bluuebunny/crossref_metadata_2025 huggingface dataset:
Continuous real-time measurement of fluorescence lifetimes
Authors: Trabesinger W. , Hübner C. G. , Hecht B. , Wild U. P.
Published: September 2002

We report on the continuous measurement of fluorescence lifetimes at low light levels. Fluorescence photons following pulsed excitation generate a pulse sequence with exponentially distributed amplitudes and interphoton times at the output of a time-to-amplitude converter. This sequence is turned into a continuous step function and is time averaged with an adjustable bandwidth. For a single-exponential decay, our approach yields identical results as would be obtained from fitting fluorescence decays, while being a real-time technique. The proposed technique performs especially well at low count rates. We demonstrate the applicability of the method at the example of confocal fluorescence lifetime imaging of single molecules.
Characterization and optimization of scan speed for tapping-mode atomic force microscopy
Authors: Sulchek T. , Yaralioglu G. G. , Quate C. F. , Minne S. C.
Published: September 2002

Increasing the imaging speed of tapping mode atomic force microscopy (AFM) has important practical and scientific applications. The scan speed of tapping-mode AFMs is limited by the speed of the feedback loop that maintains a constant tapping amplitude. This article seeks to illuminate these limits to scanning speed. The limits to the feedback loop are: (1) slow transient response of probe; (2) instability limitations of high-quality factor (Q) systems; (3) feedback actuator bandwidth; (4) error signal saturation; and the (5) rms-to-dc converter. The article will also suggest solutions to mitigate these limitations. These limitations can be addressed through integrating a faster feedback actuator as well as active control of the dynamics of the cantilever.
Are members of the family of hydrogen-bonded complexes formed by furan with the hydrogen halides isostructural? An answer from the rotational spectrum of furan⋯HBr
Authors: Cole G. C. , Legon A. C. , Ottaviani P.
Published: September 2002

The ground-state rotational spectra of the five isotopomers C4H4O⋯H79Br, C4H4O⋯H81Br, C4D4O⋯H79Br, C4H4O⋯D79Br, and C4H4O⋯D81Br of a weakly bound complex formed by furan with hydrogen bromide in the gas phase have been observed with a pulsed-jet, Fourier-transform instrument. Each spectrum was analyzed and fitted to give rotational constants A0, B0, and C0, centrifugal distortion constants ΔJ and ΔJK, and components χaa, χbb-χcc, and χab (or χac in the case of C4D4O⋯H79Br) of the bromine nuclear quadrupole coupling tensor. A detailed analysis of the spectroscopic constants reveals that the observed complex does not have C2v symmetry, with HBr lying along the C2 axis of furan. Instead, the geometry is of the face-on type, with the Br atom of HBr lying close to the perpendicular drawn through the center of the mass of the furan ring. The H atom of HBr lies between the Br atom and the face of the furan ring. The angles αaz made by the HBr internuclear axis z with the a axis has the two possible values ±11.929°. The preferred structure is that generated when the positive value of the angle is chosen and has the HBr subunit pointing in the direction of the O atom of furan. The determined geometrical parameters are r(O⋯H)=2.599(3) Å, φ=112.90(14)°, and θ=6.05(4)°, where φ is the angle made by the O⋯H internuclear line with the local C2 axis of furan and θ is the angular deviation of the O⋯H–Br nuclei from collinearity. Reasons why furan⋯HCl, but not furan⋯HBr, obeys some simple rules for predicting angular geometries are discussed.
Influence of overlayers on determination of the optical constants of ZnSe thin films
Authors: Franta Daniel , Ohlídal Ivan , Klapetek Petr , Montaigne-Ramil Alberto , Bonanni Alberta , Stifter David , Sitter Helmut
Published: September 2002

In this article a multisample modification of variable angle spectroscopic ellipsometry is used to characterize ZnSe thin films prepared by molecular beam epitaxy on substrates formed by GaAs single crystals. Atomic force microscopy (AFM) is employed to characterize the morphology of the upper boundaries of these films. To interpret the ellipsometric data a relatively complicated physical model that contains a rough overlayer between the ambient and the ZnSe film and a transition layer between the GaAs substrate and the ZnSe film is employed. Several models of dispersion of the optical constants of the overlayers are examined to interpret the ellipsometric data. It is shown that the choice of overlayer dispersion model has a strong influence on determining the optical constants and dielectric function of the ZnSe films in the near-UV region. Within the visible region there are no differences between the overlayer dispersion models regarding determination of the ZnSe optical constants. The spectral dependences of the ZnSe dielectric function obtained are compared with those presented by other researchers. Further, by AFM it is shown that the upper boundaries of the ZnSe films are randomly rough and partially covered with small objects.
Fusion bases as facets of polytopes
Authors: Bégin L. , Cummins C. , Lapointe L. , Mathieu P.
Published: September 2002

A new way of constructing fusion bases (i.e., the set of inequalities governing fusion rules) out of fusion elementary couplings is presented. It relies on a polytope reinterpretation of the problem: the elementary couplings are associated with the vertices of the polytope while the inequalities defining the fusion basis are the facets. The symmetry group of the polytope associated with the lowest rank affine Lie algebras is found; it has order 24 for su∧(2), 432 for su∧(3) and quite surprisingly, it reduces to 36 for su∧(4), while it is only of order 4 for sp∧(4). This drastic reduction in the order of the symmetry group as the algebra gets more complicated is rooted in the presence of many linear relations between the elementary couplings that break most of the potential symmetries. For su∧(2) and su∧(3), it is shown that the fusion-basis defining inequalities can be generated from few (one and two, respectively) elementary ones. For su∧(3), new symmetries of the fusion coefficients are found.
Linear Paul trap for strontium ions
Authors: Berkeland D. J.
Published: September 2002

We demonstrate a linear radio-frequency trap for confining strontium ions that is simply constructed nearly entirely with commercial off-the-shelf components. The electrodes of this trap are spaced to allow tight radial confinement, and segmented to give tight axial confinement with minimal distortion of the radio-frequency field. The resulting secular frequencies of the trap are 1 MHz in the radial direction and 100 kHz in the axial direction, the ions can be imaged with 2–3 μm resolution, and we detect over 20 000 photons per second per ion. This article describes the construction of the vital parts of the trap system, the laser system and optics used to Doppler cool the ions and to observe quantum jumps, and the operation and behavior of the complete system.
Effect of the magnetohydrodynamic modes on generation of the nonthermal electrons in high density tokamak plasma
Authors: Savrukhin P. V.
Published: September 2002

Various types of nonthermal x-ray bursts are observed in the central part of plasma with high density in the T-10 tokamak [Alikaev et al., Plasma Phys. Controlled Fusion 30, 381 (1988)] with the use of a tangential view x-ray imaging system. In contrast with the “classical” hard x-ray bursts produced during the interaction of the runaway electron beams (Eγ∼0.5–3 MeV) with the tokamak wall, the “internal” bursts are associated with forward bremsstrahlung emission of the suprathermal electrons (Eγ∼25–100 keV) from the plasma area around the q=1 and q=2 magnetic surfaces. It is speculated that localized beams of the suprathermal electrons can be induced during magnetic reconnection around the X points of the m=1, n=1, and m=2,n=1 magnetic islands. Numerical simulations indicate that generation and losses of the suprathermal electrons in plasma with high density depend critically on the rate of the reconnection and amplitude of the magnetohydrodynamic modes.
Probing luminescence from nonspherical bubble collapse
Authors: Ohl Claus-Dieter
Published: September 2002

The luminescence from single laser produced cavitation bubbles for varying degrees of asphericity is investigated temporally, spatially, and spectrally. The degree of asphericity is controlled with an adjustable rigid boundary near the bubble. Temporally, single and multiple light emission events happen during a time interval of 80 ns. The luminescence duration increases with increasing asphericity. Spatially, the emissions from nonspherically collapsing bubbles display a pronounced halo around the central spot several times larger than the luminescence from spherical collapse. Spectrally, the ratio of the line to continuum emission of the sodium doublet is enhanced, whereas the total emitted energy decreases by four orders of magnitude as compared to the spherical collapse. These findings point towards emission not only from the bubble interior but also from the liquid surrounding the bubble.
The rotational dynamics of N2–HF and OC–HF in helium nanodroplets
Authors: Nauta K. , Miller R. E.
Published: September 2002

Infrared spectra are reported for the N2–HF and OC–HF complexes formed in helium nanodroplets. The spectra are rotationally resolved and show that the structures of the complexes are essentially the same (linear) as determined previously from gas phase studies. The rotational constants of the helium solvated complexes are smaller than the gas phase values, in agreement with what has been previously observed for several other systems. N2–HF is particularly interesting when studied in the presence of a large electric field, given that the change in the dipole moment upon vibrational excitation is large, so that the individual transitions in the pendular spectrum can be resolved. This enables us to monitor the change in the helium environment around the molecule as it goes from rotational to pendular motion. The results also reveal that the relaxation lifetime of these pendular states in helium is comparable to the flight time of the droplets through the electric field and depends upon the droplet size.
Numerical study of wave propagation in a nonuniform compressible flow
Authors: Povitsky Alex
Published: September 2002

The propagation of acoustic waves originating from cylindrical and spherical pulses in a nonuniform mean flow, and in the presence of a reflecting wall, is investigated by solving linearized Euler equations in terms of disturbances using high-order compact approximation of spatial derivatives. The two-dimensional and three-dimensional stagnation flows and a flow around a cylinder are taken as prototypes of real-world flows with strong gradients of mean pressure and velocity. The intensity and directivity of acoustic wave patterns appear to be quite different from the benchmark solutions obtained in a static environment for the same geometry. The physical reasons for amplification of sound are discussed in terms of redistribution of acoustic energy and its potential and kinetic components. The role of acoustic energy exchange with the mean flow is investigated. Compressibility of the background mean flow is taken into account and its effect on amplification of acoustic pressure is discussed.
Characterization of photorefractive GaAlAs multiple quantum well structures by ac photoelectromotive force and modulated photocarrier grating techniques
Authors: Stepanov S. , Seres I. , Mansurova S. , Nolte D.
Published: September 2002

Simultaneous utilization of ac photoelectromotive (photo-EMF) and modulated photocarrier grating (MPG) techniques for characterization of bipolar transport in semi-insulating photorefractive GaAlAs multiple quantum well structure is reported. From the ac photo-EMF experiment the electrons were determined as the dominant photocarriers for interband optical excitation at λ=633 nm. From the self-consistent data obtained from photo-EMF and MPG experiments the diffusion length of the photoelectrons was evaluated as LD≃1.4±0.2 μm, and that of the photoholes (minority carriers)—as LDm≃0.6±0.1 μm. In the MPG configuration, the contrast of the illuminating interference pattern was controlled by electro-optic modulation of the signal beam polarization, which allowed us to measure dielectric relaxation time of the photoconductor as well. As evaluated by both techniques it proved to be about τdi≃0.05 μs for the average light intensity I0≃0.3 mW/mm2. The proposed combination of the characterization techniques seems to be especially promising for amorphous a–Si and polymer films.
Correlated motion of small Ag clusters and Ge dimer-buckling on Ge(001)
Authors: Naitoh Y. , Nakatsuji K. , Komori F.
Published: September 2002

We study submonolayer Ag deposited Ge(001) surfaces at 90 K by scanning tunneling microscopy (STM). Silver atoms and their small aggregates on the surface are found as bright dots on the surface at 78 K. The Ag aggregates form small clusters, and move with the change of the buckling phase in the adjacent Ge dimer row at 180 K as observed in successive STM images. These are transient processes to the formation of a surface alloy between Ag and Ge.
Amorphous solid water’s isotopic exchange kinetics
Authors: Johari G. P.
Published: September 2002

The mechanism for isotopic exchange by proton hopping and subsequent reorientation of H2O molecules in vapor-deposited, 0.1 mol % 2-naphthol containing amorphous solid water [M. Fischer and J. P. Devlin, J. Phys. Chem. 99, 11584 (1995)] has been reconsidered and an alternative mechanism in terms of diffusion controlled reactions is proposed. In this mechanism, H2O molecules diffuse within the H-bonded clusters, with or without a net increase in the number of H bonds, and isotopic exchange occurs by two processes: (i) Random diffusion of molecules in two clusters, one containing D2O and the other H3O+, leads to formation of an intercluster H bond, which in turn provides a path for proton hopping and converts one D2O (plus one H2O) to two coupled HODs by proton translocation and subsequent orientation. (ii) One H bond between two HOD neighbors in a cluster breaks and reforms with another H2O in the same cluster or in a different cluster, and hence a coupled HOD is converted to an uncoupled HOD. The decrease in D2O and the increase in HOD concentrations with time follow a stretched exponential kinetics, with exponent of 0.65 for the former and 0.54 for the latter process at 122 K. This is equivalent to the time-dependent rate constant in Plonka’s formalism [J. Chem. Phys. 96, 1128 (1992)] and is seen as characteristic of dispersive kinetics. Because fluctuation of the environment is slower than the time scale of overall barrier crossing, the mass-controlled kinetics equations do not apply to a consecutive reaction scheme. The known variation of the isotopic species concentration with time seems to be consistent with this reaction kinetics.
Magnetic and structural characterization of Mn-implanted, single-crystal ZnGeSiN2
Authors: Pearton S. J. , Overberg M. E. , Abernathy C. R. , Theodoropoulou N. A. , Hebard A. F. , Chu S. N. G. , Osinsky A. , Fuflyigin V. , Zhu L. D. , Polyakov A. Y. , Wilson R. G.
Published: September 2002

Epitaxial layers of ZnSiN2, ZnGe0.65Si0.35N2, and ZnGe0.31Si0.69N2 grown on Al2O3 substrates were implanted at 350 °C with high doses (5×1016 cm−2) of Mn+ ions and annealed at 700 °C. The implanted region did not appear to become amorphous and showed strong selected area diffraction patterns. Hysteresis was observed in magnetization versus field curves from all of the implanted samples. Differences in field-cooled and zero field-cooled magnetization persisted to temperatures of ∼200 K for ZnSiN2, and ∼280 K for both ZnGe0.31Si0.69N2 and ZnGe0.69Si0.31N2. The results are consistent with recent magnetic data from (ZnxMn1−x)GeP2, ZnSnAs2 and (CdxMn1−x)GeP2 and suggest that this class of materials may be promising for dilute magnetic semiconductor applications.
Time correlation function approach to liquid phase vibrational energy relaxation: H2 and D2 solutes in Ar solvent
Authors: Miller David W. , Adelman Steven A.
Published: September 2002

The theoretical treatment in Paper I [D. W. Miller and S. A. Adelman, J. Chem. Phys. 117, 2672, (2002), preceding paper] of the vibrational energy relaxation (VER) of low-frequency, large mass dihalogen solutes is extended to the VER of the high-frequency, small mass molecular hydrogen solutes H2 and D2 in a Lennard-Jones argon-like solvent. As in Paper I, values of the relaxation times T1 predicted by the theory are tested against molecular dynamics (MD) results and are found to be of semiquantitative accuracy. To start, it is noted that standard Lennard-Jones site–site potentials derived from macroscopic data can be very inaccurate in the steep repulsive slope region crucial for T1. Thus, the H–Ar Lennard-Jones diameter σUV is not taken from literature values but rather is chosen as σUV=1.39 Å, the value needed to make the theory reproduce the experimental H2/Ar gas phase VER rate constant. Next, by MD simulation it is shown that the vibrational coordinate fluctuating force autocorrelation function 〈F̃(t)F̃〉0 of Paper I decays roughly an order of magnitude more rapidly for the molecular hydrogen solutions than for the dihalogen solutions. This result implies a relatively slow decay for the molecular hydrogen friction kernels β(ω)=(kBT)−1∫0∞〈F̃(t)F̃〉0 cos ω tdt, yielding for the H2/Ar and D2/Ar systems at T=150 K physical millisecond values for T1=β−1(ωl) despite the high liquid phase vibrational frequencies ωl of H2 and D2. The rapid decay of 〈F̃(t)F̃〉0 is due to both the steepness of the repulsive slope of the H–Ar potential and the small masses of H and D. Thus, the small value chosen for σUV is needed to avoid unphysically long T1’s. Next, an analytical treatment of the H2/D2 isotope effect on T1, based on the theory, is found to predict that the H2/Ar and D2/Ar T1’s are close in value due to the compensating effects of lower ωl but slower decay of 〈F̃(t)F̃〉0 for D2/Ar, a result in qualitative agreement with experiments. Applying the theory to numerically study the isothermal ρ dependencies of the VER rate constant k(T,ρ)=T1−1 at 150 K reveals that for both H2/Ar and D2/Ar, as for the solutions of Paper I, k(T,ρ) can be factorized as in the isolated binary collision (IBC) model. Moreover, the molecular theory and IBC rate isotherms differ only slightly for both solutions, a result interpreted in terms of the form of the H–Ar pair correlation function. The theoretical and experimental rate isotherms at 150 K are then compared. Agreement is very good for the H2/Ar solution, but for the D2/Ar solution the theoretical rates are about four times too large. Finally, the isochoric T dependencies of k(T,ρ) in the range 200–1000 K are found for both solutions to conform to an Arrhenius rate law.
Energy dependence of forward scattering in the differential cross section of the H+D2→HD(v′=3,j′=0)+D reaction
Authors: Aoiz F. J. , Bañares L. , Castillo J. F. , Sokolovski D.
Published: September 2002

Extensive time-independent quantum mechanical scattering calculations for the H+D2(v=0,j=0) reaction have been performed in the collision energy range 1.39–2.20 eV on the Boothroyd–Keogh–Martin–Peterson potential energy surface. The theoretical differential cross sections (DCS) obtained for the H+D2→HD(v′=3,j′=0)+D channel of the reaction have been compared with recent measurements by Zare and co-workers over the collision energy range 1.39–1.85 eV using the photoloc technique [S. C. Althorpe et al., Nature (London) 416, 67 (2002)]. An excellent agreement between experiment and theory has been found for most of the collision energies studied. In particular, the appearance and evolution of forward scattering with collision energy observed experimentally has been quantitatively reproduced by the theoretical calculations. An analysis of the theoretical results, including a semiclassical complex angular momentum analysis, have been performed in order to ascertain the origin of the sharp forward peaks in the DCS.
Electronic photodissociation spectroscopy of isolated IrX62− (X=Cl,Br)
Authors: Friedrich Jochen , Gilb Stefan , Ehrler Oli T. , Behrendt Anja , Kappes Manfred M.
Published: September 2002

Photodissociation spectra of free doubly charged anions IrX62− (X=Cl,Br) were measured in the photon energy range from 1.5 to 2.9 eV. Both data sets show the same features as the spectra of the respective aqueous solutions. Compared to solution, the gas phase absorption bands of IrBr62− are redshifted by 0.01–0.15 eV. For IrCl62− no such shift could be observed. Photodissociation of IrBr62− results in the formation of Br−, IrBr4−, and IrBr5−. Fluence dependent measurements of fragment formation as well as parent ion depletion, allowed inferences regarding the dissociation pathway and the inner barrier height for the dissociation process which was estimated to be 1.6±0.2 eV. From measurements of the kinetic energy released upon fragmentation into monoanions, we estimate the outer barrier height to be 2.2±0.2 eV.
Simple, compact source for low-temperature air plasmas
Authors: Sheehan D. P. , Lawson J. , Sosa M. , Long R. A.
Published: September 2002

A simple, compact source of low-temperature, spatially and temporally uniform air plasma using a Telsa induction coil driver is described. The low-power ionization discharge plasma is localized (2 cm×0.5 cm×0.1 cm) and essentially free of arc channels. A Teflon coated rolling cylindrical electrode and dielectric coated ground plate are essential to the source’s operation and allow flat test samples to be readily exposed to the plasma. The plasma is a copious source of ozone and nitrogen oxides. Its effects on various microbes are discussed.
Nonequilibrium Reynolds stress for the dispersed phase of solid particles in turbulent flows
Authors: Shin Mansoo , Lee Jin W.
Published: September 2002

The nonequilibrium turbulent transport of dispersed-solid phase is investigated theoretically, and confirmed with simple numerical experiments. The nonequilibrium form of the particle Reynolds stress was obtained as a function of Stokes number, Stτ (=τp/τ), where τp is the particle relaxation time and τ the characteristic time scale of particle–turbulence interaction. The nonequilibrium memory effect could be explicitly expressed as a damped residual contribution of initial particle Reynolds stress. Major interest is given to the region of τ∼O(τL), where τL is the integral time scale associated with the Lagrangian autocorrelation of fluctuating velocities of carrier turbulence. In two-dimensional (2D) simple shear flows with stationary homogeneous turbulence, explicit expressions for the equilibrium and nonequilibrium particle Reynolds stress were obtained, and their characteristics were analyzed extensively for a wide range of particle relaxation time and various conditions of mean shearing of the carrier phase. The reliability of theoretical predictions was tested through a comparison with results of stochastic simulation of particle motion. It was shown that the particle inertia and the mean shearing of the carrier phase seriously affect the relaxation time of particle Reynolds stress required to reach its equilibrium state in such a way that the relaxation time to equilibrium increases in proportional to the increase of τp and this trend is augmented by increasing the mean shearing of the carrier phase. Finally, a nonequilibrium constitutive equation for the dispersed phase Reynolds stress was obtained in a form usable in the two-fluid Eulerian approach for two-phase turbulent flows.
Evolution of mean and fluctuating velocity components in the laminar–turbulent transition of spherical Couette flow
Authors: Nakabayashi Koichi , Zheng Zhiming , Tsuchida Yoichi
Published: September 2002

In the laminar–turbulent transition of spherical Couette flow with only the inner sphere rotating in the case of gap ratio 0.14, we have studied experimentally the evolution of mean and fluctuating velocities with increasing Reynolds number at two meridian angles, θ=60° and 90° (the equator), respectively. For a reduced Reynolds number, R*=1.2, the phase of a fluctuating velocity component in the main-flow direction advances radially inward from the outer to the inner sphere at θ=90°, but delays radially inward at θ=60°. When R* is 4.2 and 6.0, the phase slightly advances radially inward at θ=90°, but does not change so much at θ=60°. The phase-averaged profile of fluctuating velocity in the presence of spiral Taylor–Görtler (TG) vortices differs from that in the presence of traveling azimuthal waves. Spiral TG vortices make the amplitude of fluctuating velocity large in the central part of the gap. But traveling azimuthal waves make it large somewhat near the inner sphere. For the flow with spiral TG vortices and/or traveling waves at low R*, the root-mean square value of fluctuating velocity tends to be large on the outer-sphere side at θ=60°, while on the inner-sphere side at θ=90°. In the fully turbulent flow for R*≧20, the rms value of fluctuation velocity becomes large near the fixed outer sphere at both θ=60° and 90°, and log-laws on mean azimuthal velocity profiles hold there.
Turbulent microwave plasma thermodynamics for fundamental fluctuation modes
Authors: Podder N. K. , Johnson J. A. , Mezonlin E. D.
Published: September 2002

Microwave plasmas are generated in helium, neon, argon, krypton, and xenon at a range of microwave powers from 300 to 1800 W. A floating Langmuir double probe is employed to determine plasma electron density and temperature for all five species. The standard turbulence analysis is carried out by using time resolved neutral line emission data form these gases at a sampling rate of 100 MHz. From the Fourier power spectrum of the data, the strongest fluctuation frequency is found to be consistently the fundamental or a second harmonic of a turbulence characteristic frequency in the spectra. In all five species the strongest frequency is not influenced by increased microwave power even though other thermodynamic parameters are changed. The low chaotic dimension for all species seems independent of microwave power and of turbulent fluctuation energy. The phase space trajectories show simplicity and periodicities are consistent with the low chaotic dimension and with the peak frequencies obtained from the fluctuation spectra. The deterministic cyclic evolution of the phase space trajectories with low chaotic dimension for our plasma suggests underlying simplicity in the local turbulence physics. The average value of the strongest fluctuation frequency decreases with increasing atomic weight suggesting that the fundamental frequency is a characterizing parameter for the turbulent system.
Formation of curvature singularity along vortex line in an axisymmetric, swirling vortex sheet
Authors: Sakajo Takashi
Published: September 2002

We consider an axisymmetric, swirling vortex sheet in an inviscid and incompressible flow. Caflisch et al. pointed out that the vortex sheet acquired a singularity in finite time, but the property of the singularity was not revealed. In the present paper we show convincing numerical evidences of the singularity formation by applying the same numerical methods as what was used in the study of a two-dimensional (2D) vortex sheet. We find that the radial and axial components of the axisymmetric vortex sheet behave like the 2D singularity that has been observed in many vortex-sheet motions, while the azimuthal component of the sheet behaves differently. Furthermore, the singularity appears along the vortex line and the first derivative of the vortex sheet strength forms a cusp, while the known singularities are associated with the curvature along curves perpendicular to the vortex lines and the sheet strength has a cusp.
Pseudopotential hole–particle formalism for excitations in xenon molecules and clusters. II. The electronic structure of Xe2*
Authors: Jonin C. , Spiegelmann F.
Published: September 2002

The Ω electronic states of the Xe2* excimer dissociating into Xe(5p6, 1S0)+Xe* (5p56s, 5p56p, 5p55d, 5p57s, and 5p57p) are determined using a one-electron hole–particle formalism including spin–orbit coupling. A partially diabatic correlation of the states is achieved. The content of the states in terms of the ionic cores and the Rydberg electron is analyzed theoretically by means of molecular natural orbitals and their overlaps with orbitals of the separated atoms. The spectroscopic molecular constants of all bound states dissociating up to the Xe+Xe*(7s) limit are determined. This theoretical determination of the electronic structure is used to survey and discuss a wide pannel of experimental data characterizing the spectroscopy of the Xe2* excimer and involving both the gerade and ungerade states.
Structural investigation of thiophene thiol adsorption on Au nanoclusters: Influence of back bonds
Authors: Majumder Chiranjib , Briere Tina M. , Mizuseki Hiroshi , Kawazoe Yoshiyuki
Published: September 2002

The adsorption of thiolate radicals on the Au24 nanocluster truncated from the Au (111) surface is investigated using first principles electronic structure calculations under the density functional theory formalism. Particular emphasis is given to understanding the chemical interactions at the gold-sulfur interface. In order to describe the influence of the back bonds at the thiolate sites we have carried out adsorption studies with thiophene 2-thiolate (-ST) and thiophen–2-yl-methanethiolate (-SCH2T) along with atomic sulfur (S), mercapto (-SH), and methylthiolate (-SCH3). The results suggest that the adsorption geometry at the gold-sulfur interface is strongly dependent on the local environment of the terminal sulfur atom. The interfacial charge transfer is found to be localized along the Au-S bonds and does not influence the molecular structure of the thiophene ring.
Transfer matrix approach to quantum conductivity calculations in single-wall carbon nanotubes
Authors: Andriotis Antonis N. , Menon Madhu , Srivastava Deepak
Published: September 2002

We present an efficient transfer matrix formalism for obtaining the quantum conductivity of single-wall carbon nanotubes (SWCN’s) based on a nonorthogonal tight-binding scheme. The formalism is used to calculate conductivity in the presence of topological defects and H adsorbates. I-V characteristics show large oscillatory behavior as a function of the number of H adatoms for both (10,0) and (5,5) SWCN’s. Furthermore, the conductivity is found to depend sensitively on structural relaxation.
Electron (hole) paramagnetic resonance of spherical CdSe nanocrystals
Authors: Gokhberg K. , Glozman A. , Lifshitz E. , Maniv T. , Schlamp M. C. , Alivisatos P.
Published: September 2002

A new mechanism of electron paramagnetic resonance in spherical zinc-blende semiconductor nanocrystals, based on the extended orbital motion of electrons in the entire nanocrystal, is presented. Quantum confinement plays a crucial role in making the resonance signal observable. The mechanism remains operative in nanocrystals with uniaxially distorted shape. A theoretical model based on the proposed mechanism is in good quantitative agreement with unusual ODMR spectra observed in nearly spherical CdSe nanocrystals.
Stabilization of impurity states in crossed magnetic and electric fields
Authors: Krajewska K. , Kamiński J. Z.
Published: September 2002

It is shown that the renormalizability of the zero-range interaction in the two-dimensional space is always followed by the existence of a bound state, which is not true for odd-dimensional spaces. A renormalization procedure is defined and the exact retarded Green’s function for electrons moving in two dimensions and interacting with both crossed magnetic and electric fields and an attractive zero-range interaction is constructed. Imaginary parts of poles of this Green’s function determine lifetimes of quasibound (resonance) states. It is shown that for some particular parameters the stabilization against decay occurs even for strong electric fields.
Guarded needle for “charge injection” measurement
Authors: Cao Yang , Jiang Grace Ge , Boggs Steven
Published: September 2002

The ability to measure resistive current resulting from high-field carrier mobility in dielectrics provides insight into transport phenomena and high-voltage performance. The threshold electric field for the formation of a space-charge limited field region under highly inhomogeneous field conditions has fundamental significance in that above this field, rapid degradation takes place from hot electrons and UV photons generated by carrier recombination. A guarded needle electrode with a tip radius in the range of 5 μm and an electrical guard which extends to within about 35 μm of the tip provides the basis for measuring high-field mobility as the resistive current can be measured independent of the displacement current. The residual displacement current resulting from the fF capacitance from the needle tip to the opposite plane electrode is compensated out of the measurement. In this article, we provide construction details for such a guarded needle.
Ion-induced nucleation: Measurement of the effect of embryo’s size and charge state on the critical supersaturation
Authors: Gamero-Castaño Manuel , de la Mora Juan Fernández
Published: September 2002

We use 42 different charged nanoparticles generated by electrospray ionization and a differential mobility analyzer of unusual resolving power, as well as a condensation chamber of the turbulent-mixing type to study the dependence between the diameter, charge state, and critical supersaturation of embryos promoting heterogeneous nucleation. The nanoparticles investigated have diameters ranging from 0.43 to 6.51 nm, and positive charge states varying between 1 and 5 elementary units. We find that the critical supersaturation of small singly charged ions (mobility diameters bellow 1.01 nm) is independent of their size and its value coincides with the result anticipated by Thompson in his theory of ion-induced nucleation. On the other hand, the supersaturation required to activate multiply charged embryos is consistently higher than Thomson’s prediction. In fact, the reduction of the critical supersaturation induced by electrification is much smaller than expected for large and multiply charged embryos, and their critical radius is estimated better by Kelvin’s criterion. We speculate that this discrepancy is due to the geometrical differences between the actual nucleation sites and the idealized embryo considered in Thomson’s model.
Compressional Alfvén eigenmode dispersion in low aspect ratio plasmas
Authors: Gorelenkov N. N. , Cheng C. Z. , Fredrickson E.
Published: September 2002

A new theory of compressional Alfvén eigenmodes (CAEs) applicable to low aspect ratio toroidal plasmas is developed. The theory is based on the ballooning representation of the poloidal dependence of the perturbed quantities. The theory provides localized CAE solution with discrete eigenfrequencies determined by the Alfvén velocity of the plasma. The eigenmode localization is determined by the effective “potential” well given by the minimum of the magnetic field on a magnetic surface, so that the plasma equilibrium is essential to accurately determine the dispersion of CAEs. The mode structure is localized in both minor radius and poloidal angle directions near the low magnetic field side. Properties of recently observed fast ion beam driven instabilities in MHz frequency range in National Spherical Torus experiments (NSTX) [Kaye, et al., Fusion Technol. 36, 16 (1999)] are consistent with ones of CAEs.
Ion-beam doping of silver in amorphous As2S3 thin films
Authors: Suzuki Takenobu , Hosono Hideo
Published: September 2002

Double layers of metallic Ag/amorphous (a-)As2S3 on silica substrates were irradiated with 300 keV He+ ions to examine doping of Ag+ into a-As2S3 by dense electronic excitation. The irradiating ions passed through the double layers and came to rest in the silica substrates. Thus, energy deposition from energetic He+ ions into the double layers was completely almost due to electronic processes (99.4%). Ionized silver found to diffuse into a-As2S3 as a result of electronic excitation by ion-beam irradiation. Rutherford backscattering spectroscopy revealed that the highest concentration of silver doped into a-As2S3 was 35 at. %. When the resulting Ag-doped a-As2S3 was illuminated by band gap light, photo-surface-deposition of metallic silver particles, which was not observed for Ag photo-doped specimens, was observed.
Near-dissociation states and coupled potential curves for the HeN+ complex
Authors: Soldán Pavel , Hutson Jeremy M.
Published: September 2002

The near-dissociation microwave rovibronic spectra of HeN+ [Carrington et al., Chem. Phys. Lett. 262, 598 (1996)] are used to obtain coupled potential energy curves for the six electronic states correlating with He+N+ 3P0, 3P1, and 3P2. High-quality ab initio calculations are carried out, using a spin-restricted open-shell coupled-cluster method with an augmented correlation-consistent quintuple-zeta basis set (aug-cc-pV5Z). Fully coupled calculations of bound and quasibound states are performed, including all six electronic states, and suggest two possible assignments of the observed transitions. The potentials are then morphed (scaled) to reproduce the experimental frequencies. One of the two assignments, designated SH1, is preferred because it gives a more satisfactory explanation of the observed hyperfine splittings. The corresponding morphed potential has well depths of 1954 cm−1 and 192 cm−1 for the spin-free 3Σ− and 3Π curves, respectively.
State-to-state rate constants for the rotational relaxation of NH(A 3Π,v=0) radicals in inelastic collisions with NH3
Authors: Hake A. , Stuhl F.
Published: September 2002

A state-resolved experimental investigation of the rotational relaxation of the NH(A 3Π,v=0) radical in inelastic collisions with ammonia is presented. Fine structure and Λ-doublet resolved, state-to-state rate constants were extracted from NH(A 3Π,v=0→X 3Σ−,v=0) fluorescence spectra recorded upon laser excitation to individual levels (N,Fj,e/f ) (j=1–3) in the A 3Π,v=0 state. Relaxation trends are reported with regard to Hund’s coupling (a), (b), and intermediate cases. At low N, the most dominant process is the Λ-doublet mixing. For high N, rotational relaxation proceeds predominantly via ΔN=−1 transitions at conservation of the spin unit and the A′/A″ symmetry. A general tendency of conservation of the spin unit (ΔΩ=0) is observed. A comparison of the dipolar collision partner NH3 with the spherical He is performed.
Modeling plasma contact with the main vessel walls of a divertor tokamak
Authors: Stangeby P. C.
Published: September 2002

Simple, analytic models are developed to estimate the plasma particle fluxes to the main chamber walls of a divertor tokamak in terms of assumed values of the cross-field transport coefficients, D⊥, v⊥, and of the wall–gap size. The wall–gap size is the cross-field width of the gap between the last closed flux surface, LCFS, defined by magnetic (divertor) separatrix and the wall of the main chamber, the “wall-limiter.” The neutrals recycling from the wall may be ionized at, or inboard of, the LCFS—which is a key feature of the so-called “simple SOL” and which is referred to here as the pure transport case. On the other hand, the neutrals may be ionized within the scrape-off layer, SOL, in the main chamber, with the ions returning to the wall—referred to here as the SOL ionization case. SOL ionization gives rise to a radial flux amplification effect, in the sense that the plasma flux to the walls is larger than for the “simple SOL”—all else held equal, including the separatrix density, wall–gap, and the transport coefficients. The amplification factor is found to be only modest, however, up to ∼2x. Primarily, plasma–wall contact is therefore just simply a function of cross-field plasma transport, which is strongly anomalous in the SOL, probably due to turbulence, and possibly intermittent, large-event activity in the outer regions. The experimental database for D⊥SOL, v⊥SOL is weak, particularly for the far SOL, a situation calling for increased experimental and theoretical effort.
Competitive effects of grooves and photoalignment on nematic liquid-crystal alignment using azobenzene polymer
Authors: Chung Doo-Han , Fukuda Takashi , Takanishi Yoichi , Ishikawa Ken , Matsuda Hiro , Takezoe Hideo , Osipov Mikhail A.
Published: September 2002

Effect of an anisotropic photoalignment layer and microgrooves on nematic liquid-crystal (LC) alignment was quantitatively examined using azobenzene polymer thin film with surface relief grating (SRG) of about 1 μm pitch. The SRG with various modulation depths was treated with polarized light irradiation to align molecules at 45° from the groove. Nematic LC molecules, 4′-n-pentyl-4-cyanobiphenyl, orient to the photoaligned direction on the SRG being shallower than 200 nm. The orientation rather sharply deviates from the photoaligned direction toward the groove direction with increasing grating depth into the deeper region than 200 nm and finally becomes parallel to the grooves on the SRG of 400 nm deep. This behavior is successfully simulated by the consideration of anisotropic surface interaction and an elastic energy of LCs.
Thomson-scattering observation of non-Maxwellian electron energy distribution in inductively coupled C4F8/Ar and CF4/Ar plasmas and the effect of vibrational excitation and nonlocal electron kinetics
Authors: Kono A. , Funahashi H.
Published: September 2002

Laser Thomson-scattering diagnostics of electron energy distribution function (EEDF) was carried out for inductively coupled cyclo–C4F8/Ar and CF4/Ar plasmas at 25 mTorr. Care was taken to ascertain that the observed Thomson spectra are unaffected by other laser-induced effects, in particular by Raman scattering. Admixing of C4F8 and CF4 with Ar resulted in marked deviation of EEDF from Maxwellian. The mechanism leading to non-Maxwellian EEDF was also studied by a Monte Carlo simulation. The results suggest that the major factors responsible are rapid electron energy loss due to vibrational excitation and the effect nonlocal electron kinetics associated with localized electron heating.
Formation of cellular defect structure on GaSb ion-implanted at low temperature
Authors: Nitta Noriko , Taniwaki Masafumi , Hayashi Yoshihiko , Yoshiie Toshimasa
Published: September 2002

Formation of the anomalous cellular structure in (100) GaSb with Sn ion-implantation at a low temperature is investigated by cross-sectional scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. A fine structure consisting of many cells was formed on GaSb surface implanted by 60 keV Sn. The cell diameter and the thickness of the walls partitioning the cells were about 50 and 10 nm, respectively, which are almost constant in the range of the ion dose 4.0×1014–8.9×1014 ions/cm2. The depth of the cells increased linearly with increasing ion dose, from 100 nm in the sample implanted with a dose of 4.0×1014 ions/cm2 to 220 nm in that implanted with a dose of 8.9×1014 ions/cm2. From the experimental results, it is concluded that the development of the cellular structure originates in formation of the voids. An improved defect formation mechanism based on movement of the implantation-induced point defects is discussed.
Orientational and induced contributions to the depolarized Rayleigh spectra of liquid and supercooled ortho-terphenyl
Authors: Mossa S. , Ruocco G. , Sampoli M.
Published: September 2002

The depolarized light scattering spectra of the glassforming liquid ortho-terphenyl have been calculated in the low frequency region using molecular dynamics simulation. Realistic system configurations are produced by using a recent flexible molecular model and combined with two limiting polarizability schemes, both of them using the dipole-induced-dipole contributions at first and second order. The calculated Rayleigh spectral shape are in good agreement with the experimental results in a large temperature range. The analysis of the different contributions to the Rayleigh spectra emphasizes that the orientational and the collision-induced (translational) terms lie on the same time scale and are of comparable intensity. Moreover, the cross correlation terms are always found to be an important contribution to the scattering intensity.
Transition from unbounded to bounded plasma whistler wave dispersion
Authors: Franck Christian M. , Grulke Olaf , Klinger Thomas
Published: September 2002

Whistler wave dispersion measurements are done in a linear magnetized helicon plasma experiment. The waves are excited by an induction loop and detected by movable magnetic probes for a frequency range of 100–800 MHz, corresponding to 0.05–0.9 ωce. The dispersion of whistler waves is measured for various plasma densities and magnetic field strengths. A key issue is to study the transition from an unbounded to bounded plasma wave dispersion. A comparison with theoretically derived dispersion relations is made. For small wavelengths, the dispersion can be described with whistler wave theory for unbounded plasmas whereas for larger wavelengths, the bounded geometry must be taken into consideration. The experimental results agree with theoretical dispersion relations derived for the bounded and the unbounded situation.
Comment on “Rapid calculation of the Coulomb component of the stress tensor for three-dimensional systems with two-dimensional periodicity” [J. Chem. Phys. 115, 4457 (2001)]
Authors: Mazars Martial
Published: September 2002

The Ewald-like method for quasi-two dimensional systems proposed by M. Kawata and co-workers [J. Chem. Phys. 115, 4457 (2001)] is examined and compared to the method proposed by Sphor, Yeh, Berkowitz and others. Both methods are found numerically equivalent.
Response to “Comment on ‘Rapid calculation of the Coulomb component of the stress tensor for three-dimensional systems with two-dimensional periodicity’ ” [J. Chem. Phys. 117, 3524 (2002)]
Authors: Kawata Masaaki , Mikami Masuhiro , Nagashima Umpei
Published: September 2002

The difference between the Ewald method for systems with three-dimensional periodicity and for three-dimensional systems with two-dimensional periodicity [J. Chem. Phys. 115, 4457 (2001)] is described.
Characterizing entanglement
Authors: Bruß Dagmar
Published: September 2002

Quantum entanglement is at the heart of many tasks in quantum information. Apart from simple cases (low dimensions, few particles, pure states), however, the mathematical structure of entanglement is not yet fully understood. This tutorial is an introduction to our present knowledge about how to decide whether a given state is separable or entangled, how to characterize entanglement via witness operators, how to classify entangled states according to their usefulness (i.e., distillability), and how to quantify entanglement with appropriate measures.
Generalized semirigid vibrating rotor target model for atom–poly reaction: Inclusion of umbrella mode for H+CH4 reaction
Authors: Wang Ming L. , Zhang J. Z. H.
Published: September 2002

In this work, we present a generalized version of the semirigid vibrating rotor target (SVRT) model by including additional vibrational modes explicitly in the SVRT Hamiltonian. The inclusion of additional vibrational modes eliminates the uncertainty of fixing certain geometries of the target molecule as required in the basic SVRT model. This generalized SVRT (GSVRT) model was employed to study the benchmark reaction H+CH4 by including the umbrella mode of CH4. Influence of the umbrella mode of the reagent on reactivity is investigated. It is concluded that the inclusion of the umbrella vibrational mode of CH4 has only a small effect on the reaction from the ground state of the reagent, and essentially no effect from the excited C–H stretching vibrational state of the reagent. However, the initial excitation of the umbrella mode does give a sizable enhancement of reaction and reduces the reaction barrier by about 1.1 kcal/mol.