bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature dependence of the vibrational relaxation processes in natural and isotopically pure 32S8: Effect of the isotopic impurities on infrared phonon lifetimes
Authors: Bini R. , Califano S. , Eckert B. , Jodl H. J.
Published: July 2002

We have measured the temperature dependence of the infrared bandwidth of several lattice phonons and internal vibrons in natural and 32S isotopically pure orthorhombic sulfur crystals, using a high resolution FTIR interferometer. The experimental data were analyzed in terms of anharmonic phonon–phonon coupling processes and of contributions to the bandwidth due to the presence of randomly distributed isotopic impurities. The same three-phonon processes that contribute to the lifetime of the Raman bands were found to be active also for the infrared ones in the isotopically pure crystal. In parallel to the increase of the two-phonon density of states, the third-order anharmonic coupling coefficients for phonon decay processes were found to increase from the lowest to the highest lattice mode. The effect of the isotopic impurities was explained in terms of two contributions, the first, which is temperature independent, due to pure harmonic scattering processes and the second due to the combined effect of anharmonicity and scattering processes, which is instead dependent on the temperature. The contribution due to the presence of impurities was found to dominate the width of the lattice modes in the natural crystal at low temperatures. This purely harmonic contribution can be in some cases almost one order of magnitude larger than that due to the normal anharmonic decay processes in the isotopically pure crystal. Only the width of the factor group component of infrared and Raman vibrons close to the frequency of the isolated molecule was found to be affected by the presence of impurities through a resonant scattering mechanism.
Vibrational relaxation of multiphonon bound states in crystalline HCl
Authors: Gellini Cristina , Salvi Pier Remigio , Schettino Vincenzo
Published: July 2002

The vibrational properties of multiphonon bound states in the region of the first and second overtone of orthorhombic HCl crystal have been studied by means of infrared spectroscopy. Triphonons have been observed and assigned on the basis of the available theories of multiphonon crystal excitations. The Lorentzian widths of biphonon and triphonon bands have been determined from observed band profiles by band deconvolution as a function of temperature in the range 20–90 K. The temperature dependent bandwidths of biphonons and triphonons are compared and discussed in terms of depopulation and dephasing processes of the bound state.
Stochastic dynamics simulation of surfactant self-assembly
Authors: von Gottberg Friedrich K. , Smith Kenneth A. , Hatton T. Alan
Published: July 2002

The self-assembly of short amphiphilic molecules of type A2B2 (A=hydrophilic, B=hydrophobic) is investigated using Stochastic Dynamics simulations with a scalar frictional coefficient. Equilibrium properties were calculated and explained in the context of existing thermodynamic theories. Spherical micelles are observed to form and the effect of temperature and total surfactant concentration on the structural properties are investigated. Above the critical micelle concentration, a decline in the free surfactant concentration is observed, contrary to existing theories dealing with micelle formation. The observed behavior can be explained in terms of excluded volume effects. Excellent agreement is obtained between the resulting predictions and the simulation results over the entire concentration range.
Benchmark quality total atomization energies of small polyatomic molecules
Authors: Martin Jan M. L. , Taylor Peter R.
Published: July 2002

Successive coupled-cluster [CCSD(T)] calculations in basis sets of spdf, spdfg, and spdfgh quality, combined with separate Schwartz-type extrapolations A+B/(l+1/2)α of the self-consistent field (SCF) and correlation energies, permit the calculations of molecular total atomization energies (TAEs) with a mean absolute error of as low as 0.12 kcal/mol. For the largest molecule treated, C2H4, we find ∑D0=532.0 kcal/mol, in perfect agreement with experiment. The aug-cc-pV5Z basis set recovers on average about 99% of the valence correlation contribution to the TAE, and essentially the entire SCF contribution.
Micron-gap rheo-optics with parallel plates
Authors: Dhinojwala Ali , Granick Steve
Published: July 2002

Solid planar plates with area up to several square centimeters can be made parallel at controllable separations from ca. 0.1 (if airborne dust is eliminated) to >500 μm. Apart from dust and surface roughness, which set the lower bound of plate-plate separation, there is no other fundamental constraint on the type of surface (metallic or dielectric; opaque or translucent) that can be studied. When conducting plates are employed, it is possible to apply an electric field in the direction normal to the plates and observe the competition between shear fields and electric fields in orthogonal directions. The large surface area should afford sufficient quantity of sample to make possible various spectroscopic and scattering experiments (especially infrared and dielectric spectroscopy in the direction normal to the plates, and x-ray and neutron reflectivity).
Effect of guest–host interaction on Raman spectrum of a CO2 clathrate hydrate single crystal
Authors: Ikeda Tomoko , Mae Shinji , Uchida Tsutomu
Published: July 2002

The polarized Raman spectra of an artificial CO2 clathrate hydrate single crystal have been measured in order to examine the crystal-orientation dependence of the Raman spectra. Since the crystal had crystallographic facets, the orientation of the crystal was determined by using the Miller indices of the facets. When the angle θ between the polarization plane of the incident laser beam and the direction of one of the 〈110〉 axes of the single crystal varied, it was observed that the intensities of the peaks, which were caused by the Fermi resonance of the symmetric stretching mode and the overtone of the bending mode of CO2, and the O–H symmetric stretching vibration mode, varied with θ. Since the tetrakaidecahedron cage in the CO2 clathrate hydrate is distorted along the 〈100〉 axis, the variations of the scattering intensities of the CO2 have been calculated by using a simple model that assumes that the CO2 rotates on the {100} plane in the tetrakaidecahedron cage. The results obtained from the experiments are consistent with the calculations made by using this model. It has been concluded that the anisotropy of the peak intensities of the CO2 show the influence of the cage geometry on the motion of the guest molecule. The anisotropy of the O–H symmetric stretching vibration mode was interpreted with a five-body structure model. As the calculation with the model was consistent with the result obtained from the experiment, it was found that the anisotropy of the peak intensity of the O–H symmetric stretching vibration mode was related to the arrangement of the water molecules. We consider that the result indicates the influence of the motion of the guest molecule on the surrounding hydrogen-bonded network.
Atomic clusters and nanoscale particles: From coarse-grained dynamics to optimized annealing schedules
Authors: Kunz Ralph E. , Blaudeck Peter , Hoffmann Karl Heinz , Berry R. Stephen
Published: July 2002

An adaptive method is presented to optimize schedules for the simulated annealing of clusters and nanoscale particles. The method, based on both molecular-dynamics simulations and a set of master equations, is applied to a model configuration space for which the exact optimal schedule can also be found. The adaptive method is demonstrably suitable for optimizing larger and more realistic systems than can be treated by an exact method, even one based on a statistical-sample master equation.
Temporal coherent control in the photoionization of Cs2: Theory and experiment
Authors: Blanchet Valérie , Bouchène Mohamed Aziz , Girard Bertrand
Published: July 2002

Two identical femtosecond pulses are used to create a coherent superposition of two vibrational wave packets in a bound electronic state of cesium dimers. The oscillations of these two wave packets are further detected after photoionization of the system. Quantum interferences between the two wave packets result in a temporal coherent control of the ionization probability. The interferogram exhibits the following features as a function of the time delay between the two laser pulses: high-frequency oscillation corresponding to Ramsey fringes (at the Bohr frequency of the transition) modulated by a slow envelope corresponding to the oscillations of vibrational wave packets (vibrational recurrences). Here the control parameter is the time delay between the two laser pulses which can be used to control the preparation of a wave packet in a quantum system and monitor its evolution. The detailed theory of this experiment is presented and compared with the pump-probe experiment. The temporal coherent control experiment is performed in the B state of Cs2, at 768 nm, with 150 fs pulses. We finally analyze quantitatively and discuss the effects of a thermal distribution of initial states on the temporal coherent control signal.
Elevated temperature scanning tunneling microscopy study of formic acid adsorption and reaction on oxygen (2×1) covered Cu(110)
Authors: Bennett Roger A. , Poulston Stephen , Bowker Michael
Published: July 2002

The adsorption and reaction of formic acid on a fully oxygen (2×1) covered Cu(110) surface maintained at 353 K has been studied by scanning tunneling microscopy (STM). Formic acid adsorption is followed in a sequence of images that show the removal of oxygen (2×1) rows preferentially in the 〈001〉 direction. The added copper atoms which were associated with the oxygen reconstruction are liberated and diffuse to create significant areas of the high oxygen coverage O-c(6×2) structure. The adsorbed formate produces a (4×1) structure. Analysis of the STM images allows limits to be placed upon the surface structure of the final formate (4×1) surface revealing a lower local formate density than produced for room temperature adsorption. These two observations lead to an understanding of the significant drop in formic acid uptake which has been reported for a narrow temperature window between 350–420 K on this surface.
Random sequential adsorption on partially covered surfaces
Authors: Adamczyk Zbigniew , Weroński Paweł
Published: July 2002

The random sequential adsorption (RSA) approach was used to analyze adsorption of hard spheres at surfaces precovered with smaller sized particles. Numerical simulations were performed to determine the available surface function φl of larger particles for various particle size ratios λ=al/as and surface concentration of smaller particles θs. It was found that the numerical results were in a reasonable agreement with the formula stemming from the scaled particle theory with the modification for the sphere/sphere geometry. Particle adsorption kinetics was also determined in terms of the RSA simulations. By extrapolating the θl vs τ−1/2 dependencies, the jamming concentrations of larger spheres θl∞ were determined as a function of the initial smaller sphere concentration. It was found that θl∞ were considerably reduced by the presence of smaller sized particles, especially for λ≫1. The pair correlation function g of larger particles in the jamming state was also determined, showing more short range ordering (at the same θl) in comparison with monodisperse systems. The theoretical predictions stemming from our calculations suggest that the presence of trace amounts of very small particles may exert a decisive influence on adsorption of larger particles.
Laser spectroscopy of free pentacene molecules (I): The rotational structure of the vibrationless S1←S transition
Authors: Heinecke E. , Hartmann D. , Müller R. , Hese A.
Published: July 2002

The rotational structure of the vibrationless S1←S0 transition of pentacene has been investigated using a strongly collimated seeded supersonic argon beam. Because single rotational lines could not be completely resolved, a band contour analysis was performed. The rotational constants of the electronic ground state X 1A1g were found to be under the asymmetric rotor approximation A″=1320.6(9), B″=117.97(9), C″=108.28(15) MHz, whereas the differences to the first electronic excited state A 1B2u are ΔA=A′−A″=13.2(3), ΔB=−0.764(45), ΔC=−0.54(6) MHz. A new value of the band origin was determined to be ν00=18 648.996(4) cm−1 and the band type was confirmed to be of type b as proposed by symmetry arguments. Good agreement between observed and calculated spectra was obtained assuming planarity in both ground and excited state. From the fit procedure a rotational temperature of about 7 K was deduced. The nuclear statistical weights of the electronic ground state are reported.
Statistical modeling of competitive threshold collision-induced dissociation
Authors: Rodgers M. T. , Armentrout P. B.
Published: July 2002

Collision-induced dissociation of (R1OH)Li+(R2OH) with xenon is studied using guided ion beam mass spectrometry. R1OH and R2OH include the following molecules: water, methanol, ethanol, 1-propanol, 2-propanol, and 1-butanol. In all cases, the primary products formed correspond to endothermic loss of one of the neutral alcohols, with minor products that include those formed by ligand exchange and loss of both ligands. The cross-section thresholds are interpreted to yield 0 and 298 K bond energies for (R1OH)Li+–R2OH and relative Li+ binding affinities of the R1OH and R2OH ligands after accounting for the effects of multiple ion–molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. We introduce a means to simultaneously analyze the cross sections for these competitive dissociations using statistical theories to predict the energy dependent branching ratio. Thermochemistry in good agreement with previous work is obtained in all cases. In essence, this statistical approach provides a detailed means of correcting for the “competitive shift” inherent in multichannel processes.
Surface properties of diluted aqueous solutions of normal short-chained alcohols
Authors: Gliński Jacek , Chavepeyer Guy , Platten Jean-Karl , Smet Phillippe
Published: July 2002

The surface tension σ of aqueous solutions of normal alcohols (from methanol to n-butanol) was measured in the temperature range between 278 and 303 K using the Wilhelmy plate method. The surface entropies are calculated. The results are interpreted in terms of specific interactions of the hydrophobic parts of the molecules adsorbed at the interface. It is concluded that increasing surface concentration of alcohol molecules causes their rearrangement at the surface, similar to phase transition. This process depends on the symmetry of molecules.
Spreading of perfluoropolyalkylether films on amorphous carbon surfaces
Authors: Ma X. , Gui J. , Smoliar L. , Grannen K. , Marchon B. , Jhon M. S. , Bauer C. L.
Published: July 2002

Spreading of perfluoropolyalkylether thin films on amorphous carbon surfaces has been studied by scanning microellipsometry. Two types of perfluoropolyalkylethers with the same main-chain structure and various molecular weights (between 1000 and 6000 g/mol) were used: Zdol, with OH functional end groups, and Z, with nonfunctional CF3 groups. For Zdol, the thickness of the molecular layers in the spreading profile increase as Mn0.6, where Mn is the mean molecular weight, with the second layer being nearly twice as thick as the first layer. This layered structure was not observed for Z in the molecular weight range under study. As expected, the thickness-dependent diffusion coefficient D(h) was found to decrease with increased molecular weight. Possible molecular conformations near the solid surface are discussed. The spreading of binary blends of Zdol–Zdol, Z–Z, and Zdol–Z were also studied. The results show that the spreading of the binary blend of the same kind of polymer with different molecular weight behaved like that of a lubricant with an intermediate molecular weight. The diffusion coefficient of a blend was found to obey the additivity of viscosity. For Zdol–Z blends, however, the faster moving Z molecules migrate through the network of the slower moving Zdol molecules, and form a monolayer ahead of Zdol.
Solvent electronic polarization effect on the electronic transitions in solution: Charge polarizable reference interaction site model self-consistent field approach
Authors: Naka Kazunari , Morita Akihiro , Kato Shigeki
Published: July 2002

Effect of the electronic polarization in solvent on the solvatochromic shift for the excitation energy of solute molecule is studied by the reference interaction site model self-consistent field (RISM-SCF) method. The electronic polarization in solvent molecule is represented by the charge response kernel (CRK) obtained by ab initio calculations. Employing the CRK, a charge polarizable RISM-SCF method is proposed for describing the electronic structure of solute molecules in solution and estimating the excitation energies. The excitation energy for nπ* transition of acetone is calculated in CH3CN, CHCl3, CCl4, and CS2 solvents and the solvent electronic polarization effect on the solvation shift is examined. As a result, a blue shifts of 1225, 675, 166, and 92 cm−1 is obtained in those solvents. Furthermore, the solvation shifts in the transitions to the 11B1, 11B2, and 21A1 states of pyridine are evaluated in the same solvents. A blue shift is observed for the 11B1 ← 11A1 transition in all the solvent, while the 21A1 ← 11A1 one shows a red shift.
The O–H stretching vibrations of glycine trapped in rare gas matrices and helium clusters
Authors: Huisken F. , Werhahn O. , Ivanov A. Yu. , Krasnokutski S. A.
Published: July 2002

Employing molecular beam depletion spectroscopy and Fourier transform infrared matrix spectroscopy, respectively, we have studied the O–H stretching vibrations of the glycine conformers I, II, and III. The glycine molecules were either deposited into large liquid helium clusters HeN, N̄=11000, THe=0.4 K) or trapped in various rare gas matrices (Ne, Ar, Kr) at temperatures below 12 K. By extrapolating the experimental data plotted as a function of the square root of the critical temperature of the matrix material, the positions of the gas phase absorption bands were estimated to be 3585±2 cm−1 (conformer I), 3295±3 cm−1 (conformer II), and 3580±5 cm−1 (conformer III). The experimental results are compared with recent ab initio calculations.
Lennard-Jones as a model for argon and test of extended renormalization group calculations
Authors: White John A.
Published: July 2002

Renormalization group (RG) procedures have been extended recently in phase-space cell approximation to predict, in addition to universal thermal properties observed asymptotically close to the gas-liquid critical point of fluids, also nonuniversal and nonasymptotic properties. This “globalized” RG theory is applied here, using a Lennard-Jones potential, to calculate the temperature, density, and pressure at the critical point of argon and to calculate pressures for a wide range of densities at temperatures close to, below, and considerably above that at the argon critical point. Choices required for the Lennard-Jones parameters and the quality of fit to experimental data suggest some of the strengths and limitations of the global RG theory.
Structure and thermodynamics of anisotropic polymer fluids
Authors: Pickett Galen T. , Schweizer Kenneth S.
Published: July 2002

We investigate the structure and thermodynamics of anisotropic polymer fluids, focusing on the nematic phases of flexible polymers. The chains interact only through monomer–monomer excluded-volume interactions. As a function of an externally provided alignment along a fixed nematic director, we calculate the anisotropic pair correlation function, and demonstrate the existence of two density correlation lengths, ξ⊥ and ξz, controlling transverse and longitudinal density fluctuations, respectively. We allow the possibility that the chains align either along the director (nematic conformations), or are anti-aligned in a “discotic-like” configuration. The cohesive contribution to the free energy is established in a high-temperature approximation, and its sensitivity to the orientation of the chains is probed. Our approach is not limited to homogeneous liquid crystalline phases, but applies in any circumstance when the orientation of otherwise disordered polymers is the physically controlling effect, e.g., confinement in thin films or pores, shear-alignment of flexible polymers, or straining a cross-linked rubber network.
Picosecond polarized fluorescence studies of anisotropic fluid media. II. Experimental studies of molecular order and motion in jet aligned rhodamine 6G and resorufin solutions
Authors: Bain A. J. , Chandna P. , Butcher G. , Bryant J.
Published: July 2002

New polarized time resolved fluorescence techniques are implemented to determine the full angular motion of a probe molecule in an anisotropic environment. Studies of rhodamine 6G and resorufin molecules aligned in a free ethylene glycol jet show that the presence of net molecular order is accompanied by a distinct anisotropy in alignment relaxation following photoselection. Diffusion coefficients for φ and θ motion (D∥ and D⊥) in a jet fixed axis system are determined from the cylindrically symmetric and asymmetric alignment relaxation rates for the isotropic and anisotropic regions of the jet. The presence of net negative molecular alignment as the free jet is formed is seen to correspond to restricted φ motion (D∥<D⊥), with a net positive steady state alignment the anisotropy in the diffusion dynamics is reversed. The differences in D∥ and D⊥ are attributed to anisotropy in the solvent viscosity as a consequence of flow. The combination of linear and circular polarization techniques is seen to provide useful information on cylindrical asymmetry and relaxation dynamics hitherto unobserved by conventional fluorescence polarization techniques.
Molecular dynamics study of screening in ionic fluids
Authors: Keblinski P. , Eggebrecht J. , Wolf D. , Phillpot S. R.
Published: July 2002

Molecular-dynamics simulations of molten and gaseous NaCl are used to probe the entire range of screening behaviors including weakly and strongly-coupled regimes as well as the transition between them. Our simulations confirm that, in accordance with the Debye–Hückel theory, in the weakly-coupled regime (corresponding to the ionic gas) the spatial correlations between the ions decay monotonically, with a screening length, λQ, given by the Debye length, λD, which decreases with increasing ionic coupling strength. By contrast, in the strongly-coupled regime (corresponding to the melt) the spatial correlations between the ions decay in an oscillatory manner, λQ then being given by the envelope of the damped oscillations. In accordance with theoretical predictions, in the strongly-coupled λQ increases with increasing coupling strength. Also in agreement with existing theories our simulations show that the transition from monotonic to oscillatory screening occurs at the shortest possible screening length, which is on the order of the average ion diameter, σ; also, the simulated values of λQ across the entire range can be represented by a universal function of the ionic-strength parameter, σ/λD.
Symmetric-tensor eigenspectrum of the Laplacian on n-spheres
Authors: Rubin Mark A. , Ordóñez Carlos R.
Published: July 2002

The eigenvalues and degeneracies of the covariant Laplacian acting on symmetric tensors of rank m≤2 defined on n-spheres with n≥3 are given.
A search for bilinear equations passing Hirota’s three-soliton condition. II. mKdV-type bilinear equations
Authors: Hietarinta Jarmo
Published: July 2002

In this paper (second in a series) [for part I, see J. Math. Phys. 30, 1732 (1987)] the search for bilinear equations having three-soliton solutions continues. This time pairs of bilinear equations of the type P1(Dx,Dt)F⋅G=0, P2(Dx,Dt)F⋅G=0, where P1 is an odd polynomial and P2 is quadratic, are considered. The main results are the following new bilinear systems: P1=aDx7+bDx5 +Dx2Dt+Dy, P2=Dx2; P1=aDx3+bDt3 +Dy, P2=DxDt; and P1=DxDtDy +aDx+bDt, P2=DxDt. In addition to these, several models with linear dispersion manifolds were obtained, as before.
A search for bilinear equations passing Hirota’s three-soliton condition. III. Sine–Gordon-type bilinear equations
Authors: Hietarinta Jarmo
Published: July 2002

In this paper the results of a search for pairs of bilinear equations of the type Ai(Dx,Dt)F⋅F +Bi(Dx,Dt)G⋅F +Ci(Dx,Dt)G⋅G=0, i=1,2, which have standard type three-soliton solutions, are presented. The freedom to rotate in (F,G) space is fixed by the one-soliton ansatz F=1, G=en, then the Bi determine the dispersion manifold while Ai and Ci are auxiliary functions. In this paper it is assumed that B1 and B2 are even and proportional, and that Ai and Ci are quadratic. As new results, B1=aD3x Dt +DtDy+b, A2=−C2=DxDt, and generalizations of the sine–Gordon model B1=DxDt+a with a family of auxiliary functions Ai and Ci are obtained.
Multi-Hamiltonian structure of the Born–Infeld equation
Authors: Arik Metin , Neyzi Fahrünisa , Nutku Yavuz , Olver Peter J. , Verosky John M.
Published: July 2002

The multi-Hamiltonian structure, conservation laws, and higher order symmetries for the Born–Infeld equation are exhibited. A new transformation of the Born-Infeld equation to the equations of a Chaplygin gas is presented and explored. The Born–Infeld equation is distinguished among two-dimensional hyperbolic systems by its wealth of such multi-Hamiltonian structures.
On equations of type u
x
t=F(u,u
x) which describe pseudospherical surfaces
Authors: Rabelo Mauro L. , Tenenblat Keti
Published: July 2002

The equations uxt=F(u,ux), which describe η-pseudospherical surfaces, are characterized. In particular, when F does not depend on ux, the sine–Gordon, sinh–Gordon, and Liouville equations are essentially obtained. Moreover, it is shown that an equation uxt=F(u) has a self-Bäclund transformation if and only if it describes an η-pseudospherical surface.
Solution of the central field problem for a Duffin–Kemmer–Petiau vector boson
Authors: Nedjadi Y. , Barrett R. C.
Published: July 2002

In view of current interest in the use of the Duffin–Kemmer–Petiau (DKP) relativistic equation, the problem of the vector DKP boson in a central field is resolved and the system of first-order coupled differential radial equations needed for an exact calculation of the eigenvalues as well as the full ten-component spinor is derived. This is of practical importance for problems involving massive vector bosons in central fields. This formalism is applied to the free-particle, spherically symmetric square well and Coulomb problems.
Anomaly cancellation in six dimensions
Authors: Erler Jens
Published: July 2002

It is shown that anomaly cancellation conditions are sufficient to determine the two most important topological numbers relevant for Calabi–Yau (CY) compactification to six dimensions. This reflects the fact that K3 is the only nontrivial CY manifold in two complex dimensions. The Green–Schwarz counterterms are explicitly constructed and sum rules for charges of additional enhanced U(1) factors are derived and the results with all possible Abelian orbifold constructions of K3 are compared. This includes asymmetric orbifolds as well, showing that it is possible to regain a geometrical interpretation for this class of models. Finally, some models with a broken E7 gauge group which will be useful for more phenomenological applications are discussed herein.
Long-range properties of spanning trees
Authors: Kenyon Richard
Published: July 2002

We compute some large-scale properties of the uniform spanning tree process on bounded regions in Z2. In particular, we compute the distribution of the meeting point of the branches of the tree issued from three boundary points. We also compute the crossing probabilities of branches of the tree on rectangular and annular regions, as well as the winding number of the branches of the tree.
EPR studies of N, H, and D atoms trapped in the matrix of solid molecular nitrogen
Authors: Dmitriev Yu. A. , Zhitnikov R. A.
Published: July 2002

The behavior of nitrogen, hydrogen, and deuterium atoms trapped from the gaseous phase and matrix-isolated in solid molecular hydrogen are studied by the EPR method. It is established that all these atoms are localized in the substitutional position of the matrix crystal lattice. The linewidths in the EPR spectrum of nitrogen are found to be one fourth of the linewidths recorded by most authors. It is shown that a version of theoretical calculation gives a linewidth matching with the experimental value. It is proposed that two different types of matrix surroundings are possible for a nitrogen atom trapped in the substitutional position, one of which corresponding to the undistorted crystal lattice of N2. The EPR line broadening under the action of zero-point atomic vibrations is observed for hydrogen and deuterium atoms. The magnitude and temperature dependence of the spin-lattice relaxation time for N, H, and D atoms in the solid N2 matrix are obtained experimentally.
Structure of partly dispersed normal shock waves in vapor-droplet flows
Authors: Guha A.
Published: July 2002

A numerical and analytical study has been made to reveal the internal structures of partly dispersed shock waves in pure vapor-droplet media. The results clearly demonstrate the effects of different relaxation phenomena present and their associated time scales on the structure. The study systematically analyzes the relative effects of different flow parameters on the structure and thickness of the shock wave. Wherever possible, the structure in a vapor-droplet medium is contrasted against similar structures in more familiar solid particle-laden gas flow. The case of complete evaporation through which a two-phase vapor-droplet medium reverts to a single-phase nonrelaxing one is discussed. Although linearized analyses are often presented for relaxing flows downstream of frozen shocks, they are of limited applicability to vapor-droplet flows. The paper gives many examples of why a linearized analysis is unlikely to be successful in such cases.
Study of three-dimensional Rayleigh–Taylor instability in compressible fluids through level set method and parallel computation
Authors: Li X. L.
Published: July 2002

Computation of three-dimensional (3-D) Rayleigh–Taylor instability in compressible fluids is performed on a MIMD computer. A second-order TVD scheme is applied with a fully parallelized algorithm to the 3-D Euler equations. The computational program is implemented for a 3-D study of bubble evolution in the Rayleigh–Taylor instability with varying bubble aspect ratio and for large-scale simulation of a 3-D random fluid interface. The numerical solution is compared with the experimental results by Taylor.
A program toward a fusion reactor
Authors: Rebut P.-H. , Watkins M. L. , Gambier D. J. , Boucher D.
Published: July 2002

Near breakeven conditions have been attained in the JET tokamak [Fusion Technol. 11, 13 (1987)], with beryllium as the first-wall material. A fusion triple product (nDτETi) of 8–9×1020 m−3 sec keV has been reached (within a factor of 8 of that required in a fusion reactor). However, this has only been achieved transiently. At high heating powers, an influx of impurities still limits the achievement of better performance and steady-state operation. In parallel, an improved quantitative understanding of fusion plasmas has emerged from the development of a particular plasma model. Good quantitative agreement is obtained between the model and JET data. The main predictions are also consistent with statistical scaling laws. With such a model, a predictive capability begins to emerge to define the parameters and operating conditions of a DEMO, including impurity levels. Present experimental results and model predictions suggest that impurity dilution is a major threat to a reactor. A divertor concept must be developed further to ensure impurity control before a DEMO can be constructed. A New Phase for JET is planned in which an axisymmetric pumped divertor configuration will be used to address the problems of impurity control, plasma fueling, and helium ash exhaust. It should demonstrate a concept of impurity control and the operational domain for such a device. A single Next Step facility (ITER) is a high risk strategy in terms of physics, technology, and management, since it does not provide a sufficiently sound foundation for a DEMO. A Next Step program is proposed, which could comprise several complementary facilities, each optimized with respect to specific clear objectives. In a minimum program, there could be two Next Step tokamaks, and a Materials Test Facility. Such a program would allow division of effort and sharing of risk across the various scientific and technical problems, permit cross comparison, and ensure continuity of results. It could even be accomplished without a significant increase in world funding.
Excitation of magnetohydrodynamic modes by energetic trapped particles in reactive and dissipative systems
Authors: Liljeström M. , Weiland J.
Published: July 2002

The excitation of magnetohydrodynamic modes by hot trapped particles is studied using different kinetic distribution functions and, for the first time, a fluid model, valid in both the adiabatic and isothermal limits of the perpendicular motion, an unexpanded fluid description for the hot particles. The fluid model gives zero threshold for continuum modes but also a significant reduction in the kinetic threshold can be obtained for particular equilibria. In addition, a comparison with global modes has been made.
Continuum damping of low-n toroidicity-induced shear Alfvén eigenmodes
Authors: Berk H. L. , Van Dam J. W. , Guo Z. , Lindberg D. M.
Published: July 2002

The effect of resonant continuum damping is investigated for the low-mode-number, toroidicity-induced, global shear Alfvén eigenmodes, which can be self-excited by energetic circulating alpha particles in an ignited tokamak plasma. Resonant interaction with the shear Alfvén continuum is possible for these eigenmodes, especially near the plasma periphery, leading to significant dissipation, which is typically larger than direct bulk plasma dissipation rates. Two perturbation methods are developed for obtaining the Alfvén resonance damping rate from the ideal fluid zeroth-order shear Alfvén eigenvalue and eigenfunction. In both methods the real part of the frequency is estimated to zeroth order, and the imaginary part, which includes the damping rate, is then obtained by perturbation theory. One method, which is applicable when the eigenfunction is nearly real, can readily be incorporated into general magnetohydrodynamic (MHD) codes. In the second method, the zeroth-order eigenfunctions may be complex; however, the application of this method to general MHD codes needs more detailed development. Also, an analytical estimate is found for the next-order real frequency shift of the fluid global Alfvén mode. Analytical and numerical studies of this continuum damping effect indicate that it can substantially reduce the alpha particle-induced growth rate. Thus, either it is possible to prevent instability or, if unstable, to use the Alfvén resonance damping to estimate the saturation amplitude level predicted from quasilinear theory.
Measurements of electron parallel-momentum distributions using cyclotron wave transmission*
Authors: Skiff F. , Boyd D. A. , Colborn J. A.
Published: July 2002

Practical aspects of the implementation of cyclotron wave transmission diagnostics are considered in three specific areas. Questions raised by the calculation of the diagnostic resolution and sensitivity, elimination of reflections through swept frequency and phase-coherent detection, and correcting for the effects of wave refraction through the use of equivalent rays are addressed. Results of preliminary tests and experiments as well as comments on future implementations are used to illustrate the discussion.
Laminar-turbulent transition in a slowly pulsating pipe flow
Authors: Shemer Lev
Published: July 2002

Transitional pulsating flow in a pipe is investigated experimentally in the frequency region where its behavior is assumed to be quasisteady. Instantaneous velocity measurements were performed at several radial locations at the exit plane of the pipe. The output of flush-mounted hot wires at two upstream positions was also recorded simultaneously. The results indicate that the quasisteady assumption is valid in general. The flow behavior is the function primarily of the instantaneous Reynolds number, although the laminarization process and retransition to turbulence are qualitatively different. The laminarization at subcritical instantaneous Reynolds numbers in a time-dependent flow is a gradual process, similar to the observed turbulence decay in flow geometries where Re is varied spatially. The retransition to turbulence occurs by means of the generation of turbulent slugs at various locations along the pipe. These slugs are convected downstream and coalesce, eventually resulting in fully developed turbulent flow in the whole pipe at higher instantaneous Reynolds numbers.
The unsteady oblique stagnation point flow
Authors: Wang C. Y.
Published: July 2002

An exact solution of the unsteady Navier–Stokes equations is found. The solution describes the stagnation region of an accelerating circular jet impinging obliquely on a flat plate.
The limiting configuration of interfacial gravity waves
Authors: Turner R. E. L. , Vanden-Broeck J. -M.
Published: July 2002

Progressive gravity waves at the interface between two unbounded fluids are considered. The flow in each fluid is taken to be potential flow. The problem is converted into a set of integrodifferential equations, reduced to a set of algebraic equations by discretization, and solved by Newton’s method together with parameter variation. Meiron and Saffman’s [J. Fluid Mech. 129, 213 (1983)] calculations showing the existence of overhanging waves are confirmed. However, the present calculations do not support Saffman and Yuen’s [J. Fluid Mech. 123, 459 (1982)] conjecture that the waves are geometrically limited (i.e., that solutions exist until the interface intersects itself). It is proposed that along a solution branch starting with sinusoidal waves of small amplitude, one reaches solutions with vertical streamlines and then overhanging waves. Continuing on this branch, one returns to nonoverhanging waves and then back toward a wave with vertical streamlines. It is suggested that this succession of patterns and accompanying oscillation in wave characteristics is repeated indefinitely. Graphs of the results are included.
Chaotic advection in a Stokes flow
Authors: Aref H. , Balachandar S.
Published: July 2002

Chaotic advection can be produced whenever the kinematic equations of motion for passively advected particles give rise to a nonintegrable dynamical system. Although this interpretation of the phenomenon immediately shows that it is possible for flows at any value of Reynolds number, the notion of stochastic particle motion within laminar flows runs counter to common intuition to such a degree that the range of applicability of early model results has been questioned. To dispel lingering doubts of this type a study of advection in a two-dimensional Stokes flow slowly modulated in time is presented. Even for this very low Reynolds number, manifestly ‘‘laminar’’ flow chaotic particle motion is readily realizable. Standard diagnostics of chaos are computed for various methods of time modulation. Relations to the general ideas of parametric resonance and adiabatic invariance are pointed out.
Bénard–Marangoni convection with a deformable interface and poorly conducting boundaries
Authors: Garcia-Ybarra P. L. , Castillo J. L. , Velarde M. G.
Published: July 2002

Dispersion relations and threshold values for the onset of buoyancy–thermocapillary instability are given for the case of prescribed heat flux on the boundaries of a liquid layer heated from below when the upper boundary is a deformable surface open to the ambient air. The nonlinear evolution equations of this free surface under various circumstances are also provided (without and with buoyancy, for microgravity or standard ground conditions).
A unified theory of aerodynamic and condensation shock waves in vapor-droplet flows with or without a carrier gas
Authors: Guha Abhijit
Published: July 2002

A unified theory for aerodynamic and condensation shock waves in vapor-droplet flows in the presence of an inert carrier gas is presented. Same conservation equations apply across discontinuous models for both types of wave. Exact (as well as approximate), explicit analytical jump conditions across such discontinuities are derived subject to several boundary conditions. Collectively they may be called the generalized Rankine–Hugoniot equations for vapor-droplet mixtures. All the equations derived are general and can be applied in the case of a pure vapor-droplet flow by letting the mass fraction of the carrier gas go to zero. Much physical insight may be obtained from this integral analysis. It is shown that four types of aerodynamic shock waves (viz., equilibrium partly dispersed, equilibrium fully dispersed, partly dispersed with complete evaporation, and fully dispersed with complete evaporation) may occur. Conditions for each type of these waves to occur are specified and the appropriate jump conditions are derived. A flow map for different types of condensation discontinuities to occur is deduced. It is shown that the same jump conditions are applicable for most supersonic and subsonic condensations—both homogeneous and heterogeneous. However, for certain types of condensation shocks predicted by the integral jump conditions, consideration of nonequilibrium gas dynamics must be called for. As a sequel to the integral analysis, time-marching solutions for different types of condensation shock waves in a convergent–divergent nozzle are presented, which include some novel solutions. Isentropic exponents for gas–vapor-droplet flow under frozen and equilibrium conditions are formulated. Gasdynamic equations for vapor-droplet flow, including area variation and interphase transport of mass, momentum, and energy, are derived. It is shown that equations in this full form are to be considered for making correct physical interpretations, e.g., determining the conditions for thermal choking.
Measurements of the Kolmogorov constant and intermittency exponent at very high Reynolds numbers
Authors: Praskovsky Alexander , Oncley Steven
Published: July 2002

Characteristics of turbulence in the inertial range are experimentally studied in the atmospheric surface layer over the range of the Taylor microscale based Reynolds number Rλ≊(2.8–12.7)×103 and in a large wind tunnel (in a mixing layer at Rλ≊2.0×103 and a return channel at Rλ≊3.2×103). The intermittency exponent μ, estimated from the correlation function of energy dissipation Rεε(r)=〈ε(x)ε(x+r)〉∝r−μ, is found to be independent of Reynolds number and approximately equal to 0.20. No ‘‘measurable’’ deviation from the −5/3 exponent in the ‘‘five-thirds’’ law is found. On the other hand, the Kolmogorov constant C in this law is found to be weakly dependent on Rλ. This dependence is well described by the power law C∝R−μ/2λ≊R−0.10λ at μ≊0.20.
Preferential concentration of heavy particles in a turbulent channel flow
Authors: Fessler John R. , Kulick Jonathan D. , Eaton John K.
Published: July 2002

An investigation of the instantaneous particle concentration at the centerline of a turbulent channel flow has been conducted. The concentration field was obtained by digitizing photographs of particles illuminated by a spanwise laser sheet and identifying individual particles. The resulting distribution was then compared to the expected distribution for the same number of particles randomly distributed throughout the volume. Significant departures from randomness have been found and the differences are strongly dependent on the time constants of the particles. Five different particle classes were investigated and the maximum departure from randomness was found when the ratio of the particle’s aerodynamic response time to the Kolmogorov time scale of the flow was approximately one. The length scales of the particle clusters were found to change with the particle size. The correlation dimension was used to produce a single parameter describing the degree of concentration regardless of the scale on which it occurs. The spacing between particle clusters was also investigated and found to be much larger than the scales on which concentration occurs.
Asymptotic study and weakly nonlinear analysis at the onset of Rayleigh–Bénard convection in Hele–Shaw cell
Authors: Aniss Saïd , Souhar Mohamed , Brancher Jean Pierre
Published: July 2002

The aim of this paper is the derivation of the Ginzburg–Landau equation [as introduced by A. C. Newell and J. A. Whitehead, J. Fluid Mech. 38, 279 (1969)] from the hydrodynamic equations for an infinite Hele–Shaw cell. The dimensional analysis and the asymptotic study allow one to distinguish two nonlinear formulations, each one depends on the order of magnitude of the Prandtl number. The first formulation corresponds to the case Pr=O(1) or Pr≫1, whereas the second corresponds to the case Pr=O(ε*2), where ε*≪1 denotes the aspect ratio of the cell. Here a weakly nonlinear analysis is performed for the two formulations.
Dynamics of heavy particles in a Burgers vortex
Authors: Marcu B. , Meiburg E. , Newton P. K.
Published: July 2002

This paper presents a linear stability analysis as well as some numerical results for the motion of heavy particles in the flow field of a Burgers vortex, under the combined effects of particle inertia, Stokes drag, and gravity. By rendering the particle motion equations dimensionless, the particle Stokes number, a Froude number, and a vortex Reynolds number are obtained as the governing three parameters. In the absence of gravity, the vortex center represents a stable equilibrium point for particles up to a critical value of the Stokes number, as the inward drag overcomes the destabilizing centrifugal force on the particle. Particles exceeding the critical Stokes number value asymptotically approach closed circular orbits. Under the influence of gravity, one or three equilibrium points appear away from the vortex center. Both their locations and their stability characteristics are derived analytically. These stability characteristics can furthermore be related to the nature of the critical points in a related directional force field. These findings are expected to be applicable to the coupling between the small-scale turbulent flow structures and the motion of suspended particles.