bluuebunny/crossref_metadata_2025 huggingface dataset:
Epitaxial pulsed laser crystallization of amorphous germanium on GaAs
Authors: Santos P. V. , Trampert A. , Dondeo F. , Comedi D. , Zhu H. J. , Ploog K. H. , Zanatta A. R. , Chambouleyron I.
Published: July 2002

We have investigated the crystallization of amorphous germanium films on GaAs crystals using nanosecond laser pulses. The structure and composition of the crystallized layers is dominated by nonequilibrium effects induced by the fast cooling process following laser irradiation. Perfect epitaxial films are obtained for fluencies that completely melt the Ge film, but not the substrate. For higher fluencies, partial melting of the substrate leads to the formation of a (GaAs)1−xGe2x epitaxial alloy with a graded composition profile at the interface with the substrate. Since Ge and GaAs are thermodynamically immiscible in the solid phase, the formation of the alloy is attributed to the suppression of phase separation during the fast cooling process. Lower laser fluencies lead to polycrystalline layers with a patterned surface structure. The latter is attributed to the freeze-in of instabilities in the melt during the fast solidification process.
Physical mechanisms of photoluminescence of chlorine-doped ZnSe epilayers grown by molecular beam epitaxy
Authors: Wang S. Z. , Yoon S. F. , He L. , Shen X. C.
Published: July 2002

Reported here are some temperature-dependent and excitation-dependent photoluminescence (PL) results from chlorine-doped ZnSe layers grown by molecular beam epitaxy. The PL spectrum is characterized by an overwhelming exciton recombination emission at 2.797 eV (10 K) near the band edge, and no other PL features are found. This overwhelming neutral donor-bound exciton (Cl0X) emission line at 2.797 eV (10 K) with a full width at half maximum (FWHM) of ∼13 meV reveals the high crystalline quality of the samples. The quick quenching of this exciton line above 200 K is due to the presence of a nonradiative center with a thermal activation energy of ∼90 meV. The decrease of the Cl0X line over the temperature range from 10 to 200 K is due to the thermal activation of Cl0X bound excitons to free excitons with an activation energy of ∼9.0 meV. The change of the emission nature occurs at about ∼200 K from Cl0X bound exciton recombination to free exciton recombination; accordingly, a kink appears on the temperature-dependent FWHM curve at ∼190 K. The broadening behavior of the Cl0X line over the temperature range 10–200 K can be understood within the theoretical framework of the phonon scattering model, while the impurity scattering mechanisms are responsible for the broadening of the exciton line above 200 K. This PL peak does not shift with the change in excitation power, while the integrated intensity increases exponentially with the increase in excitation power, i.e., IPL∝Iex1.21, indicating the competition between the radical recombination process and the nonradical recombination process of photogenerated carriers. The FWHM remains unchanged when the excitation power was varied from 0.5 to 50 W cm−2, and then increases exponentially when the excitation power exceeds 50 W cm−2. However, the PL peak consists of four exciton subpeaks, and these subpeaks do not broaden. The PL peak is found to broaden due to recombination competition among the four kinds of excitons.
Foreign body induced kinetic roughening: Kinetics and observations
Authors: Liu Xiang Yang , Bennema P.
Published: July 2002

A new and novel phenomenon: kinetic roughening induced by foreign particles at a growing crystal face was observed for the first time. Beginning with relatively low supercoolings, when foreign particles are in contact with the crystal surface, the faceted growth of the {110} faces of naphthalene crystals from its melt reveals a transition from smooth growth to rough growth, followed by an increase in the growth rate. This is due to the vanishing of the two-dimensional (2-D) nucleation barrier at the particle contact area. At high supercoolings, the particle-induced kinetic roughening will be suppressed. To describe this unusual phenomenon, the thermodynamic and kinetic criteria for this type of kinetic roughening were derived. The effect of particles on kinetic roughening is determined in terms of the step correlation parameter f, which is 0.035 for the naphthalene crystallization system. This is the first quantitative measurement for the interaction (or contact) parameters between the crystal steps and foreign particles.
Power plant exhaust gas simulator for the development of optical diagnostics
Authors: Spelman J. , Skrien S. , Parker T. E.
Published: July 2002

A power plant simulator was designed and built for development and calibration of optical diagnostics in power plant flows. This article describes the important features of this simulator as well as it performance. The simulator was constructed using a 120-mm-i.d., approximately 3-m-long quartz tube enclosed in a custom manufactured multizone furnace. Simulation of power plant flows requires combustion effluent; this is provided by a propane burner. Additional gases may be seeded into the flow using ports positioned before the entry into the 3 m quartz tube. Thus, flow with specified concentrations of pollutants of interest (i.e., NO, NH3, SO2, etc.) can be produced. Temperature control for the gas flow entering the quartz tube is provided by a 4-m-long heat exchanger positioned between the burner and the 3-m-long quartz tube. Optical access is provided at the tube ends using NaCl windows, which were chosen for their transmissive characteristics in the visible and infrared. These windows are protected thermally and from H2O with a recirculating nitrogen flow. Capabilities of the simulator are a high-temperature optical pathlength of 2.60 m with maximum temperatures up to 800 °C.
Ultrafast dynamics of excitations in conjugated polymers: A spectroscopic study
Authors: Chang R. , Hayashi M. , Lin S. H. , Hsu J.-H. , Fann W. S.
Published: July 2002

We construct a microscopic model to describe the excited states of poly(2-methoxy, 5-(2′-ethylhexoxy)-p-(phenylenevinylene) in thin film. Within this model, we deduce that in the high energy region, the nature of excited states in the film is very similar to the species observed in solution phase. Moreover, we propose that the decay process of these excited states involves energy transfer, vibrational relaxation, and dissipation simultaneously, in contrast to the usual argument that assumes exciton migration occurs after vibrational motion reaches thermal equilibrium. As a result, the simulation of time-resolved photoluminescence spectra is in agreement with the experiment.
Coupled diffusion of segments and counterions in polyelectrolyte gels and solutions
Authors: Sasaki Shigeo , Schipper F. J. M.
Published: July 2002

Pulsed field gradient NMR and dynamic light scattering results revealed that the dynamical properties of chain segments and counterions in a polyelectrolyte gel are almost identical to those in a polyelectrolyte solution. The NMR results show that the dependence of the self-diffusion of a counterion on the degree of chain neutralization in the poly(acrylate) gel hardly differs from that in a normal poly(acrylate) solution. Dynamic light scattering of the poly(acrylate) gel demonstrates the existence of the fast dynamic mode whose characteristic time is of the same order as that observed in the poly(acrylate) solution. It is shown that the formalism of coupling theory, coupling the dynamics of the polyelectrolyte segments and the counterions, is valid both for polyelectrolyte solutions and for the gel system. The observed similarity of the dynamics in solutions and in gel systems is consistent with this theoretical concurrence.
Vibrational relaxation of Ne, Ar, Kr–HF (v=1) binary complexes in helium nanodroplets
Authors: Nauta K. , Miller R. E.
Published: July 2002

We report high-resolution infrared laser spectra of the Ne, Ar, Kr–HF binary complexes solvated in liquid helium droplets. The present experiments show that the vibrational relaxation rates become progressively faster in the series Ne, Ar, and Kr–HF. Ar–HF is an intermediate case, with vibrational relaxation occurring on the time scale of the flight time of the droplets through the apparatus (0.5 ms). In this case, the data show that the relaxation rate is also dependent upon the size of the helium droplet in which the complex is solvated.
Fluoroalcohols as nucleating agents in supersaturated vapors: Efficient clustering with water in the vapor phase
Authors: Abedalsayed V. , Ibrahim Y. , Rusyniak M. , Rabeony M. , El-Shall M. S.
Published: July 2002

Fluoroalcohols in small concentrations in the vapor phase display striking enhancing effects on homogeneous nucleation of supersaturated aliphatic alcohols, and on the formation of water clusters by supersonic expansion. The enhanced nucleation effects are attributed to the surfactant properties of fluoroalcohols, which lower the surface tension of the growing droplets, and therefore lower the barrier to nucleation.
Measurements of C V flows from thermal charge-exchange excitation in divertor plasmas
Authors: Zaniol B. , Isler R. C. , Brooks N. H. , West W. P. , Olson R. E.
Published: July 2002

Certain transitions of C IV (C3+) from n=7 to n=6 (≈7226 Å) and from n=6 to n=5 (≈4660 Å) sometimes appear much brighter in tokamak divertors than expected for electron-impact excitation from the ground state. This situation occurs because of charge exchange between C V (C4+) and recycling thermal deuterium atoms in the n=2 level. As a result, it is possible to extend parallel flow measurements of carbon, which have previously been performed on C II–C IV ions using Doppler shift spectroscopy, to include flows of the He-like C V ions. The work described here includes modeling of the spectral features, correlation of state populations with classical Monte Carlo trajectory (CTMC) predictions, and applications to flow measurements in the DIII-D divertor [Plasma Physics Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159; Proceedings of the 18th IEEE/NPSS Symposium on Fusion Engineering, Albuquerque (Institute of Electrical and Electronic Engineers, Piscataway, 1999), p. 515].
Coupled cluster study of the X̃ 2Π and Ã 2Σ+ electronic states of the HCGe radical: Renner–Teller splitting and the effects of relativistic corrections
Authors: Sari Levent , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

The X̃ 2Π and Ã 2Σ+ states of the germanium methylidyne radical (HCGe) have been investigated at the SCF, CISD, CCSD, and CCSD(T) levels of theory. The total energies, equilibrium geometries, dipole moments, harmonic vibrational frequencies, infrared intensities, and Renner–Teller splitting are reported. The relativistic one-electron Darwin and mass-velocity terms are calculated using first-order perturbation theory and the effects of these corrections on energetics, harmonic vibrational frequencies, and Renner–Teller splitting are discussed. At our highest level of theory [CCSD(T)/cc-pVQZ], the ground electronic state (X̃ 2Π) has a linear geometry with re(CH)=1.079 Å and re(CGe)=1.769 Å, in good agreement with the experimental values of r0(CH)=1.067 Å and r0(CGe)=1.776 Å. In the electronically excited Ã 2Σ+ state, HCGe is also found to have a linear geometry with re(CH)=1.074 Å and a much shorter re(CGe)=1.669 Å at the [CCSD(T)/cc-pVQZ] level, in agreement with experimental values of r0(CH)=1.059 Å, r0(CGe)=1.674 Å, and the observation of a C–Ge triple bond character. The Ã 2Σ+ state C–Ge stretching vibrational frequency is determined to be ω3=990.2 cm−1 at the CCSD(T)/cc-pVTZ level with the inclusion of relativistic effects, which is in essentially perfect agreement with the experimental value of ν3=990 cm−1. With the same method, the X̃ 2Π state harmonic vibrational frequencies are predicted to be 846 cm−1 for the C–Ge stretching, 443 and 506 cm−1 for the two nondegenerate bending Renner–Teller components, and 3249 cm−1 for the C–H stretching modes for which experimental values are not available. The quantum mechanical splitting (T0 value) was determined to be 38.0 kcal/mol at the nonrelativistic CCSD(T)/TZ3P(2 f,2d)+2diff level, while it is found to be 39.9 kcal/mol with the inclusion of relativity, in very good agreement with the experimental value of 39.8 kcal/mol. The Renner parameter (ε) is determined to be −0.1386, and the effects of relativity were seen to produce a smaller Renner parameter of −0.1329.
Alcator C-Mod predictive modeling
Authors: Pankin Alexei , Bateman Glenn , Kritz Arnold , Greenwald Martin , Snipes Joseph , Fredian Thomas
Published: July 2002

Predictive simulations for the Alcator C-mod tokamak [I. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are carried out using the BALDUR integrated modeling code [C. E. Singer et al., Comput. Phys. Commun. 49, 275 (1988)]. The results are obtained for temperature and density profiles using the Multi-Mode transport model [G. Bateman et al., Phys. Plasmas 5, 1793 (1998)] as well as the mixed-Bohm/gyro-Bohm transport model [M. Erba et al., Plasma Phys. Controlled Fusion 39, 261 (1997)]. The simulated discharges are characterized by very high plasma density in both low and high modes of confinement. The predicted profiles for each of the transport models match the experimental data about equally well in spite of the fact that the two models have different dimensionless scalings. Average relative rms deviations are less than 8% for the electron density profiles and 16% for the electron and ion temperature profiles.
A fluid-kinetic model for the two plasmon decay instability
Authors: Machacek A. C. , Wark J. S.
Published: July 2002

A solution for the gain rates and plasmon frequencies of the two plasmon decay instability is obtained using a fluid-kinetic model. This work is of relevance to high temperature, short-pulse laser plasma interactions where appreciable instability is expected in regions of plasma where the phase velocity of the electrostatic waves may not be assumed to be much larger than the thermal velocity of the electrons. The model is evaluated by comparing its results for the stimulated Raman scattering process with a fully kinetic treatment.
Diffraction of electromagnetic waves by small scale geomagnetic field-aligned density striations
Authors: Istomin Ya. N. , Leyser T. B.
Published: July 2002

A powerful electromagnetic wave in the ordinary mode transmitted near vertically into the ionospheric F region excites small scale density filaments which are stretched along the geomagnetic field. These striations are associated with a number of electromagnetically driven plasma phenomena, such as anomalous absorption of electromagnetic waves, anomalous heating, and stimulation of electromagnetic emissions. The diffraction of an electromagnetic wave is studied theoretically in a cold plasma as it propagates at small angles to striations which have a small scale across the ambient magnetic field. For a sufficiently high number density of striations the stationary self-consistent wave electric field distribution consists of several independent beams. As a result the pump-induced striations are grouped in independent patches in the plane across the magnetic field. Such bunches of striations may have been detected in in situ measurements from a sounding rocket at Arecibo and as large scale density irregularities observed at Tromso/.
Recurrence in plasma edge turbulence
Authors: Baptista M. S. , Caldas I. L. , Heller M. V. A. P. , Ferreira A. A. , Bengtson R. D. , Stöckel J.
Published: July 2002

Common statistics of turbulent electrostatic fluctuations observed at the plasma edge and scrape-off layer are analyzed in three tokamak devices that have different configurations. The statistics of experimental data collected using fixed sampling time is the same than the statistics of the time for which the oscillation return to a specified reference interval of values. This observation, in addition to the finding of power-scaling laws for some average quantities with respect to either the sampling time or the size of the reference interval, suggests that turbulence on tokamaks have recurrent characteristics, typical of a recurrent chaotic low-dimensional system. Furthermore, the first Poincaré recurrence time and other dynamical tools are used to simulate the mentioned fluctuation statistical properties.
Rotationally resolved infrared spectrum of the Br−−D2 anion complex
Authors: Wild D. A. , Weiser P. S. , Bieske E. J.
Published: July 2002

The midinfrared spectrum of the Br−79−D2 anion complex is measured in the D2 stretch region by monitoring the production of Br− photofragments in a tandem mass spectrometer. The rotationally resolved spectrum comprises two overlapping Σ−Σ subbands, red-shifted by ≈85 cm−1 from the free D2 vibrational frequency. These subbands are assigned to absorptions by Br−−D2 complexes containing para and ortho forms of the D2 molecule. The Br−−D2 complex is deduced to possess a linear equilibrium geometry, although the zero-point bending excursion is expected to be substantial. The rotational constants are consistent with vibrationally averaged intermolecular separations between the Br− anion and D2 center of mass of 3.414(4) Å for Br−−D2(p) and 3.413(1) Å for Br−−D2(o). The intermolecular bond contracts by 0.076 Å following vibrational excitation of the D2 diatomic molecule. Effective one-dimensional radial potential energy curves are developed through Rydberg–Klein–Rees inversion of the spectroscopic data and consideration of the long-range electrostatic and induction interaction between the D2 molecule and a point charge. On the basis of these potential energy curves the binding energies of Br−−D2(p) and Br−−D2(o) are estimated as 364 and 418 cm−1, respectively.
Single electron tunneling detected by electrostatic force
Authors: Klein Levente J. , Williams Clayton C.
Published: July 2002

Single electron tunneling events between a specially fabricated scanning probe and a conducting surface are demonstrated. The probe is an oxidized silicon atomic force microscope tip with an electrically isolated metallic dot at its apex. A voltage applied to the silicon tip produces an electrostatic force on the probe, which depends upon the charge on the metallic dot. Single electron tunneling events are observed in both the electrostatic force amplitude and phase signal. Electrostatic modeling of the probe response to single tunneling events is in good agreement with measured results.
Investigation of the Ne-NH3 van der Waals complex: Rotational spectrum and ab initio calculations
Authors: van Wijngaarden Jennifer , Jäger Wolfgang
Published: July 2002

The Ne-NH3 van der Waals complex was investigated through analysis of microwave spectra and the construction of ab initio potential energy surfaces. Rotational spectra of the ground internal rotor state of Ne-NH3 were recorded between 7 and 24 GHz using a Balle–Flygare-type Fourier transform microwave spectrometer. In total, ten isotopomers were studied, which include Ne20 and Ne22 paired with NH3, NH315, ND3, ND2H, and NDH2. The spectroscopic constants including the N14 quadrupole coupling constants were determined and used to estimate structural and dynamical details of the van der Waals complex. For each deuterium-substituted isotopomer, a tunneling splitting was observed due to the inversion of NH3 within the ground state of the complex. One of these inversion components is missing for the NH3 and NH315 containing isotopomers for spin statistical reasons. Ab initio potential energy surfaces were constructed for three different umbrella angles of NH3 within the complex to provide information about the interaction along the NH3 inversion pathway. The interaction energies were calculated using Møller–Plesset perturbation theory to fourth order. All three surfaces have minima corresponding to structures where the van der Waals bond (the line from the center of mass of NH3 to the Ne atom) lies perpendicular to the C3 axis of NH3, with the neon atom sitting between two hydrogen atoms. At this orientation, the potential well depth is 62.66 cm−1 (285.5 μhartree) at a van der Waals bond length of 3.30 Å (6.24 bohr) for the experimental equilibrium NH3 monomer geometry.
Two-stream sausage and hollowing instabilities in high-intensity particle beams
Authors: Uhm Han S. , Davidson Ronald C. , Kaganovich Igor
Published: July 2002

Axisymmetric two-stream instabilities in high-intensity particle beams are investigated analytically by making use of the Vlasov–Maxwell equations in the smooth-focusing approximation. The eigenfunctions for the axisymmetric radial modes are calculated self-consistently in order to determine the dispersion relation describing collective stability properties. Stability properties for the sausage and hollowing modes, characterized by radial mode numbers n=1 and n=2, respectively, are investigated, and the dispersion relations are obtained for the complex eigenfrequency ω in terms of the axial wavenumber k and other system parameters. The eigenfunctions obtained self-consistently for the sausage and hollowing modes indicate that the perturbations exist only inside the beam. Therefore, the location of the conducting wall does not have an effect on stability behavior. The growth rates of the sausage and hollowing modes are of the same order of magnitude as that of the hose (dipole-mode) instability. Therefore, it is concluded that the axisymmetric sausage and hollowing instabilities may also be deleterious to intense ion beam propagation when a background component of electrons is presented.
Dynamics of periodic ion holes in a forced beam–plasma experiment
Authors: Franck C. , Klinger T. , Piel A. , Schamel H.
Published: July 2002

Experimental observation of the propagation of periodic ion phase space vortices is reported. Density fluctuation measurements in a double plasma device show an apparently spontaneous acceleration of these periodic structures from ion thermal to ion acoustic velocity. A nonlinear kinetic description explains this as a transition from a new type of nonlinear electrostatic mode (periodic ion holes) to an ion acoustic mode which is caused by trapped particle scattering.
Kerr relaxation of anisotropic dielectric particles and Néel relaxation of anisotropic magnetic particles after a sudden change of field
Authors: Felderhof B. U. , Jones R. B.
Published: July 2002

Kerr relaxation of anisotropic dielectric particles and Néel relaxation of anisotropic magnetic particles after a sudden change of electric or magnetic field are studied on the basis of the Smoluchowski equation for the orientation distribution function. The equilibrium average of a Legendre moment of any order is evaluated. The time dependence of the relaxation of polarization (first-order Legendre moment) and order parameter (second-order Legendre moment) after a sudden change of field is determined by a fast and accurate numerical method.
Frequency conversion in coplanar waveguide based on BaxSr1−xTiO3 film
Authors: Samoilova T. B. , Astafiev K. F. , Rivkin T. , Ginley D. S.
Published: July 2002

We report on both a model and experimental verification of the high frequency nonlinearity for coplanar waveguides (CPW) composed of a dielectric substrate coated with nonlinear dielectric Ba0.4Sr0.6TiO3 films. For the CPW with a propagating microwave signal (f0≈30 GHz), modulation of Ba0.4Sr0.6TiO3 film dielectric constant by a high frequency (fp≈20 MHz) pump signal results in the generation of sidebands with frequency (f0±Nfp). Experimental and simulated spectra of CPW output signals are presented as a function of the direct current bias voltage, pump voltage amplitude, CPW length, and CPW nonlinearity.
Phase equilibria of a near-critical ionic system. Critical exponent of the order parameter
Authors: Bianchi Hugo L. , Japas M. L.
Published: July 2002

The density ρ of coexisting phases of the ternary system water+1,4-dioxane+potassium chloride was investigated along the liquid–liquid–solid coexistence curve near the critical end-point using a vibrating tube densimeter. By visual determination, this lower critical end-point was located at 311.026±0.010 K with a mole ratio dioxane (D) to water (W) rc=nD/nW=0.418±0.004. Density measurements were carried out in the range 0.01 K<(T−Tc)<31.5 K (i.e., 3×10−5<t=(T−Tc)/Tc<0.1). The obtained coexistence curve displays an apparent sharp crossover at reduced temperatures t≃10−2 from a nonclassical (Ising) to a classical (mean field) exponent. In ancillary experiments, the critical salt concentration was determined. Reduced critical values were calculated and compared with those predicted by the simplest ionic model (RPM, Restricted Primitive Model). The relation between critical behavior and reduced critical parameters in the present and other non-Coulombic systems is discussed.
Switching kinetics of 180° domains in telluric acid ammonium phosphate crystals with large unipolarity
Authors: Matyjasek K.
Published: July 2002

The switching process in unipolar telluric acid ammonium phosphate crystal has been studied by a combination of optical observation of the domain structure in low electric fields and electrical switching measurements in high fields. The stabilization mechanism of 180° domains appears to have a correlation to a defect-induced internal field Eb in the crystal. In crystal sample with Eb>Ec (Ec, coercive field measured from D–E hysteresis loop), the spontaneous repolarization of the crystal occurs after the removal of the applied field. It has been found that Eb acts as a driving field causing nucleation and growth of back-switching domains.
Time-resolved fluorescence of polarizable chromophores
Authors: Matyushov Dmitry V.
Published: July 2002

The paper reports an exact solution for spectral profiles of time-resolved fluorescence following a broad-band excitation producing changes in both the chromophore’s dipole moment and the polarizability. The problem is formulated in terms of a two-state chromophore with a bilinear coupling to a Gaussian solvent mode. Closed-form solutions for the time-dependent spectral shift and width are obtained. These two transient spectral moments are sufficient to generate transient, non-Gaussian band-shapes by using the proposed formalism. An approximately linear relation between the normalized shift and width time correlation functions is found. The Stokes shift correlation function is virtually insensitive to the nonlinear solute–solvent effects, in spite of dramatic transient changes in the spectral width. Depending on chromophore’s parameters, the transient line shape can be well approximated by a nonstationary Gaussian process or shows clearly non-Gaussian statistics. The model is extended to treat heterogeneous solvent dynamics.
Temperature-dependent behavior of low-temperature-grown GaAs nonalloyed ohmic contacts
Authors: Ueng H. J. , Chen N.-P. , Janes D. B. , Webb K. J. , McInturff D. T. , Melloch M. R.
Published: July 2002

A study of nonalloyed ohmic contact structures consisting of Au/Ti metallization deposited on a thin (3.5–5 nm) layer of low-temperature-grown GaAs (LTG:GaAs) on a thin (10 nm) layer of heavily doped n-type GaAs is summarized. We demonstrate that this Au/Ti:LTG:GaAs/n+GaAs contact structure has a stable specific contact resistance between 40 and 300 K, with measured contact resistance as low as 2×10−6 Ω cm2 at 40 K. Based on comparisons of the measured data with calculations using a uniformly doped Schottky model, we infer that the activation doping density in these structures is higher than 5×1018 cm−3, and that the surface potential barrier height is lower than 0.7 eV (midgap). The characteristic current–voltage curves of the nonalloyed contact show that tunneling is the primary conduction mechanism.
Photodissociation of Br2 at 234 and 265 nm: Imaging studies of one and two photon excitation
Authors: Jee Yun-Jung , Jung Young-Jae , Jung Kyung-Hoon
Published: July 2002

The photodissociation of Br2 at 234 and 265 nm was investigated using a velocity map imaging technique via one- and two-photon excitation. The speed and angular distributions of the bromine ions obtained via one-photon excitation following state selective ionization reveal a unique transition to the 1441 3Σu+(1u) state. This state dissociates further into Br(4p 2P3/2)+Br(4p 2P1/2) at 234 nm, and into Br(4p 2P3/2)+Br(4p 2P1/2) and Br(4p 2P3/2)+Br(4p 2P3/2) at 265 nm, with relative quantum yields of Φ(Br(4p 2P3/2)+Br(4p 2P1/2))=0.96 and Φ(Br(4p 2P3/2)+Br(4p 2P3/2))=0.04. The proportionality constant k, related to the relative [2+1] REMPI strengths of atomic bromine (Br 2P1/2/Br 2P3/2) was determined to be 0.42±0.02 for Br(4p 2P1/2) detected at 234.0 nm and for Br(4p 2P3/2) at 233.7 nm, and 0.73±0.02 for Br 2P1/2/Br 2P3/2 at 264.9 nm to detect Br(4p 2P1/2) and 264.8 nm to detect Br(4p 2P3/2). Image analysis of the ions and photoelectrons obtained by two-photon excitation at various wavelengths in the range of 229.4–237.6 nm shows that some fragmented bromine atoms are in highly excited Br(5s 4PJ:J=5/2, 3/2, 1/2) states, and that there exists superexcited Rydberg states converging to the asymptotes, Br(4p 2P)+Br(5s 4P) near the ionization threshold.
Electron field emission from sulfur-incorporated nanocrystalline carbon thin films
Authors: Gupta S. , Weiss B. L. , Weiner B. R. , Morell G.
Published: July 2002

Results are reported on the electron field emission properties of sulfur-incorporated nanocrystalline carbon (n-C:S) thin films grown by hot-filament chemical vapor deposition technique. The lowest turn-on field values observed were around 4.0–4.5 V/μm, which are about half of those measured for films grown without sulfur. Associated to the effect of addition of sulfur on field emission properties, there are interesting microstructural changes, as characterized with scanning electron microscopy, atomic force microscopy, and Raman spectroscopy techniques. The sulfur-incorporated films show smoother and finer-grained surfaces than those grown without sulfur. These results are similar to those found for the introduction of nitrogen, but different to those produced by oxygen addition to the chemical vapor deposition process. These findings are attributed to changes in the electronic band structure.
Dynamics of exhaust gas generated by arc extinction
Authors: Hayashi Yasushi , Watanabe Masato , Okino Akitoshi , Hotta Eiki
Published: July 2002

We report an analytical study on hot gas exhaust process of a SF6 gas circuit breaker (GCB), after current interruption. The behavior of the hot gas has been studied based on measured gas temperature and simulation results of gas composition. We also propose a mechanism of interaction between the hot gas and pressure waves, which causes a self-blocking of the exhaust gas. During the heavy current interruption, the flow model suggests that the dielectric strength of the hot gas is affected by the pressure waves that are generated by the hot gas exhaustion. We believe that the results reported in this article provide guidance for the optimum structure of the exhaust chamber for small size GCB, operating at very high interrupting current.
Emission of radiation from square arrays of stacked Josephson junctions
Authors: Filatrella G. , Straughn B. , Barbara P.
Published: July 2002

We propose a design for two-dimensional arrays of stacked Josephson junctions and study their properties as coherent sources of radiation in the mm and sub-mm wavelength range. This design can be implemented by using either mesas of intrinsic Josephson junctions, from crystals of high TC superconductors, or artificial stacks of low TC junctions. We outline a possible Nb/AlOx/Nb trilayer fabrication process for such devices. Our model for the dynamics of the system predicts the range of parameters over which the junctions synchronize.
Phase-matched second harmonic generation and cascaded nonlinearity in a Langmuir–Blodgett inverted waveguide of 2-docosylamino-5-nitropyridine
Authors: Flueraru C. , Schrader S. , Dietzel B. , Motschmann H.
Published: July 2002

We discuss the results of our second harmonic generation (SHG) and nonlinear cascading experiments in organic waveguide. The waveguide is made of two identical Langmuir–Blodgett films with 2-docosylamino-5-nitropyridine. The thin film and waveguide characterization is presented with a detailed description of the experimental setup. An inverted waveguide is produced by a proper arrangement of the thin organic films. Using a tunable laser source the phase match at λ=1063 nm of the SHG is proved. The SHG power-corrected efficiency of 8.9% W−1 was measured. The cascading of second-order nonlinearities is characterized by means of interferometric measurements which consist of detecting a nonlinear phase shift of the fundamental in dependence on input intensity. The measured value of cascading n2 is 2.68×10−13 cm2/W.
Tunneling current and thickness inhomogeneities of ultrathin aluminum oxide films in magnetic tunneling junctions
Authors: Luo E. Z. , Wong S. K. , Pakhomov A. B. , Xu J. B. , Wilson I. H. , Wong C. Y.
Published: July 2002

Tunneling current and thickness inhomogeneities of ultrathin aluminum oxide layers of magnetic tunnel junctions are studied by conducting atomic force microscopy (CAFM). The current inhomogeneities are attributed to thickness inhomogeneities on a nanometer scale. Thickness distributions are derived from the current distributions, using the Simmons’ tunneling model by assuming a constant and uniform barrier height. It is shown that dielectric breakdown at high voltages can cause modifications of the AlOx layer during CAFM measurements, characterized by the irreversibility of both current images and local I–V characteristics. Working at low voltage and low current is a necessary condition for applicability of the CAFM method for testing the insulator layers. The effect of the method of film preparation on the film dielectric strength is studied.
Annealing effects on structures and optical properties of silicon nanostructured films prepared by pulsed-laser ablation in inert background gas
Authors: Makino Toshiharu , Yamada Yuka , Suzuki Nobuyasu , Yoshida Takehito , Onari Seinosuke
Published: July 2002

We studied the annealing effects on the structures and optical properties of silicon (Si) nanostructured films. The Si nanostructured films were synthesized by pulsed-laser ablation in inert background gas. It was found that the Si nanostructured films partially include an amorphous-like structure under the as-deposited condition. After annealing in nitrogen gas, the crystallinities of the Si nanoparticles recovered, and four visible photoluminescence (PL) bands (1.7, 2.2, 2.7, and 3.1 eV) appeared at room temperature. Furthermore, upon subsequent annealing in oxygen gas, strong quantum confinement effects for both phonons and carriers of the Si nanoparticles appeared, and the relative intensity of the 2.7 eV blue band increased. We fabricated electroluminescent (EL) diodes with active layers of annealed Si nanostructured films, and verified visible EL spectra at room temperature, which exhibited the same trends as the PL spectra. A possible explanation for the increase of relative intensity of the 2.7 eV blue band is the increase of the amount of silicon dioxide, which contains neutral oxygen vacancy defects, in the Si nanostructured films.
Comparison between thermal sampling and numerical analysis of thermally stimulated depolarization current peaks
Authors: Laredo E. , Bello A. , Hernández M. C. , Grimau M.
Published: July 2002

Thermal sampling peaks recorded after windowing polarization are studied for the segmental mode in poly(ε-caprolactone). Also, numerical decompositions of the global thermally stimulated depolarization current peak into pure Debye contributions are performed with direct signal analysis (DSA) and simulated annealing direct signal analysis procedures for Arrhenius and Vogel–Tammann–Fulcher (VTF) temperature dependences, respectively. It is found that the results between the experimental and the numerical procedures agree very well and the approximations made in the analysis of the experimental curves are thus validated, despite the unphysical values for the relaxation parameters found by both methods when using Arrhenius relaxation times. On the contrary, when VTF relaxation times are used for the numerical decomposition, agreement is found with the results of isothermal dielectric absorption as a function of frequency, together with reasonable values for reorientation energies, pre-exponential factors and VTF temperature. Thermal sampling and DSA also compare well when studying the departure from the zero-entropy line which indicates the onset of a cooperative character in the dynamics of molecular motion. Compensation is found whenever the primary relaxation is analyzed with Arrhenius or Eyring relaxation times and does not appear when VTF relaxation times are used in the numerical decomposition.
Low-energy excitations of vapor-deposited amorphous ice and its annealing and methanol-doping effects studied by inelastic neutron scattering
Authors: Yamamuro Osamu , Madokoro Yasushi , Yamasaki Hirotaka , Matsuo Takasuke , Tsukushi Itaru , Takeda Kiyoshi
Published: July 2002

A novel cryostat was developed for in situ neutron scattering studies on vapor-deposited amorphous samples. By the use of this cryostat, vapor-deposited amorphous ice was prepared at ca. 8 K. The neutron scattering spectra of the as-deposited sample and those annealed at ca. 120 K, 160 K, and 250 K were measured at 50 K in energy range below 100 meV. The four samples studied corresponded to vapor-deposited amorphous solid water (ASW), hyperquenched glassy water (HGW) (approximately), ice Ic, and ice Ih, respectively (according to historical nomenclature). The librational frequency of the water molecule is smaller in the order of ASW<HGWHGW>ASW. The absolute value of the vibrational density of states G(E) was obtained from the analysis combining the heat capacity and neutron scattering data. G(E) of ASW and HGW integrated below 6 meV were larger than that of ice Ih(≈Ic) by 0.060 and 0.039 degrees of freedom per water molecule, respectively. Similar experiments were performed on the vapor-deposited amorphous ice doped with 5% and 10% of methanol (CD3OH). The magnitude of G(E) below 6 meV increased with increasing fraction of methanol. All of the results in this study indicate that the low energy excitation [G(E) below 6 meV] of amorphous ice is enhanced by the defects and distortion of the hydrogen bonds.
The Rydberg states of NO2: Vibrational autoionization of the ndσ states
Authors: Petsalakis Ioannis D. , Theodorakopoulos Giannoula , Child Mark S.
Published: July 2002

Ab initio configuration interaction calculations were carried out on the potential-energy surfaces of the ground and Rydberg excited electronic states of NO2. The results show that potential-energy curves with typically Rydberg form are obtained for most of the excited states at linear geometries, similar to the ground-state potential of NO2+. At nonlinear geometries valence-Rydberg interactions complicate the potential-energy surfaces of the excited states. Quantum defect functions have been determined from the ab initio results on the 3dσ Rydberg state and vibrational autoionization widths have been calculated for excited vibrational levels of members of the ndσ series.
Ultrafast laser control of ionic-bond formation: ClF in argon solids
Authors: Bargheer M. , Pietzner J. , Dietrich P. , Schwentner N.
Published: July 2002

The formation of the two ionic products of Cl+F− versus Arn+F− is controlled by variation of the time delay between two ultrafast UV pulses. The Arn+F− exciplex is characterized by a fluorescence band at 355 nm and the Cl+F− product by the D′ emission at 420 nm. In this Tannor–Kosloff–Rice control scheme, the pump pulse dissociates ClF in a solid argon matrix. During the early dynamics in the matrix cage the F fragments have sufficient kinetic energy to closely approach the Ar atoms. Here a control pulse, which is delayed by typically 3 ps, induces the ionic bond Arn+F−. Energy loss by collisions with the cage leads to recombination and vibrational relaxation in the B state of ClF. A control pulse delayed by 30–100 ps leads to exclusive formation of the ionic bond Cl+F−. A switching contrast better than 1 to 10 can be achieved in both directions. The control scheme makes use of the time scale of vibrational relaxation. By femtosecond pump–probe spectroscopy we show the wave packet oscillations from the recombination dynamics in the cage lasting for 2 ps and determine the subsequent decay of vibrational energy.
Zimm model for a copolymer chain under an alternating field in θ solvents
Authors: Wenczel Rob , Shew Chwen-Yang
Published: July 2002

We investigate the dynamics of a copolymer molecule in a θ-solvent under a weak alternating field using the Zimm model. We first consider a diblock copolymer with two blocks of same force constant, where the monomers on the two different blocks interact with the applied field differently. The theory predicts that the center-of-mass and the chain conformation of copolymer molecules oscillate in response to the sinusoidal electric field. For the center-of-mass, the oscillation becomes more pronounced as the charge fraction is increased. However, the oscillation of chain conformation is not observed for neutral or fully charged homopolymers, but is enhanced when the fraction of the two blocks becomes equal. In highly viscous solutions, the strength of the oscillation of the center-of-mass and conformation is weakened, and the phase of oscillation is shifted. Moreover, the oscillatory behavior diminishes at higher frequencies. For comparison, we also study a special copolymer model where the two monomers alternate along the polymer backbone. Again the oscillation emerges for the molecular motion and conformation, but the oscillation of chain properties becomes much weaker than diblock copolymers. These results disclose the effect of the chemical architectures of copolymers on their chain dynamic behavior under an alternating field.
On the microscopic interaction of para-chlorofluorobenzene with water
Authors: Riehn C. , Reimann B. , Buchhold K. , Vaupel S. , Barth H.-D. , Brutschy B. , Tarakashwar P. , Kim Kwang S.
Published: July 2002

The isolated (1:1) complex of para-chlorofluorobenzene with water has been investigated by IR/R2PI depletion spectroscopy and ab initio calculations on the geometry and IR spectra. Whereas three different isomers (σF, σCl, π) were located by theory, in the molecular beam experiments only the two σ isomers have been observed.
Erbium-doped yttrium aluminum garnet as a magnetic refrigerant for low temperature x-ray detectors
Authors: Kushino Akihiro , Aoki Yuji , Yamasaki Noriko Y. , Namiki Takahiro , Ishisaki Yoshitaka , Matsuda Tatsuma D. , Ohashi Takaya , Mitsuda Kazuhisa , Yazawa Takashi
Published: July 2002

Garnets doped with rare-earth elements can be used in adiabatic demagnetization refrigerators. We have measured the specific heat and magnetization of a single crystal yttrium aluminum garnet (YAG) doped with 30% Er3+ ion at temperatures between 93 mK and 8 K under magnetic fields up to 8.0 T along the 〈111〉 crystal axis. From the specific heat and magnetization, we derived consistent temperature and magnetic-field dependence of the magnetic entropy. Under zero magnetic field, the magnetic entropy begins to decrease below 2 K and becomes half of R ln 2 at ∼160 mK. This decrease is considered to be due to an antiferromagnetic short-range ordering among Er3+ ions. This behavior of the specific heat in the measured temperature range can be explained by a model in which both the crystalline-electric-field ground state and the first excited state are included. The operating temperature of the Er3+-doped YAG as a magnetic coolant is estimated to extend down to ∼100 mK, which is lower than those with nonsubstituted garnets such as gallium–gadolinium–garnet used in the range ∼4.2–15 K. With a doping level of 30%, we estimate that ∼6 kg of Er3+-doped YAG exhibits the same cooling performance at 60 mK as the 916 g of ferric–ammonium–alum salt used for the x-ray spectrometer (microcalorimeter detectors) on the Astro-E satellite.
Cavitation in two-dimensional metallo-supramolecular coordination polyelectrolyte amphiphile complexes
Authors: Khattari Z. , Hatta E. , Kurth D. G. , Fischer Th. M.
Published: July 2002

Cavitation bubbles in a two-dimensional metallo-supramolecular coordination polyelectrolyte amphiphile complex at the air–water interface are formed using laser heating. The cavitation threshold, the rupture kinetics, and the radius of the stationary cavitation gas bubble is measured with Brewster angle microscopy. The tensile strength of the two dimensional complex is estimated and characterized by a critical cavitation pressure.
Dynamical body frames, orientation-shape variables and canonical spin bases for the nonrelativistic N-body problem
Authors: Alba David , Lusanna Luca , Pauri Massimo
Published: July 2002

After the separation of the center-of-mass motion, a new privileged class of canonical Darboux bases is proposed for the nonrelativistic N-body problem by exploiting a geometrical and group theoretical approach to the definition of body frame for deformable bodies. This basis is adapted to the rotation group SO(3), whose canonical realization is associated with a symmetry Hamiltonian left action. The analysis of the SO(3) coadjoint orbits contained in the N-body phase space implies the existence of a spin frame for the N-body system. Then, the existence of appropriate nonsymmetry Hamiltonian right actions for nonrigid systems leads to the construction of an N-dependent discrete number of dynamical body frames for the N-body system, hence to the associated notions of dynamical and measurable orientation and shape variables, angular velocity, rotational and vibrational configurations. For N=3 the dynamical body frame turns out to be unique and our approach reproduces the xxzz gauge of the gauge theory associated with the orientation-shape SO(3) principal bundle approach of Littlejohn and Reinsch. For N⩾4 our description is different, since the dynamical body frames turn out to be momentum dependent. The resulting Darboux bases for N⩾4 are connected to the coupling of the spins of particle clusters rather than the coupling of the centers of mass (based on Jacobi relative normal coordinates). One of the advantages of the spin coupling is that, unlike the center-of-mass coupling, it admits a relativistic generalization.
Singularity formation in 2+1 wave maps
Authors: Isenberg James , Liebling Steven L.
Published: July 2002

We present numerical evidence that singularities form in finite time during the evolution of 2+1 wave maps from spherically equivariant initial data of sufficient energy.
Theoretical determination of magnetic properties of planar benzene isomers
Authors: Ligabue Andrea , Lazzeretti Paolo
Published: July 2002

Four theoretical methods, using continuous transformation of the origin of the current density (CTOCD) to annihilate either the diamagnetic or the paramagnetic contribution, have been employed at the coupled Hartree–Fock level of accuracy to evaluate magnetic susceptibility and nuclear magnetic shielding of five planar molecules with the chemical formula C6H6. The results have been compared with corresponding estimates from basis sets of London orbitals, showing the near Hartree–Fock quality of calculated values and the practicality of CTOCD approaches. Maps of streamlines and intensity of the current density induced in the π-electrons by a stationary magnetic- field perpendicular to the molecular plane are shown to interpret the different magnetic response properties of benzene isomers by means of the Ring–Current model. The plots show delocalized flow in benzene, determining its enhanced diamagnetism. Diamagnetic loops, localized in the region of carbon–carbon bonds, and a paramagnetic vortex in the vicinity of electronic centroid, are found in 3,4-dimethylenecyclobutene and trimethylenecyclopropane. This pattern is typical of conjugated nonaromatic molecules. Delocalized diamagnetic circulation observed in fulvene explains its exalted magnetic susceptibility.