weighted by an entire cell of nearby trajectories (Filinov transform). But even in this approach the exponential part of the propagator typically becomes large and positive over time. Here the cellularization (Filinov) parameter is subject to constraints which make it time dependent and trajectory dependent. It also depends on dimensionality, so it ends up as a matrix. Physically, the Filinov transform is done differently in different directions associated with the stability matrix for the phase—essentially a more confined integration in directions where the matrix diverges and a wider integration in other directions. This squelches the contribution from any part of a trajectory that becomes excessively chaotic. A trajectory-dependent cellurized frozen Gaussian is applied here within the Herman–Kluk semiclassical approach. It is tested by looking at a single-particle three-dimensional problem, He attached to a rigid immovable naphtalene, where it is shown to be more accurate than the original HK approach, without the divergence of the correlation function common in the usual cellular dynamics (HK) formulation, and is able to separate a low-lying excited state from the ground state.
Single-pulse coherent anti-Stokes Raman spectroscopy in the fingerprint spectral region
Authors: Dudovich Nirit , Oron Dan , Silberberg Yaron
Published: May 2003

Quantum coherent control techniques are applied to achieve high spectral resolution nonlinear vibrational spectroscopy using a single ultrashort laser source. By controlling the spectral phase of ∼10 fs pulses, we are able to obtain detailed coherent anti-Stokes Raman (CARS) spectra in the important fingerprint spectral region, which reflects the structural chemical information. A full theoretical analysis and an experimental demonstration of two alternative schemes leading to spectral resolution two orders of magnitude better than the pulse bandwidth are presented. The first involves selective excitation of vibrational levels within the pulse bandwidth by periodic modulation of the spectral phase of the pulse. In the second scheme an effective narrow probing of the vibrational level has been achieved by phase shifting of a narrow spectral band. Single-pulse CARS offers an attractive alternative to conventional multibeam nonlinear vibrational spectroscopy techniques.
Structural study of the hydrogen-bonded 1-naphthol⋅(NH3)2 cluster
Authors: Tanner Christian , Henseler Debora , Leutwyler Samuel , Connell Leslie L. , Felker Peter M.
Published: May 2003

The structure of the 1-naphthol⋅(NH3)2 cluster was investigated by rotational coherence spectroscopy (RCS), mass selective one- and two-color resonant two-photon ionization (R2PI) experiments and ab initio calculations. RCS measurements yielded rotational constants of 1-naphthol⋅(NH3)2 as A=1197, B=500, and C=413 MHz, as well as those for several isotopomers. The counterpoise-corrected second-order Møller–Plesset perturbation theory (MP2) method predicts two isomers A and B. Both structures have hydrogen bonded naphthol–OH⋯NH3⋯NH3 chains, with the second NH3 bent above the proximal aromatic ring and pointing towards the π-electron system and have nearly the same binding energy. The experimental rotational constants agree better with those calculated for structure B. The B3LYP and PW91 density functional methods also predict two isomers A, B with the rotational constants of B in acceptable agreement with experiment. Based on two-color R2PI experiments using low ionization frequency to suppress cluster fragmentation, the S1←S0 electronic origin region of the 1-naphthol⋅(NH3)(2–4) cluster series was reassigned, in agreement with the work of Dedonder-Lardeux et al. [Phys. Chem. Chem. Phys. 3, 4316 (2001)]. In one-color experiments, the 1-naphthol⋅(NH3)3 cluster fragments with nearly 100% efficiency into the 1-naphthol⋅(NH3)2+ mass channel.
Ab initio potential-energy surfaces for the X̃ 2B1, Ã 2A1, and B̃ 2B2 states of the H2S+ molecular ion
Authors: Hirst David M.
Published: May 2003

Multireference configuration interaction calculations with the cc-pVQZ basis set are reported for the potential-energy surfaces of the X̃ 2B1, Ã 2A1, B̃ 2B2, 1 4A2, and 1 4B1 states of H2S+. Seams of intersection between the states have been characterized and the conical intersection between the Ã 2A1 and B̃ 2B2 states has been located. Optimized geometries and vertical and adiabatic ionization energies are in good agreement with the available experimental data. The X̃ 2B1 state is strongly bound with an equilibrium geometry very similar to that of ground-state H2S. The equilibrium bond angle for the Ã 2A1 state is 126.4°. The dissociation of this state to S+(4S)+H2 is interpreted in terms of an initial increase in bond angle on formation and a transition to the X̃ 2B1 surface at linearity. As the bond angle decreases the intersection with the 1 4A2 state is reached and a nonadiabatic transition to this state results in dissociation. It is suggested that the dissociation of the B̃ 2B2 state to SH+(3∑−)+H occurs via asymmetric stretching followed by a nonadiabatic transition to the dissociative 1 4A″ surface.
Density functional and Monte Carlo studies of sulfur. I. Structure and bonding in Sn rings and chains (n=2–18)
Authors: Jones R. O. , Ballone P.
Published: May 2003

Density functional calculations have been performed for ring isomers of sulfur with up to 18 atoms, and for chains with up to ten atoms. There are many isomers of both types, and the calculations predict the existence of new forms. Larger rings and chains are very flexible, with numerous local energy minima. Apart from a small, but consistent overestimate in the bond lengths, the results reproduce experimental structures where known. Calculations are also performed on the energy surfaces of S8 rings, on the interaction between a pair of such rings, and the reaction between one S8 ring and the triplet diradical S8 chain. The results for potential energies, vibrational frequencies, and reaction mechanisms in sulfur rings and chains provide essential ingredients for Monte Carlo simulations of the liquid–liquid phase transition. The results of these simulations will be presented in Part II.
Kinetics and chemomechanical properties of the F1-ATPase molecular motor
Authors: Liu Ming S. , Todd B. D. , Sadus Richard J.
Published: May 2003

F 1 - ATPase hydrolyzes ATP into ADP and Pi and converts chemical energy into mechanical rotation with exceptionally high efficiency. This energy-transducing molecular motor increasingly attracts interest for its unique cellular functions and promising application in nanobiotechnology. To better understand the chemomechanics of rotation and loading dynamics of F1-ATPase, we propose a computational model based on enzyme kinetics and Langevin dynamics. We show that the torsional energy and stepwise rotation can be regulated by a series of near-equilibrium reactions when nucleotides bind or unbind, as well as characterized by an effective “ratchet” drag coefficient and a fitting chemomechanic coefficient. For the case of driving an actin filament, the theoretical load-rotation profile is analyzed and comparison with experimental data indicates reasonable agreement. The chemomechanics described in this work is of fundamental importance to all ATP-fueled motor proteins.
Aperture shape effect on the performance of very small aperture lasers
Authors: Chen Fang , Stancil D. D. , Schlesinger T. E.
Published: May 2003

We report on the effect of different aperture shapes on the power output of a very small aperture laser (VSAL) fabricated from commercial edge-emitting laser diodes and correlate the results to the layer structure and polarization of the optical field. A waveguide theory is used to explain the experimental observations. We show that the shape of the aperture has a significant effect on the VSAL output power. In particular shapes exploiting the asymmetry of the laser can achieve much higher throughput over square apertures while keeping the aperture area constant. This work also indicates the validity of analyzing near-field small apertures as cutoff waveguide structures.
Spectroscopic characterization of chromium (IV, V, VI) in Cr:Li2MSiO4 (M=Mg,Zn)
Authors: Jousseaume C. , Vivien D. , Kahn-Harari A. , Derouet J. , Ribot F. , Villain F.
Published: May 2003

To understand the exceptionally long fluorescence lifetime of CrIV in Li2MSiO4 (M=Mg,Zn), several spectroscopic investigations are performed on Cr:Li2MSiO4 (M=Mg,Zn) crystals. X-ray absorption near-edge structure investigations attest that chromium is localized in tetrahedral sites and that CrVI is the major species while CrIV is the minor one in both compounds. Electron paramagnetic resonance studies confirm the occurrence of CrV in elongated tetrahedral environment (dx2−y2 ground state), corresponding probably to the silicon site, and suggest the presence of several charge compensation schemes. Fluorescence and fluorescence dynamics of CrIV:Li2MSiO4 are reported. The very long CrIV excited state lifetime previously observed on powders samples is confirmed for the crystals (117 μs at room temperature, 305 μs at 30 K for Cr:Li2MgSiO4) and is explained by combined contributions to the emission from both E1 level and the lowest component of the T23 level, in thermal equilibrium. Contrary to the situation found in most CrIV activated compounds, the E1 level lies below the T23 one.
Femtosecond laser ablation of polytetrafluoroethylene (Teflon) in ambient air
Authors: Wang Z. B. , Hong M. H. , Lu Y. F. , Wu D. J. , Lan B. , Chong T. C.
Published: May 2003

Teflon, polytetrafluorethylene (PTFE), is an important material in bioscience and medical application due to its special characteristics (bio-compatible, nonflammable, antiadhesive, and heat resistant). The advantages of ultrashort laser processing of Teflon include a minimal thermal penetration region and low processing temperatures, precision removal of material, and good-quality feature definition. In this paper, laser processing of PTFE in ambient air by a Ti:sapphire femtosecond laser (780 nm, 110 fs) is investigated. It is found that the pulse number on each irradiated surface area must be large enough for a clear edge definition and the ablated depth increases with the pulse number. The air ionization effect at high laser fluences not only degrades the ablated structures quality but also reduces the ablation efficiency. High quality microstructures are demonstrated with controlling laser fluence below a critical fluence to exclude the air ionization effect. The ablated microstructures show strong adhesion property to liquids and clear edges that are suitable for bio-implantation applications. Theoretical calculation is used to analyze the evolution of the ablated width and depth at various laser fluences.
Tunneling barrier in nanoparticle junctions of La2/3(Ca,Sr)1/3MnO3: Nonlinear current–voltage characteristics
Authors: Niebieskikwiat D. , Sánchez R. D. , Lamas D. G. , Caneiro A. , Hueso L. E. , Rivas J.
Published: May 2003

We study the nonlinear current–voltage (I–V) characteristics and analyze the voltage-dependent tunneling conductance in nanoparticles of La2/3A1/3MnO3 (A=Ca, Sr). The powders were prepared by different wet-chemical routes and low calcination temperatures were used to obtain an average particle size D≈30 nm. The data are comprehensively explained in terms of the tunneling picture, which allows one to estimate the height of the grain boundary insulating barrier (φ) for each sample. For constant D, our results show that the sample preparation route is mainly responsible for the value of φ in nanoparticles, while the Coulomb gap in the Coulomb blockade regime is ∼3 times higher for Sr- than for Ca-doping. We also show that a small fraction of the barriers contribute to the nonlinear transport, and the current is mainly carried through low-resistive percolated paths. In addition, despite the different barrier strengths, the low-field magnetoresistance (LFMR) is similar for all samples, implying that φ is not the fundamental parameter determining the LFMR.