bluuebunny/crossref_metadata_2025 huggingface dataset:
Helical organizing centers in excitable media
Authors: Henze C. , Lugosi E. , Winfree A. T.
Published: April 2011

Numerical simulations of three distinct models of excitable media (two-variable Oregonator, with both variables diffusing equally; and piecewise linear A and B kinetics, with only the propagator variable diffusing) are used here to investigate the dynamics of scroll filaments, with emphasis on the role of twist. In all three models, initially uncurved filaments uniformly twisted more than a threshold amount 'sproing' into radially expanding helices, some of which stabilize at finite radius. By monitoring the evolution of these helices with a differential geometry 'tool kit', we attempt to discern the presumed causal dependence of filament motion (resolved into velocities along the normal and binormal directions of the local Frenet frame) and rotor spin rate, on local filament geometry (curvature and torsion) and twist. Twist can reverse the direction of normal velocity from that seen in untwisted scroll rings; binormal velocity is either reversed (A kinetics), abolished (B kinetics), or engendered (Oregonator). In all cases twist increased spin rate, as predicted by theory. Our efforts to perceive a functional dependence of the dynamical variables on filament geometry and twist have met with limited success, but we believe our attempts have uncovered some useful methodology and exposed some pitfalls relevant to any such investigation. The existence and properties of a novel class of stable organizing centers, as well as the existence of a threshold of twist for their induction, should be of interest to theorists. In addition, our findings here based on models suggest new phenomena to look for in various excitable media, such as the Belousov–Zhabotinsky reagent or heart muscle.
Recovering the attractor: A review of chaotic time-series analysis
Authors: Frank G. W. , Lookman T. , Nerenberg M. A. H.
Published: April 2011

Previous applications of chaotic time-series analysis have focussed on either abstract model systems or experimental systems with unknown or poorly understood equations of motion. We present an analysis of experimental time series obtained from a driven nonlinear pendulum with accurately known governing equations. The method of delays provided faithful attractor recoveries in both periodic and chaotic states, showing that high-quality recoveries are possible from experimental time series. Methods for determining the time series' embedding dimension and delay time are also reviewed.
Boson Hamiltonians and stochasticity for the vorticity equation
Authors: Shen Hubert H.
Published: April 2011

The evolution of the vorticity in time for 2D inviscid flow and in Lagrangian time for 3D viscous flow is written in Hamiltonian form by introducing Bose operators. The addition of the viscous and convective terms, respectively, leads to an interpretation of the Hamiltonian contribution to the evolution as Langevin noise.
Nonlinear phase-space diffusion approach to turbulent organized structures
Authors: Shen Hubert H.
Published: April 2011

The spatial evolution of the velocity in a turbulent fluid is shown to consist of a mean strain rate plus a noise term whose mean-square amplitude is dependent upon the local velocity. Assuming the independence of different length scales allows one to eliminate the effect of cubic and higher moments of the velocity upon the evolution of the two-point probability density function for the velocity, analogous to the system-size expansion of the master equation (N. G. van Kampen. Adv. Chem. Phys. 34, 245 (1976); R. Kubo, K. Matsuo, and K. Kitahara. J. Stat. Phys. 9, 51 (1973); M. Suzuki. Adv. Chem. Phys. 46, 198 (1981)). Furthermore, for the velocity structures that exhibit the most internal correlation (i.e., the flow fields that are most likely to be observed) the mean strain rate drops out, leaving only the nonlinear diffusion term. In other words, the dependence of the two-point velocity statistics upon length scale or separation is found to be governed by the gradients of the stress (rather than of the velocity, as is usually assumed.) This provides a new approach for predicting the dominant patterns observed in turbulent flows and a new means of characterizing or classifying different structures, namely, by their nonlinear diffusion coefficient. As a physically relevant example, the isotropic case with algebraic nonlinearity is considered.
Time-resolved spin dynamics in diluted magnetic semiconductor quantum wells
Authors: Freeman M. R. , Awschalom D. D.
Published: April 2011

Ultrashort laser pulses were used to prepare charge-carrier populations of known initial energy and spin polarization in two-dimensional quantum wells of the diluted magnetic semiconductor Cd1−xMnxTe. The evolutions of the carrier spin polarization and of magnetization transferred to the magnetic ions were observed using time-resolved optical techniques.
Pragmatic information and dynamical instabilities in a multimode continuous-wave dye laser
Authors: Atmanspacher H. , Scheingraber H.
Published: April 2011

Observations of the behavior of a multimode continuous-wave dye laser around instabilities are interpreted using the concept of pragmatic information. It is demonstrated that this concept contains aspects of two different approaches toward the description of self-organizing system, namely, those of synergetics and of nonequilibrium thermodynamics. Both of them prove to be particularly relevant in the considered example. The key quantities of the two approaches (efficiency and entropy production) are related to a measure of pragmatic information. The significance of this measure for general issues referring to meaning, complexity, and evolution in self-organizing processes is indicated.
Structural instability and sustained oscillations in an extended Brusselator model
Authors: Otwinowski M. , Laidlaw W. G. , Paul R.
Published: April 2011

When all reactions in the "Brusselator" kinetic scheme are allowed to be reversible one can demonstrate, by analyzing the focal values, that, for a range of rate constants, a unique limit cycle is created from a multiple focus. A simple modification of the kinetic scheme leads to a model that has two saddle nodes in addition to the well-known unstable focus and a stable limit cycle. In the modified system, for some parameter values, the limit cycle disappears after evolving into a saddle-node connection. For some initial conditions the modified system has unbounded solutions that describe a possible explosion. The analysis that yields these results is based on a general, constructive procedure, which can be applied to higher order physical and chemical systems.
Pattern formation and pattern selection in the Landau–Ginzburg model of critical phenomena
Authors: Tuszynski J. A. , Otwinowski M.
Published: April 2011

In this paper we investigate the family of nonlinear partial differential equations used to describe the kinetics of critical phenomena within the Landau–Ginzburg model. An analysis of the recently obtained symmetry-reduction results for a number of such equations is provided from the point of view of pattern formation at criticality. Various possibilities occur depending on the choice of control parameters. An illustration is provided using several physical examples such as metamagnets and liquid crystals.
On the hexagonal structure of cellular flames
Authors: Shtilman L. , Sivashinsky G.
Published: April 2011

The effect of hexagonal pattern formation in steady spherical flames is studied both analytically and numerically. It is shown that the hexagonal cells emerge as a subcritical bifurcation from the basic state provided the flame radius exceeds a certain critical value.
Reaction diffusion modelling of biological pattern formation: Application to the embryogenesis of Drosophila melanogaster
Authors: Lyons M. J. , Harrison L. G. , Lakowski B. C. , Lacalli T. C.
Published: April 2011

The general problem of pattern formation in biological development is described. One possible type of general solution, reaction–diffusion theory, is outlined, together with the difficulties involved in definite experimental confirmation or rejection of it. A specific biological problem, the origin of the segmentation pattern in the fruit fly Drosophila melanogaster, is discussed. The general plan of our program of computational work to study how this could arise by reaction–diffusion is indicated, with some early results.
Mechanisms of microstructural pattern formation in irradiated solids
Authors: Abromeit C. , Trinkaus H. , Wollenberger H.
Published: April 2011

Metals and alloys under irradiation with energetic particles such as electrons, neutrons, or ions are open dissipative systems far from thermodynamic equilibrium. The continuous production, diffusion, and annihilation of point defects result in microstructural changes in the irradiated materials. Under proper conditions, self-organization of the microstructure is experimentally observed. Examples are the void lattice, periodic concentration fluctuations of dislocation loops, and irradiation-induced homogeneous precipitation in undersaturated alloys. The theoretical description rests on a coupling of the point defects with the microstructure by nonlinear reactions. In general, a complicated reaction scheme has to be investigated. In practice, however, a simplified reaction model is applicable from which the minimum requirements for self-organization can be derived by using a linear stability analysis. In a special case of the reaction scheme it is possible to evaluate exact stationary solutions of the appropriate diffusion-reaction equations. They show the stabilizing effect of the annihilation of the point defects by recombination and at neutral sinks against pattern formation. A stability diagram for irradiation-induced periodic structures is developed that gives the temperature and displacement range where self-organization of loop arrangement is possible, and that is in good accordance with the experimental results.
Solid state kinetic processes with distribution of reactivity
Authors: Machida Masataka , Senna Mamoru
Published: April 2011

A concept of the distribution of the local reactivity is combined with a topochemical distribution of the activity in a solid particle by using the local potential energy, EL. The EL of the constituent atoms was calculated from the summation of the pair potential. The distribution of the activity was then linked to the kinetic process by repeating the deletion of the atoms with highest EL and then recalculating EL. The apparent variation of the exponent, n, of the Johnson–Mehl–Avrami (JMA) equation is possibly explained by the distribution of EL, and hence the distribution of the local rate constant. Refinement for the better fitting of the calculated JMA equation from the atomic model and the conceptual distribution of the rate constant may be achieved by accounting for the time-dependent change in the distribution of the rate constant.
The nonlinear dynamics of solidification of a binary melt with a quasi-equilibrium mushy region
Authors: Buyevich Yu. A. , Iskakova L. Y. , Mansurov V. V.
Published: April 2011

A mushy region (a two-phase zone) between the solid and liquid phases occurs often in the process of solidification of a binary melt. An analysis of the structure of the mushy region, which includes the liquid, solid particles, and dendrites extending from the bulk solid surface, is suggested. The processes of heat and mass transfer in the mushy region are considered on the basis of the small parameter method. The analysis leads to equations governing unsteady heat and mass transfer with internal heat, and mass sources within the mushy region, and it includes the condition for the absence of supercooling (the condition for the zone quasi-equilibrium), convection being neglected. The temperature, concentration of solute, and solid phase volume fraction are found. On the basis of this solution a new model of the process is formulated. Within the scope of this model the mushy region is replaced by a liquid–solid interface with discontinuous boundary conditions.
Origin and evolution of the large-scale structure of the universe
Authors: Silk Joseph
Published: April 2011

Ever since the epoch of the spontaneous breaking of grand unification symmetry between the nuclear and electromagnetic interactions, the universe has expanded under the imprint of a spectrum of density fluctuations that is generally considered to have originated in this phase transition. I will discuss various possibilities for the form of the primordial fluctuation spectrum, spanning the range of adiabatic fluctuations, isocurvature fluctuations, and cosmic strings. Growth of the seed fluctuations by gravitational instability generates the formation of large-scale structures, from the scale of galaxies to that of clusters and superclusters of galaxies. There are three areas of confrontation with observational cosmology that will be reviewed. The large-scale distribution of the galaxies, including the apparent voids, sheets and filaments, and the coherent peculiar velocity field on scales of several tens of megaparsecs, probe the primordial fluctuation spectrum on scales that are only mildly nonlinear. Even larger scales are probed by study of the anisotropy of the cosmic microwave background radiation, which provides a direct glimpse of the primordial fluctuations that existed about 106 years or so after the initial big bang singularity. Galaxy formation is the process by which the building blocks of the universe have formed, involving a complex interaction between hydrodynamical and dynamical processes in a collapsing gas cloud. Both by detection of forming galaxies in the most remote regions of the universe and by study of the fundamental morphological characteristics of galaxies, which provide a fossilized memory of their past, can one relate the origin of galaxies to the same primordial fluctuation spectrum that gave rise' to the large-scale structure of the universe.
Formation of structure in star-forming clouds
Authors: Pudritz Ralph E.
Published: April 2011

Star formation occurs in massive, dense, molecular clouds in the interstellar medium. These clouds have a rich substructure consisting of dense clumps and extended filaments. Since stars only form within these dense clumps, any fundamental theory of star formation must predict their physical properties. This review focusses on the physics of molecular clouds and discusses in this context a particular mechanism for the formation of structure that is well supported by the observations. Strong hydromagnetic waves are likely to be excited in molecular clouds since it is observed that cloud magnetic fields have energy densities close to gravity. These waves support the cloud against global gravitational collapse by providing an effective wave "pressure". This review also shows that waves may control the formation of structure in molecular clouds.
Inhomogeneous solutions in cosmology
Authors: Wesson Paul S.
Published: April 2011

The standard cosmological solutions of Einstein's equations of general relativity describe a fluid that is homogeneous and isotropic in density and pressure. These solutions, often called the Friedmann–Robertson–Walker solutions, are believed to be good descriptions of the universe at the present time. But early on, processes connected with particle physics and quantum field theory may have caused localized inhomogeneities, and recently some new kinds of solution of Einstein's equations have been found, which may describe such regions. In one solution being studied by Wesson and Ponce de Leon (Phys. Rev. D: Part. Fields, 39, 420 (1989)), the density is still uniform but the pressure is nonuniform about a centre. The mass is given by a relation that looks like the familiar Newtonian relation m = (4/3)πR3ρ. However, the solution has other properties that are quite strange (e.g. a region of negative pressure and a kind of dipolar geometry). It is not known if solutions like this are merely mathematical curiosities or imply something about the behaviour of real matter in extreme situations.
Angular pair correlations of observed and simulated galaxy distributions
Authors: Wiedenmann G. , Atmanspacher H. , Scheingraber H.
Published: April 2011

The main body of quantitative information about galaxy statistics is obtained from correlation studies. It has recently turned out that a modified correlation formalism can provide details about large-scale structure in the galaxy distribution, which are obscured by artefacts of the conventional correlation function. The modified pair correlation function, as applied to the Zwicky catalogue of galaxies, shows two distinct power-law regimes at small scales (< 1°) and large scales (around 10°). Based on the comparison of simulated bubblelike large-scale structures with the Zwicky sample, these regimes are interpreted to correspond to the distribution of galaxies within the shells of the bubbles (small scale), and the distribution of the bubbles themselves (large scale).
Nonhomogeneous processes in radiation research: Radical diffusion models
Authors: Magee John L.
Published: April 2011

Radiation research has come to mean investigations of the nature of penetrating radiations and their effects on the materials they traverse. The radiations themselves are fast charged particles (including atomic nuclei and electrons) and short wave length electromagnetic photons (X and γ rays). These radiations have the common property that they deposit their energy nonhomogeneously (in tracks) as they are stopped in matter. Chemically reactive species (excited molecules, ions, radicals, etc.) are created in the tracks. Irradiated systems are the earliest examples of systems recognized to be nonhomogeneous. The tracks formed by radiations develop by expansion and reaction of the newly created reactive species among themselves and with any molecules of the stopping medium. The radiation chemical yields of products depend on the nature of the track, which in turn depends on the radiation. Radiation chemists have been successful in the construction of models that describe track expansion and the prediction of radiation chemical yields. The development of this field, over the years, is reviewed.
Kinetics of nonhomogeneous chemical processes in tracks of high-energy charged particles
Authors: Hummel A.
Published: April 2011

High-energy charged particles, when slowing down in a molecular medium, lose their energy by electronic excitations and ionizations of molecules along their paths. If the secondary electrons that are formed as a result of the ionizations have sufficient energy, they give rise to further excitations and ionizations. In this way tracks of excited states, positive ions, and electrons are formed. The spatial distribution of the species initially formed in the track will change in time owing to diffusion; the charged species will also drift in each other's Coulomb field. In nonpolar systems the range of the Coulomb forces is very large (30 nm) and neutralization of the oppositely charged species in the track is a dominant process, which in turn leads to formation of excited molecules that generally decompose into reactive fragments. In polar liquids, like water, neutralization is less prevalent and a relatively large fraction of the charged species escapes from the Coulombic attraction. The transient species formed may react with one another and with molecules of the medium, either solvent molecules or solute molecules. The probability of the occurrence of these reactions depends on the initial spatial distribution of the reactive species in the track. The present state of the theory of the kinetics of the nonhomogeneous processes in tracks of high-energy charged particles, which relates the initial spatial distribution of the transient species in the track to the various experimental observables, will be discussed.
Defect reactions and clustering in irradiated solids
Authors: Mansur L. K.
Published: April 2011

Irradiation of solid materials with energetic neutrons or charged particles can lead to profound changes in defect structure, microcomposition, and macroscopic properties. Such changes occur by atomic and microstructural mechanisms, some of which are familiar in "classical" physical metallurgy and materials science. However, other cases appear to be unique to irradiation. Irradiation has considerably broadened and indeed provided an entirely new dimension in materials science, since the energetic displacement of atoms potentially reaches to every property or process. The initial damaging events leading to the creation of point defects are generally complete in times of order 10−11 s. Subsequent changes in structure, composition, and properties take place in a span of much longer time scales corresponding to interstitial and vacancy diffusion, clustering, solute segregation, and precipitation. An extensive theoretical framework has been developed to understand the kinetics of these processes. Emphasis has been placed on both steady cumulative processes and on fluctuations, and on the appropriate application of stochastic and deterministic descriptions. Parallel and interactive experimental activities for both applied and basic programs over the past two decades have increased the level of phenomenological knowledge enormously. Much of the work has emphasized either high-dose phenomena such as irradiation-induced swelling, creep, embrittlement, phase instability, and solute segregation relevant to materials applications, or the properties, structures, and interactions of defects, which underlie more fundamental issues.
A description of random walks with collision anisotropy and with a nonconstant mean free path
Authors: Goulet Thomas , Mattél Isabelle , Jay-Gerin Jean-Paul
Published: April 2011

We study various characteristics of a particle's random walk both analytically and with the help of Monte-Carlo simulation techniques. With the analytical approach, we derive the expression[Formula: see text]which relates the mean-square displacement [Formula: see text] to (i) the number of steps N in the walk, (ii) the mean-square displacement [Formula: see text] on each of the steps, and (iii) a coefficient of collision anisotropy A defined as the average value of the cosine of the scattering angle θ. This expression is general in the sense that it holds for any value of N and A. It is, however, restricted to cases where the mean free path is constant throughout the random walk. The results of the simulations allow a further generalization to random walks with a nonconstant mean free path. They also allow the study of the radial distribution f(r) of particles after the walk. We find that a set of six functions fi(r) is necessary to give a satisfactory description of the particles' radial distribution for arbitrary values of N and A.
Geminate-ion kinetics with competing ion fragmentation in nonpolar liquids with halocarbons
Authors: Bühler Rolf E.
Published: April 2011

The semi-empirical rate law for geminate-ion recombination by van den Ende, Warman, and Hummel, which predicts a linear dependence of the ion concentration with t−0.6, is modified to include simultaneous ion fragmentation. The theory is applied to the kinetics, as observed by pulse radiolysis of liquid methylcyclohexane (MCH) solutions of N2O, CHCl3, or tert-butylchloride (t-BuCl) at low temperatures. In MCH saturated with N2O (−130 °C), the solvent cation (MCH+, λmax = 550 nm) moves about 400 times faster than prediced by diffusion. With the known conductivity data at room temperature, an activation energy of about 2.7 kJ/mol can be derived. The solvent cation MCH+ does not appear to fragment. With t-BuCl added to MCH (−134 °C), MCH+ (λmax = 550 nm) and t-BuCl− (λmax = 450 nm) are observed simultaneously. The initial kinetics corresponds to the parent ion (MCH+) recombination with t-BuCl−. Then the MCH+ fragmentation with k1(−134 °C) = 3 × 105 s−1 is observed, followed by the geminate recombination of some fragment cation with t-BuCl−. The fragment cation recombines 300 times slower than the parent cation. With CHCl3 added to MCH (−130 °C), the MCH+ absorption is hidden within the [Formula: see text] band (λmax = 470 nm); however, the fragmentation is detected from kinetic analysis to occur in about 2 × 106 s−1. The modified t−0.6 rate law appears to be a very useful tool to study simultaneous ion recombination and ion fragmentation.
The mechanism of electron thermalization in H2O, D2O, and HDO
Authors: Jonah Charles D. , Chernovitz Anita C.
Published: April 2011

The yield of the hydrated electron, [Formula: see text], was measured as a function of time in H2O, D2O, and mixtures of the two solvents. The results of these measurements showed that the decay of [Formula: see text] was considerably slower in D2O than in H2O. These results suggest that the electron travels a longer distance in D2O than in H2O. In H2O–D2O mixtures there had to be at least 50% D2O in the solution before the decay was appreciably different from the pure H2O solution. The decay in 80% D2O is still considerably different than the decay in pure D2O solution. From these results we draw the following conclusions. (i) The distance an electron travels before solvation is greater in D2O than in H2O. (ii) Much of the travel of the electron prior to thermalization occurs when the energy of the electron is less than the excitation energy of the solvent. (iii) A major mode of energy loss is due to energy transfer to the stretching vibrational modes of the water molecules.
Solvent structure effects on solvated electron reactions in mixed solvents: Negative ions in 1-propanol–water and 2-propanol–water
Authors: Sedigallage Annesley Peiris , Freeman Gordon R.
Published: April 2011

In models of the kinetics of chemical reactions in solution the solvent is commonly assumed to be a uniform continuum. An example is the Smoluchowski–Debye–Stokes–Einstein equation for the rate constant k2 of a bimolecular reaction between charged or polar species: k2 = κRTfrr/1.5ηrd where κ = probability that a reactant encounter pair will react, R = gas constant, T = temperature, f = Coulombic interaction factor, rr = effective radius for reaction, η = solvent viscosity, and rd = effective radius for mutual diffusion. The equation is useful in evaluating effects of bulk-fluid properties on reaction rates. Residual effects are attributed to more specific solvent behaviour. Rate constants and activation energies E2 of reactions of solvated electrons [Formula: see text] with [Formula: see text] and [Formula: see text] ions vary with the composition of 1-propanol–water and 2-propanol–water mixed solvents. Plots of k2η/fT against solvent composition are nonlinear and change with solvent pair and with reactant pair. Measured molar conductivities Λ0(Li+, [Formula: see text]) and Λ0(2Li+, [Formula: see text]) indicate the solvent dependence of rd for the mutual diffusion of Li+ and [Formula: see text] or [Formula: see text]. The liquid structure influences both the rate of diffusion of the reactants and the probability of reaction of a reactant encounter pair.
Nonhomogeneous distribution of muonium and other paramagnetic products following positive muon radiolysis of water
Authors: Leung Siu-Keung , Brodovitch Jean-Claude , Percival Paul W. , Yu Dake
Published: April 2011

When energetic positive muons (μ+) stop in water roughly 40% form the hydrogen-like atom muonium (Mu = μ+e−) and the remainder are incorporated in diamagnetic compounds, such as MuH and MuOH. The two fractions can be distinguished experimentally by the precession of spin-polarized muons in a transverse magnetic field, but the muonium signal is only one-half the intensity expected. Previous studies have shown that spin-dependent interactions between Mu and hydrated electrons formed at the end of the muon track can result in the loss of muon polarization, via electron spin exchange or chemical reaction, to give MuH + OH−. These interactions are nonhomogeneous and the study of their kinetics should provide information on the distribution of radiolysis transients at the end of the track. Some relevant data have arisen from a study of the free radical products of the reaction between Mu and pyrogallol (1,2,3-trihydroxybenzene) in aqueous solution. For low concentrations of pyrogallol (< 3 mM) the radical signal amplitudes, measured by muon level-crossing resonance, are consistent with radical formation by addition of thermal Mu to one or other of the unsubstituted positions of the benzene ring. At higher concentrations this reaction competes with the muonium depolarization process and results in increased signal amplitude.
Nonhomogeneous processes as seen by muonium reactions in micelles
Authors: Venkateswaran Krishnan , Barnabas Mary V. , Stadlbauer John M. , Walker David C.
Published: April 2011

The addition of micelles to solutions of solutes in water with which muonium reacts cause a wide variety of effects. Micelle-induced enhancement ratios have been found to vary from less than 1 to greater than 104, depending on the properties of the solute and the type of chemical reaction involved. A kinetic analysis is presented here that seems appropriate for most of the mixtures studied so far. These are nonhomogeneous systems, with separate phases involved, but the effects do not arise simply from confined diffusion because the mean residence time of muonium is only 2 ns. The possibility that the largest enhancements arise from quantum mechanical tunneling, and therefore peculiar to muonium, cannot be ruled out.
Loss of muonium in nonhomogeneous processes studied by level-crossing resonance
Authors: Venkateswaran Krishnan , Barnabas Mary V. , Wu Zhennan , Walker David C.
Published: April 2011

Level-crossing resonance spectroscopy has been used to determine the yield of muonium atoms formed in water and in hexane through production of muonated free radicals. In the presence of high concentrations of solutes, which yield radicals, all muons injected into the solutions are found either in diamagnetic molecules or free radicals. However, at low concentrations, the free-radical yield was smaller and a "missing fraction" appeared. Inverse yield against inverse concentration plots were not linear, indicating that reactions leading to loss occurred with nonhomogeneously distributed species from the muon's expanding track.
Radical yields in DNA exposed to ionizing radiation: Role of energy and charge transfer
Authors: Miller John H. , Swenberg Charles E.
Published: April 2011

Theoretical and experimental studies of free-radical yields in oriented DNA samples exposed to ionizing radiation with high linear energy transfer at 77 K are discussed. The dependence of radical yields on the orientation of DNA chains relative to the particle flux is being investigated to gain insight into the role of intramolecular energy and charge transfer processes in radical production and decay. Model calculations based on a thermal-spike approximation are presented and their limitations for predicting the orientation dependence of radical yields observed after neutron irradiation (see C. M. Arroyo et al. Int. J. Radiat. Biol. 50, 789 (1986)) are discussed. A more mechanistic model based on the high mobility of excess electrons in hydrated DNA (D. van Lith et al. J. Chem. Soc. Faraday Trans. 1, 82, 2933 (1986)) is outlined.
An initial life-threatening effect of ionizing radiation: Hypotension
Authors: Senanayake P. C. , Goddard C. M. , Yacyshyn E. A. , Hulbert W. B. , Ruth R. F.
Published: April 2011

Radiation hypotension (RH) is a severe decrease in arterial blood pressure during the 48 h after irradiation. RH is the presumed cause of three human deaths in reactor accidents and occurs after clinical irradiation. RH is described more fully in the rhesus monkey and the rabbit. This report describes prevention of RH in the rabbit by the cutting of two nerves of the sympathetic nervous system.
A holistic view of low-level radiation effects in biological systems
Authors: Oftedal Per
Published: April 2011

Living organisms have been exposed to background radiation during all their evolution. Repair systems suited to counteract the inflicted damage have developed. It is hypothesized that the level of background radiation is "sensed" by organisms via the low-energy thermalized part of the absorbed energy, thus allowing for long-term "pre-emptive" adjustment of inducible repair capacity. The high-energy (> 100 eV) absorption events cause damage that must be accommodated post hoc by means of stabilizing elements (heat-shock proteins?) and subsequent palliative or restorative repair. The complexity of the reactions and interactions may lead to nonhomogeneous dose–effect curves in the low-dose range. The analysis is developed and discussed on the basis of experimental and epidemiological data that are informative in this context. Dose rates of 0.25 mGy/day and less appear to cause little change in repair capacity. Acute doses of 4 mGy and higher lead to increased repair. Acute doses of up to 250 mGy cause abruptly increasing damage with dose. A plateau of damage, independent of doses in the range following the abrupt increase, indicates a prompt stabilization of damage and subsequent repair. Thus, the reaction pattern is influenced by dose, dose rate, and radiation quality, as well as biological factors.
Reaction kinetics in micellar solutions
Authors: Tachiya M.
Published: April 2011

The stochastic treatment of reaction kinetics in micellar solutions is presented by taking, as a typical example of a reaction, the quenching of an excited probe by a quencher, both solubilized in a micelle. Three cases are considered concerning migration of probes and quenchers between micelles, namely, (i) the case where only quenchers migrate, (ii) the case where only probes migrate, and (iii) the case where micelles exchange solubilizates by fusion–fission processes. The decay curves of the excited probes are calculated for the three cases and compared with each other. Whether it is possible to distinguish between the three cases on the basis of the experimentally obtained decay curves is discussed.
Picosecond dynamics of excitation-energy relaxation of dye molecules in Langmuir–Blodgett monolayers
Authors: Tamai Naoto , Yamazaki Tomoko , Yamazaki Iwao
Published: April 2011

Fluorescence-decay curves and time-resolved fluorescence spectra of N, N′-dioctadecylrhodamine B in Langmuir–Blodgett monolayers have been measured by means of a picosecond time-resolved fluorescence spectrophotometer. The excitation energy relaxation of the dye molecule in the monolayers was compared at 295 and 80 K in the concentration range from 0.02 to 31 mol%. The relaxation of the dynamic Stokes shift was found to decay logarithmically with time in longer time regions when the concentration of N, N′-dioctadecylrhodamine B < 5 mol%. The fluorescence decays can be analyzed by an equation including a stretched exponential function. The mechanisms of singlet excitation-energy relaxation were interpreted in terms of (i) the energy migration among energetically disordered monomer sites represented by ultrametric space (hierarchical energy distribution) or the Gaussian distribution of the density of excited states and (ii) the energy trapping by two-dimensional aggregates. It was found that the relaxation of the dynamic Stokes shift and the fluorescence decay can be estimated by the diffusion length of the excitation-energy transport.
Measurements of energy dispersion at liquid–solid interfaces: Fluorescence quenching of pyrene bound to fumed silica
Authors: Wong A. L. , Harris J. M. , Marshall D. B.
Published: April 2011

The observation of a stretched exponential response in chemical kinetics at solid–liquid interfaces is an indicator of solid surface disorder. In this contribution, we review earlier relaxation kinetics studies of solid surface disorder and the statistical criteria analyzing kinetic data that do not fit single exponential. We draw on these concepts to interpret the fluorescence decay kinetics of pyrene covalently attached to the surface of fumed silica particles that are suspended in methanol. This surface probe exhibits stretched exponential behavior of the form f(t) = exp[−(kt)β] in the decay of its excited-state populations. The addition of iodine quencher increases the average decay rate, but does not alter the nonlinear exponent β. These results, along with the diffusional length of the iodine quencher, the photophysics of the probe, and the chemistry of the interface, indicate that the kinetic inhomogeneity is dominated by dispersion of surface energies, rather than by diffusional excursions of the quencher on a fractally aggregated surface. Numerical Laplace inversion of the experimental decay curves is used to determine the distribution of rates which reveal that the quenching process is inhomogeneous and correlated with the unquenched decay rates of the probe.
Different degrees of hydration in water–oil microemulsions by low-resolution 1H magnetic relaxation analysis
Authors: Senatra Donatella , Lendinara Laura , Giri M. Grazia
Published: April 2011

The purpose of the research was to establish whether the water configurations, detected by previous differential scanning calorimetry (DSC) analysis, could give rise to different or characteristic nuclear magnetic resonance relaxations. We report the results of a study on water–hexadecane microemulsions at different water contents. The 1H spin–spin and spin–lattice relaxation curves were obtained at 20 MHz and 310 K. The trends of all best-fit relaxation parameters versus water concentration were compared with those obtained for D2O microemulsions with the same experimental conditions and fitting procedures. Two different H2O microemulsion "states" were identified: (i) all the systems without "free water" (defined by DSC as water melting at 273 K) were characterized by 1H spin–spin and spin–lattice relaxation curves well described by two exponential functions and (ii) all the systems with free water were characterized by relaxation curves well described by three exponential functions. The identification of the main phases of the microemulsions interphasal region and continuous medium, besides free water, seems possible on the basis of the 1H magnetic relaxation characteristics.
Simple views on adhesion and fracture
Authors: Gennes P. G. de
Published: April 2011

The industrial importance of adhesives is constantly increasing. Yet it is difficult to systematize the vast amount of practical knowledge, which has accumulated, covering chemistry, interfacial physics, and mechanics. This review describes an attempt to bridge the gap between polymer science and fracture mechanics. It focuses on weak mechanical junctions. Examples can be found at glass–rubber interfaces or at glass–plastic interfaces, where the glass has been grafted with polymer chains that promote adhesion. When a fracture propagates along such a junction, the dissipation tends to be localized in the junction region. We present a phenomenological description of this process in terms of two ingredients: (i) a threshold stress σc associated with chemical scission or with plastic flow; (ii) a "suction" process with a suction velocity proportional to the local stress σ, which ends when the volume transfer (per unit area) has reached a certain limit hf. Assuming no cavitation (no crazes), we are led to expect two fracture regimes: (a) at low-fracture velocities V, the process is quasi-static and the fracture energy G scales like σchf and (b) beyond a velocity V*, the width of the suction region is very much spread out, and G increases linearly with V. On the whole, these ideas can put into perspective a number of existing data, for instance, we may understand why adhesive elastomers become poorer when their level of cross linking is increased.
Development and application of a new pulsed-field gel-electrophoresis process based on molecular dynamics
Authors: Noolandi Jaan
Published: April 2011

Separation of DNA molecules according to size is an important step in molecular biology. Pulsed-field gel electrophoresis is the main experimental tool for carrying out this separation, and is one of the techniques used in the human genome megaproject. We have developed a new approach to the separation of biological molecules by choosing electric field pulses to activate specific stretching and relaxation modes of charged molecules moving through a gel. In our process, a particular sequence of electric field pulses provides the "code" (specific instructions) for the migration of a DNA fragment of a given size to a designated position on the gel. The entire pulse train (for the duration of the experiment) can then be used to predetermine the separation pattern of a large number of DNA molecules of different sizes. This allows, for example, for the movement of only nucleic acid fragments below a certain size through the gel, and simplifies DNA preparation techniques. We have developed new hardware and software to carry out this process in a routine fashion in a laboratory environment. In this paper we review the theoretical concepts that form the basis of this separation technique, and discuss its applicability to human chromosomes and proteins.
Adsorption of linear polymers
Authors: Daoud M. , Farnoux B. , Jannink G. , Johner A.
Published: April 2011

Linear polymer chains usually interact strongly with surfaces. Various regimes are obtained when one varies the bulk monomer concentration C or the interaction δ of a monomer with the surface. We discuss the recent results that were obtained theoretically by scaling arguments and compare them with hydrodynamic, neutron-scattering, and reflectivity measurements. Although the agreement is satisfactory in some cases, no general agreement has been reached so far. We also discuss the relaxation of the streched chain when the fixed end constraint is released. Possible experimental realizations are given.
Heavy-oil cracking: The case for nonhomogenous kinetics
Authors: Overend R. P. , Chornet E.
Published: April 2011

The thermal cracking of heavy oils has been modelled on the basis of combinations of first-order reactions in order to describe the behaviour of pseudocomponents with time and temperature of reaction. An alternative formulation uses the concept of a time-varying rate constant of the form[Formula: see text]where β is the rate constant and γ is an exponent that has values of 0 < γ ≤ 1.0. This was used to derive a transform of temperature and time that we have called the reaction ordinate Rω, which enables the correlation of extensive kinetic data sets by means of three constants: β, γ, and the mean activation energy Ea. We demonstrate this correlation for two sets of published data. The values of γ were less than unity and are interpreted in terms of the Kohlrausch function, leading to the hypothesis of a continuous distribution of activation energies in the system. This form of correlation is similar to that of the continuous-time random-walk relaxation phenomena that has been explained in many ways including fractal time, and thus we propose that this is indirect evidence for the nonhomogenous nature of heavy-oil-cracking kinetics.
A new approach to gate/n− overlapped lightly doped drain structures: added gate after implantation of n− (AGAIN)
Authors: Wylie Ian W. , Tarr N. Garry
Published: April 2011

A new lightly doped drain (LDD) metal oxide semiconductor field effect transistor structure is presented that provides substantial overlap of the gate over the n− region independent of the n− junction depth. This structure uses polysilicon spacers to replace the oxide sidewall spacers used in a conventional LDD device. The structure has been given the acronym "AGAIN," for added gate after implantation of n−.
A novel millimetre-wave gallium arsenide beam-lead mixer diode with cutoff frequency > 2 THz
Authors: Huang Hsien-Ching , Swarup Arvind , Henderson Richard
Published: April 2011

A low-parasitic-capacitance beam-lead Schottky diode has been developed for millimetre-wavelength mixer applications from 30–110 GHz. It has a novel structure that incorporates a polyimide film to support the beam leads and the GaAs active area. Typical devices have an ideality factor of 1.12, a parasitic capacitance of 10 fF, a zero-bias-junction capacitance of 20 fF, a series resistance of 2.5 Ω, and hence a figure of merit cutoff frequency of over 3 THz. Diodes tested in production mixers at 36 GHz have yielded a conversion loss of 5.14 dB. The devices, fabricated by repeatable production techniques, are mechanically rugged and suitable for qualification to space and defence standards.
Deposition of silicon nitride by glow-discharge decomposition of silane–nitrogen mixtures in a saddle-field plasma
Authors: Ukah C. I. , Meh R. , Zukotynski S. , Kruzelecky R. V. , Chakrabarti A.
Published: April 2011

High-quality silicon nitride films have been deposited onto a variety of substrates including c-Si, GaAs, and InP at temperatures in the range 50–350 °C by the dc glow-discharge decomposition of silane–nitrogen mixtures in a saddle-field cavity. The effect of the N2–SiH4 flow ratio [Formula: see text] and the effect of a dc bias Vb applied to the substrate holder on the properties of the resulting films were systematically investigated. The N/Si ratio in the resulting films, as determined by Auger analysis, could be varied from 0 to 1.8 by adjusting [Formula: see text], with [Formula: see text] yielding stoichiometric silicon nitride. The total hydrogen content, as determined by 15N profiling, decreased linearly with [Formula: see text]. Far infrared absorption measurements indicated that hydrogen was incorporated mainly into Si–H sites at lower N2–SiH4 flow ratios in the range 1–2; higher N2–SiH4 flow ratios of 5–15 resulted in approximately equal concentrations of Si–H and N–H sites with a total bonded hydrogen content of about 12 at.% for films deposited at 250 °C. Ellipsometry measurements indicated that a refractive index as low as 1.82 at 632.8 nm was achieved. Refractive index and thickness variations of less than 5% were observed in films deposited on 4 in (1 in = 2.54 cm) diameter c-Si wafers. Scanning electron microscopy and photoluminescence studies of the thermal stability of SiNH films deposited onto semiinsulating GaAs indicated improved annealing behaviour with positive substrate bias.
A Mg acceptor level in Si
Authors: Shih I. , Wu S. L. , Li L. , Qiu C. X. , Grant P. , Denhoff M. W. , Phong L. Ngo
Published: April 2011

Thin films of MgO were deposited on p-type monocrystalline Si substrates as buffer layers for high-Tc superconductor/Si hybrid devices. After heat treatment at temperatures in the range from 700 to 1100 °C, the MgO films were removed and Ag/p-Si Schottky junctions formed. Deep-level transient spectroscopy measurements were made on the treated junctions. The sample treated at 1000 °C showed a single acceptor level located at 240 meV from the valence band edge.
Etching of polymers in microwave/radio-frequency O2–CF4 plasma
Authors: Lamontagne B. , Küttel O. M. , Wertheimer M. R.
Published: April 2011

We have studied O2–CF4 plasma etching of commercial polymers, particularly KaptonR polyimide, using a reactor in which the plasma can be excited by radio-frequency (rf, 13.56 MHz), microwave power (MW, 2.45 GHz), or mixed frequency (MW/rf) excitation. For the case of rf plasma etching of polyimide, a marked effect of dc self-bias voltage Vb on the etch rate R has been observed; Vb is found to vary systematically with pressure and with CF4 concentration in the etch gas, [CF4]. Beside the well-documented maximum in R at low-[CF4] values, the measurement of ion flux allows us to attribute a second peak in R (near [CF4] = 65%) to ion bombardment. We also report observations regarding the dependence of R on polymer structure.