Sur la mobilité atomique accélérée liée à l'irradiation au cours d'une abrasion ionique

We propose several simple models to study the physical origin and respective importance in surface analysis of different mechanisms of enhanced atomic mobility in sputtered samples, and their influence on the depth resolution of the analysis. We study successively the effects of point defects created by the irradiation, collision cascades (effects of atomic displacements and also thermal spikes) and knock-on. The enhanced diffusion due to point defects is shown to be dominant in low melting point elements where vacancies are mobile. In other systems where vacancies are not mobile during analysis the effect of knock-on by primary ions is shown to be the most important.     

Theoretical models for activation of CO2 towards hydration (CO2 + H2O → H2CO3) by cationic binding sites

Theoretical models are studied which illustrate how the hydration reaction CO₂ + H₂O → H₂CO₃ can be catalysed by one or two cationic binding sites (NH₄⁺). In the latter, the arrangement of the two bindings sites is held during the course of the reaction, simulating a rigid molecular receptor. Two different arrangements of the binding sites are studied, and their relative abilities to lower the activation energy of the hydration reaction are studied.     

Molecular and crystal structure of (+)-tetracarbonyl(methyl-1-naphthylphenylgermyl)-cobalt

The crystal and molecular structure of (+)-tetracarbonyl(methyl-1-naphthyl- phenylgermyl)cobalt was solved from three dimensional X-ray diffraction data by Patterson and Fourier syntheses and refined by full-matrix least-squares method to a final unweighted R of 0.047, excluding zeroes. The crystal is mono-clinic. space group P2₁, with a = 9.950(4), b = 6.860(3), c = 14.792(6) », β = 99.89(5)°, Z = 2. 1990 independent reflexions were recorded at room temperature with Ni-filtered Cu-K_α radiation. The determination of absolute configuration confirmed chemical assumptions.     

A practical implementation of turbulence models for the computation of three-dimensional separated flows

An upwind MUSCL-type implicit scheme for the three-dimensional Navier-Stokes equations is presented and details on the implementation for three-dimensional flows of a ‘diagonal’ upwind implicit operator are developed. Turbulence models for separated flows are also described with an emphasis on the numerical specificities of the Johnson-King non-equilibrium model. Good predictions of separated two- and three-dimensional flows are demonstrated.     

A short and convergent synthesis of unsaturated macrodilactones

A simple and versatile approach to macrodiolides 1, using two consecutive Wittig reactions, is described.     

An improvement of low frequency spectroscopic techniques applying fourier transform processes

An FT transfer into time-domain, followed by a double FT deconvolution, allows us to analyze dispersion or absorption curves similar to those obtained in low frequency dielectric or ultrasonic experiments.     

Amino-acids as chiral synthons : synthesis of enantiomerically pure α-hydroxy esters.

Reaction of lithium homocuprates with α-hydroxy β-bromo ester derivated from aspartic acid affords α-hydroxy esters of high enantiomerical purity.     

Internal dynamics of nonrigid molecules. I. Application to acetone

The group theory for nonrigid molecules is used for studying the internal dynamics of the two equivalent C_3v rotor “bent” molecules. Special emphasis is given to the deduction of the symmetry basis vectors which represent in box form the Hamiltonian operator. It is shown that these basis vectors may be advantageously employed in order to simplify the resolution of the two-rotor equation. The procedure is applied to the acetone molecule. It is found that the lowest solutions are clustered into groups of four. The four lowest levels are related to vibrational states, the upper 64 to vibro–rotational states, in which the rotors are rotating in a restricted manner. Only few states show some cogwheel effect. Internal rotation contributions to the principal thermodynamic parameters of acetone are also computed.     

A possible mechanism in heavy ion induced reactions: “Fast fission process”

The influence of the orbital angular momentuml on the mass distribution of fission fragments is studied, both on previously available data on heavy ion induced fission and in new specifically planned experiments: systems⁴⁰Ar+¹⁶⁵Ho and²⁴Mg+¹⁸¹Ta at bombarding energies ranging from 180 up to 391 MeV and leading to the same fissioning nucleus²⁰⁵At with differentl distributions. Whenl values corresponding to a vanished fission barrier are reached, the mass distribution broadens. This suggest the existence of a specific process, “fast fission”, atl-values intermediate betweenl-values leading to compound nucleus formation and deep inelastic collisions, respectively. This process and its conditions of occurence are discussed; of special interest are the correlated differences between the limitations to the fusion cross-section and the fission mass distributions broadenings, respectively, for the Ar+Ho and Mg+Ta systems.     

Propriétés de contraction d’un semi-groupe de matrices inversibles. Coefficients de Liapunoff d’un produit de matrices aléatoires indépendantes

We study in this paper contraction properties of a matrix semi-groupT ⊂GL(d,R) acting on the flag space ofR ^d ; then we obtain properties of the Liapunoff exponents of theT-valued products of random matrices. The principal result is that, in this study, we can replaceT by its algebraic closureH inGL(d,R). This implies a “decomposition” of the action ofT in a proximal part and an isometric part; then we can write, modulo cohomology, the corresponding cocycle in a block-diagonal form, the blocks being similarities. In fact, we can express the multiplicities of the exponents in terms of the diagonal part of a conjugate of the groupH. So we obtain an extension of a recent result of Goldsheid and Margulis about the simplicity of Liapunoff’s spectrum [5]; this work uses their ideas as well as those of previous work [6].