The temperature dependence of the relaxation rates in YbCl 3 .6H 2 O up to 45 K can be explained by the Raman pro cess if phonon dispersion is considered. The analysis of the measured Mossbauer spectra has been performed by means of the field up-down model. It is pointed out that there are found dispersionlike terms in the spectra in addition to the usual Lorentzians.

Cells studied in electron microscopy show in their fine structure a series of definite morphological patterns : equidistant parallel membranes, hexagonal arrays of tubes, cubic lattices etc. All these structures come from the folding and from the topological rehandling of bilayered fluid membranes. Phospholipids are essential component of membranes and are at the origin of the bilayered and fluid structure of the cell membrane. These phospholipids, when purified and in presence of water and oily components, form liquid crystalline phases, with similar geometrical structures (equidistant parallel surfaces, hexagonally packed cylinders, cubic arrays etc.) but there are several differences separating them from their biological counterpart. The proportion of water is much higher in cell membrane systems. The distribution of oily components often has a distinct topology. The periodicities observed in these patterns are reduced by a factor of about ten, when prepared in vitro, relative to those observed in cells. It is shown that these differences originate from the asymmetry of cell membranes and that several factors may be involved in saddle-shaped membranes.

A u s t r a l i aRh,un&-La contrainte globale spatiale imposke par la composition d'un systeme binaire surfactant-eau peut etre exprimbe en fonction de la g8ombtrie locale intrinsbque du film de surfactant, si le film est suppose suffisamment homog&ne, c'est &-dire de faible Bnergie de courbure.Ces relations sont calcul6es pour des gkom6tries (quasi-)homog&nes hyperboliques, elliptiques (sphbres) et parabolique (cylindres).I1 est sugg6re que la cristallinite de certaines mksophases de surfactants est le rksultat de la contrainte homog&ne.Une thkorie dktaillke des structures rencontrkes dans les phases cubiques bicontinues, ainsi que des techniques experimentales pour determiner la microstructure de ces phases est prksentke.

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The analyzing power for quasi-free nucleon-nucleon scattering has been found to differ substantially from that for the free interaction. This medium modification was first noticed in inclusive scattering experiments. In the present work, it has been demonstrated that kinematically complete exclusive measurements are capable of demonstrating this effect more clearly. This advantage arises by virtue of the possibility of selecting nucleons bound in states localized in the nuclear interior.

research documents, whether they are published or not.The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

The amplitude corresponding to the lowest order diagram for photonuclear reactions is calculated in the framework of Dirac phenomenology. The nucleon motion is described using the Dirac equation and distortion effects are taken into account. Calculations are carried out for the polarization observables in the reaction and some comparison with experiment is discussed.