bluuebunny/crossref_metadata_2025 huggingface dataset:
Solute–solvent interaction dynamics studied by photon echo spectroscopies in polymer glasses
Authors: Nagasawa Yutaka , Yu Jae-Young , Fleming Graham R.
Published: July 2002

The influence of coupling strength and temperature on the solute–solvent interaction of two chromophores in polymer glass is reported. The three-pulse photon echo peak shift method was used to study the dye IR144 in polyvinylformal (PVF) and the dye DTTCI (3,3′-diethylthiatricarbocyanine iodide) in polymethylmethacrylate (PMMA). IR144 is more strongly coupled (larger reorganization energy) to both its intramolecular modes and to the solvent than is DTTCI. Our results can be well described by the linearly coupled harmonic bath model over the range 300 to 30 K. In particular, the strikingly different temperature sensitivities of the long-time (asymptotic) peak shift are well described by the theory. Temperature-independent spectral densities and inhomogeneous widths suffice to quantitatively describe the peak shift data over this temperature range and a number of numerical predictions based on the theoretical model are experimentally confirmed. An ultrafast component corresponding to a decay of ∼100 fs time scale in the transition frequency correlation function is found in all cases, though the amplitude is small for the DTTCI solutions. This ultrafast response is assigned to the inertial response of the solvent. The inertial response measured for DTTCI in PMMA is very similar to our previous measurements of the inertial response for IR144 in PMMA, suggesting that a general characteristic of the solvent is being probed. The weaker coupling of DTTCI produces a drastic increase in the width of the photon echo signal as predicted by theory and strong vibrational quantum beats at 30 K. By contrast, such beats are not observed in the echo signal for IR144.
Three-body collision contributions to recombination and collision-induced dissociation. I. Cross sections
Authors: T Pack Russell , Walker Robert B. , Kendrick Brian K.
Published: July 2002

Many of the current chemical kinetics textbooks and kinetics papers treat atomic and molecular recombination and collision-induced dissociation (CID) as occurring only via sequences of two-body collisions. Actually, there is considerable evidence from experiment and classical trajectory calculations for contributions by true three-body collisions to the recombination of atomic and diatomic radicals, and that evidence is reviewed. Then, an approximate quantum method treating both two-body and three-body collisions simultaneously and on equal footing is used to calculate cross sections for the reaction Ne2+H⇌Ne+Ne+H. The results provide clear quantum evidence that direct three-body collisions do contribute significantly to recombination and CID.
Coherence in polyatomic photodissociation: Aligned O(3P) from photodissociation of NO2 at 212.8 nm
Authors: Ahmed Musahid , Peterka Darcy S. , Bracker Allan S. , Vasyutinskii Oleg S. , Suits Arthur G.
Published: July 2002

Strong orbital alignment is observed in the ground-state oxygen atom following photodissociation of NO2 at 212.8 nm using ion imaging. The imaging method allows for investigation of the angular distribution of this alignment, providing insight into the dynamics in the frame of the molecule. The results are analyzed using a rigorous quantum mechanical theory yielding alignment parameters having direct physical significance. This alignment is dominated by a strong incoherent parallel contribution. In addition, the results reveal direct evidence of coherence between parallel and perpendicular contributions to the excitation of a polyatomic molecule, showing that the electron cloud in the recoiling atom “remembers” the original molecular plane.
Highly accurate ab initio calculation of the interaction potential for two sodium atoms with parallel spins
Authors: Gutowski Maciej
Published: July 2002

The potential energy curve for Na2 in the a 3Σu+ state is characterized by the Re and De parameters which are found to be equal to 5.192 Å and 177.7 cm−1, respectively, at the coupled cluster level of theory with single, double, and noniterative triple excitations. It is argued that the theoretical value of Re may be more accurate than that extracted from the experimental data [Friedman-Hill and Field, J. Chem. Phys. 96, 2444 (1992)].
Time-dependent quantum dynamics in a Gauss–Hermite basis
Authors: Billing Gert D.
Published: July 2002

We formulate time-dependent quantum dynamics with a basis set in which the classical limit arises in a natural fashion. The basis set is time-dependent and can be used either for all degrees of freedom or together with time-independent basis functions, grids, etc. The basis-set is driven by classical mechanical equations of motion governed by an effective potential derived from the Dirac–Frenkel variational principle. We furthermore formulate an operator version of the theory. Here the coupling between the basis set functions located around the classical trajectories is obtained by solving the quantum matrix problem in a second quantization frame. In this approximation the quantum theory scales as (3N)2, where N is the number of particles.
The electronic spectra of jet-cooled 5-phenyltropolone: Phenyl torsion and its effect on proton tunneling
Authors: Tsuji Takeshi , Hayashi Yoshiyuki , Hamabe Hidenori , Kawakami Hiroko , Mori Akira , Nishimura Yukio , Sekiya Hiroshi
Published: July 2002

The S1–S0 electronic spectra of jet-cooled 5-phenyltropolone were measured to elucidate the features of phenyl torsion and its effect on intramolecular proton transfer. Several bands in the fluorescence excitation spectra were assigned to phenyl torsion and other low-frequency modes with the aid of the ab initio calculation. The torsional potential energy function was investigated with the one-dimensional torsional potential function analysis for the S0 state. Presence of significant potential barriers due to the large H-atom steric repulsion between the phenyl ring and the tropolone ring were anticipated at the torsional angle φ=0° in both the S0 and S1 states. It was suggested that the large steric repulsion significantly influenced both phenyl torsion and proton transfer.
Dynamics of the O(1D)+CH4 reaction: Atomic hydrogen channel vs molecular hydrogen channel
Authors: Lin J. J. , Harich S. , Lee Y. T. , Yang X.
Published: July 2002

The O(1D)+CH4 reaction has been investigated using a new universal crossed molecular beam apparatus. Both the atomic hydrogen channel (CH3O/CH2OH+H) and the molecular hydrogen channel (H2CO/HCOH+H2) have been experimentally observed in this reaction. The experimental results suggest that the main atomic hydrogen channel in the O(1D)+CH4 reaction should be CH2OH (hydroxymethyl)+H, while the CH3O (methoxy)+H channel is at most a minor process. From the product angular distribution measurements, it is clear that the radical products (CH2OH and/or CH3O) in the hydrogen atom channel are only slightly backward scattered relative to the O(1D) beam direction, indicating that this product channel mainly goes through a long-lived intermediate pathway. The slightly backward scattered products are possibly due to other reaction mechanisms. For the molecular hydrogen channel, the product angular distribution obtained from simulation also seems isotropic, implying that this channel also likely goes through a long-lived intermediate pathway.
Anisotropic intermolecular interactions in van der Waals and hydrogen-bonded complexes: What can we get from density functional calculations?
Authors: Milet Anne , Korona Tatiana , Moszynski Robert , Kochanski Elise
Published: July 2002

The applicability of various density functional theory (DFT) methods to describe the anisotropy of the intermolecular potential energy surfaces of hydrogen-bonded [OH−–H2O, (H2O)2] and van der Waals [CO–H2O, He–CO2] complexes has been tested by comparison with supermolecule CCSD(T) (coupled-cluster method restricted to single, double, and noniterative triple excitations) and perturbational SAPT (symmetry-adapted perturbation theory) results computed for the same geometries and with the same basis sets. It is shown that for strongly bound ionic hydrogen-bonded complexes, like OH−–H2O, hybrid approaches provide accurate results. For other systems, including the water dimer, the DFT calculations fail to reproduce the correct angular dependence of the potential surfaces. It is also shown that a hybrid functional adjusted to reproduce the CCSD(T) value of the binding energy for the water dimer produces results worse than the standard hybrid functionals for OH−–H2O, and fails to describe the correct anisotropy of the CO–H2O interaction.
Molecular properties from combined QM/MM methods. I. Analytical second derivative and vibrational calculations
Authors: Cui Qiang , Karplus Martin
Published: July 2002

Analytical second derivatives for combined QM/MM calculations have been formulated and implemented in the CHARMM program interfaced with the ab initio quantum mechanical GAMESS and CADPAC programs. This makes possible evaluation of vibrational frequencies and infrared intensities in large systems that cannot be treated effectively by QM or MM alone; examples are polarizable molecules in solution and substrates or transition states in enzymes. Test calculations on a number of systems, including formamide in water, butanol, a model transition state structure for triosephosphate isomerase and the active site model of myoglobin, show that the MM description of the environment can capture much of its polarization effects on the QM region. Thus the implementation of analytical second derivatives within the QM/MM framework has considerable potential for the study of large systems.
Photoexcitation and predissociation intensities of the c′ 1∑u+ (v=3 and 4), c 1∏u (v=3 and 4), and b′ 1∑u+ (v=10, 12, 13, and 15) states of N2
Authors: Walter C. W. , Cosby P. C. , Helm H.
Published: July 2002

Photofragment spectroscopy is applied to investigate perturbations in the photoexcitation and predissociation of excited singlet states of N2. Discrete rovibrational levels in the Rydberg states c′ 1∑u+ (v=3 and 4) and c 1∏u (v=3 and 4) and in the valence state b′ 1∑u+ (v=10, 12, 13, and 15) are prepared by laser excitation from the metastable a″ 1∑g+ (v=0) state in a fast (3 keV) molecular beam. Fragment atoms produced by predissociation are monitored as a function of exciting laser frequency using a position- and time-sensitive detector to measure the photofragment intensity distribution in the rovibrational bands. The photofragment spectra show extreme departures from normal rovibronic intensity distributions due to strong perturbations in the two highly mixed complexes: c′(3)/c(3)/b′(10) and c′(4)/c(4)/b′(13). The measured photofragment intensities are compared to photoexcitation line strengths calculated using a comprehensive model of the Rydberg-valence state mixing [Stahel, Leoni, and Dressler, J. Chem. Phys. 79, 2541 (1983)]. This theoretical model accurately predicts most of the variations in the photofragment intensities, and reveals the causes of the perturbations. For the states investigated in the present study, the primary factor that determines the photofragment intensity is the degree of b′ character of the state, which favorably affects both photoexcitation and predissociation. Interference effects are found to be important in determining absorption intensities in N2.
Infrared spectra of the CS2−, CS2+, and C2S4+ molecular ions in solid neon and argon
Authors: Zhou Mingfei , Andrews Lester
Published: July 2002

Laser ablation of transition metal targets with concurrent code position of CS2/Ne and CS2/Ar mixtures produces metal independent absorptions at 1206.8 and 1159.2 cm−1 in neon and 1200.5 and 1160.4 cm−1 in argon due to CS2+ and CS2−. Additional metal independent absorptions at 1385.2 cm−1 in neon and 1379.7 cm−1 in argon increase on annealing. Isotopic substitutions show that this vibration involves two equivalent CS2 subunits. Based on density functional theory calculations of structure and vibrational frequencies, the 1385.2 and 1379.7 cm−1 bands are assigned to the C2S4+ cation in solid neon and argon. Identical experiments using matrix samples doped with the electron trapping molecule CCl4 enhance the 1385.2 and 1379.7 cm−1 absorptions and further support the cation assignment. No evidence was found for the (CS2)2− anion in these experiments.
The structure of O3–CH4 and the implications for the O+CH4 precursor-initiated reaction
Authors: Hight Walker A. R. , Fraser G. T. , Suenram R. D. , Lovas F. J.
Published: July 2002

The rotational spectrum of the O3–CH4 complex has been measured in a molecular beam using a pulsed-nozzle Fourier-transform microwave spectrometer. An a-type pure-rotation and a c-type rotation-inversion electric-dipole spectrum is observed, complicated by the nearly free internal rotation of the CH4 top and the inversion tunneling of the O3. The nuclear-spin statistics of the equivalent oxygen nuclei leads to only one tunneling component existing for each rotation–internal-rotation state, indicating that the transition state has a heavy-atom, C2v-symmetry geometry. The tunneling splitting is determined to be 30 to 40 MHz, dependent on the CH4 internal-rotor state. Only two of the three methane internal-rotor states have been assigned. These two states of A and F symmetry have asymmetric-rotor energy-level structures, weakly perturbed by the ozone-inversion tunneling. The zero-point structure of the complex has a heavy-atom plane of symmetry with the two terminal O atoms equidistant above and below this plane. The angle between the line joining the center of masses of the two subunits and the O3, C2 axis is 118.2(5)°, with the central O directed away from the CH4. The shortest O–C separation is 3.57 Å. The geometry of the complex suggests two outcomes for the reaction of an O atom produced by 267 nm photolysis of O3 in the complex (assuming that the initial O3 photodissociation dynamics are not perturbed by complexation), either nonreaction or reaction by stripping of a hydrogen atom at high impact parameters, leading to fast, highly rotationally excited, OH product.
Energy levels of a two-dimensional anharmonic oscillator: Characteristic function approach
Authors: Ari Naşit , Demiralp Metin
Published: July 2002

The eigenlevels of a two-dimensional quartic anharmonic oscillator are identified as the zeros of a ‘‘characteristic function’’ derived from the resolvent. Numerical values are obtained by means of Padé approximants.
Recurrence relations for the analytic calculation of Feynman integrals in the axial gauge: The case of massless particles
Authors: Slim H. A.
Published: July 2002

An algorithm is described which enables the calculation of Feynman integrals in the axial gauge. The algorithm is based on recurrence relations, which are used to simplify integrals associated with both massless and massive particles. Detailed formulas are presented for the case of massless particles. They can be used to tabulate all Feynman integrals that are relevant for the calculation of propagators in axial gauge quantum chromodynamics and quantum gravity.
Plane-fronted gravitational and electromagnetic waves in spaces with cosmological constant
Authors: Ozsváth István , Robinson Ivor , Rózga Krzysztof
Published: July 2002

Plane-fronted waves in spaces with nonzero cosmological constant are studied. In particular their complete classification, which depends essentially on the sign of the cosmological constant and that of some second-order invariant determined by the congruence of null rays, is provided.
The density of occupied nearest neighbor pairs on a rectangular 2×N lattice space
Authors: McQuistan R. B. , Hock J. L.
Published: July 2002

Utilizing a 15-term recursion that describes exactly the composite nearest neighbor degeneracy for simple, indistinguishable particles distributed on a 2×N lattice space, 〈θ11〉, the expectation of the normalized number of occupied nearest neighbor pairs, is calculated as a function of coverage and the reduced interaction potential.
On the bi-Hamiltonian structures of the sKdV
Authors: Barcelos-Neto J. , Das Ashok
Published: July 2002

The first and the second Hamiltonian structures of the supersymmetric KdV equation are derived, following the conventional approach of Gel’fand and Dickey. In particular, it is shown that the nonlocal nature of the first bracket is a direct consequence of the constrained nature of the Lax operator.
A simple dynamical system that mimics open-flow turbulence
Authors: Bhat G. S. , Narasimha R. , Wiggins S.
Published: July 2002

The possible relevance of recent theories concerning the chaotic behavior of nonlinear dynamical systems to turbulence, especially in open flows, has frequently been questioned. Here, the issues that have led to this skepticism are investigated by studying a simple system that has been devised to include, albeit in an impressionistic way, the major mechanisms that are widely considered to operate in a broad class of turbulent flows. The variables in the system seek to represent the amplitudes of large- and small-eddy motion, respectively, and are governed by equations that allow for a Landau–Stuart nonlinear growth, a one-step Richardson cascade, and a specified time-dependent driving force. It is found that the critical value (at the onset of chaos) of the Reynolds-number-like control parameter (ν−1) in the system depends on the character and magnitude of the driving force; and it is analytically demonstrated using the Melnikov technique that, with an appropriate choice of model parameters, chaos can persist at all sufficiently high values of the model Reynolds number (unlike as in most other low-dimensional models). The routes to chaos in the system when the forcing is increased at fixed ν are different from those when ν is decreased at fixed forcing, the latter being found to be more relevant to the case of streamwise-developing flows like a boundary layer. The observed routes are sensitive to the presence of even small stochastic components in the forcing. Computed spectral evolutions in the model show qualitative similarities with observations in boundary layer flow under different disturbance environments. It is concluded that many of the gross features of open-flow turbulence can be understood as dynamical chaos.
Study of the parametric oscillator driven by narrow-band noise to model the response of a fluid surface to time-dependent accelerations
Authors: Zhang Wenbin , Casademunt Jaume , Viñals Jorge
Published: July 2002

A stochastic formulation is introduced to study the large amplitude and high-frequency components of residual accelerations found in a typical microgravity environment (or g-jitter). The linear response of a fluid surface to such residual accelerations is discussed in detail. The analysis of the stability of a free fluid surface can be reduced in the underdamped limit to studying the equation of the parametric harmonic oscillator for each of the Fourier components of the surface displacement. A narrow-band noise is introduced to describe a realistic spectrum of accelerations, that interpolates between white noise and monochromatic noise. Analytic results for the stability of the second moments of the stochastic parametric oscillator are presented in the limits of low-frequency oscillations, and near the region of subharmonic parametric resonance. Based upon simple physical considerations, an explicit form of the stability boundary valid for arbitrary frequencies is proposed, which interpolates smoothly between the low frequency and the near resonance limits with no adjustable parameter, and extrapolates to higher frequencies. A second-order numerical algorithm has also been implemented to simulate the parametric stochastic oscillator driven with narrow-band noise. The simulations are in excellent agreement with our theoretical predictions for a very wide range of noise parameters. The validity of previous approximate theories for the particular case of Ornstein–Uhlenbeck noise is also checked numerically. Finally, the results obtained are applied to typical microgravity conditions to determine the characteristic wavelength for instability of a fluid surface as a function of the intensity of residual acceleration and its spectral width.
Preferential electron-cyclotron heating of hot electrons and formation of overdense plasmas
Authors: Quon B. H. , Dandl R. A.
Published: July 2002

Three electron-cyclotron-heating techniques, which preferentially couple microwave power to different energy segments of the electron distribution function, have been experimentally investigated in the AMPHED facility [C. Bodeldijk, Special Supplement, Nucl. Fusion 26, 184 (1986)]. Whistler waves launched from the high-field mirror throat are strongly absorbed in a single pass across the resonant interaction layer, producing highly overdense cold background plasma but no relativistic hot particles. On the other hand, ordinary waves launched from the system side wall are only weakly damped, giving rise to local cylindrical cavity modes and preferential coupling to hot electrons in the 100 keV region. Low levels (≤5%) of upper-off-resonance heating power were shown to be most effective for preferential hot-electron plasma formation, with ∼100% of the injected power being absorbed by the energetic electrons.
Note on detailed models for trapped electron transport in tokamaks
Authors: Waltz R. E. , Dominguez R. R.
Published: July 2002

Detailed quasilinear trapped electron transport expressions are compared with the simple scaling forms of the extreme dissipative regime used in recent modeling studies. The problem of obtaining a consistent plasma pinch in the dissipative regime is discussed.
Diffusion of passive scalars under stochastic convection
Authors: Careta A. , Sagués F. , Sancho J. M.
Published: July 2002

The diffusion of passive scalars convected by turbulent flows is addressed here. A practical procedure to obtain stochastic velocity fields with well-defined energy spectrum functions is also presented. Analytical results are derived, based on the use of stochastic differential equations, where the basic hypothesis involved refers to a rapidly decaying turbulence. These predictions are favorable compared with direct computer simulations of stochastic differential equations containing multiplicative space-time correlated noise.
Competition between subharmonic and sideband secondary instabilities on a falling film
Authors: Cheng Minquan , Chang Hsueh-Chia
Published: July 2002

The secondary instabilities on a falling film which cause a monochromatic wave to evolve into solitary waves are examined with a weakly nonlinear theory. The unique phase speed dispersion relation dictated by inertia and capillarity is found to favor a nonlinear, two-wave subharmonic resonance that causes neighboring crests to coalesce. This occurs when the fundamental wave frequency is below a critical value ωc which can be approximated by a nonlinear resonant frequency. An entire band of secondary waves are excited by this static subharmonic mechanism such that the coalescence occurs nonuniformly. The lower half of this band of secondary waves travels faster than the fundamental and induces the coalesced waves to move faster than the slower fundamental. For monochromatic waves with frequencies above ωc, a three-wave oscillatory sideband instability is triggered which generates two secondary waves that are slower than the fundamental. This sideband instability involves a long-wave modulation that causes several crests to coalesce simultaneously. Both secondary transitions are important intermediate stages of the route to spatiotemporal chaos involving solitary waves.
On the stability of non-columnar swirling flows
Authors: Wang S. , Rusak Z.
Published: July 2002

The linear stability of an inviscid, axisymmetric and non-columnar swirling flow in a finite length pipe is studied. A novel linearized set of equations for the development of infinitesimal axially-symmetric disturbances imposed on a base non-columnar rotating flow is derived. Then, a general mode of an axisymmetric disturbance, that is not limited to the axially-periodic mode, is introduced and an eigenvalue problem is obtained. A neutral mode of disturbance exists at the critical state. The asymptotic behavior of the branches of non-columnar solutions that bifurcate at the critical state is given. Using asymptotic techniques, it is shown that the critical state is a point of exchange of stability for these branches of solutions. This result, together with a previous result of Wang and Rusak [Phys. Fluids 8, 1007 (1996)] on the stability of columnar vortex flows, completes the investigation on the stability of all branches of solutions near the critical state. Results reveal the important relation between stability of vortex flows and the physical mechanism leading to the axisymmetric vortex breakdown phenomenon.
Correlations of vorticity and material line elements with strain in decaying turbulence
Authors: Huang Mei-Jiau
Published: July 2002

Correlations of the vorticity and the passive material line element with strains in decaying turbulence are investigated. The mean principal rates of the strain and the growth rates of the vorticity and the material line element due to the nonlinear stretching are all found to be proportional to the square-root-mean enstrophy. The ratios of the principal rates are 4:1:−5. It is shown how these ratios help determining the values of free parameters in the phenomenological models of turbulence such as the Burgers vortex model proposed by Ashurst (private communication) and the Lundgren–Townsend vortex model by Pullin and Saffman [Phys. Fluids A 5, 126 (1993); 6, 3010 (1994)]. The mean growth rate of material line elements is slightly larger than the intermediate principal rate, but much less than the most positive one. Numerical results show that the material line element has nearly the same preferences in aligning with the principal directions of the strain corresponding to the mostly positive and the intermediate principal rates. The growth rate is approximately linearly correlated with the strain magnitude but almost uncorrelated with the vorticity magnitude. Conditional statistics and a simple model flow have been used to explain the above observations.
Experimental measurement of shear-induced diffusion in suspensions using long time data
Authors: Madanshetty Sameer I. , Nadim Ali , Stone H. A.
Published: July 2002

A method based upon Taylor dispersion theory is used to determine the shear-induced diffusion coefficient in concentrated suspensions. The experiments are performed in a cylindrical Couette device with a suspension consisting of polystyrene spheres in a density-matched solution of glycerin and water. A sequence of several hundred transit times for a single tagged sphere to complete successive orbits within the device is measured. The data are analyzed to compute the azimuthal Taylor dispersion coefficient from which the coefficient of shear-induced diffusivity is obtained. In our experiments the particle Reynolds numbers are 𝒪(10−1). The experimental results are compared to the existing measurements of the shear-induced diffusion coefficient obtained at lower particle Reynolds numbers and based upon short-time data. We find a shear-enhanced diffusion coefficient D⊥/γ̇a2=𝒪(0.1) for a volume fraction of φ≊0.25; this is comparable to existing results from previous low particle Reynolds number studies (ℛ<10−3).
Representation of core dynamics in slender vortex filament simulations
Authors: Klein Rupert , Knio Omar M. , Ting Lu
Published: July 2002

The numerical description of slender vortex motion faces several major obstacles: (i) The stiffness induced by the rapid rotatory motion in the vortex core, where peak velocities are an order of magnitude larger than the filament velocity. In a vorticity-velocity formulation, this stiffness is reflected by the singular behavior of the line-Biot-Savart integral as one approaches the vortex geometry. Regularization occurs physically by viscous smoothing of the vorticity. (ii) The vortex core vorticity distribution has a crucial influence on the vortex filament motion. Thus, an accurate description of the core structure evolution due to vortex stretching and vorticity diffusion is necessary. We propose a numerical scheme that allows an accurate description of the effects of axial flow in the core, viscosity and vortex stretching on slender vortex filament motion. The approach is based on incorporating the detailed asymptotic analyses of the vortex core structure evolution by Callegari and Ting [SIAM J. Appl. Math. 15, 148 (1978)] and Klein and Ting [Appl. Math. Lett. 8, 45 (1995)] for stretched viscous slender vortices into the improved thin-tube vortex element schemes of Klein and Knio (1995). The resulting schemes overcome the difficulties mentioned above except for the issue of temporal stiffness, which we leave for future work.
Langevin approach to generate synthetic turbulent flows
Authors: Martí A. C. , Sancho J. M. , Sagués F. , Careta A.
Published: July 2002

We present an analytical scheme, easily implemented numerically, to generate synthetic Gaussian turbulent flows by using a linear Langevin equation, where the noise term acts as a stochastic stirring force. The characteristic parameters of the velocity field are well introduced, in particular the kinematic viscosity and the spectrum of energy. As an application, the diffusion of a passive scalar is studied for two different energy spectra. Numerical results are compared favorably with analytical calculations.
Plasma and neutral dynamics in a simulated tokamak gas target divertor
Authors: Schmitz L. , Merriman B. , Blush L. , Lehmer R. , Conn R. W. , Doerner R. , Grossman A. , Najmabadi F.
Published: July 2002

A stationary, detached ionization front is observed in an experimentally simulated divertor plasma (n≤3×1019 m−3, kTe≤20 eV) interacting with a hydrogen gas target. With a neutral hydrogen density, n0≊2×1021 m−3, the electron temperature at the simulated divertor target is reduced to kTe target≊2.5 eV. Up to 97% of the electron heat flux (≤7 MW/m2) is dissipated by dissociation and ionization losses and hydrogen line radiation. The plasma pressure is observed to peak near the ionization front, and a plasma flow reversal is observed in the region of reversed pressure gradient. Classical momentum flow parallel to the magnetic field and anomalous cross-field particle transport are found. The plasma flow is strongly damped by ion–neutral collisions and is subsonic. Numerical results from a one-and-one-half dimensional (11/2-D) coupled plasma–neutral fluid model (incorporating radial particle transport, recycling, and neutral gas injection) agree well with the experimental data, and indicate that the electron heat flow is classical and well described by a harmonic flux limit. The scale length of the parallel plasma pressure gradient in a gas target is found to depend on the neutral density, the electron temperature, and the cross-field diffusion coefficient.
Multidimensional autoresonant mode conversion
Authors: Friedland L.
Published: July 2002

It is shown that the autoresonance effect is characteristic of wave interactions in slowly varying weakly nonlinear multidimensional media. The theory of the phenomenon is presented for the mode conversion case and illustrated in two dimensions. It is demonstrated that multidimensional autoresonance is stable with respect to weak damping and transverse nonuniformity of the incident wave.
Resistive magnetohydrodynamic equilibria in a torus
Authors: Montgomery David , Bates Jason W. , Lewis H. Ralph
Published: July 2002

It was recently demonstrated that static, resistive, magnetohydrodynamic equilibria, in the presence of spatially uniform electrical conductivity, do not exist in a torus under a standard set of assumed symmetries and boundary conditions. The difficulty, which goes away in the “periodic straight cylinder approximation,” is associated with the necessarily non-vanishing character of the curl of the Lorentz force, j×B. Here, we ask if there exists a spatial profile of electrical conductivity that permits the existence of zero-flow, axisymmetric resistive equilibria in a torus, and answer the question in the affirmative. However, the physical properties of the conductivity profile are unusual (the conductivity cannot be constant on a magnetic surface, for example) and whether such equilibria are to be considered physically possible remains an open question.
Long-range correlations in dusty plasmas
Authors: Tsytovich V. N. , de Angelis U. , Bingham R. , Resendes D.
Published: July 2002

The correlations in dusty plasmas in the gaseous state is considered. It is shown that the long-ranged correlations of dust particles, contrary to usual matter, start to form in a weakly correlated state, leading finally to the formation of dust crystals and dust liquids with not strong but intermediate strength correlations. The physical mechanism leading to long-ranged correlations is the dust attraction due to shadow effects of the plasma particle fluxes. An analytic theory for the long-ranged dust density correlation function is developed and the numerical results for a broad range of dust densities and plasma temperatures are presented. The long-ranged correlations induced by dust for electron densities, ion densities, and dust charges are also investigated, and the results for electron density correlations are presented in a way that permits measurement of the long-ranged correlation function in laser and radio-wave scattering experiments.
K-shell photoionizations in strongly coupled plasmas
Authors: Jung Young-Dae
Published: July 2002

In strongly coupled plasmas, the photoionizations from the 1s ground state of hydrogenic ions are investigated using the ion-sphere model potential. The initial bound wave function and energy eigenvalue of the target ion are modified in the ion-sphere potential using the Ritz variation method. The final state of the ejected photoelectron is represented by a plane wave solution. The Coulomb correction is considered using the screened Coulomb wave function with the appropriate effective charge. The K-shell photoionization cross section is obtained for the acceleration form of the matrix element because of the convenience of the calculations. The plasma screening effect is obtained as a function of the ion-sphere radius and photon energy. The retardation correction is also considered in obtaining the total K-shell photoionization cross section in strongly coupled plasmas. The plasma screening effects on the total K-shell photoionization cross section for the ion-sphere radius, RZ⩾2aZ, are found to be less than about 25%. It is also found that the plasma screening effect is almost independent of the incident photon energy.
Particle acceleration in the dynamic magnetotail
Authors: Birn J. , Thomsen M. F. , Borovsky J. E. , Reeves G. D. , Hesse M.
Published: July 2002

The acceleration of charged particles to suprathermal energies is investigated in the context of magnetospheric substorms. Ion and electron test particle orbits are studied in dynamically evolving fields obtained from a three-dimensional resistive magnetohydrodynamic (MHD) simulation of reconnection in the near magnetotail. The simulation leads to plasmoid formation and ejection into the distant tail and an earthward collapse of the field in the inner tail. Energization of particles takes place predominantly in the inner tail region earthward of the neutral line, rather than in the vicinity of the neutral line. The test particle studies reproduce major observed characteristics of energetic particle flux increases (“injections”) in the inner magnetotail; a fast rise, a limited energy range of the flux increases, and spatially varying delays between the onsets of ion and electron injections. Acceleration mechanisms include a “quasipotential” acceleration, resulting from nonadiabatic particle motion in the direction of the cross-tail electric field, as well as betatron and Fermi-type acceleration. The major source region for accelerated ions (electrons) in the hundreds of keV range is the central plasma sheet at the dawn (dusk) flank, outside the reconnection site. Since this source plasma is already hot and dense, a moderate energization by a factor of approximately 2 is sufficient to explain the observed increases in the energetic particle fluxes. Differences between ions and electrons at energies of a few tens of keV are found to be associated with differences in the bounce periods, which enables ions to circumvent the acceleration region, so that only electron fluxes in that energy range become enhanced.
Rutherford backscattering study of the photodissolution of Ag in amorphous GeSe2
Authors: Rennie J. , Elliott S. R. , Jeynes C.
Published: July 2002

Amorphous GeSe2 films with Ag overlayers have been illuminated in situ in a Rutherford backscattering chamber and the evolution of the silver depth profile as a function of illumination time has been studied. The silver distribution in the doped region is found to be steplike as had earlier been seen in the system Ag/As2S3 and the composition of the photodoped product is approximately Ag1.1GexSe3−x. During the course of the reaction the silver/glass interface develops irregularities possibly due to the formation of silver islands.
New avalanche multiplication phenomenon in quantum well superlattices: Evidence of impact ionization across the band-edge discontinuity
Authors: Capasso F. , Allam J. , Cho A. Y. , Mohammed K. , Malik R. J. , Hutchinson A. L. , Sivco D.
Published: July 2002

In suitably designed superlattice structures hot carriers in the barrier layers can collide with carriers confined or dynamically stored in the wells and impact ionize them out, across the band-edge discontinuity. Photomultiplication and spectral response measurements as a function of temperature and chopping frequency in Al0.48 In0.52 As/Ga0.47 In0.53As and AlSb/GaSb superlattices provide strong evidence of this effect. The observed phenomenon is characterized by a large ratio of the multiplication factors for holes and electrons implying that β≫α and can be the basis for a new class of very low noise avalanche photodiodes and solid-state photomultipliers.
Subpicosecond optoelectronic study of resistive and superconductive transmission lines
Authors: Gallagher W. J. , Chi C.-C. , Duling I. N. , Grischkowsky D. , Halas N. J. , Ketchen M. B. , Kleinsasser A. W.
Published: July 2002

We have studed the propagation of subpicosecond electrical pulses on coplanar resistive and superconductive Nb transmission lines. Pulses with 0.9 ps full width at half maximum were generated and detected by shorting fast photoconductive switches with 80 fs laser pulses. Dramatic improvements in propagation characteristics were achieved when the Nb was superconductive. We observed the strong dispersion and attenuation predicted to occur for frequency components near the superconducting enery gap frequency.
Transmission gratings that diffract 8 keV x rays
Authors: Bionta R. M.
Published: July 2002

We have created and characterized a transmissive diffraction grating that disperses 8 keV x rays. The grating has a period of 7077±70 Å and consists of opaque zones of Ta separated by transparent zones of Al. It was fabricated by sputtering alternating layers of Al and Ta onto a glass optical flat and then slicing the coating into a 17-μm-thick slab. We tested the performance of the grating against a theoretical prediction by exposing the grating to 8 keV radiation from a Cu x-ray source and observing the interference pattern.
The immunoprotective MHC II epitope of a chemically induced tumor harbors a unique mutation in a ribosomal protein
Authors: Matsutake Toyoshi , Srivastava Pramod K.
Published: July 2002

CD4+T lymphocyte clones, generated from mice immunized with the methylcholanthrene-induced fibrosarcoma Meth A (H-2d), are restricted by I-Edand recognize a unique antigen on Meth A. The antigen has been purified and characterized as the ribosomal protein L11. The antigenic epitope is contained within the sequence EYELRKHNFSDTG and is generated by substitution of Asn by His (italic) caused by a single point mutation. The tumor contains the wild-type and the mutated alleles. Immunization of BALB/cJ mice with the mutated epitope but not with the wild-type epitope protects mice against a subsequent challenge with the Meth A sarcoma. Adoptive transfer of CD4+clones into BALB/c mice renders the mice specifically resistant to Meth A sarcoma. The mutated L11 epitope is thus shown to be an immunoprotective epitopein vivoby several criteria.
Influence of alkyltransferase activity and chromosomal locus on mutational hotspots in Chinese hamster ovary cells.
Authors: Belouchi A , Ouimet M , Dion P , Gaudreault N , Bradley W E
Published: July 2002

High-density mutational spectra have been established for exon 3 of the gene encoding adenine phosphoribosyltransferase (APRT) of the Chinese hamster ovary (CHO) cell line derivative D422 and closely related and/or modified lines by using the mutagen ethyl methanesulfonate (EMS). The total number of selectable sites (GC-->AT transitions yielding a selectable APRT- phenotype) was estimated at 31 based on our own accumulated data base of 136 sequenced exon 3 mutations and on literature reports. D422 and two other APRT hemizygous lines each yielded very similar spectra and showed two populations of mutable sites: (i) 24 "baseline" sites that followed the Poisson distribution and therefore were equally susceptible to mutation and (ii) two hotspots, one comprising a cluster at nucleotides 1293-1309 and the other at nucleotide 1365. Collectively, the latter sites were about 10-fold more frequently mutated than the others. CHO cells are mer- as they lack the repair enzyme O6-methylguanidine methyltransferase (EC 2.1.1.63). In modified repair-proficient CHO cells, the distribution of mutations among all of the 31 sites was random, with only 3 of the 19 GC-->AT transitions in the above hotspots. To determine whether the distribution was locus-dependent, two independent lines carrying single copies of transfected APRT genes were generated from a derivative of D422 carrying a deletion in the endogenous APRT gene. Nucleotides 1293-1309 were again no longer preferentially mutated, but the site at nucleotide 1365 was still a hotspot. We conclude that mutational spectra in mer- cells are at least in part locus dependent and that some sequences are particularly susceptible to EMS mutagenesis and perhaps also to methyltransferase repair.
Excitation transfer spectroscopy with two metastable 138Ba+ ions in same trap.
Authors: Dehmelt H , Zhao X , Yu N , Nagourney W
Published: July 2002

In the absence of lasers approaching trapped ion clock transitions in sharpness we propose to replace the 12.49 m laser field exciting the D3/2-D5/2 transition of the single Ba+ ion A in D3/2 with the near-field of a close by identical ion B in the excited D5/2 state. We tune the frequency of the near-field by the differential Stark shift generated when the center of mass of the tuned ions is slightly moved out of the trap center by a small bias voltage. We demonstrate that the resultant resonant energy exchange can be made considerably faster than the natural lifetime of either metastable level and show how it might be detected.
A Ds insertion alters the nuclear localization of the maize transcriptional activator R.
Authors: Liu Y , Alleman M , Wessler S R
Published: July 2002

The R-sc gene of maize is a member of the R gene family of transcriptional activators that regulate anthocyanin biosynthesis. A derivative of R-sc, r-m9 conditions a reduced level of aleurone pigmentation due to the presence of a 2.1-kb Ds insertion near the 3' end of the coding region. Excision of Ds from r-m9 leaves a 7-bp insertion in the darker but still mutant v24 derivative. Both the 7-bp insertion in v24 and the 2.1-kb Ds in r-m9 are predicted to truncate their respective R proteins proximal to the carboxyl terminus, which was shown previously to contain one of three nuclear localization sequences. We find that the reduced expression of r-m9 and v24 are not due to mRNA or protein instability, but most likely reflect the inefficient localization of truncated R proteins to the nucleus. To our knowledge this is the first example of a transposable element insertion that alters gene expression by affecting nuclear localization. In addition, our data indicate that the carboxyl terminus of the R protein is far more important than previously suspected and illustrates the utility of natural mutations for defining functional domains in proteins.
Trials of the beta model for complex inheritance.
Authors: Collins A , MacLean C J , Morton N E
Published: July 2002

Theoretical advantages of nonparametric logarithm of odds to map polygenic diseases are supported by tests of the beta model that depends on a single logistic parameter and is the only model under which paternal and maternal transmissions to sibs of specified phenotypes are independent. Although it does not precisely describe recurrence risks in monozygous twins, the beta model has greater power to detect family resemblance or linkage than the more general delta model which describes the probability of 0, 1, or 2 alleles identical by descent (ibd) with two parameters. Available data on ibd in sibs are consistent with the beta model, but not with the equally parsimonious but less powerful gamma model that assumes a fixed probability of 1/2 for 1 allele ibd. Additivity of loci on the liability scale is not disproven. A simple equivalence extends the beta model to multipoint analysis.
Endogenous c-src as a determinant of the tumorigenicity of src oncogenes.
Authors: Halpern M S , England J M , Kopen G C , Christou A A , Taylor R L
Published: July 2002

We have compared the tumorigenicity of two src oncogenes, v-src and c-src(527), whose respective protein products pp60v-src and pp60c-src(527) show a different spectrum of amino acid substitutions vis-à-vis the c-src protooncogene-encoded product pp60c-src. Whereas the extent of primary tumor growth induced by c-src(527) was quite similar in the two chicken lines tested, the extent of v-src-induced tumor growth showed a marked line dependence. As examined with a line of chickens that shows immune-mediated regression of v-src-induced tumors, a weaker tumor immunity, as correlated with a greater level of primary tumor growth, resulted from inoculation of c-src(527) DNA than of v-src DNA. These observations indicated that the v-src-specific amino acid substitutions define a major tumor antigenicity. That a separate src-associated antigenicity is also targetable by the tumor immune response followed from the finding that the level of protective immunity against the growth of c-src(527) DNA-induced tumors was augmented under conditions of the prior regression of v-src DNA-induced tumors. As this latter antigenicity may include one or more c-src(527)-encoded peptides that are equivalent to c-src-encoded self peptides, these observations suggest that a host tolerance to pp60c-src can be broken so as to permit a tumor immune response based on recognition of self peptides of pp60c-src(527).