bluuebunny/crossref_metadata_2025 huggingface dataset:
Nucleation and growth kinetics of AlN films on atomically smooth 6H–SiC (0001) surfaces
Authors: Yamada Satoshi , Kato Jun-ichi , Tanaka Satoru , Suemune Ikuo , Avramescu Adrian , Aoyagi Yoshinobu , Teraguchi Nobuaki , Suzuki Akira
Published: July 2002

Nucleation and growth kinetics of AlN films on atomically smooth 6H–SiC (0001) surfaces, which were obtained by HCl etching at elevated temperatures prior to growth, were investigated. The surface morphology and the defect density of AlN films on such surfaces were significantly improved compared to those on as-received SiC surfaces. This is due to enhanced diffusion length and reduced incoherent boundaries at the coalescence regions of the AlN islands. AlN nuclei on the as-received SiC surface were crystallographically misaligned and thus induced incoherent boundaries at the coalescence stage, resulting in the delay of the two-dimensional growth mode transition and defect formation in AlN films.
Reduction in transport by the parallel velocity shear instability due to reversed magnetic shear
Authors: McCarthy D. R. , Fuselier E. J. , Sen S.
Published: July 2002

A nonlocal theory of the electrostatic parallel velocity shear instability in a three-dimensional slab with a uniformly sheared magnetic field has been developed. It is shown that in the limit of a weak parallel velocity gradient, the linear growth rate can be increased depending upon the direction of the magnetic shear (ŝ) with respect to the radial curvature of the parallel velocity profile (d2v∥/dx2). When these parameters have the same sign, the growth rate can actually be stronger than in the limit of no magnetic shear. In this limit of increased instability, the eigenmode is broadened, thus producing enhanced transport. This effect should be observable when the scale length of the curvature is of order ∼Lsρs. For strong parallel velocity gradients that are more typical of flows in tokamaks, the effect of the varying Doppler shift becomes more prominent on the stability of the mode, the net result being that the sensitivity of the growth rates on the sign of the magnetic shear becomes insignificant. This effect, however, is effectively offset when a finite density gradient is included. When the density scale length is of order the scale length of v∥, the growth rate is moderately reduced, but becomes dependent again upon the sign of the magnetic shear.
Pulse duration effect on the distribution of energetic particles produced by intense femtosecond laser pulses irradiating solids
Authors: Zhidkov A. G. , Sasaki A. , Fukumoto I. , Tajima T. , Auguste T. , D’Oliveira P. , Hulin S. , Monot P. , Faenov A. Ya. , Pikuz T. A. , Skobelev I. Yu.
Published: July 2002

The energy distribution of hot electrons produced by a very short, intense laser pulse (I=2/4×1018 W/cm2, 60 fs, λ=800 nm, obliquely incident p polarized) is investigated theoretically via particle simulation and experimentally via measurements of the electron distribution in the MeV region and the Doppler-shifted emission spectrum of fast ions. This energy distribution is shown to be greatly different from the known two-temperature distribution. The hot electrons with energies near the maximal (∼2 MeV) constitute the distribution with an effective temperature Th considerably higher than that of lower-energy electrons, which dominate the emission of energetic ions. The temperature scaling with the laser intensity differs from the known Th∼I1/2.
Eddy viscosity and laminarization of sheared flow in three dimensional reduced magnetohydrodynamic turbulence
Authors: Kim Eun-jin , Hahm T. S. , Diamond P. H.
Published: July 2002

The effect of Alfvén waves on turbulent momentum transport is studied in three-dimensional reduced magnetohydrodynamic turbulence. A strong external uniform magnetic field B0 is assumed to be present orthogonal to both background shear flow and its inhomogeneity. Energy is injected into the fluid and/or magnetic field on small scales. It is shown analytically that eddy viscosity is reduced as 1/B02 for a strong B0, due to the cancellation of the Reynolds stress by Maxwell stress. The sign of eddy viscosity is found to depend on the detailed properties of forcings. Specifically, it is positive for fluid forcing but depends on the anisotropy of the forcing in the case of magnetic forcing. Furthermore, it is indicated that a magnetic field tends to laminarize a mean shear flow. The possible implication of these results for the dynamics of the zonal flows in tokamaks is discussed in view of the cancellation of stresses.
Impact of electron–electron cusp on configuration interaction energies
Authors: Prendergast David , Nolan M. , Filippi Claudia , Fahy Stephen , Greer J. C.
Published: July 2002

The effect of the electron–electron cusp on the convergence of configuration interaction (CI) wave functions is examined. By analogy with the pseudopotential approach for electron–ion interactions, an effective electron–electron interaction is developed which closely reproduces the scattering of the Coulomb interaction but is smooth and finite at zero electron–electron separation. The exact many-electron wave function for this smooth effective interaction has no cusp at zero electron–electron separation. We perform CI and quantum Monte Carlo calculations for He and Be atoms, both with the Coulomb electron–electron interaction and with the smooth effective electron–electron interaction. We find that convergence of the CI expansion of the wave function for the smooth electron–electron interaction is not significantly improved compared with that for the divergent Coulomb interaction for energy differences on the order of 1 mHartree. This shows that, contrary to popular belief, description of the electron–electron cusp is not a limiting factor, to within chemical accuracy, for CI calculations.
Quantitative comparison of three calibration techniques for the lateral force microscope
Authors: Cain Robert G. , Reitsma Mark G. , Biggs Simon , Page Neil W.
Published: July 2002

The quantitative use of atomic force microscopes in lateral mode for friction measurements has been limited by uncertainty about reliable calibration techniques. This article describes a comparison of three methods that have been proposed for the calibration of the lateral sensitivity of atomic force microscopes: (a) one based on movement of the photodiode assembly, (b) one based on the slope of the friction-loop while the contacting surfaces are in static contact, and (c) one based on a comparison of the lateral force signal on a surface with changing slopes of known orientation. All three methods gave comparable results thereby confirming their robust nature, and also confirming the validity of atomic force microscope methods for lateral force measurement. However, (b) indicated that for the commercial instrument used here, the lateral signal sensitivity is load dependent. A simple extension to (a) revealed the nature of this dependence: a misalignment of the four-quadrant photodiode detection system with respect to the alignment of the reflected beam path resulting in a coupling of the normal and lateral signals. The result is that the lateral signal does not scale directly with friction, requiring that for friction versus applied load studies, lateral signal calibration be performed across the full range of applied loads of interest. To a greater or lesser extent, this shortcoming will be evident in a wide range of commercial instruments. All three methods studied here have special advantages: (a) provides the most complete information about the detector response, (b) provides a calibration method when friction characteristics of colloid probes of a wide range of possible materials are of interest, and (c) has the advantage of also providing information about the state of the tip geometry.
Corrugation induced rotational excitation in photon/electron-induced desorption of ammonia: A three-dimensional quantum study
Authors: Li Shenmin , Guo Hua
Published: July 2002

The photon/electron-stimulated desorption dynamics of ammonia from the Cu(111) and Cu(100) surfaces is investigated using a three-dimensional two-state quantum model, which includes the desorption and ammonia inversion coordinates as well as the “spinning” rotational degree of freedom. The desorption dynamics is followed by time-dependent wave packet on the ground or excited electronic state. Rotational state populations of both NH3 and ND3, obtained from the asymptotic wave packet, can be approximated by Boltzmann distributions and they show strong dependence on the symmetry and corrugation of the Cu surface. The NH3 rotational temperature of Trot=205 K on Cu(111) and 372 K on Cu(100) agree well with experimental observations. The desorbed NH3 is found to be rotational hotter than ND3, also in agreement with experiment. The addition of the rotational degree of freedom has little impact on other aspects of the dynamics, including desorption yield and vibrational excitation.
Basis set superposition error free self-consistent field method for molecular interaction in multi-component systems: Projection operator formalism
Authors: Nagata Takeshi , Takahashi Osamu , Saito Ko , Iwata Suehiro
Published: July 2002

The self-consistent field method for molecular interaction (SCF MI) by Gianinetti, Raimondi, and Tornaghi is extended to multi-component systems. A set of equations are written with projection operators, and the accurate approximate equations are derived. The method is applied to water clusters and to a fluoride anion complex with a water dimer. The calculated interaction energies are compared with those estimated with the counterpoise method, and they converge to smaller values for extensive basis sets. The underestimation of the binding energy results from the omission of the most part of charge transfer contribution in the wave function.
Two-dimensional arrays of magnetic nanostructures characterized using an underlying two-dimensional electron gas
Authors: Skuras E. , Long A. R. , Chowdhury S. , Rahman M. , Kirk K. J. , Davies J. H.
Published: July 2002

We have studied the magnetoresistance in an inclined magnetic field of a two-dimensional electron gas modulated by a two-dimensional array of thin cobalt nanostructures. At low fields, hysteretic behavior due to the magnetization of the elements was observed, and at higher fields magnetic commensurability (Weiss) oscillations were clearly seen. These magnetoresistance features only occurred when the in-plane component of the field was in the current flow direction. The commensurability oscillations behave exactly as predicted in a recent theory of two-dimensional surface superlattices.
Population properties and carrier dynamics in a GaAs/(Al,Ga)As double-quantum-well superlattice investigated by time-resolved photoluminescence spectroscopy
Authors: Schrottke L. , Hey R. , Grahn H. T.
Published: July 2002

We have analyzed the electric-field-dependent subband population as well as the carrier dynamics in a double-quantum-well GaAs/(Al,Ga)As superlattice using time-resolved photoluminescence (PL) spectroscopy. Applying a rate equation model, the steady-state subband population of the majority carriers in the two quantum wells and the transfer coefficients for the minority carriers can be directly determined from measured time-dependent PL spectra. A comparison with results derived from steady-state PL investigations demonstrates that the dynamics of the minority carriers are essential in order to determine the population of the majority carriers. In the experiments, we used an n–i–n structure, in which electrons are the majority and holes are the minority carriers.
Cosmic Hot Interstellar Plasma Spectrometer external stop delay board: An alternative to bulky and massive coax for long (>100 ns) time delays
Authors: Marckwordt Mario
Published: July 2002

The Cosmic Hot Interstellar Plasma Spectrometer (CHIPS) utilizes a typical microchannel plate detector with a cross delay line event readout anode. A CHIPS delay line was predicted to approach a time delay of 80 ns. Therefore an added time delay of 100 ns was required for the stop channels. Implementation of such a delay using conventional coax cable was deemed too bulky and massive. After exploring other alternatives, which failed to meet specific requirements, a stripline delay printed circuit board was developed. This approach eventually proved to be a superior alternative to coax cable for the CHIPS detector. This article outlines and details the development of the CHIPS external stop delay board from basic microwave circuit theory to design and testing. Discussion of the mathematical and computer models used for design are contained in the Appendices.
Accurate first-derivative nonadiabatic couplings for the H3 system
Authors: Abrol Ravinder , Shaw Amy , Kuppermann Aron , Yarkony David R.
Published: July 2002

A conical intersection exists between the ground (1 A′2) and the first-excited (2  A′2) electronic potential energy surfaces (PESs) of the H3 system for C3v geometries. This intersection induces a geometric phase effect, an important factor in accurate quantum mechanical reactive scattering calculations, which at low energies can be performed using the ground PES only, together with appropriate nuclear motion boundary conditions. At higher energies, however, such calculations require the inclusion of both the 1 2A′ and 2 2A′ electronic PESs and the corresponding nuclear derivative couplings. Here we present ab initio first-derivative couplings for these states obtained by analytic gradient techniques and a fit to these results. We also present a fit to the corresponding 1 2A′ and 2 2A′ adiabatic electronic PESs, obtained from the ab initio electronic energies. The first-derivative couplings are compared with their approximate analytical counterparts obtained by Varandas et al. [J. Chem. Phys. 86, 6258 (1987)] using the double many-body expansion method. As expected, the latter are accurate close to conical intersection configurations but not elsewhere. We also present the contour integrals of the ab initio couplings along closed loops around the above-mentioned conical intersection, which contain information about possible interactions between the 2 2A′ and 3 2A′ states.
Energy versus amplitude corrected coupled-cluster approaches. II. Breaking the triple bond
Authors: Li Xiangzhu , Paldus Josef
Published: July 2002

We examine the effectiveness of various energy corrections to the standard CCSD and to the reduced multireference (RMR) CCSD methods. These corrections are based on the asymmetric energy formula, but instead of projecting onto the reference configuration, as in the standard CCSD method, we employ for this purpose either the MR CISD wave function that is based on a suitable model space of the kind used in RMR CCSD, or simply the zero-order wave function in that model space. Both full complete-active-space and severely-truncated model spaces are employed. The method is applied to the prototypical case of the triple-bond dissociation, namely, to the exactly solvable double-zeta model of the N2 molecule. It is shown that in this way we can eliminate the breakdown of the standard CCSD method in the region of highly stretched geometries and obtain reliable potential energy curves. The comparison with the recently proposed renormalized CCSD(T) and variational CCD methods is also briefly addressed.
Observation of CH4 (v2=1 or v4=1) in the reaction Cl+CH4 with time-resolved Fourier-transform infrared absorption spectroscopy
Authors: Chen Yu-Ju , Chu Li-Kung , Lin Shiaw-Ruey , Lee Yuan-Pern
Published: July 2002

The reaction Cl(2P)+CH4 was initiated on laser irradiation of a flowing mixture containing Cl2, CH4, and Ar at 355 nm; reaction products were monitored with a step-scan time-resolved Fourier-transform absorption spectrometer coupled with a multipass absorption cell. Not only loss of CH4 but also production of HCl, CH3Cl, highly rotationally excited CH4 [designated as CH4(J*)], and vibrationally excited CH4 (v2=1 or v4=1), designated as CH4(v*), was observed after laser irradiation. Absorption lines of CH4(J*) and CH4(v*) are assigned according to published spectral parameters. Rates of formation and decay of CH4(v*) are derived on fitting observed temporal profiles with a simple kinetic model. A bimolecular rate coefficient for formation of CH4(v*) is determined to be (1.1±0.2)×10−14 cm3 molecule−1 s−1, nearly identical to that reported for the reaction Cl+CH4. Experimental evidence indicates that the reaction Cl+CH4 is rate determining to formation of CH4(v*). CH4(v*) is likely produced through energy transfer from vibrationally excited CH3Cl that is produced via secondary reactions. A rate coefficient for relaxation of CH4* by collision with Ar is determined to be (2.2±0.1)×10−15cm3 molecule−1 s−1, consistent with previous results. The proportion of CH4(v*) in the system is estimated to be ∼1.4% in CH4. According to theoretical calculations reported previously, the rate coefficient for the reaction Cl+CH4(v*) is much greater than that for Cl+CH4 at 298 K, especially at low temperatures (10–235 times at 200 K); formation of CH4(v*) in the Cl+CH4 system can thus explain why rate coefficients determined previously through flash photolysis near 220 K are ∼20% greater than those determined in a discharge-flow system.
Multidensity integral equation theory for a sticky hard sphere-hard sphere heteronuclear dimer fluid: Thermodynamic and structural properties
Authors: Wu Ning , Chiew Y. C.
Published: July 2002

The thermodynamic and structural properties of a sticky hard sphere-hard sphere heteronuclear dimer fluid was studied using the multidensity Ornstein–Zernike integral equation theory. In this formalism, a monodisperse system of the heteronuclear dimer was modeled as an equal molar binary mixture of associating sticky hard sphere and hard sphere particles in the complete association limit. A general analytical solution to the model was obtained within the Percus–Yevick and polymer Percus–Yevick approximations for any degree of association including the complete association limit. Explicit analytical expressions for the contact values of correlation functions were obtained. Further, correlation functions beyond the hard core region were also calculated. The Helmholtz energy and pressure of the heteronuclear dimer are obtained via the energy route. The critical temperature, critical density, and phase coexistence of the fluid are also obtained.
Control of domain structure of epitaxial PbZr0.2Ti0.8O3 thin films grown on vicinal (001) SrTiO3 substrates
Authors: Nagarajan V. , Ganpule C. S. , Li H. , Salamanca-Riba L. , Roytburd A. L. , Williams E. D. , Ramesh R.
Published: July 2002

We have investigated the polydomain formation in 100–200-nm-thick PbZr0.2Ti0.8O3 epitaxial thin films on vicinally cut (100) oriented SrTiO3 substrates. Our results show that there is a preferential location of the nucleation of the a domains along the step edges of the underlying substrate. By piezo-response microscopy, we show that all a domains have their polarization aligned along the same direction. This result is in contrast to flat substrates where fourfold symmetry of a domains is observed. We observe that the critical thickness for a domain formation is much lower than that for PbZr0.2Ti0.8O3 films grown on flat substrates. We have developed a model based on minimization of elastic energy to describe the effect of localized stresses at step edges on the formation of a domains in the ferroelectric layer.
Nanostructuring of tips for scanning probe microscopy by ion sputtering: Control of the apex ratio and the tip radius
Authors: Hoffrogge P. , Kopf H. , Reichelt R.
Published: July 2002

Ion etching under well-defined conditions represents a very powerful tool to fabricate tips in a controlled and reproducible manner for Scanning Probe Microscopy which possess clean, relatively smooth, and oxide-free surfaces. The possibilities and limitations of ion etching are demonstrated thoroughly for tungsten, which is of particular interest for scanning tunneling microscopy. Iterative computer simulations and experimental studies are provided. The simulation of the etching process at the atomic level is based on the Monte Carlo program (Transport of Ions in Matter), which provides reliable values for the key input parameters such as the sputtering yield Y(θ,Eion) and the mean ion range in the material for ion energies between 1 and 6 keV. The simulation program starts with a large tip radius as a crude form and asymptotically approaches a “final form” of a very sharp tip in the course of ion etching. Our experimental results with argon ions in the energy range up to 6 keV and crude electrochemically etched polycrystalline tungsten wire agreed very reasonably with the simulated results. For example, the minimum tip radius obtained experimentally amounts to approximately 5 nm. This value corresponds closely to the ion range, which obviously sets a limit to the smallest achievable tip radius by the effect of transmission sputtering. By variation of the angle between the ion beam and the macroscopic tip, the tip angle can be controlled in the range between 12° and 35°.
Stability of angular confinement and rotational acceleration of a diatomic molecule in an optical centrifuge
Authors: Spanner Michael , Davitt Kristina M. , Ivanov Misha Yu.
Published: July 2002

Modern femtosecond technology can be used to create laser pulses that induce controlled spinning of anisotropic molecules to very high angular momentum states (“optical centrifuge”). In this paper we extend our previous study [M. Spanner and M. Ivanov, J. Chem. Phys. 114, 3456 (2001)] and focus on the stability of angular trapping and forced rotational acceleration of a diatomic molecule in an optical centrifuge. The effects of laser intensity modulations and rovibrational coupling are analyzed in detail, classically and quantum mechanically. The numerical simulations show excellent qualitative agreement between the quantum and classical systems. Forced rotations of the classical system can exhibit chaotic behavior, which becomes rather unique when the accelerating rotation of the angular trapping potential combines with efficient rovibrational coupling. In this regime the Lyapunov exponent becomes time-dependent and the trajectories separate as exp(λF(t)).
Electronic structure and spectroscopy of copper dichloride
Authors: Lorenz Martin , Smith Alice M. , Bondybey Vladimir E.
Published: July 2002

Adiabatic energies of the low lying, so-called “ligand field” electronic states of copper halides derived from the recently reported gas phase photodetachment spectra [Wang et al., J. Chem. Phys. 7114, 7388 (2001)], exhibit a very good agreement with neon matrix data and ab initio theory. While the gas phase provides verification that the halides are not seriously perturbed by the solid, the higher resolution matrix study yields also detailed information about their vibrational structures.
On asymptotic perturbation theory for quantum mechanics: Almost invariant subspaces and gauge invariant magnetic perturbation theory
Authors: Nenciu G.
Published: July 2002

Singular perturbation theory for quantum mechanics is considered in a framework generalizing the spectral concentration theory. Under very general conditions, asymptotic estimations on the Rayleigh–Schrödinger expansions of the perturbed spectral projections are obtained. As a consequence almost invariant subspaces of exponential order are constructed. The results cover practically all singular perturbations considered in nonrelativistic quantum mechanics. In the magnetic field case, under the condition that the magnetic field does not increase at infinity, a gauge invariant perturbation theory leading to convergent series with field-dependent coefficients is developed.
Inverse bremstrahlung in cold dense plasmas
Authors: Hazak G. , Metzler N. , Klapisch M. , Gardner John
Published: July 2002

A formula for inverse bremstrahlung process in quantum dense plasmas is derived. The formula is implemented in the Naval Research Laboratory FAST2D hydro code [J. H. Gardner et al., Phys. Plasmas 5, 1935 (1998)] and run for some simple test cases.
Isotropic Raman line shapes of N2 and O2 along their liquid–gas coexistence lines
Authors: Everitt K. F. , Skinner J. L.
Published: July 2002

Isotropic Raman line shapes provide information about molecular interactions, structure, and dynamics. Such line shapes have been measured experimentally along the liquid–gas coexistence lines for both nitrogen and oxygen. We extend previous theoretical studies of nitrogen Raman line shapes by including in a systematic way the dependence of the bond lengths and dispersion and repulsive force parameters on vibrational coordinates. In so doing we include the effects of vibration-rotation and resonant vibrational intermolecular interactions. The dispersion and repulsive force parameter dependences are crucial for obtaining a quantitative description (and even the correct sign) of the line shift. Using a recently developed intermolecular potential, we perform similar calculations for oxygen. For both oxygen and nitrogen agreement with experimental Raman line shifts and line widths along the liquid–gas coexistence lines is reasonably good. One interesting feature of our results is that the dependence of the dispersion and repulsive force parameters on the vibrational coordinates is developed in such a way as to be directly useful in calculations of vibrational lifetimes.
An x-ray extensional flow cell
Authors: Kisilak Marsha , Anderson Heather , Babcock Nathan S. , Stetzer MacKenzie R. , Idziak Stefan H. J. , Sirota Eric B.
Published: July 2002

We have designed an x-ray extensional flow cell based on the crossed-slot geometry that is compatible with x-ray diffraction. Extensional flow within the cell was both confirmed and characterized by examining the trajectories of small latex spheres suspended in water via video microscopy. The cell was then used to examine extensional-flow-induced structural changes in a well-known lamellar surfactant system, and the alignment of the surfactant domains was observed to increase with increasing extension rate.
Spectroscopic absorption measurement of a low-Z plasma
Authors: Yang Jiamin , Ding Yaonan , Yan Jun , Zheng Zhijan , Li Jiaming , Zhang Baohan , Yang Guohong , Zhang Wenhai , Wang Yaomei
Published: July 2002

Low-Z plasmas have been produced by x-ray radiative heating of a low-Z sample on the “Xingguang II” laser facility. High-resolution transmission spectra of the low-Z plasma (CHO) have been measured by using a flat field grating spectrometer. Absorption lines of oxygen and carbon ions in the region from 1.6 nm to 5.0 nm have been observed clearly and identified. The unresolved transition array model (UTA) has been introduced to calculate the transmission spectra of the CHO plasma. The measured transmission spectra have been compared with the calculated ones and the results of other works.
Scaling laws in granular flows down a rough plane
Authors: Andreotti B. , Daerr A. , Douady S.
Published: July 2002

The scaling properties of granular flows down an inclined plane are investigated in a model previously proposed to describe surface flows on a sandpile. Introducing a depth dependent friction, we are able to reproduce the results obtained experimentally by Pouliquen [Phys. Fluids 11, 542 (1999); 11, 1956 (1999)] on both the fronts velocities and their shapes.
Nonstationary closure relations of the collisionless fluid equations
Authors: Bendib A. , Matthieussent G. , Bouzid F.
Published: July 2002

An analytical method to solve the time-dependent linearized Vlasov equation is carried out by making use of the method developed recently in the literature [K. Bendib and A. Bendib, Phys. Plasmas 6, 1500 (1999)]. The distribution function is computed with respect to the continued fractions and the collisionless transport coefficients are deduced. These transport coefficients have been used to close the fluid equations and it has been checked that the fluid and the kinetic response functions coincide very accurately for arbitrary normalized phase velocities ξ=ω/2kvt, where ω and k are the frequency and the wave number of the plasma modes and vt is the thermal velocity. The collisionless fluid equations have been expressed with respect to a phenomenological ratio of specific heats Γ(ξ) and a fluid damping rate ν(ξ), which include the kinetic effects. They are used to study the dispersion relation of the Langmuir waves and of the thermal filamentation instability.
On the problem of algebraic completeness for the invariants of the Riemann tensor. II
Authors: Carminati J. , Zakhary E. , McLenaghan R. G.
Published: July 2002

We study the set of invariants CZ [E. Zakhary and J. Carminati, J. Math. Phys. 42, 1474 (2001)] for the class of space–times whose Ricci tensors do not possess a null eigenvector. We show that all cases are completely backsolvable in terms of sets of invariants from CZ. We provide algebraically complete sets for each canonically different space–time.
Vibrational energy relaxation in liquid oxygen (revisited) and in liquid nitrogen
Authors: Everitt K. F. , Skinner J. L. , Ladanyi Branka M.
Published: July 2002

The lifetimes of the first excited vibrational states in liquid oxygen and nitrogen are very long: 2.5 ms and 56 s, respectively. The theoretical calculation of these long lifetimes is challenging, ultimately because in each case the vibrational energy gap is much higher both than kT and than characteristic translational and rotational frequencies of the molecules in the liquid. A few years ago we presented a theoretical calculation for the vibrational lifetime in liquid oxygen based on a semiclassical implementation of Fermi’s golden rule. In this paper we improve upon that work, primarily by using an improved intermolecular potential, and what we believe is a more reliable quantum correction factor. Our theoretical result for the lifetime at 77 K is in reasonable agreement with experiment. We also present a calculation for liquid nitrogen at 77 K. In this case our result for the (nonradiative) lifetime is about a factor of 35 longer than the experimental lifetime, lending support to the belief that in liquid nitrogen the vibrational lifetime is in fact radiative.
Linear instability of dilute particle-laden compressible mixing layers
Authors: Thevand N. , Daniel E.
Published: July 2002

The effects of fluid–particle interaction on the temporal development of the compressible mixing layer is investigated using a linearization of the set of equations used to describe dilute two-phase flows with an Eulerian approach. The evolution of the characteristics of the most unstable perturbation as a function of the convective Mach number is studied, including the variation of the wave number, the maximum growth rate, and the obliquity of the disturbance. A parametric investigation is conducted on the influence of the particle inertia, mass loading, and specific heat. The results show the important influence of the particles on the three-dimensional perturbation amplification and the growth rate attenuation when compressibility effects increase.
Rotationally resolved spectra of transitions involving methyl torsion and C–C–O bend of acetaldehyde in the system of Ã1A″–X̃1A′
Authors: Chou Yung-Ching , Huang Cheng-Liang , Chen I-Chia , Ni Chi-Kung , Kung A. H.
Published: July 2002

In the fluorescence excitation spectrum of acetaldehyde cooled in a supersonic jet, we performed a full rotational analysis of combination bands 10011400+150n and 10011400−150n, n=0–4 in the system Ã1A″–X̃1A′. The vibrational frequency of the C–C–O bending mode is determined to be ν10′=373.163(3) cm−1. The rotational structures of combination bands 10011400+1502, 10011400−1502, 10011400+1503, and 10011400−1504 resemble the structures of 1400+1502, 1400−1502, 1400+1503, and 1400−1504, respectively, but the intense E lines observed for 1400−1503 are not found in 10011400−1503. Torsional spacings observed in the C–C–O bend series are slightly smaller than those in the pure torsional series 140+ and 140−; these result from a decreased torsional barrier due to the C–C–O bending motion. Inversion spacings exhibit a pattern similar to those in the series 140+ and 140−. Reversed abnormal torsional sublevel A/E splittings are found for states n=0–2 of the 101140− series, similar to those in the 140− series. For states n=3, the K rotational structures between the 140+ and 101140+ series and for n=4, the 140− and 101140− series are more similar than those between the pure torsion-inversion series 140+ and 140−. Hence, these experimental data imply that the interaction of the C–C–O bend with rotational structures of torsional states is smaller than that resulting from the aldehyde inversion.
Effects of a uniform magnetic field on a growing or collapsing bubble in a weakly viscous conducting fluid
Authors: Kang K. H. , Kang I. S. , Lee C. M.
Published: July 2002

The effects of a uniform magnetic field on a growing or collapsing bubble are investigated. The governing equations for the volume and shape modes of oscillation are derived. To obtain the pressure correction due to the electromagnetic field, the perturbed flow by the electromagnetic force is analyzed with the aid of the domain perturbation method. The viscous effect is assumed to be confined to a thin layer adjacent to the bubble surface. It is shown that the electromagnetic field exerts a damping force on the volume mode so that both the growing and collapsing speeds of a bubble are reduced. The magnetic field also affects the shape mode by contributing to a forcing term. Due to the forcing term, the shape of a growing–collapsing bubble becomes unstable even in the case of no initial disturbance.
High temperature electrochemical scanning tunneling microscope instrument
Authors: Shkurankov Andrei , Endres Frank , Freyland Werner
Published: July 2002

We present a novel construction of a scanning tunneling microscope (STM) for investigations of fluid/solid interfaces and, in particular, for in situ electrochemical measurements at elevated temperatures. A special feature of this instrument is a vacuum tight connection of the electrochemical cell with the STM scanner via a flexible metal bellow. This enables measurements with highly reactive and volatile fluids at high temperatures. Details of the mechanical and electronic parts of this setup are described. Test measurements on the electrodeposition of metals from molten salt electrolytes have been performed. The Ag deposition has been studied in an acidic room temperature molten salt composed of 1-butyl-3-methyl-imidazoliumchloride and AlCl3 up to 355 K. As a second example the Al deposition from molten AlCl3–NaCl has been tested up to 500 K. First results of these experiments are briefly presented.
Bottom-loading dilution refrigerator with ultrahigh vacuum deposition capability
Authors: Hernandez L. M. , Goldman A. M.
Published: July 2002

A Kelvinox 400 dilution refrigerator with the ability to load samples onto the mixing chamber from the bottom of the cryostat has been combined with an ultrahigh-vacuum (UHV) deposition chamber equipped with molecular beam sources. The liquid-helium-cooled sample transfer mechanism is used in a manner that allows films to be grown on substrates which are kept at temperatures of order 8 K with chamber pressures in the 10−9 to 10−10 Torr range. This system facilitates the growth of quench-condensed ultrathin films which must always be kept below ∼12 K in an UHV environment during and after growth. Measurements can be made on the films down to milliKelvin temperatures and in magnetic fields up to 15 T.
Ion plasma waves induced by frustrated Debye shielding
Authors: Drake R. P. , Marjoribanks R. S.
Published: July 2002

The oscillation of electrons, in a sufficiently intense pump wave, frustrates Debye shielding in the direction of the oscillation. One finds that such oscillating electrons cannot shield charge fluctuations over distances smaller than the distance they sample in a plasma period. One consequence is that the frequency of ion waves can be increased from the ion acoustic frequency to the ion plasma frequency in the presence of large enough oscillations. This may explain a number of observations in laser experiments. More generally, any phenomenon involving Debye shielding will be altered by an intense pump wave.
The effect of heat conduction in the vapor on the dynamics of downflowing condensate
Authors: Kliakhandler Igor L. , Davis Stephen H. , Bankoff S. George
Published: July 2002

A vapor fills the gap between two vertical plates, one hot and one cold. The temperatures are adjusted so that condensate forms on the cold wall. It is the dynamics of the system that is examined. The paper extends the one-sided model of evaporation–condensation to account the heat conduction in the vapor phase, which turns out to be important in many condensation problems. For the considered flow, both vapor recoil and Marangoni effect are stabilizing; as a result, the condensate becomes unstable at nonzero Reynolds numbers in contrast to the usual film flow down a vertical wall. A nonlinear evolution equation is derived and analyzed for the interaction of viscous shear and evaporation–condensation. It turns out that the one-sided model of heat and mass transfer gives a very good description of the initial stage of thin-film growth; in later stages, however, the heat conduction through the vapor becomes important when the film is sufficiently thick.
An all-cryogenic THz transmission spectrometer
Authors: Burke P. J. , Eisenstein J. P. , Pfeiffer L. N. , West K. W.
Published: July 2002

This article describes a THz transmission spectrometer for the spectral range of 2–65 cm−1 (100 GHz to 2 THz) with a spectral resolution of at least 1.8 cm−1 (50 GHz) where the source, sample, and detector are all fully contained in a cryogenic environment. Cyclotron emission from a two-dimensional electron gas (2DEG) heated with an electrical current serves as a magnetic-field tunable source. The spectrometer is demonstrated at 4.2 K by measuring the resonant cyclotron absorption of a second 2DEG. Unique aspects of the spectrometer are that (1) an ultrabroadband detector is used, and (2) the emitter is run quasicontinuously with a chopping frequency of only 1 Hz. Since optical coupling to room temperature components is not necessary, this technique is compatible with ultralow temperature (sub-100 mK) operation.
Multichannel rf-compensated Langmuir probe array driven by a single bias supply
Authors: Jeon S. J. , Eom G. S. , Kim J. H. , Choe W.
Published: July 2002

A multichannel Langmuir probe array driven by a single bias supply was constructed. For the probes to be used in harsh radio frequency (rf) environments, the individual probe has a rf-compensation circuit. For simultaneously obtaining I–V curves from the probe array, shunt resistors were placed between the probe tips and the active terminal of the bias supply. The pickup signal due to the stray capacitance of the signal lines and the shunt resistance is discussed. Modification of the probe bias circuit by utilizing lock-in amplifiers was attempted to obtain the electron energy distribution function. The multichannel probe array and the relevant circuit driven by a single bias supply were successfully tested to obtain plasma parameters from various plasma conditions not only in rf plasma but also in tokamak ohmic plasma.
High temperature ion sources with ion confinement
Authors: Panteleev V. N. , Barzakh A. E. , Fedorov D. V. , Moroz F. V. , Orlov S. Yu. , Seliverstov M. D. , Volkov Yu. M. , Tecchio L. , Andrighetto A.
Published: July 2002

Different types of high temperature ion sources such as surface, laser, and electron beam ionization ion sources have been developed and tested in off-line and on-line experiments. All types of developed ion sources have an essential common feature: an existence of the ion confinement inside the ion source cavity.
Simulations of intense pulsed ion beam uniformity in a planar magnetically insulated ion diode
Authors: Xiang W. , Zhao W. J. , Yan S. , Zeng B. Q.
Published: July 2002

Using the self-consistent, 2-1/2-dimensional particle-in-cell MAGIC code, based upon the explosive emission model, we have simulated the characteristics of the proton flux extracted from a planar magnetically insulated diode with a 200-mm-wide and an 8 mm anode-cathode gap. Results of numerical calculation, e.g., the spatial distributions of the extracted proton current density vs time and the spatial distribution of particles in diode gap are presented at a diode voltage of 300 kV with 40 ns pulse duration and an externally insulating magnetic field of 0–4Bcrit. With the increase of the magnetic field, the uniformity of the extracted proton flux is significantly improved; however, the maximum of the proton current density and one of the proton energy density decrease from 150 to 45 A/cm2 and from 1.0 to 0.3 J/cm2, respectively, when B varied from 0 to 4Bcrit. The effects of the magnetic field on uniformity of the extracted proton flux are discussed in this article.
Photodissociation of O2 in the Herzberg continuum. II. Calculation of fragment polarization and angular distribution
Authors: van Vroonhoven Mirjam C. G. N. , Groenenboom Gerrit C.
Published: July 2002

Parallel and perpendicular components of the Herzberg I, II, and III transitions contribute to the photodissociation of O2 in the Herzberg continuum. The photodissociation dynamics determines the O(3Pj), j=0,1, and 2 atomic fine-structure branching ratios and angular distributions, which were determined in ion imaging experiments at λ=236, 226, and 204 nm by Buijsse et al. [J. Chem. Phys. 108, 7229 (1998)]. In the preceding paper we presented potential energy curves for all eight ungerade O2 states that correlate with the O(3P)+O(3P) dissociation limit, and the R-dependent spin–orbit and the nonadiabatic radial derivative couplings between these states. Here, we employ these potentials and couplings in a semiclassical calculation of the fine-structure branching ratios, atomic polarizations, and fine-structure resolved anisotropy parameters. We discuss the adiabaticity of the dissociation by comparing the results with adiabatic and diabatic models. The O(3Pj) 2+1 REMPI detection scheme used in the experiment is sensitive to the polarization of the atomic fragments. We predict an important effect of the polarization on the anisotropy of the j=1 and j=2 ion images at low energies (λ>236 nm). The agreement between the semiclassical calculations and experiment is reasonable, possible explanations for the remaining differences are discussed.
Major enhancement of extra-low-frequency radiation by increasing the high-frequency heating wave power in electrojet modulation
Authors: Kuo S. P. , Lee S. H. , Kossey Paul
Published: July 2002

Extra-low-frequency (ELF) wave generation by modulated polar electrojet currents is studied. The amplitude-modulated high-frequency (HF) heating wave excites a stimulated thermal instability to enhance the electrojet current modulation by the passive Ohmic heating process. Inelastic collisions of electrons with neutral particles (mainly due to vibrational excitation of N2) damp nonlinearly this instability, which is normally saturated at low levels. However, the electron’s inelastic collision loss rate drops rapidly to a low value in the energy regime from 3.5 to 6 eV. As the power of the modulated HF heating wave exceeds a threshold level, it is shown that significant electron heating enhanced by the stimulated thermal instability can indeed cause a steep drop in the electron inelastic collision loss rate. Consequently, this instability saturates at a much higher level, resulting to a near step increase (of about 10–13 dB, depending on the modulation wave form) in the spectral intensity of ELF radiation. The dependence of the threshold power of the HF heating wave on the modulation frequency is determined.
Kinetic electron closures for electromagnetic simulation of drift and shear-Alfvén waves. I.
Authors: Cohen Bruce I. , Dimits Andris M. , Nevins William M. , Chen Yang , Parker Scott
Published: July 2002

The electromagnetic hybrid scheme of Chen and Parker (fluid electrons and gyrokinetic ions) [Phys. Plasmas 8, 441 (2001)] is extended to include a kinetic electron closure valid for βe>me/mi (βe is the ratio of the plasma electron pressure to the magnetic field energy density). The new schemes incorporate partially linearized (δf ) drift-kinetic electrons whose pressure and number density moments are used to close the fluid momentum equation for the electron fluid (Ohm’s law) using the departure of the perturbed δf kinetic pressure from the isothermal perturbed pressure response. Comparisons are made between the results of the hybrid schemes with kinetic electron closure and a conventional δf algorithm for drift-kinetic electrons and gyrokinetic ions in a two-dimensional slab model. The test cases used are small-amplitude kinetic shear-Alfvén waves with electron Landau damping, the ion-temperature-gradient instability, and the collisionless drift instability (universal mode) in an unsheared slab as a function of the plasma βe. The hybrid schemes have the desirable properties that they do not require that the mesh size perpendicular to the applied magnetic field be smaller than the collisionless skin depth c/ωpe and naturally accommodate zonal flow physics (radial modes) with nonadiabatic electron effects. The most successful of the new algorithms introduced gives very good results for βe>me/mi.
Characteristics of plasmas in a compact ion source using high-frequency microwave electron cyclotron resonance
Authors: Suzuki T. , Fujii Y.
Published: July 2002

A compact electron cyclotron resonance (ECR) ion source (compact ECRIS) for the mass spectrometer was tested. A feature of the compact ECRIS is its size, i.e., inner diameter: 22.5 mm (1 in. tube), length: 56 mm. ECR plasmas in this ion source using argon gas were discharged by high-frequency microwaves, 7–10 GHz. It was confirmed that the plasma was generated in the pressure range 30–410 mPa at the low input power of 5 W.
Small anode source for efficient ion production
Authors: Dudnikov V. , Dudnikova G.
Published: July 2002

Ion source modification is proposed for efficient production of ion beam and extending of operating lifetime. Ionization efficiency of the Bernas type ion source has been improved by using a small anode-thin rod, oriented along the magnetic field. The transverse electric field of small anode transport plasma by drift in crossed field to the emission slit. Optimization of the cathode material recycling is used to increase the operating lifetime. Optimization of the wall potential is used for suppression of flakes formation. A three-electrode extraction system was optimized for low energy beam production and efficient space charge neutralization. An ion beam with emission current density up to 60 mA/cm2 has been extracted from discharge in BF3 gas. Ion beams of B11 isotope with intensity up to 6 mA for 3 keV, up to 11 mA for 5 keV, 18 mA for 15 keV have been transported through the analyzer magnet.