bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of spin viscosity on ferrofluid flow profiles in alternating and rotating magnetic fields
Authors: Rinaldi Carlos , Zahn Markus
Published: September 2002

Previous models of plane-Poiseuille flow of ferrofluids in alternating and rotating magnetic fields are extended by including the effects of spin diffusion and planar Couette flow. Accurate modeling of this problem is required for the design of experiments to determine key parameters for previously reported anomalous forward and backward ferrofluid pumping behavior in alternating and rotating magnetic fields. The previously derived zero-spin-viscosity analysis predicted multi-valued and singular flow solutions with possible zero and negative effective magnetoviscosity. This analysis calculates analytical expressions for the translational and spin velocity profiles, vorticity profiles, volumetric flow rate, and the shear force on a moving duct surface, comparing the effects of boundary conditions of zero spin velocity and spin velocity equal to half the vorticity at the duct walls. The analysis shows that the single singularity in flow behavior corresponding to zero magnetoviscosity in the zero-spin-viscosity analysis expands to multiple possible flow singularities for the nonzero spin viscosity case. Simple representative shearing experiments are proposed to differentiate between the zero and nonzero spin viscosity solutions, to calculate the values of key viscous parameters, and to show how wall boundary conditions can be determined from shear stress measurements.
Experimental study of the DCCl X̃1A′ state vibrational structure by dispersed fluorescence spectroscopy
Authors: Lee Chia-Ling , Liu Mei-Lung , Chang Bor-Chen
Published: September 2002

Dispersed fluorescence spectra following excitation of the DCCl Ã-X̃ vibronic bands between 570 and 620 nm were successfully recorded for the first time using a combination of direct current discharge and supersonic free jet expansion techniques to produce the radical. This is the first experimental study on the DCCl X̃1A′ state vibrational structure. Analysis of the dispersed fluorescence spectra reveals details of the vibrational structure of the DCCl X̃1A′ state. Emission bands to all three vibrational modes were observed and the fundamental vibrational frequencies were determined: 2078 cm−1 (C–D stretch), 893 cm−1 (bend), and 801 cm−1 (C–Cl stretch). Vibrational parameters including anharmonicities and the bend-stretch coupling constant for the DCCl X̃1A′ state were also determined using a least-square fit. In addition, based on our results, the triplet-singlet energy gap (the zero-point level energy gap) could be estimated to be approximately 11±2 kcal/mol.
Noninvasive, low-noise, fast imaging of blood volume and deoxygenation changes in muscles using light-emitting diode continuous-wave imager
Authors: Lin Yuanqing , Lech Gwen , Nioka Shoko , Intes Xavier , Chance Britton
Published: September 2002

This article focuses on optimizing the signal to noise ratio (SNR) of a three-wavelength light-emitting diode (LED) near-infrared continuous-wave (cw) imager and its application to in vivo muscle metabolism measurement. The shot-noise limited SNR is derived and calculated to be 2×104 for the physiological blood concentrations of muscle. Aiming at shot-noise limited SNR performance and fast imaging, we utilize sample and hold circuits to reduce high-frequency noise. These circuits have also been designed to be parallel integrating, through which SNR of 2×103 and 2 Hz imaging acquisition rate have been achieved when the probe is placed on a muscle model. The noise corresponds to 2×10−4 optical density error, which suggests an in vitro resolution of 15. 4 nM blood volume and 46.8 nM deoxygenation changes. A 48 dB digital gain control circuit with 256 steps is employed to enlarge the dynamic range of the imager. We utilize cuff ischemia as a living model demonstration and its results are reported. The instrument is applied during exercise to measure the changes of blood volume and deoxygenation, which provides important information about muscle metabolism. We find that the primary source of noise encountered during exercise experiment is from the random motion of muscle. The results demonstrate that the LED cw imager is ideal for the noninvasive study of muscle metabolism.
A synchronized rotating crystal x-ray beam chopper
Authors: McPherson Armon , Lee Wah-Keat , Mills Dennis M.
Published: September 2002

A new x-ray beam chopper has been designed and tested at the Advanced Photon Source. This beam chopper uses the diffractive properties of a rotating crystal to produce transmission window times as short as about 10 ns from a single diffracting face for 10 keV radiation. Precision control of the crystal rotation allows phase locking with the orbital frequency of the synchrotron storage ring; hence, the transmission of short x-ray pulses, corresponding to the 50 ps full width at half maximum charge bunch length, is possible with any fill pattern presently used at the Advanced Photon Source storage ring.
The propagator of the radial Dirac equation
Authors: Ichinose Takashi , Jefferies Brian
Published: September 2002

The propagator for the radial Dirac equation is explicitly constructed. It turns out to be a distribution of order zero, but it is shown that there exists no path-space measure associated with this equation.
Br
−
-
H
2
and I−-H2 anion complexes: Infrared spectra and radial intermolecular potential energy curves
Authors: Wild D. A. , Loh Z. M. , Wilson R. L. , Bieske E. J.
Published: September 2002

Midinfrared spectra of the Br−81-H2 and I−-H2 anion complexes are measured in the H-H stretch region by monitoring the production of halide anion photofragments. The spectra, which are assigned to complexes containing ortho H2, exhibit rotationally resolved ∑-∑ bands whose origins are redshifted from the molecular hydrogen Q1(1) transition by 110.8 cm−1 (Br−-H2) and 74.1 cm−1 (I−-H2). The complexes are deduced to possess linear equilibrium structures, with vibrationally averaged intermolecular separations between the halide anion and H2 center of mass of 3.461 Å (Br−-H2) and 3.851 Å (I−-H2). Vibrational excitation of the H2 subunit causes the intermolecular bond to stiffen and contract by 0.115 Å (Br−-H2) and 0.112 Å (I−-H2). Rydberg–Klein–Rees inversion of the spectroscopic data is used to generate effective radial potential energy curves near the potential minimum that are joined to long-range potential energy curves describing the interaction between an H2 molecule and a point negative charge. From these curves the dissociation energies of Br−-H2 and I−-H2 with respect to isolated H2 (j=1) and halide fragments are estimated as 365 and 253 cm−1, respectively.
Investigation of coupling of magnetohydrodynamic modes by soft x-ray computer tomography on the WT-3 tokamak
Authors: Yoshimura Satoru , Maekawa Takashi , Terumichi Yasushi
Published: September 2002

The internal structure of the stationary m=1 and m=2 modes in an ohmic heating plasma and the double m=1 mode structure in a lower hybrid current drive plasma are investigated on the WT-3 tokamak [Maehara et al., Nucl. Fusion 38, 39 (1998)] using computer tomography after the application of the singular value decomposition to the soft x-ray signals. The results show that, in both cases, two coexisting modes have the same frequency and have a fixed mutual phase relation, indicating that two modes are coupled and rotate as one body in the toroidal direction. It is found that the mutual inductance of two loops of helical current filaments for producing magnetic islands always takes the maximum at the experimentally observed positions of two-mode structures. This result means not only that the electromagnetic coupling of two current loops is at the maximum, but also that the two loops are in the dynamically stable position.
Boundary effects and self-organization in dense granular flows
Authors: Silbert Leonardo E. , Grest Gary S. , Plimpton Steven J. , Levine Dov
Published: September 2002

Boundary effects in gravity-driven, dense granular flows down inclined planes are studied using large-scale molecular dynamics simulations. We find that the flow behavior and structure of the flowing pile changes dramatically as we vary the roughness of the supporting base. For a rough, bumpy base, there are three principal flow regimes that depend on the inclination angle θ: at small angles θ<θr, where θr is the angle of repose, the system does not flow; for large angles θ>θmax, where θmax is the maximum angle for which stable, steady state flow exists, the flow is unstable; and for θr<θ<θmax, the energy input from gravity is balanced by that dissipated through friction and the system reaches a stable, steady state flow. In the stable regime, we find no slip boundary conditions with a bulk density that is independent of the height above the base. For a chute base that is ordered, the steady state regime splits into a further three distinct flow regimes: at lower angles, the flowing system self-organizes into a state of low-dissipation flow consisting of in-plane ordering in the bulk; at higher angles, a high-dissipation regime similar to that for a rough base but with considerable slip at the bottom is observed; and between these two sub-regions we observe a transitional flow regime characterized by large oscillations in the bulk averaged kinetic energy due to the spontaneous ordering and disordering of the system as a function of time.
The XXZ spin chain at Δ=−1/2: Bethe roots, symmetric functions, and determinants
Authors: de Gier J. , Batchelor M. T. , Nienhuis B. , Mitra S.
Published: September 2002

A number of conjectures have been given recently concerning the connection between the antiferromagnetic XXZ spin chain at Δ=−1/2 and various symmetry classes of alternating sign matrices. Here we use the integrability of the XXZ chain to gain further insight into these developments. In doing so we obtain a number of new results using Baxter’s Q function for the XXZ chain for periodic, twisted and open boundary conditions. These include expressions for the elementary symmetric functions evaluated at the ground state solution of the Bethe roots. In this approach Schur functions play a central role and enable us to derive determinant expressions which appear in certain natural double products over the Bethe roots. When evaluated these give rise to the numbers counting different symmetry classes of alternating sign matrices.
Violating Suydam criterion produces feeble instabilities
Authors: Gupta Sangeeta , Callen J. D. , Hegna C. C.
Published: September 2002

Eigenmode analysis of a magnetic shear localized ideal interchange mode indicates a marginal stability condition consistent with the Suydam (and Mercier) criterion. However, the linear eigenmode analysis shows that it is not a very restrictive condition because the mode growth rate is exponentially small just above the marginal stability condition and the modes are easily stabilized by finite ion Larmor radius (FLR) effects. This suggests that for practical applications to toroidal confinement experiments, the Suydam–Mercier criterion does not introduce a restrictive condition for operational limits. The criterion for robust growth appears to be almost a factor of two greater than the Suydam criterion. It is also shown that even the presence of usual nonideal effects (FLR, electron diamagnetic flow and resistivity) does not alter this higher criterion for robust growth.
Envelope solitons and nonstationary shocks created by intense neutrino beams in dense plasmas
Authors: Shukla P. K. , Stenflo L. , Tsintsadze L. N. , Tsintsadze N. L.
Published: September 2002

The nonlinear interaction between intense neutrino beams and sound waves in dense plasmas is considered. Accordingly, a Schrödinger-type equation is derived for the amplitude wave function of neutrinos interacting with a background plasma via the weak Fermi force. The neutrino driving (ponderomotive) force pushes the plasma electrons locally, thereby creating space charge electric fields through which the inertial ions are reinforced. It is shown that the nonlinearly coupled neutrinos and the ion sound perturbations are responsible for the formation of stationary envelope solitons and nonstationary shocks. The relevance of these nonlinear structures to supernova explosions is pointed out.
Solutocapillary instabilities in liquid bridges
Authors: Witkowski L. Martin , Walker J. S.
Published: September 2002

The stability of a steady, axisymmetric flow driven by a surface-tension variation due to a solute-concentration gradient is studied with a linear analysis. There is a liquid bridge with a cylindrical free surface between two collinear solid cylinders with the same radius. One cylinder injects the solute into the liquid as it melts at a velocity Vg*, and the other cylinder preferentially absorbs the solute from the liquid as it solidifies at the same velocity. The surface tension of the free surface is assumed to be a linear function of the solute concentration. For given values of the Schmidt number Sc and of Vg*, the first instability is a Hopf bifurcation at a critical value of a solutal Marangoni number. The present problem is related to the thermocapillary convection in the half-zone problem for which the geometry is the same, the two solids are isothermal at different temperatures and the surface tension is a linear function of temperature. Differences arise because the solute concentration distribution here is different from the temperature distribution in the half-zone problem, and Vg* is the variable which controls these differences. Results are presented for a range of values of Vg* for two aspect ratios and for Sc=0.5–20.
Simulations of the instability of the m=1 self-shielding diocotron mode in finite-length non-neutral plasmas
Authors: Mason Grant W. , Spencer Ross L.
Published: September 2002

The “self-shielding” m=1 diocotron mode in Malmberg–Penning traps has been known for over a decade to be unstable for finite length non-neutral plasmas with hollow density profiles. Early theoretical efforts were unsuccessful in accounting for the exponential growth and/or the magnitude of the growth rate. Recent theoretical work has sought to resolve the discrepancy either as a consequence of the shape of the plasma ends or as a kinetic effect resulting from a modified distribution function as a consequence of the protocol used to form the hollow profiles in experiments. Both of these finite length mechanisms have been investigated in selected test cases using a three-dimensional particle-in-cell code that allows realistic treatment of shape and kinetic effects. A persistent discrepancy of a factor of 2–3 remains between simulation and experimental values of the growth rate. Simulations reported here are more in agreement with theoretical predictions and fail to explain the discrepancy.
Inhomogeneous magnetic-field-aligned ion flow measured in a Q machine
Authors: Koepke M. E. , Zintl M. W. , Teodorescu C. , Reynolds E. W. , Wang G. , Good T. N.
Published: September 2002

Radial profiles of ion flow vd(r) are measured with laser-induced fluorescence for cases in which the flow direction is parallel (vd>0) and/or antiparallel (vd<0) to the equilibrium magnetic field. Experiments are conducted in the barium-ion plasma of a double-ended Q machine. In cases where the ionizers associated with the two ends are not biased relative to each other, two distinct, counterstreaming ion-beam populations are evident. The insertion of blocking electrodes introduces inhomogeneity into the density profiles of the ion populations without effecting the homogeneity of the radial profile of each population’s drift velocity. In cases where the two ionizers are biased relative to each other, a single ion population exists. Variation in the radial profile of the ion population’s parallel drift velocity vd is produced such that (dvd/dr) can be negative or positive with magnitudes 0–70% of the ion gyrofrequency ωci. These results are discussed in the context of beam-driven and velocity-shear-driven instabilities. Laboratory and space measurements of sheared parallel flow and counterstreaming ion beams are compared.
Symmetry breaking regime in the nonlinear Hartree equation
Authors: Aschbacher W. H. , Fröhlich J. , Graf G. M. , Schnee K. , Troyer M.
Published: September 2002

The present article is concerned with minimizers of the attractive Hartree energy functional Hg[ψ̄,ψ]=12‖∇ψ‖22+(ψ,vψ)2+g(ψ,V * |ψ|2ψ)2 on Rd, d⩾2, for a general class of external potentials v and two-body interactions V of positive type, with g<0. We prove spontaneous symmetry breaking in the large coupling limit. A numerical investigation visualizes this regime in the example of an external double well potential.
A magnetized torch module for plasma generation
Authors: Kuo S. P. , Bivolaru D. , Orlick Lester
Published: September 2002

Remodeling components from two commercially available spark plugs and adding a tungsten wire as the central electrode construct a cylindrical-shaped magnetized plasma torch module. A ring-shaped permanent magnet is used to provide an axial magnetic field, which rotates the discharge to prevent the formation of hot spots. The electric properties of this magnetized torch module are studied and compared with those of the unmagnetized one.
Solid-state halogen atom source for chemical dynamics and etching
Authors: Hess Wayne P. , Joly Alan G. , Beck Kenneth M. , Gerrity Daniel P. , Sushko Peter V. , Shluger Alexander L.
Published: September 2002

We describe a solid-state Br atom source for surface etching, kinetics, and reaction dynamics studies. Pulsed laser irradiation of crystalline KBr, near the bulk absorption threshold at 6 eV, produces hyperthermal Br atoms in dense plumes. The Br atom density and velocities may be controlled by choice of laser pulse power and photon energy. Single and multiple pulse excitation of KBr produces Br and Br* in controllable quantities and velocities, thus providing an attractive ultrahigh vacuum compatible solid-state radical atom source. The solid-state atom source is in principle extendable to other halogens using other alkali halides and perhaps other materials.
On the plug potential formation mechanism in a tandem mirror
Authors: Katanuma I. , Tatematsu Y. , Ishii K. , Saito T. , Yatsu K.
Published: September 2002

A formation mechanism of the plug potential in a tandem mirror is proposed. The orbits for ions, which are accelerated by the thermal barrier potential, are calculated numerically in a magnetic mirror field. A non-Maxwellian electron distribution function, which leads to a modified Boltzmann law, is assumed in order to determine the electrostatic potential profile. Monte Carlo simulation is carried out for ion dynamics to include the effects of Coulomb collisions and ion radial losses. It is found that the plug potential is formed under the condition that the ions trapped in the thermal barrier region are few.
Anomalous behavior of the target decay on small world networks
Authors: Jasch F. , Blumen A.
Published: September 2002

We investigate on small world networks (SWN’s) the survival probability of immobile targets, which get annihilated by random walkers at first encounter. On SWN’s we find (distinct from regular lattices, Cayley trees, and regular ultrametric spaces) that in general the survival probability cannot be directly related to the average number of distinct sites visited. We underline this finding with arguments related to the structural disorder of SWN’s and through the derivation of a lower bound for the targets’ decay.
Theoretical study of infrared and Raman spectra of hydrated magnesium sulfate salts
Authors: Chaban Galina M. , Huo Winifred M. , Lee Timothy J.
Published: September 2002

Harmonic and anharmonic vibrational frequencies, as well as infrared and Raman intensities, are calculated for MgSO4⋅nH2O (n=1–3). Electronic structure theory at the second order Møller–Plesset perturbation theory level with a triple-ζ+polarization basis set is used to determine the geometry, energetics, charge distributions, and vibrational spectra of pure and hydrated MgSO4 salts. The direct vibrational self-consistent field method and its correlation corrected extension are used to determine anharmonic corrections to vibrational frequencies and infrared intensities for the pure MgSO4 and its complex with one water molecule. Very significant differences are found between vibrational spectra of water molecules in complexes with MgSO4 and pure water. Some of the O–H stretching frequencies are shifted to the red very significantly (by up to 2000 cm−1) upon complexation with magnesium sulfate. They should be observed between 1700 and 3000 cm−1, in a region very different from the corresponding O–H stretch frequency region of pure water (3700–3800 cm−1). In addition, the SO2 stretching vibrations are found at lower frequency regions than the water vibrations. They can serve as unique identifiers for the presence of sulfate salts. The predicted infrared and Raman spectra should be of valuable help in the design of future missions and analysis of observed data from the ice surface of Jupiter’s moon Europa that possibly contains hydrated MgSO4 salts.
H
2
S
gas sensing mechanism of SnO2 films with ultrathin CuO dotted islands
Authors: Chowdhuri Arijit , Sharma Parmanand , Gupta Vinay , Sreenivas K. , Rao K. V.
Published: September 2002

H 2 S gas interaction mechanisms of sputtered SnO2 and SnO2–CuO bilayer sensors with a varying distribution of the Cu catalyst on SnO2 are studied using Pt interdigital electrodes within the sensing film. Sensitivity to H2S gas is investigated in the range 20–1200 ppm. Changes induced on the surface, the SnO2–CuO interface, and the internal bulk region of the sensing SnO2 film upon exposure to H2S have been analyzed to explain the increasing sensitivity of three different sensors SnO2, SnO2–CuO, and SnO2 with CuO islands. SnO2 film covered with 0.6 mm diameter ultrathin (∼10 nm) CuO dots is found to exhibit a high sensitivity of 7.3×103 at a low operating temperature of 150 °C. A response speed of 14 s for 20 ppm of H2S, and a fast recovery time of 118 s in flowing air have been measured. The presence of ultrathin CuO dotted islands allow effective removal of adsorbed oxygen from the uncovered SnO2 surface due to spillover of hydrogen dissociated from the H2S–CuO interaction, and the spillover mechanism is sensed through the observed fast response characteristics, and the high sensitivity of the SnO2–CuO-dot sensor.
Generalization of collisional fluid theory to long mean-free-path and relativistic motion
Authors: Hazeltine R. D. , Mahajan S. M.
Published: September 2002

The influence of collisions on a magnetized plasma at arbitrary collisionality is modeled by fluid equations that include the self-consistent moments of the collision operator. A key feature of the derivation is its basis in Lorentz-invariant equations, using the most general form of the energy-momentum tensor in a magnetized plasma. The collisional limit of the resulting description is shown to agree qualitatively with well-known short mean-free-path equations. In particular it reproduces exactly the correct symmetry of the transport matrix. Previously the long mean-free-path limit has been shown to reproduce collisionless physics more accurately than other collisionless fluid models.
Improved temporal resolution in junction-mixing ultrafast scanning tunneling microscopy
Authors: Yarotski D. A. , Taylor A. J.
Published: September 2002

We demonstrate ultrafast dynamical imaging of surfaces using a junction-mixing scanning tunneling microscope. We detect picosecond transient voltage pulses on a microstrip transmission line and demonstrate a temporal resolution (full width at half maximum) of 8 ps. We show that the time resolution achieved in these experiments is limited mostly by the microstrip geometry and can be significantly improved by reducing the value of the transmission line impedance. By dynamically imaging the microstrip line and detecting picosecond voltage pulses propagating on a patterned metal-on-metal (Ti/Pt) structure, we demonstrate that 1 nm spatial resolution is achievable for a 13 ps (full width at half maximum) transient correlated signal.
Development of a versatile SMOKE system with electrochemical applications
Authors: Hampton Jennifer R. , Martı́nez-Albertos José-Luis , Abruña Héctor D.
Published: September 2002

We describe the design, construction, and implementation of a simple and inexpensive, yet versatile surface magneto-optic Kerr effect (SMOKE) setup designed to operate in conjunction with the electrodeposition of magnetic layers both in situ and ex situ. The system is based on a homemade electromagnet and commercially available components. The sensitivity of the system is demonstrated by measuring ex situ SMOKE hysteresis loops of Co thin films (down to three monolayers thick) electrodeposited onto a Au(111) electrode substrate.
Minimization of fixed pattern noise in photon event counting imaging
Authors: Suhling Klaus , Airey Robert W. , Morgan Brian L.
Published: September 2002

Low light-level ultraviolet and optical imaging with a photon counting image intensifier coupled to a charge coupled device camera generally results in varying levels of fixed pattern noise in the image. Here, we demonstrate that this can be minimized by the appropriate choice of photon event centroiding algorithm. We compare the fixed pattern noise generated by a center of gravity centroiding algorithm, a Gaussian centroiding algorithm, and a hybrid centroiding algorithm which uses center of gravity centroiding when one wing is zero, and Gaussian centroiding otherwise. This approach yields the best image quality with a lower fixed pattern noise parameter (9.99%) than the sole use of Gaussian centroiding (16.4%), and there is no need for a look-up table correction. In addition, the hybrid algorithm also yields maximum detective quantum efficiency by overcoming small pulse centroiding failure associated with Gaussian centroiding. The digitization error when recording the events is modeled with a Monte Carlo simulation and discussed. It is found that a center of gravity algorithm produces not only significant fixed pattern noise, but also pulse height dependent x̄ positions. For a Gaussian centroiding algorithm the x̄ positions are independent of the pulse height, the fixed pattern noise is low and the digitization error only yields a small increase of the fixed pattern noise parameter. This shows that while there is a limit to centroiding accuracy due to the digitization error, the appropriate choice of centroiding algorithm is a much more important factor to minimize fixed pattern noise.
X-ray absorption spectroscopy studies of ionic association in aqueous solutions of zinc bromide from normal to critical conditions
Authors: Simonet V. , Calzavara Y. , Hazemann J. L. , Argoud R. , Geaymond O. , Raoux D.
Published: September 2002

Ion-pairing and dehydration phenomena occurring in ZnBr2 aqueous solutions from normal to critical T, P conditions were investigated by x-ray absorption spectroscopy. The respective influences of temperature, pressure, and concentration were studied. The evolution of the density of solute ions, probed by the height of the absorption edge, allowed us to get information on phase diagrams and salt precipitation. The average structural evolution deduced from extended x-ray absorption fine structure was related to the formation of complexes identified from x-ray absorption near edge structure analysis. Consequently, in noncritical conditions, an increase of temperature or concentration produces dehydration and ion-pairing, while a rise of pressure destroys the ion-pairs. In contrast, concentration and pressure have weaker effects on the local order in high P, T conditions. Moreover, ion pairing formation is found not to be specifically enhanced when the fluid is close to supercritical conditions as it also occurs at lower temperatures. In a discussion, the modifications induced by a variation of the different structural parameters are related to the macroscopic properties of the solvent.
Time correlation function approach to liquid phase vibrational energy relaxation: Dihalogen solutes in rare gas solvents
Authors: Miller David W. , Adelman Steven A.
Published: September 2002

A molecular theory of liquid phase vibrational energy relaxation (VER) [S. A. Adelman et al., Adv. Chem. Phys. 84, 73 (1993)] is applied to study the temperature T and density ρ dependencies of the VER rate constant k(T,ρ)=T1−1, where T1 is the energy relaxation time, of model Lennard-Jones systems that roughly simulate solutions of high-mass, low-frequency dihalogen solutes in rare gas solvents; specifically the I2/Xe, I2/Ar, and ICI/Xe solutions. For selected states of these systems, the theory’s assumptions are tested against molecular dynamics (MD) results. The theory is based on the expression T1=β−1(ωl), where ωl and β(ω) are, respectively, the solute’s liquid phase vibrational frequency and vibrational coordinate friction kernel. The friction kernel is evaluated as a cosine transform of the fluctuating force autocorrelation function of the solute vibrational coordinate, conditional that this coordinate is fixed at equilibrium. Additionally, the early-time decay of the force autocorrelation function is approximated by a Gaussian function which is exact to order t2. This Gaussian approximation permits evaluation of T1 in terms of integrals over equilibrium solute–solvent pair correlation functions. The pair correlation function formulas yield T1’s in semiquantitative agreement with those found by MD evaluations of the Gaussian approximation, but with three orders of magnitude less computational effort. For the isothermal ρ dependencies of k(T,ρ), the theory predicts for all systems that the Gaussian decay time τ is nearly independent of ρ. This in turn implies that k(T,ρ) factorizes into a liquid phase structural contribution and a gas phase dynamical contribution, yielding a first-principles form for k(T,ρ) similar to that postulated by the isolated binary collision model. Also, the theory predicts both “classical” superlinear rate isotherms, and “nonclassical” sublinear isotherms similar to those recently observed by Troe and co-workers for azulene relaxation in supercritical fluids. The isochoric T dependencies of k(T,ρ) are studied in the range 300 to 1000 K. For none of the solutions are the rate isochores found to accurately conform to either Arrhenius or Landau–Teller kinetics.
Multistate vibronic interactions in the benzene radical cation. I. Electronic structure calculations
Authors: Döscher Martina , Köppel Horst , Szalay Péter G.
Published: September 2002

The multistate vibronic interactions in the benzene radical cation are investigated theoretically, employing the framework of a linear vibronic coupling scheme. The five lowest electronic states are included in the treatment; in view of the degeneracy of some states, this amounts to eight coupled potential energy surfaces. Different types of ab initio calculations have been performed for the system parameters and been found to be in good mutual agreement, thus supporting each other. The calculations reveal a whole sequence of low-energy conical intersections between the potential energy surfaces of different states. Their importance for the nuclear dynamics in this prototypical organic radical cation is pointed out. Wave-packet dynamical simulations for these coupled potential energy surfaces will be presented in the following paper (Paper II).
Multistate vibronic interactions in the benzene radical cation. II. Quantum dynamical simulations
Authors: Köppel H. , Döscher M. , Bâldea I. , Meyer H.-D. , Szalay P. G.
Published: September 2002

The multistate vibronic dynamics in the X̃ 2E1g-Ẽ 2B2u electronic states of the benzene radical cation is investigated theoretically by an ab initio quantum-dynamical approach. The vibronic coupling scheme and the ab initio values of the system parameters are adopted from the previous Paper I. Vibronic line spectra are obtained with the Lanczos procedure. Extensive calculations on wave-packet propagation have been performed with the aid of the multiconfiguration time-dependent Hartree method. Up to five coupled electronic potential energy surfaces and 13 vibrational degrees of freedom have been included in these calculations. As a result, the impact of a third electronic state (X̃ or B̃) on a strongly coupled manifold (B̃-C̃ or D̃-Ẽ states) is quantitatively assessed. It leads to a restructuring of the spectral envelope which is stronger for the B̃-D̃-Ẽ than for the X̃-B̃-C̃ system. The internal conversion dynamics is characterized by a stepwise transfer of electronic population to the lowest electronic state on a time scale of ∼100 fs, if the system is prepared initially on the highest potential energy surface. Companion calculations have also been performed for the case when the system is prepared in the intermediate state at t=0; they show a branching of the electronic populations. These are all novel findings which are discussed in terms of a series of conical intersections between the various potential energy surfaces. The importance of such multistate vibronic interactions for the photophysics and photochemistry of medium-sized systems is pointed out.
Electron trapping at neutral divacancy sites on the MgO surface
Authors: Ricci Davide , Pacchioni Gianfranco , Sushko Peter V. , Shluger Alexander L.
Published: September 2002

The electronic properties of Mg-O divacancy defects at the MgO surface obtained by removing of a pair of O and Mg ions from terrace, step, or corner sites have been investigated using an embedded cluster model. Long-range polarization and lattice relaxation effects have been included through a shell model approach. It is demonstrated that all these defects are electron traps: an addition of one electron to a neutral precursor results in a stable paramagnetic center. We calculate relaxed electron affinities, vertical ionization energies, formation energies, and hyperfine coupling constants of these defects and discuss their relevance for the interpretation of experimental results on the nature of paramagnetic electronic defects at the surface of MgO. These results further extend a concept of surface electron traps beyond simple anion vacancies to more general structural features.
On the origin of stationary concentration patterns in the H2+O2 reaction on a microstructured Rh(110)/Pt surface with potassium
Authors: Günther S. , Marbach H. , Hoyer R. , Imbihl R. , Gregoratti L. , Barinov A. , Kiskinova M.
Published: September 2002

The formation of stationary concentration patterns that develop in the O2+H2 reaction on a microstructured Rh(110)/Pt surface predosed with potassium (θK≈0.08) has been investigated with photoelectron emission microscopy and scanning photoelectron microscopy. The reaction was studied in the 10−7 and 10−6 mbar range between T≈500–600 K on a Rh(110) surface onto which a square-shaped Pt domain of 70×70 μm2 size and 70 Å thickness had been deposited in UHV. Two types of Pt domains, one with pure Pt and one with Rh alloying, were prepared. The chemical state of the metallic substrate and of the adsorbate layers were characterized in situ by local (spot size 0.15 μm) photoelectron spectra of the Pt 4f, Rh 3d, K 2p and O 1s levels. Stationary concentration patterns consisting of K+O condensation islands of macroscopic size developed on both types of Pt domains under reaction conditions. On the alloyed Pt patch the formation of the large K+O condensation islands was associated with the reversible local segregation of Rh. The K+O condensation is interpreted as primary pattern forming process, while the segregation of Rh is a secondary process, which follows the distribution of oxygen resulting from the primary process.
Discovery of long-lived excited electronic states of chlorobenzene, bromobenzene, benzonitrile, and phenyl acetylene cations
Authors: Youn Yeu Young , Kwon Chan Ho , Choe Joong Chul , Kim Myung Soo
Published: September 2002

Previously developed technique of charge exchange ionization in a collision cell outside the ion source, which detects ions originating from the collision gas in the cell, was used to find long-lived excited electronic states of monosubstituted benzene cations. The technique is based on the criterion that charge exchange between polyatomic species is efficient only when the energy of reaction is close to zero or negative (ΔE⩽0), or the exoergicity rule. The B̃ 2B2 states of chlorobenzene, bromobenzene, benzonitrile, and phenyl acetylene cations were found to have long lifetimes (10 microseconds or longer) while excited electronic states with long lifetime were not detected for fluorobenzene, iodobenzene, toluene, nitrobenzene, and styrene cations. The long-lived states found were those displaying well-resolved vibrational structures in the photoelectron spectra. In particular, these were the states generated by removal of an electron from the in-plane nonbonding p orbitals of halogens or in-plane π orbitals of the triple bonds.
Spectrum of transition radiation from hot electrons generated in ultra-intense laser plasma interaction
Authors: Zheng Jian , Tanaka K. A. , Miyakoshi T. , Kitagawa Y. , Kodama R. , Kurahashi T. , Yamanaka T.
Published: September 2002

The spectrum of transition radiation from a relativistic hot electron beam with the Boltzmann energy distribution is theoretically studied. The spectrum of transition radiation is separated into two parts: one is the incoherent spectrum, the other is the coherent spectrum. The results show that the temperature and total number of hot electrons can be obtained from the incoherent spectrum, and that the perturbation wave number in an electron bunch can be deduced from the coherent spectrum, from which the dominant heating process can be studied.
Heterodyned impulsive stimulated Raman scattering of phonon–polaritons in LiTaO3 and LiNbO3
Authors: Crimmins Timothy F. , Stoyanov Nikolay S. , Nelson Keith A.
Published: September 2002

Phonon-polariton dispersion is characterized in ferroelectric lithium tantalate and lithium niobate through femtosecond time-resolved impulsive stimulated Raman scattering (ISRS). An improvement in the ISRS setup permits optical heterodyne detection of the signals. In addition to substantially increasing the sensitivity and accuracy of the measurements, the phase sensitivity of heterodyne detection makes it possible to fully characterize the polariton wave after it has propagated outside of the excitation region. The detection of propagating responses with heterodyned ISRS is explored theoretically and experimentally. Discrepancies in earlier results reported for these materials are resolved.
Electrostatics in periodic slab geometries. II
Authors: de Joannis Jason , Arnold Axel , Holm Christian
Published: September 2002

In our preceeding Paper I [Ref. 16] a method was developed to subtract the interactions due to periodically replicated charges (or other long-range entities) in one spatial dimension. The method constitutes a generalized “electrostatic layer correction” which adapts any standard three-dimensional summation method to slab-like conditions. Here the implementation of the layer correction is considered in detail for the standard Ewald (EW3DLC) and the P3M mesh Ewald (P3MLC) methods. In particular this method offers a strong control on the accuracy and an improved computational complexity of O(N log N) for mesh-based implementations. We derive anisotropic Ewald error formulas and give some fundamental guidelines for optimization. A demonstration of the accuracy, error formulas and computation times for typical systems is also presented.
Structural and dielectric characterization of the (Ba1−xSrx)(Ti0.9Sn0.1)O3 thin films deposited on Pt/Ti/SiO2/Si substrate by radio frequency magnetron sputtering
Authors: Wang Moo-Chin , Tsai Cheng-Chi , Wu Nan-Chung , Hung Kun-Ming
Published: September 2002

( Ba 1−x Sr x )( Ti 0.9 Sn 0.1 ) O 3  (BSxTS) thin films prepared by rf magnetron sputtering have been characterized as a function of temperature, applied voltage, and electric field. The BSxTS thin films have been confirmed with x-ray diffraction and electron diffraction analysis. The BSxTS thin films show a strong (111) preferred orientation for Sr content 0.1≦x≦0.3. Grain size increases with increasing deposition temperature and is correlated to high dielectric constants. Leakage current density at 1 kV/cm varies from below 10−7 to mid 10−9 A/cm2 for the O2/(O2+Ar) ratio varying from 5/(5+5) to 1/(1+9). A large and clear hysteresis shows ferroelectricity at 25 °C for all BSxTS thin films. The remnant polarization increases with increasing Sr content, which is preseemably caused by the lattice mismatch between BSxTS thin films and Pt layers.
Comparison of various L2 methods for calculations of radiative association cross sections: Application to collisions of Li with H+
Authors: Baccarelli I. , Andric L. , Grozdanov T. P. , McCarroll R.
Published: September 2002

The performance of three different L2 methods are tested in calculations of radiative association cross sections in collisions of Li with H+. The first two methods, the smooth-exterior-complex-scaling method and a method using complex absorbing potentials, are based on the direct determination of poles of the Green function and are superior at low collision energies where narrow shape resonances dominate. The third method, based on the iterative Chebyshev-polynomial expansion of the Green function, is efficient only at higher energies where the cross sections are smooth functions of the collision energy.
Yellow-green ZnCdSe/BeZnTe II-VI laser diodes grown on InP substrates
Authors: Che Song-Bek , Nomura Ichirou , Kikuchi Akihiko , Kishino Katsumi
Published: September 2002

Yellow-green (560 nm) II-VI laser diodes on InP substrates were successfully operated under the pulsed current injection at 77 K. A separate confinement heterostructure was formed by employing MgSe/BeZnTe:N superlattices (SL) as p-cladding layers and MgSe/ZnCdSe:Cl SL as n-cladding layers. The threshold current density was about 2.5 kA/cm2.
Theory of near-field optical imaging with a single molecule as light source
Authors: Colas des Francs Gérard , Girard Christian , Dereux Alain
Published: September 2002

A fluorescing structure (made of one or several molecules) can be considered as an ideal point source of light for use in a near-field optical microscope [Michaelis et al., Nature 405, 325 (2000)]. This paper discusses how the image–object relationship is made easier to understand by introducing the concept of photon local density of states (LDOS). Specifically, we show that the LDOS determined by Green’s dyadic calculations provide the relevant interpretation of the images recorded with a single fluorescing molecule as source of light. Recent experimental results are analyzed on this basis.
A gas-spring system for optimizing loudspeakers in thermoacoustic refrigerators
Authors: Tijani M. E. H. , Zeegers J. C. H. , de Waele A. T. A. M.
Published: September 2002

Moving-coil loudspeakers are appropriate drivers for thermoacoustic refrigeration. They are cheap, commercially available, compact, light, and can be adapted to meet specific requirements. This paper deals with the optimization of loudspeakers for thermoacoustic refrigeration. Using an electrical model that describes the refrigerator, it is concluded that the electroacoustic efficiency can be maximized over a wider frequency range by matching the mechanical resonance frequency of the driver to the acoustic resonance frequency of the resonator. A gas-spring system is introduced as a practical tool to shift the mechanical resonance frequency of the driver. An electroacoustic efficiency of 35% is obtained when the mechanical resonance frequency of the driver and the acoustic resonance frequency are equal. Additionally, the efficiency is constant over a relatively wide frequency range. This has advantages for thermoacoustic refrigeration. During cool-down, the operating acoustic frequency decreases so that the refrigerator will keep near the optimum performance.
Can coupled cluster singles and doubles be approximated by a valence active space model?
Authors: Beran Gregory J. O. , Gwaltney Steven R. , Head-Gordon Martin
Published: September 2002

A new, efficient approximation for coupled cluster singles and doubles (CCSD) is proposed in which a CCSD calculation is performed in a valence active space followed by a second-order perturbative correction to account for the inactive singles and doubles cluster amplitudes. This method, denoted VCCSD(SD), satisfactorily reproduces CCSD results in a variety of test cases, including spectroscopic constants of diatomic molecules, reaction energies, the Cope rearrangement, and other relative energies. Use of VCCSD alone is significantly less satisfactory. Formally, the O2V4 scaling of CCSD is reduced to o2v2V2, where o is the number of active occupied orbitals, v is the number of active virtual orbitals, and V is the total number of virtual orbitals. We also investigate the role of orbital optimizations and the appropriate choice of an active space in such methods.
Improved heat capacity estimator for path integral simulations
Authors: Glaesemann Kurt R. , Fried Laurence E.
Published: September 2002

New path integral Monte Carlo constant volume specific heat (CV) estimators are presented that improve upon the thermodynamic, virial, and centroid virial CV estimators via a free particle projection. These projected estimators significantly reduce the numerical noise of the traditional estimators. The new projected thermodynamic estimator has particular advantages when derivatives of the potential are expensive to evaluate. A double virial estimator is derived for real space path integrals and comparisons are made to it. The centroid virial estimators are found to be significantly better than the noncentroid virial estimators.
Brownian dynamics simulation of linear polymers under elongational flow: Bead–rod model with hydrodynamic interactions
Authors: Neelov Igor M. , Adolf David B. , Lyulin Alexey V. , Davies Geoffrey R.
Published: September 2002

Brownian dynamics (BD) simulations of a linear freely jointed bead–rod polymer chain with excluded volume (EV) interaction have been performed under elongational flow with and without the use of fluctuating hydrodynamic interactions (HI). The dependence of the chain size, shape and intrinsic elongational viscosity on the elongational rate ε̇ are reported. A sharp coil–stretch transition is observed when ε̇ exceeds a critical value, ε̇c. The inclusion of the HI leads to a shift in the coil–stretch transition to higher flow values. Chain deformation due to elongational flow is observed to first consist of the alignment of the chain with the direction of flow without significant chain extension followed by additional alignment of the bond vectors with the flow direction and chain extension as flow rate is increased further. The distribution function for the chain’s radius of gyration becomes significantly broader within the transition region which implies an increase in fluctuations in the chain size in this region. The structure factors parallel and perpendicular to the flow direction illustrate different elongational rate dependencies. At high rates, the structure factor in the direction of the flow exhibits an oscillating dependence which corresponds to the theoretically predicted shape for a rigid-rod model. The mean squared orientation of each bond within the chain with respect to the flow direction as function of bond number is nearly parabolic in shape with the highest degree of orientation found within the chain’s interior. The dependence of the critical elongational rate, ε̇c, on the chain length, N, is observed to be ε̇c∼N−1.96 when hydrodynamic interactions are not employed and ε̇c∼N−1.55 when they are invoked. These scaling exponents agree well with those obtained in previous BD simulations of bead-FENE (i.e., finitely extensible nonlinear elastic) spring chains as well as with the theoretical predictions of ε̇c∼N−2 and ε̇c∼N−1.5 without and with hydrodynamic interactions based on the Rouse and Zimm models, respectively.