Showing papers by "University of Paris published in 1975"

Possibility of an inverse cascade of magnetic helicity in magnetohydrodynamic turbulence

Abstract: Some of the consequences of the conservation of magnetic helicity for incompressible three-dimensional turbulent MHD flows are investigated. Absolute equilibrium spectra for inviscid infinitely conducting flows truncated at lower and upper wavenumbers kmin and kmax are obtained. When the total magnetic helicity approaches an upper limit given by the total energy (kinetic plus magnetic) divided by kmin, the spectra of magnetic energy and helicity are strongly peaked near kmin; in addition, when the cross-correlations between the velocity and magnetic fields are small, the magnetic energy density near kmin greatly exceeds the kinetic energy density. Several arguments are presented in favour of the existence of inverse cascades of magnetic helicity towards small wavenumbers leading to the generation of large-scale magnetic energy.

Statistical mechanics of dense ionized matter. IV. Density and charge fluctuations in a simple molten salt

Abstract: The results of a molecular-dynamics study of a simple model of a molten salt are reported. The interionic pair potential which is used consists of the Coulombic term an an inverse-power repulsion which is assumed to be the same for all ions. The structure of the liquid is found to be dominated by charge-ordering effects and the calculated equilibrium properties are in good agreement with the predictions of the hypernetted-chain approximation. The relation between the self-diffusion coefficient and the electrical conductivity is discussed, and the observed deviations from the Nernst-Einstein relation in real molten salts are shown to have a natural explanation in terms of short-lived cross correlations. Data on the spectra of charge and particle density fluctuations are presented. At small wave numbers there is a propagating optic-type mode which shows a strong negative dispersion, but no Brillouin peak is seen even at the lowest wave number which is accessible. The data are analyzed in terms of a single-relaxation-time model incorporating the low-order spectral moments, for which we give explicit formulas. The fit achieved is fair, but the low-frequency behavior of the charge fluctuations at small wave numbers is incorrectly reproduced, and there is evidence for the necessity of introducing a second relaxation time. Comparison is made with results previously obtained for the classical one-component plasma.

Electron Localization Induced by Uniaxial Stress in Pure V O 2

Abstract: NMR and EPR measurements in V${\mathrm{O}}_{2}$ under uniaxial stress in the ${[110]}_{R}$ direction lead to a phase diagram entirely similar to that obtained in ${\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{2}$ alloys. Two intermediate phases ${M}_{2}$ and $T$ are observed, and, as established for ${\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{2}$, correspond to linear Heisenberg chains of spin \textonehalf{} (${M}_{2}$ phase) on one V sublattice. These chains undergo a progressive dimerization in the $T$ phase as the temperature is lowered.

Swelling and Structural Organization of Saponite

Abstract: The structural formula of Kozfikov saponite (Czechoslovakia) is as follows: $$N{a_{0.005}}C{a_{0.22}}{K_{0.01}}[S{i_{3.30}}A{l_{0.68}}F{e^{3 + }}{}_{0.02}][M{g_{2.50}}F{e^{2 + }}{}_{0.26}F{e^{3 + }}{}_{0.24}]{O_{10}}{(OH)_2}.$$ Saturated with 6 different cations (Li, Na, K, Ca, Mg, Ba) its swelling in ethylene glycol, glycerol and water and its homogeneous hydration extents according to the relative humidity have been studied. Our results were compared with those found for montmorillonites, beidellites and vermiculites in order to estimate the respective influence of the surface charge density and the charge localization on the swelling properties. The three-dimensional organization of the saponite is more or less affected by random stacking faults and by multiple b/3 translations according to the exchangeable cation, the swelling state and the nature of the solvation liquid. Our experimental results indicate that the three-dimensional order met in the hydrated saponites can be explained by an anchoring of the layers towards each other by chains made up of cation -dipole interactions and of hydrogen bonds between negatively charged surface oxygens and the interlayer water. This type of interlayer link is relatively weak. It is therefore easy to introduce stacking faults in these edifices in particular by grinding of the samples.

Cell walls of Mycobacteria and related organisms; chemistry and immunostimulant properties.

Abstract: In the following pages we intend to update and to expand a previous review 'The Mycobacterial cell wall' published in 19711 . Many new facts on the chemical structure have come to light since then and the last few years have witnessed a rapid expansion of our knowledge of the interesting biological properties of compounds derived from the cell walls of Mycobacteria and related organisms especially the Nocardiae. We shall first discuss the chemistry, then the biological properties involved.

The Thermal Cyclisation of Unsaturated Carbonyl Compounds

Abstract: When heated, in a sealed tube or in the vapour phase (250° −380°), unsaturated carbonyl compounds of suitable geometry (e-ethylenic ketones among others) undergo, in their enol form, an intramolecular ene reaction. In this reaction the enol hydrogen is shifted to the terminal carbon atom of the unsaturated center and cyclisation occurs by σ bonding between the other carbon atom of that center and the carbon atom α with respect to the carbonyl group. Such cyclisation is general, and very simple and its interest in synthesis in alicyclic chemistry is clear.

Theoretical and empirical calculations of the carbon chemical shift in terms of the electronic distribution in molecules

Abstract: The principles of the theoretical calculations of the carbon atom screening constant are reviewed. The Independent Electron, SCFMO and Valence Bond formulae are briefly discussed, and the relative importance of the different terms involved in an approximate decomposition of σ13C are pointed out. The variations of the excitation energy ΔE, the mean distance between the nucleus and 2p electrons, and the electronic charges and bond orders with the structure are presented. The nature of the steric effect is also explained. In a second part the various empirical correlations which make an evaluation of δ13C from structural and physical properties possible are shown. The actual substituent parameters of the main classes of compounds are listed and worked examples given to make the use of the tables easier. The correlations between δ13C and the electronic charge, electronegativity, Hammett-Taft constants, electric field effects, geometrical parameters, spectroscopic data (electronic transitions, screening of other nuclei) and pH are also discussed with a view to appraising the carbon screening.

Resonant Partial Wave Amplitudes in pi + n --> pi + pi + n According to the Naive Quark Pair Creation Model

Abstract: We make an extensive comparison of the naive quark-pair-creation model of strong-interaction vertices for $\ensuremath{\pi}N\ensuremath{\rightarrow}{N}^{*}\ensuremath{\rightarrow}\ensuremath{\Delta}\ensuremath{\pi}$,$\ensuremath{\rho}N$,$\ensuremath{\sigma}N\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi}N$ with recent experimental results. The model describes any vertex in terms of a quark-pair creation in the $^{3}P_{0}$ state and a rearrangement leading to final states. The hadron states are given by SU(6) harmonic-oscillator wave functions. We show that the model correctly describes, without any additional assumption, the centrifugal barrier effect and also the "anti-${\mathrm{SU}(6)}_{W}$" coupling signs, features that are present in most reactions (with the important exception of $F{P}_{15}\ensuremath{\rightarrow}\ensuremath{\Delta}\ensuremath{\pi}$ for the second feature) and are included phenomenologically in the works of Rosner et al. Both facts are related to the presence, in the limit of elementary meson emission, of recoil terms depending on internal quark momenta. Contrary to other works, the model predicts the relative coupling signs for all the two-body baryonic decays leading to $\ensuremath{\pi}\ensuremath{\pi}N$. Concerning $\ensuremath{\Delta}\ensuremath{\pi}$ and $\ensuremath{\rho}N$ channels, among 18 predictions, 15 agree with experiment, comparing our results to the last report of the Particle Data Group, and three disagree. One of these three is the very serious failure $F{P}_{15}\ensuremath{\rightarrow}\ensuremath{\Delta}\ensuremath{\pi}$, which is among the best determined experimental coupling signs. We compare our predictions with parallel works. Unlike the model of Feynman, Kislinger, and Ravndal, in the limit of elementary $\ensuremath{\rho}$ emission, our model presents a recoil term in the spin part of the interaction, $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\sigma}}(i)\ifmmode\cdot\else\textperiodcentered\fi{}[({\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{\ensuremath{\rho}}\ensuremath{-}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{i})\ifmmode\times\else\texttimes\fi{}{\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\epsilon}}}_{\ensuremath{\rho}}]$. Thus, for $F{P}_{35}\ensuremath{\rightarrow}N{\ensuremath{\rho}}_{3}$, our model predicts an "anti-${\mathrm{SU}(6)}_{W}$" sign, in agreement with experiment and contrary to the prediction of Moorhouse and Parsons. For the controversial $\ensuremath{\sigma}$ production, the model is contradicted by experiment; we show, however, that the model could be compatible with experiment if the $I=0$, ${J}^{P}={0}^{+}$ dipion observed is not really a resonance.

The Lysozyme from Asterias rubens

Abstract: Lysozyme (mucopeptide N-acetylmuramylhydrolase) from Asterias rubens was obtained in a chromatographically and electrophoretically pure state by gel filtration and affinity chromatography. The quantitative amino acid composition, the molecular weight and the N-terminal sequence determined by a sequencer are reported. This new invertebrate enzyme presents important differences when compared to previously studied lysozymes.

Lattice dynamics of several ANB8-N compounds having the zincblende structure. II. Numerical calculations

Abstract: The formalism of the deformable-bond approximation, described in the preceding article of this series, is now applied to 15 compounds with zincblende structure. The phonon dispersion curves and densities of phonon spectra are calculated for ZnS, ZnSe, GaP, GaAs, InSb, SiC, GuCl, and CuI, the method developed for determination of 15 parameters characterizing the model is discussed. Analogous results obtained with an 11-parameter version of the rigid ion model are presented for comparison. Le formalisme de l'approximation des liaisons deformables, qui a ete developpe dans le precedent article de cette serie, est applique sur 15 composes presentant la structure de la blende. Les courbes de dispersion de phonons ainsi que les densites des spectres des phonons sont calculees pour ZnS, ZnSe, GaP, GaAs, InSb, SiC, CuCl et CuI et comparees aux resultats analogues obtenus avec la version a 11 parametres du modele des ions rigides, La methode utilisee pour la determination de 15 parametres caracterisant le modele est discutee.

Collisional-radiative recombination in cold plasmas

Abstract: We have investigated the electron-ion three-body collisional-radiative recombination for electron temperatures below 4000 \ifmmode^\circ\else\textdegree\fi{}K using the Mansbach-Keck rates of electron-impact-induced transitions between hydrogenic energy levels of high principal quantum number. At such temperatures and for a wide range of electron densities, the statistical recombination process is simultaneously governed by collisional and radiative transitions between excited levels near the ionization limit, where many atomic and molecular systems possess a hydrogenic energy-level structure. In order to also take into account radiative transitions, we have numerically solved a system of coupled equations, describing the quasi-steady-state populations of 100 bound levels. These equations are expressed in, and solved for, the first differences between the reduced population densities $\ensuremath{\rho}(p)$, which improves the computing precision and establishes the location of the "bottleneck" in the recombination sequence. Our results are consistent with the following approximation for the collisional-radiative recombination rate coefficient (in ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$): ${\ensuremath{\alpha}}_{\mathrm{cr}}=1.55\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{T}^{\ensuremath{-}0.63}+6.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{T}^{\ensuremath{-}2.18}{[e]}^{0.37}+3.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{T}^{\ensuremath{-}4.5}[e]$, where the electron temperature $T$ is in \ifmmode^\circ\else\textdegree\fi{}K and the electron density [$e$] is in ${\mathrm{cm}}^{\ensuremath{-}3}$; the first and last terms describe purely radiative and collisional recombination, respectively, and the second term results from the complex interplay of collisional and radiative processes. Agreement with experimental data is reasonable.

Trigeminal representations of the masticatory and extraocular proprioceptors as revealed by horseradish peroxidase retrograde transport.

Abstract: Horseradish peroxidase has been injected in the masticatory and extraocular muscles in newborn and adult cats to identify the cells of origin for the muscle endings. Labeled motoneurons in the nuclei of the III, IV, V and VI nerves have been observed. They are the parent cells of the motor terminals taking up the enzyme in the muscle injected. Labeled ganglionic cells have been found scattered all along the ipsilateral mesencephalic nucleus of the V nerve after injection of both the jaw closing and the jaw opening muscles. Labeled cells have also been found in the ipsilateral caudal part of the same nucleus after injection of the extraocular muscles. These results are interpreted as due to enzyme uptake by the sensory endings of the muscle studied. Moreover cell bodies in the semilunar ganglion were found marked for both groups of muscles injected showing a second ganglionic representation for the sensory endings.

Nouvelles Methodes en Contrôle Impulsionnel

Abstract: On presente dans cet article une methodologie generale de traitement des problemes de controle impulsionnel, fondee sur l'etude systematique des inequations quasi variationnelles Celles-ci jouent pour le controle impulsionnel un role analogue a celui des equations de Hamilton-Jacobi-Bellman pour le controle continu

Conversion of a photon to an electrical signal by sudden polarisation in the N-retinylidene visual chromophore.

Abstract: ONE of the major unresolved problems in the chemistry of vision remains the detailed mechanism by which absorption of a photon by rhodopsin can ultimately change the permeability of the rod plasma membrane to Na+ ions1. We demonstrate here that excitation to the lowest singly-excited π,π* singlet state of a realistic model of the visual chromophore triggers a sudden, short-lived, electrical signal in which charge moves from one end of the molecule to the other. We suggest that this electrical signal is the crucial, primary event in the overall mechanism leading to vision.