bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical investigation of strain in Si-doped GaN films
Authors: Sánchez-Páramo J. , Calleja J. M. , Sánchez-Garcı́a M. A. , Calleja E.
Published: July 2002

The effects of Si doping on the growth mode and residual strain of GaN layers grown on Si(111) substrates by plasma-assisted molecular beam epitaxy are studied by Raman scattering and photoluminescence. As the Si concentration increases a progressive decrease of the high-energy E2 mode frequency is observed, together with a redshift of the excitonic emission. Both effects indicate an enhancement of the biaxial tensile strain of thermal origin for increasing doping level, which is confirmed by x-ray diffraction measurements. Beyond Si concentrations of 5×1018 cm−3 both the phonon frequency and the exciton emission energy increase again. This change indicates a partial strain relaxation due to a change in the growth mode.
Scattering of hyperthermal molecular ions from Pt(111)
Authors: Herrmann Gero , Okada Michio , Murata Yoshitada
Published: July 2002

Energy analysis of molecular ions (N2+ and CO+) scattered from Pt(111) was performed at an incident energy of 30 eV as a function of the scattering angle with a newly developed ion scattering apparatus. The kinetic energy of surviving molecular ions of N2+ and CO+ is quite different from that expected for the united-atom approximation of molecular ions with a mass of 28 and is higher than the estimated kinetic energy. The surviving molecular ions are scattered from the surface in a manner different from atomic ions. The angular distribution for both molecular ions is strongly shifted away from the specular direction toward the surface, as expected for the reactive ions. The small difference in the angular distribution between N2+ and CO+ can be understood as a result of the polarization of the heteronuclear molecular ion of CO+.
Microphase segregation in molten randomly grafted copolymers
Authors: Qi Shuyan , Chakraborty Arup K. , Balsara Nitash P.
Published: July 2002

We study microphase ordering of molten randomly grafted copolymers (RGCs) by using a mean field theory and the replica method to calculate the quenched average. Our results illustrate that in the weak segregation limit (WSI), the optimal wave vector q* of the lamellar phase formed by molten RGCs, has a temperature dependence different from either linear random copolymers (LRCs) or diblock copolymers (DCPs): when close, but below the microphase separation transition (MST) temperature, q* increases sharply with decreasing temperature; then q* gradually acquires an asymptotic value determined by the length of the branch and the average distance between branch points on the backbone. Our results are compared with recent experiments, and the effects of chain architecture on the microphase separation characteristics of RGCs are delineated. Our results suggest a new method for controlling the microphase spacing by exploiting quenched disorder.
Modeling the suppression of boron transient enhanced diffusion in silicon by substitutional carbon incorporation
Authors: Ngau Julie L. , Griffin Peter B. , Plummer James D.
Published: July 2002

Recent work has indicated that the suppression of boron transient enhanced diffusion (TED) in carbon-rich Si is caused by nonequilibrium Si point defect concentrations, specifically the undersaturation of Si self-interstitials, that result from the coupled out-diffusion of carbon interstitials via the kick-out and Frank–Turnbull reactions. This study of boron TED reduction in Si1−x−yGexCy during 750 °C inert anneals has revealed that the use of an additional reaction that further reduces the Si self-interstitial concentration is necessary to describe accurately the time evolved diffusion behavior of boron. In this article, we present a comprehensive model which includes {311} defects, boron-interstitial clusters, a carbon kick-out reaction, a carbon Frank–Turnbull reaction, and a carbon interstitial-carbon substitutional (CiCs) pairing reaction that successfully simulates carbon suppression of boron TED at 750 °C for anneal times ranging from 10 s to 60 min.
Hydrogen desorption property of mechanically prepared nanostructured graphite
Authors: Orimo S. , Matsushima T. , Fujii H. , Fukunaga T. , Majer G.
Published: July 2002

Two desorption peaks of hydrogen molecule (mass number=2), starting at about 600 and 950 K, respectively, are observed in thermal desorption mass spectroscopy of nanostructured graphite mechanically milled for 80 h under hydrogen atmosphere. It follows from a combined analysis of thermal desorption mass spectroscopy and thermogravimetry, that ∼6 mass % of hydrogen (corresponding to 80% of the total amount of hydrogen) is desorbed at the first desorption peak as a mixture of pure hydrogen and hydrocarbons. Below the temperature of the second desorption peak, at which recrystallization related desorption occurs, nanostructured graphite is expected to retain its specific defective structures mainly with carbon dangling bonds as suitable trapping sites for hydrogen storage. The formation process of the nanostructures during milling under hydrogen atmosphere is also discussed on the basis of the profile of Raman spectroscopy.
Adsorption isotherms of water in Li–, Na–, and K–montmorillonite by molecular simulation
Authors: Hensen E. J. M. , Tambach T. J. , Bliek A. , Smit B.
Published: July 2002

A biased Monte Carlo method for the insertion of water in dense clay–water systems is presented. The use of this algorithm results in a considerable increase of the success rate of insertion attempts. It allows us to compute water adsorption isotherms up to high water densities, where the conventional Monte Carlo scheme fails. The isotherms were calculated by a combination of molecular dynamics and grand-canonical Monte Carlo simulation for Li–, Na–, and K–montmorillonite at a fixed d(001) spacing of 12.0 Å. At low water pressure, the degree of clay hydration is governed by the type of counterion, Li–montmorillonite having the highest water content. Hydrogen bonding between water molecules is absent. Li+ and Na+ are small enough to be organized in two layers close to the clay mineral surfaces, whereas K+ is mainly located in the midplane. In both cases, the water molecules primarily reside in the midplane of the interlayer. Increasing the water pressure leads to water adsorption at higher energy sites closer to the surface, i.e., coordinating to the structural OH groups in the hexagonal cavities. A hydrogen bond network is formed in the clay interlayer. This points to water condensation and leads to a sharp increase in the clay water content.
Compact collimated vacuum ultraviolet diagnostics for localized impurity measurements in fusion boundary plasmas
Authors: Soukhanovskii V. A. , Stutman D. , Finkenthal M. , Moos H. W. , Kaita R. , Majeski R.
Published: July 2002

Compact vacuum ultraviolet diagnostics for impurity emission measurements in boundary plasmas of nuclear fusion plasma devices are described. The instruments are designed for monitoring intensities of resonant impurity lines between 300 and 2000 Å. The intensities are used to infer basic yet important plasma parameters, such as density of impurity charge states, radiated power, or electron temperature and density estimates. All utilized components and materials satisfy ultrahigh vacuum and high bake-out temperature requirements, enabling the instruments to qualify for vacuum port or in-vessel placement, in close proximity to emitting plasmas. The instruments have high spatial (Δl⩽1 cm) and temporal (Δτ⩽100 μs) resolution. The spectral resolution is Δλ⩽20 Å. Planar diffraction gratings at near-normal incidence are used for dispersion of incident radiation, collimated by high precision mechanical stacked grid collimators. Highly localized field of view, adequate throughput, and compactness distinguish these diagnostics from conventional slit instruments. A prototype monochromator for λ=1550 Å has been built, evaluated, radiometrically calibrated, and used on the CDX-U spherical torus for monitoring C IV emission in ohmic and high harmonic fast wave heated plasmas with Te(0)⩽100 eV. An attractive mechanical collimator based Wadsworth mount spectrometer concept is presented and its application to impurity content and transport measurements in tokamaks is discussed.
Hydrogen passivation of nitrogen acceptors confined in CdZnTe quantum well structures
Authors: Jacob A. P. , Zhao Q. X. , Willander M. , Baron T. , Magnea N.
Published: July 2002

The deactivation of nitrogen acceptors confined in Cd0.96Zn0.04Te/Cd0.86Zn0.14Te quantum well structures by hydrogen (deuterium) have been investigated by optical spectroscopy. Hydrogen (deuterium) was incorporated into the samples by annealing them in an atmosphere of hydrogen and cadmium. The annealing temperature and annealing time were varied to determine the optimum condition at which the maximum passivation is achieved without causing structural degradation. The emissions related to nitrogen acceptors were monitored in low-temperature photoluminescence measurements in order to deduce the passivation effect. The results indicate that hydrogen can effectively neutralize the nitrogen acceptors in cadmium zinc telluride quantum well structures. It is estimated that as much as 90% of the nitrogen acceptors can be passivated by this method.
Forward scattering in the H+D2→HD+D reaction: Comparison between experiment and theoretical predictions
Authors: Fernández-Alonso Félix , Bean Brian D. , Zare Richard N. , Aoiz F. J. , Bañares Luis , Castillo Jesús F.
Published: July 2002

We investigate the sensitivity of photoinitiated experiments to forward-scattering features by direct comparison of experimental angular distributions with quantum-mechanical calculations as well as by forward-convolution of theoretical and model center-of-mass differential cross sections. We find that the experimental sensitivity to forward-scattering angles depends on the instrumental velocity resolution as well as on the kinematics of the detected product channel. Explicit comparison is made between experimental HD(v′=1,2;j′) center-of-mass angular distributions at collision energies ≈1.6 eV (deduced from time-of-flight profiles using a single-laser, photolysis-probe approach) and quantum-mechanical calculations on the BKMP2 potential energy surface. The comparison takes into account the contributions from both slow and fast H atoms from the photolysis of HBr. We find that the contribution of the slow H atoms, which is the major source of experimental uncertainty, does not greatly affect the extraction of the angular distribution from the experimental time-of-flight profile for a specific HD(v′,j′) state. Except for HD(v′=1, j′=8) and HD(v′=2, j′=0), for which either slow H atoms or the presence of a narrow forward-scattering peak make the analysis more uncertain, the agreement between experiment and theoretical predictions is excellent.
Design strategies for pulse sequences in multidimensional optical spectroscopies
Authors: Scheurer C. , Mukamel S.
Published: July 2002

A unified description of resonant multiple-pulse experiments in coupled spin-12 systems in NMR spectroscopy and two-level systems in optical spectroscopy is presented. The connection between the NMR product operator formalism and the Liouville space pathways in optical spectroscopy is established. We show how the information obtained in various strong field two and three pulse NMR experiments can be extracted by combining heterodyne detected phase-controlled weak field signals generated at different directions. These results allow the design of sequences of weak optical pulses that accomplish the same goals as strong field multidimensional NMR spectroscopy.
Coronary angiography using laser plasma sources: X-ray source efficiency and optimization of a bent crystal monochromator
Authors: Andersson E. , Hölzer G. , Förster E. , Grätz M. , Kiernan L. , Sjögren A. , Svanberg S.
Published: July 2002

A monochromator has been developed for coronary angiography, comprising a single bent crystal of silicon in Laue transmission geometry. K spectra of laser irradiated solid tin and tantalum (Z=50 and 73, respectively) targets were measured. The high resolution crystal spectrometer resolve the Sn and Ta Kα doublets, allowing in a proof-of-principle experiment the absolute Kα photon numbers emitted by the source to be determined. The Ta Kα yield is measured as a function of the laser pulse energy, allowing an assessment to be made of the suitability of such sources for medical applications.
The F19–H1 coupling constants transmitted through covalent, hydrogen bond, and van der Waals interactions
Authors: Pecul Magdalena , Sadlej Joanna , Leszczynski Jerzy
Published: July 2002

The F19–H1 coupling constants were calculated on the multiconfiguration self-consistent field (MCSCF) level in several systems, ranging from covalently bonded HF, hydrogen bonded FHF− and (HF)2 complexes to weak van der Waals complex CH4–HF. The sign of the F19–H1 coupling varies in this sequence, and its absolute value decreases. Still, it is sizable even for CH4–HF. The distance dependence of F19–H1 coupling is essentially the same in all systems under study, and the calculations for FHF− with distorted geometry suggest that the value of F19–H1 coupling is determined mainly by molecular geometry. F19–F19 coupling constants were also analyzed. F19–F19 intermolecular coupling in (HF)2 is substantial but has the opposite sign to that in FHF− and its counterpart in (H2O)2.
Neoclassical poloidal plasma rotation in tokamaks during radio frequency heating
Authors: Liu Caigen , Weyssow Boris
Published: July 2002

A neoclassical theory of poloidal rotation in tokamaks is developed for rf-heated plasmas. It is based on a moment expansion but, in contrast to the usual neoclassical theory, which takes into account the strong deformation of the local equilibrium distribution function due to rf heating. This distribution function is the solution, in the banana regime, of the drift kinetic equation, which includes both collisional diffusion and quasilinear rf diffusion operators. The set of closed, self-consistent transport and rotation equations is obtained in a form similar to its usual neoclassical counterpart. The parallel components of the generalized stress tensors are calculated. This allows us to solve the transport equations for the poloidal rotation that is obtained in explicit form. Modifications of the poloidal rotation due to the rf heating are also discussed.
Electroluminescence characterization of AlGaN/GaN high-electron-mobility transistors
Authors: Shigekawa Naoteru , Shiojima Kenji , Suemitsu Tetsuya
Published: July 2002

Spectral analysis of the electroluminesence (EL) of AlGaN/GaN high-electron-mobility transistors is reported. The shape of the EL spectra is completely different from the shape of the photoluminescence spectrum. The wavelength for the peak of the EL spectrum gets shorter when the gate–bias voltage is decreased. Its intensity shows a bell shape when the gate-bias voltage is swept. These features suggest that the EL signal is due to the intraband transition of the channel electrons in the high-field region at the drain edge.
Stabilization of the resistive wall mode by flowing metal walls
Authors: Umansky M. V. , Betti R. , Freidberg J. P.
Published: July 2002

The effect of flowing metal walls on the resistive wall instabilities is analyzed for a general cylindrically symmetric diffusive pinch configuration. Two types of liquid metal flow are analyzed: a uniform flow which is poloidally symmetric, and a two-stream flow consisting of two opposite streams splitting at the top and merging at the bottom. It is found in both configurations that when the liquid wall flow velocity exceeds a critical value, the resistive wall mode is stabilized. However, for the two-stream flow the critical velocity is several times smaller than that for the uniform flow. Still in a realistic experiment one needs a flow velocity of a few tens m/s to stabilize the resistive wall mode.
Methylation effects in state resolved quenching of highly vibrationally excited azabenzenes (Evib∼38 500 cm−1). I. Collisions with water
Authors: Elioff Michael S. , Fang Maosen , Mullin Amy S.
Published: July 2002

To investigate the role of molecular structure in collisions that quench highly vibrationally excited molecules, we have performed state resolved transient infrared absorption studies of energy gain in a number of rotational levels of H2O(000) resulting from collisions of water with vibrationally excited 2-methylpyridine (2-picoline) and 2,6-dimethylpyridine (2,6-lutidine) in a low-pressure gas-phase environment at 298 K. Vibrationally excited methylpyridines were prepared with ∼38 500 cm−1 of internal energy using 266 nm ultraviolet excitation to an S1 electronic state followed by rapid radiationless decay to the S0 electronic state. Collisions that populate rotationally excited states of H2O(000) were investigated with infrared absorption by monitoring the appearance of individual rotational states of H2O(000) with energies between 1000 and 2000 cm−1. Rotational state distributions for recoiling water molecules were characterized by Boltzmann temperatures of Trot=590±90 K for quenching of hot picoline and Trot=490±80 K for lutidine quenching. Doppler-broadened transient absorption line profiles show that the scattered H2O(000) molecules have laboratory-frame translational energy distributions corresponding to Ttrans≈600 K for deactivation of picoline and Ttrans≈590 K for lutidine. Energy transfer rate constant measurements indicate that rotational excitation of H2O(000) with Evib>1000 cm−1 occurs for one in 31 picoline/water collisions and one in 17 lutidine/water collisions. Comparison with earlier quenching studies on pyrazine [M. Fraelich, M. S. Elioff, and A. S. Mullin, J. Phys. Chem. 102, 9761 (1998)] and pyridine [M. S. Elioff, M. Fraelich, R. L. Sansom, and A. S. Mullin, J. Chem. Phys. 111, 3517 (1999)] indicate that, for the same initial internal energy in the hot donor, the extent of rotational excitation in water is diminished as the number of vibrational modes in the donor increases. The energy transfer probability for this pathway exhibits opposite behavior, with the larger donor molecules being more likely to excite the high energy rotations in water. These results are interpreted using a statistical description of the high energy donors and highlight the role of low frequency vibrational modes in the vibrationally hot donor molecules. A Fermi’s golden rule approach is successful at explaining differences in the observed scattering dynamics for the various donor molecules.
Shubnikov–de Haas oscillations in layered conductors with stacking faults
Authors: Gvozdikov V. M.
Published: July 2002

The Shubnikov–de Haas (SdH) oscillations of the in-plane conductivity of layered 2D electron gas are calculated. It is shown that layer stacking faults, magnetoimpurity bound states, and electron scattering modulate the SdH oscillations via the specific factors which bear the structural information. At zero temperature the 2D SdH oscillations are strongly nonsinusoidal in shape and related by a simple equation to the derivative of the de Haas–van Alphen magnetization oscillations with respect to the magnetic field.
Giant magnetooscillations of the Josephson current
Authors: Gor’kov L. P. , Kresin V. Z.
Published: July 2002

Josephson current flowing through an antiferromagnetic metallic barrier is studied. We consider the most interesting case of the spin-valve structure when the barrier is formed by ferromagnetic layers ordered antiferromagnetically in the direction perpendicular to the current. The most remarkable feature is the onset of giant magnetooscillations of the current amplitude.
Quantum Hall effect in nonuniform two-dimensional electron systems
Authors: Shikin V. B.
Published: July 2002

The quantum Hall effect (QHE) is inherent to uniform two-dimensional (2D) electron systems. At the same time, the basic low-dimensional conducting systems in the presence of metallic contacts are “regularly nonuniform.” Nevertheless, the QHE is well observed in them. The apparent paradox is resolved in a nontrivial manner. Regularly nonuniform 2D systems under conditions of the QHE admit the existence of integer (incompressible) channels that determine the conducting properties of the 2D system. To describe such a modification of the QHE requires a special formalism that explicitly takes into account the properties of the incompressible strips. The goal of this paper is to discuss the causes for the onset of regular nonuniformity of the 2D electron density in typical low-dimensional systems and to determine the basic characteristics of the QHE for such systems in terms pertaining to the individual incompressible strips.
Spin-orbit branching in the collision-induced dissociation reaction of He(1S)+HF(X 1Σ+)→He(1S)+H(2S1/2)+F(2P3/2,1/2)
Authors: Takayanagi Toshiyuki , Wada Akira
Published: July 2002

The collision-induced dissociation process, He(1S0)+HF(X 1Σ0+)→He(1S0)+H(2S1/2)+F(2P3/2,1/2), has been studied using a three-dimensional semiclassical coupled wave packet method with a focus on electronically nonadiabatic transitions induced by spin-orbit interaction. The radial motion is described by classical mechanics and the other degrees of freedom are treated quantum mechanically. The diabatic potential energy surfaces have been constructed with the diatomics-in-molecule approach. The diatomic potential energy curves for the ground and excited states are obtained from the ab initio electronic structure calculations at the multireference configuration interaction level. The calculated spin-orbit branching fraction depends only weakly on the initial vibrational quantum number of HF and on the total angular momentum. This implies that the branching fraction is determined mainly in the large-rHF regions where the spin-orbit interaction of the F atom is dominant. We also found that the electronic anisotropy interaction between the He atom and the F atom in HF plays a less important role in collision-induced dissociation processes.
CH
2
I
2
fundamental vibrational relaxation in solution studied by transient electronic absorption spectroscopy
Authors: Cheatum Christopher M. , Heckscher Max M. , Bingemann Dieter , Crim F. Fleming
Published: July 2002

Wavelength dependent, transient, electronic absorption spectroscopy of methylene iodide (CH2I2) in CCl4, CDCl3, C6D6, and (CD3)2CO following excitation of the fundamental C–H stretching vibration reveals the time scales of intramolecular vibrational energy redistribution and energy transfer to the solvent. In contrast to the case for overtone excitation, state-specific relaxation to one or a few states that are coupled by low order interactions with the initially prepared state dominates the intramolecular vibrational energy redistribution. This mechanism is consistent with previous infrared pump–probe measurements of CH2I2 fundamental relaxation as are the measured relaxation time scales. We also find a previously unobserved relaxation pathway through weakly-coupled states that have several quanta of excitation in the Franck–Condon active modes, primarily C–I stretch and bend. Although this statistical component is a minor channel in the relaxation, it is the only contribution to the signal at the longest probe wavelengths in CCl4 and CDCl3. Time scales for both intramolecular energy redistribution and intermolecular relaxation to the solvent depend strongly on the strength of interaction with the solvent.
Effective electron mobility in Si inversion layers in metal–oxide–semiconductor systems with a high-κ insulator: The role of remote phonon scattering
Authors: Fischetti Massimo V. , Neumayer Deborah A. , Cartier Eduard A.
Published: July 2002

The high dielectric constant of insulators currently investigated as alternatives to SiO2 in metal–oxide–semiconductor structures is due to their large ionic polarizability. This is usually accompanied by the presence of soft optical phonons. We show that the long-range dipole field associated with the interface excitations resulting from these modes and from their coupling with surface plasmons, while small in the case of SiO2, for most high-κ materials causes a reduction of the effective electron mobility in the inversion layer of the Si substrate. We study the dispersion of the interfacial coupled phonon-plasmon modes, their electron-scattering strength, and their effect on the electron mobility for Si-gate structures employing films of SiO2, Al2O3, AlN, ZrO2, HfO2, and ZrSiO4 for “SiO2-equivalent” thicknesses ranging from 5 to 0.5 nm.
Effect of the magnetic-field orientation on the modulation period of the critical current of ramp-type Josephson junctions
Authors: Heinsohn J.-K. , Dittmann R. , Contreras J. Rodrı́guez , Goldobin E. , Klushin A. M. , Siegel M. , Hagedorn D. , Pöpel R. , Dolata R. , Buchholz F.-Im. , Niemeyer J.
Published: July 2002

We have investigated the dependence of the critical current IC on the value and orientation of an externally applied magnetic field H for interface-engineered YBa2Cu3O7−x ramp-type Josephson junctions. The results are compared with measurements of Nb ramp-type junctions with a PdAu interlayer. The IC versus H dependences are similar to Fraunhofer patterns and their modulation period changes several orders of magnitude with the orientation of the magnetic field. For both junction types, the dependence of the modulation period on the orientation of the magnetic field can be well described by the change of the relevant projection of the junction area and the influence of flux-focusing. Therefore the features of the IC(H) curves have to be attributed to the ramp geometry and not to specific properties of the superconducting material. © 2001 American Institute of Physics.
Atomic force microscopy inspection of the early state of formation of polymer surface relief gratings
Authors: Henneberg O. , Geue Th. , Saphiannikova M. , Pietsch U. , Chi L. F. , Rochon P. , Natansohn A. L.
Published: July 2002

The process of surface relief grating formation was inspected by atomic force microscopy after short-pulse exposure with counter-rotating circularly polarized laser light of 488 nm on a polymer film containing an azobenzene side-chain homopolymer (pDR1M, TG=129 °C). During light inscription, the grating formation was probed by time-resolved visible scattering with red laser light. The efficiency of grating formation depends on the pulse length of blue light exposure. The shortest pulse length of 2 s did not create a permanent surface relief. After 5 s, a speckled surface modification starts rising and the surface relief becomes more and more uniform with a sinusoidal shape for longer exposure. The experimental findings reveal the individual addressing of azobenzene side groups by the actinic light providing a local lateral force via molecular trans-cis and cis-trans isomerization which subsequently causes grating formation.
Isotope effects and the manifestation of 2D phase transitions in the kinetics of low-temperature (down to 5 K) hydrogen adsorption
Authors: Osovskiı̆ V. D. , Ptushinskiı̆ Yu. G. , Sukretnyi V. G. , Chuı̆kov B. A.
Published: July 2002

The adsorption–desorption characteristics of hydrogen isotopes on the (110) surfaces of W and Mo are investigated at a temperature Ts∼5 K. It is found that the characteristics for hydrogen and deuterium are very different. Isothermal desorption from W(110) at Ts∼5 K is observed in the case of H2, while that of D2 is practically absent. The desorption of H2 is suppressed as the flux of molecules to the surface increases; this, in the opinion of the authors, is a consequence of phase transition from a 2D gas to a 2D condensate. The initial sticking probability of H2 on W(110) depends strongly on the flux, while that of D2 is nearly independent of flux; in the H2 case this is explained by the formation of nuclei of the 2D condensed phase during the lifetime of the molecule in the precursor state. In the case of H2 adsorption on Mo(110) a sharp and deep minimum is observed in the dependence of the sticking probability on the coverage, which is absent in the case of D2. These isotope effects are explained by the deeper position of the levels of the zero-point vibrations of the heavier molecule D2.
Mathematical model of a nanopaddle oscillator
Authors: Dowell Earl H. , Tang Deman
Published: July 2002

A model of a nanopaddle oscillator is developed to determine the resonant frequency (or the modulus of elasticity by inference from a measurement of the resonant frequency). Also modeled is the nonlinear stiffness of the oscillator and its nonlinear response. The approach is based upon a modal expansion of the elastic deformation of the oscillator and a Lagrangian representation. A comparison with recent work by other investigators is made and it appears the present model may help resolve a question about the modulus of elasticity to be inferred from such models.
Orthogonal-to-unitary ensemble crossover in the electronic specific heat of metal nanoclusters
Authors: Mukhin S. I. , Kurapova O. V. , Moiseev I. I. , Vargaftik M. N. , Volokitin Y. E. , Mettes F. L. , de Jongh L. J.
Published: July 2002

We present a theoretical and experimental study on the influence of a magnetic, field on the energy-level statistics in metal nanoparticles. Based on the random-matrix theory, a gradual field-induced crossover behavior is predicted from the orthogonal to the unitary ensemble. Experimental data of the electronic specific heat of metal nanoparticles for different field sin the quantum size temperature regime compare favorably with these theoretical (analytical) predictions.
Approach to band gap alignment in confined semiconductors
Authors: Goldbach Andreas , Saboungi Marie-Louise , Iton Lennox E. , Price David L.
Published: July 2002

Se has been sorbed into the pore system of Cu2+-ion exchanged Y zeolite to investigate the effect of transition metal countercations on the structure and electronic properties of the incorporated semiconductor. Results from anomalous x-ray scattering experiments at the Se K-absorption edge and Raman measurements show the formation of isolated, strongly disordered Sex chains (x→∞) with an intrachain bond length of 2.39 Å. The difference pair correlation function TSe(r) exhibits a short-range pair correlation at 3.30 Å, which is attributed to specific Cu2+-Sex interactions. The optical spectrum of the composite is characterized by a strong absorption band around 400 nm with a shoulder at 450 nm and an optical band gap of 2.09 eV. The strong Cu2+-Sex correlation is explained by charge transfer from the Sex chains to the cations, and the formation of band gap states is postulated to account for the unusually long intrachain bond length and the remarkably small band gap.
Zonal and streamer structures in magnetic-curvature-driven Rayleigh–Taylor instability
Authors: Das Amita , Sen Abhijit , Mahajan Sangeeta , Kaw Predhiman
Published: July 2002

A detailed numerical investigation of the nonlinear evolution of the magnetic curvature-driven Rayleigh–Taylor instability is carried out. The evolution is found to follow one of two distinct paths—that leading to a saturated zonal flow pattern or growing streamer structures. For a fixed value of the density gradient and the magnetic field gradient, the parametric regimes for the occurrence of these two states is delineated in the form of a phase diagram in the D−μ space, where D is the diffusion coefficient and μ is the viscosity parameter. The bifurcation behavior is explained on the basis of a reduced quasilinear model and the final saturated state of zonal flows is understood from a phenomenological zero-dimensional model.
Internal rotation in high-resolution ultraviolet spectra. II. Spectrum and structure of the aniline–nitrogen van der Waals complex
Authors: Schäfer Martin , Pratt David W.
Published: July 2002

Rotationally resolved S1←S0 electronic spectra of the nitrogen complex of aniline have been observed. The spectra are split into two subbands due to internal rotation of N2. The analysis of the rotational constants reveals that N2 is located above the ring plane of aniline and in the symmetry plane of aniline in the equilibrium position. Barriers hindering internal rotation have been obtained from fitting experimental transitions frequencies using a semirigid C2v top–Cs frame internal rotation model. Upon excitation into S1, the distance of N2 to the ring decreases and the internal rotation barrier increases by a factor larger than 2. Possible reasons for this behavior are discussed.
Isomers of SNO2: Production and infrared spectra of cis- and trans-OSNO from irradiated inert matrices containing OCS and NO2
Authors: Bahou Mohammed , Lee Yuan-Pern
Published: July 2002

New species cis- and trans-OSNO, designated c-OSNO and t-OSNO, respectively, are produced and identified with infrared absorption spectra when an argon or nitrogen matrix containing OCS and NO2 is irradiated with laser emission at 248 nm. Lines at 1156.1 and 1454.4 cm−1 are assigned to c-OSNO and those at 1178.0 and 1459.0 cm−1 are assigned to t-OSNO in solid N2. Lines at 1154.9 and 1450.8 cm−1 are assigned to c-OSNO and those at 1181.2 and 1456.0 cm−1 are assigned to t-OSNO in solid Ar; further lines associated with minor matrix sites are identified. Assignments of spectral lines are based on results of both experiments with N15- and O18-isotopic substitution and theoretical calculations using density-functional theories, B3LYP with an aug-cc-pVTZ basis set; these calculations predict the geometry, energy, vibrational frequencies, and infrared intensities of SNO2 as four isomers: C2v-SNO2, t-SONO, t-OSNO, and c-OSNO, in increasing order of stability. Mechanisms are proposed to rationalize that c-OSNO and t-OSNO, rather than t-SONO or C2v-SNO2, are produced from irradiated matrices containing OCS and NO2, and that no reaction product is observed in an Ar matrix containing CS2 and NO2 after irradiation at 193 nm.
Crystallization and glass formation processes in methylcyclohexane: Vibrational dynamics as a possible molecular indicator of the liquid–glass transition
Authors: Abramczyk H. , Paradowska-Moszkowska K.
Published: July 2002

We present the result of Raman studies on vibrational dynamics and static properties of the CH2 rocking mode of methylcyclohexane as a function of temperature and cooling rate. We have found that vibrational dynamics as well as the static properties are very sensitive indicators to specify phases and phase transitions at the molecular level. It was found that methylcyclohexane in the undercooled liquid phase may form distinct thermodynamic states that strongly depend on the quenching rate. We have identified the characteristic temperature T* that is quenching rate dependent which is the no-return-point between crystallization and amorphization. Below this temperature the undercooled methylcyclohexane exists in either the liquid state that is out of equilibrium and represents nonergodic behavior that leads to glass forming or in the metastable equilibrium ergodic state that leads to crystallization.
The application of a vacuum ultraviolet Fourier transform spectrometer and synchrotron radiation source to measurements of: IV. The β(6,0) and γ(3,0) bands of NO
Authors: Cheung A. S-C. , Lo D. H-Y. , Leung K. W-S. , Yoshino K. , Thorne A. P. , Murray J. E. , Ito K. , Matsui T. , Imajo T.
Published: July 2002

The β(6,0) (B 2Πr–X 2Πr) and γ(3,0) (A 2Σ+–X 2Πr) bands of NO have been recorded using a vacuum ultraviolet Fourier transform spectrometer with synchrotron radiation as light source. The analysis of the β(6,0) and γ(3,0) bands of NO provides accurate rotational line positions and term values. Molecular constants of the v=6 level of the B 2Πr and v=3 level of the A 2Σ+ have been determined. Accurate rotational line strengths have also been obtained. The band oscillator strength of the β(6,0) and γ(3,0) bands are determined to be 0.48×10−4 and 2.69×10−4, respectively.
Modeling of laser-induced avalanche in dielectrics
Authors: Vatsya S. R. , Nikumb S. K.
Published: July 2002

A study of the dependence of the laser-induced avalanche behavior on the intensity of irradiation in dielectrics is carried out. The avalanche rate is computed directly from the reduced equations, instead of the time dependent kinetic equations, thereby requiring less computational effort. For comparison, the calculations were carried out for both the flux-doubling model and the linear Fokker–Planck type equation for fused silica material. The flux-doubling model overestimates the avalanche rate by about 8%–10%. The properties of the equations describing the avalanche behavior indicate a nonlinear dependence of the avalanche rate on the intensity. The results for fused silica show an almost linear dependence in the high intensity range from about 1 TW/cm2 to infinity, and in the low intensity range up to about 0.01 TW/cm2, but a significant departure from linearity at the intermediate intensities. For a Gaussian pulse, the exponential growth rate of the electron density distribution in the avalanche regime is still almost directly proportional to the instantaneous fluence. Although the proportionality constant increases with increasing peak intensity, the difference is negligible in the high and low intensity ranges.
Backscattering in carbon nanotubes: Role of quantum interference effects
Authors: Roche Stephan , Triozon François , Rubio Angel
Published: July 2002

The backscattering contribution to the conductivity, irrelevant for metallic single-walled carbon nanotubes, is proved to become more significant for doped semiconducting systems, in agreement with experiments. In the case of multiwalled nanotubes, the intershell coupling is further shown to enhance the contribution of backscattering for “metallic” double-walled, whereas it remains insignificant for “metallic/semiconducting” double-walled systems.
B–H
δ−
σ
bond as dihydrogen bond acceptor: Some theoretical observations and predictions
Authors: Li Jinshan , Zhao Feng , Jing Fuqian
Published: July 2002

BH 3 NH 3 dimer and BH3NH3 complexes of methane, hydrogen cyanide, ammonia, water, methanol, and hydrogen fluoride, are studied using the Møller–Plesset second-order perturbation theory and the 6-31++G** basis set to understand the features of dihydrogen bond. Complex binding energy is corrected for the basis set superposition error with the counterpoise method and for the zero point energy. Natural bond orbital analysis is used to discuss the charge transfer. Computed results indicate that dihydrogen bond does not occur in both BH3NH3…CH4 and BH3NH3…NH3 complexes. Apart from the B–Hδ−…H–N hydrogen bond (H bond) found previously in the BH3NH3 crystal, the B–H…H–X (X=C,O,F) H bonds have been observed in BH3NH3…HCN, BH3NH3…H2O, BH3NH3…CH3OH, and BH3NH3…HF complexes. As for the complexes in which only dihydrogen bonds appear the strength of dihydrogen bonds ranges from −13.9 to −20.5 kJ/mol. While the formation of BH3NH3 complexes, the weakest B–N bond of BH3NH3 contracts (its stretch appears blueshifted), and both B–H and X–H bonds in the B–H…H–X H bonds elongate slightly owing to the charge transfer from σ(B–H) to σ*(X–H), and the B–H…H tends to be bent. The charge transfer is generally proportional to the second-order perturbation energy lowering (ΔE2) due to the interaction of frontier orbitals in σ→σ* and n→σ* cases, and the threshold value (1.5 kcal/mol) of ΔE2 as one of the indices for judging the existence of dihydrogen bonds is recommended.
Electric birefringence of dilute aqueous solutions of poly(p-phenylene) polyelectrolytes
Authors: Lachenmayer Katja , Oppermann Wilhelm
Published: July 2002

Dilute aqueous solutions of stiff-chain polyelectrolytes having a poly(p-phenylene) (PPP) backbone were studied by means of electric birefringence and some supplementary conductivity measurements. The PPP polyelectrolytes form molecular solutions, whose electrically induced birefringence rises with field strength according to Kerr’s law. From an estimate of the optical anisotropy of the chain backbone, it becomes clear that the anisotropy of the ionic polarizability responsible for the orientation in an electric field is about 5–6 orders of magnitude larger than that of the electronic (optical) polarizability. It increases markedly with rising molecular weight (rod- length) of the polyelectrolyte. The anisotropy of the electric polarizability seems to be due to the ion cloud and not the condensed ions. This interpretation is obtained from a comparison of the changes of electric birefringence and electric conductivity upon addition of a low molecular weight electrolyte.
Nonlocality and optics of inhomogeneous systems: The role of quantum induction
Authors: Wijers C. M. J. , de Boeij P. L.
Published: July 2002

Nonlocal interactions play a prominent role in the optics of inhomogeneous systems. Classical discrete dipole descriptions take into account only electro-magnetic nonlocality. This is insufficient to describe correctly the inhomogeneous optical response (e.g., reflectance anisotropy) for covalently bonded systems like semiconductor surfaces. For those systems also a prominent quantum mechanical nonlocality exists. In a cellular description this can be understood easily from the behavior of the wave function. For strongly bonded systems the wave function extends across cell boundaries and when cells are polarized, neighboring cells get polarized as well. This quantum induction introduces nonlocal polarizabilities in the description. The technical details how discrete dipole models have to be adapted to use nonlocal polarizabilities in finite systems and crystalline slabs and surfaces are given in this paper. The modified method is called discrete cellular method.
Multiple surface long-range interaction potentials between C (3Pj) and closed-shell molecules
Authors: Bettens Ryan P. A. , Collins Michael A.
Published: July 2002

A method for calculating multiple long-range interaction potentials between an atom (C in this particular case), and closed-shell molecules is described. This method was recently utilized by Bettens and Collins [J. Chem. Phys. 114, 10342 (2001)] for determining capture rate coefficients for collisions of C (3Pj) and Ge (1S0) with unsaturated hydrocarbons. The method takes account of the physical size of the molecule and exactly treats the electrostatic interaction between the quadrupole of the atom and all electrostatic moments of the molecule. This method requires a molecular density matrix, which can be obtained from an ab initio calculation.
Initiation of long, free-standing z discharges by CO2 laser gas heating
Authors: Niemann C. , Tauschwitz A. , Penache D. , Neff S. , Knobloch R. , Birkner R. , Presura R. , Hoffmann D. H. H. , Yu S. S. , Sharp W. M.
Published: July 2002

High current discharge channels can neutralize both current and space charge of very intense ion beams. Therefore, they are considered an interesting solution for final focus and beam transport in a heavy ion beam fusion reactor. At the Gesellschaft fuer Schwerionenforschung accelerator facility, 50 cm long, free-standing discharge channels were created in a 60 cm diameter metallic chamber. Discharges with currents of 45 kA in 2 to 25 mbar ammonia (NH3) gas are initiated by a CO2 laser pulse along the channel axis before the capacitor bank is triggered. Resonant absorption of the laser, tuned to the v2 vibration of the ammonia molecule, causes strong gas heating. Subsequent expansion and rarefaction of the gas prepare the conditions for a stable discharge to fulfill the requirements for ion beam transport. The influence of an electric prepulse on the high current discharge was investigated. This article describes the laser–gas interaction and the discharge initiation mechanism. We found that channels are magnetohydrodynamic stable up to currents of 45 kA, measured by fast shutter and streak imaging techniques. The rarefaction of the laser heated gas is studied by means of a one-dimensional Lagrangian fluid code (CYCLOPS) and is identified as the dominant initiation mechanism of the discharge.
Effect of electron irradiation on the transformation characteristics of narrow hysteresis TiNiCu shape memory alloys
Authors: Zu X. T. , Wang L. M. , Huo Y. , Lin L. B. , Wang Z. G. , Lu T. C. , Liu L. J. , Feng X. D.
Published: July 2002

TiNiCu shape memory alloy samples were irradiated by 1.7 MeV electrons below the martensite finish temperature Mf. The transformation temperatures and the latent heat of phase transformation were measured by differential scanning calorimeter. The damage accumulation was determined by positron annihilation technology. The results indicated that the austenite transformation temperatures were raised, and the hysteresis was increased by the irradiation. The electron irradiation had a slight effect on Mf, and no detectable effect on the martensitic transformation start temperature Ms. The second lifetime of positrons were increased by the electron irradiation indicating the increase in the size and amount of vacancy clusters, which contributed to the observed change of the transformation characteristics.
The velocity distribution of the liquid sheet formed by two low-speed impinging jets
Authors: Choo Y.-J. , Kang B.-S.
Published: July 2002

In this study, the local velocities of the liquid sheet formed by two low-speed impinging jets were directly measured using a LDV (laser Doppler velocimetry) instrument. The spatial distribution of the local sheet velocities at the forward part of the sheet and the effects of the impingement angle and jet velocity on the sheet velocities were examined. A parabolic velocity profile across the pre-impinging jet greatly affected the sheet velocity distribution, causing the sheet velocities to decrease while the azimuthal angle increased. This was attributed to the movement of a high velocity core, existing in the pre-impinging jets, to the region around the axis of the sheet. In the radial direction, the local sheet velocities were a little low around the impinging point, and remained constant, but then decreased near the edge of the sheet. The dependence of sheet velocities on the impingement angle was confirmed, and the tendency towards an even distribution of sheet velocities was found to increase as the impingement angle increased. With the exception of the lowest velocities measured near the edge of the sheet, the local sheet velocities at the forward part of the sheet were higher than the mean jet velocity.
Wavelength selective excitation of surface oxygen anions on highly dispersed MgO
Authors: Diwald Oliver , Sterrer Martin , Knözinger Erich , Sushko Peter V. , Shluger Alexander L.
Published: July 2002

Monochromatic UV light in the spectral interval between 4.0 and 5.5 eV is used in order to selectively excite 3- and 4-coordinated oxygen anion sites on the surface of MgO nanoparticles exposed to O2 gas. As a result, two different paramagnetic O− surface species and also ozonide anions O3− are observed by electron paramagnetic resonance (EPR) spectroscopy. The relative abundance of each of the O− species exhibits a specific dependence on the energy of the exciting photons. EPR data together with the results of theoretical modeling suggest that both O− species are located at 3-coordinated sites having different local environments. At sufficiently high O2 pressures molecular oxygen does not only act as an electron trap, favoring the O− formation, but it also contributes to UV induced O3− formation with a maximum efficiency at 4.2 eV.
Homogeneous nucleation of n-pentanol measured in an expansion cloud chamber
Authors: Schmitt John L. , Doster G. Jay
Published: July 2002

An expansion cloud chamber was used to measure homogeneous nucleation rates for n-pentanol in argon carrier gas at four nucleation temperatures 292, 282, 272, and 252 K. The nucleation rates range from about 15000 to 400 drops/cm3 s. The data exhibits changes with time that are attributed to the removal of trace impurities by self-cleaning action in the cloud chamber. Data at the highest supersaturation ratio for a given number of drops observed is considered to be closest to true homogeneous nucleation. A comparison of these measurements with data in the literature at similar temperatures and nucleation rates shows the data from this study to be approximately three orders of magnitude lower in nucleation rate at a given supersaturation ratio.