Palaeontological data about the climatic trends from Chattian to present along the Northeastern Atlantic frontage

Climatic changes that affected the Northeastern Atlantic frontage are analyzed on the basis of the evolution of faunas and floras from the late Oligocene onwards. The study deals with calcareous nannoplankton, marine micro- and macrofaunas, some terrestrial vertebrates and vegetal assemblages. The climate, first tropical, underwent a progressive cooling (North-South thermic gradient). Notable climatic deteriorations (withdrawal towards the South or disappearance of taxa indicative of warm climate and appearance of "cold" taxa) are evidenced mainly during the Middle Miocene and the late Pliocene. Faunas and floras of modern pattern have regained, after the Pleistocene glaciations, a new climatic ranging of a temperate type in the northern part

Instability driven fragmentation of nanoscale fractal islands

Formation and evolution of fragmentation instabilities in fractal islands, obtained by deposition of silver clusters on graphite, are studied. The fragmentation dynamics and subsequent relaxation to the equilibrium shapes are controlled by the deposition conditions and cluster composition. Sharing common features with other materials' breakup phenomena, the fragmentation instability is governed by the length-to-width ratio of the fractal arms.Comment: 5 pages, 3 figures, Physical Review Letters in pres

Raies laser infrarouges dans les vapeurs de terres rares et d'alcalino-terreux

We have observed in rare earth vapors (thulium, ytterbium, samarium, europium) and in alkaline earth vapors (barium, strontium), 76 infrared laser lines. The metallic vapors were exited with short and intense electrical pulses. With a Fabry-Perot interferometer and phase sensitive detection, we have measured several isotope shifts in ytterbium vapor. Finally we have achieved a single-mode laser operating with several lines of barium and strontium vapors.Nous avons observé dans les vapeurs de terres rares (thulium, ytterbium, samarium, europium) et d'alcalino-terreux (baryum, strontium), 76 raies laser infrarouges. Les vapeurs étudiées sont excitées par des impulsions électriques brèves et intenses. A l'aide d'un interféromètre de Fabry-Perot et d'un dispositif de détection synchrone, nous avons mesuré quelques déplacements isotopiques dans la vapeur d'ytterbium. Enfin, nous avons mis au point un laser oscillant en mode unique pour plusieurs raies des vapeurs de baryum et de strontium

RAIES LASER DANS LES VAPEURS MÉTALLIQUES

En utilisant une excitation par impulsions électriques, 28 raies laser infrarouges ont été observées dans les vapeurs de thulium, d'ytterbium et de samarium. Les meilleurs résultats ont été obtenus pour une intensité de crête de 350 A et une largeur d'impulsion de µs.Using electric pulse excitation, 28 infrared laser lines have been observed in thulium, ytterbium and samarium vapors. The best results are obtained with 350 A peak current pulses of 2 µsec width

Progrès récents dans les transferts collisionnels assistés par laser

On présente dans cet exposé les différentes expériences relatives aux transferts d’excitation entre atomes, molécules ou ions en présence d’un champ électromagnétique intense, que nous avons regroupées en fonction du couplage mis en jeu au cours de la collision. Ces diverses expériences sont, dans leur ensemble, en accord avec les études théoriques correspondantes

Observation of two visible Dicke-superradiant transitions in atomic europium (*)

We report the first experimental evidence of visible superradiance for the europium lines at λ = 557.7 nm and λ = 545.3 nm. The superradiant character of these lines is checked by studying the variation of pulse heights, widths and delays versus atomic density. We show that the peculiar population inversion mechanism is essentially due to the existence of a strong superradiant infrared emission at λ = 1 759.6 nm

On the radiative-collision processes

During the last fifteen years numerous of work have been devoted to the so-called Radiative-Collision processes. The absorption or emission of a photon during collisions of atoms in the presence of a strong electromagnetic field leads to elementary processes involving a rich variety of collisional processes. Basic aspects in the theories and experiments are reviewed and commented upon

LASER INDUCED COLLISIONAL ENERGY TRANSFER : HIGH RESOLUTION STUDIES AND COHERENCE EFFECTS IN DIPOLE-DIPOLE INTERACTION

Deux méthodes complémentaires ont été utilisées pour étudier des transferts collisionnels d'énergie induits par laser dans le cas d'une interaction dipole-dipole. Une étude de profil spectral à haute résolution dans un mélange Na-Ca a confirmé l'existence d'un profil universel pour une interaction donnée, la mesure de taux de polarisation dans un mélange Eu-Sr a confirmé l'hypothèse que de tels processus préservaient partiellement les cohérences induites par un champ extérieur polarisé.Two complementary methods have been used to investigate light induced collisional energy transfers for a dipole-dipole interaction. A high resolution spectral profile study in a Na-Ca mixture has confirmed the assumption of a universal profile for a given interaction, the measurement of polarization ratios in a Eu-Sr mixture has confirmed the assumption that such processes may partially preserve the coherences induced by an external polarized field

Homogeneous and heterogeneous clustering in the accretion regime

Condensation of nano-droplets in a supersaturating vapor decomposes in two steps: the formation of a nucleation center, also called critical nuclei or nucleation seed, and the growth sequence, by accretion of further atoms on the nucleation center. These two steps have been investigated separately through the clustering of homogeneous particles Nan and heterogeneous particles NanX in a helium buffer gas (X = (Na2O)2 or (NaOH)2). The growth sequence is analyzed with preformed molecules X injected in a supersaturating sodium vapor and driving production of NanX clusters. Cluster distribution mean sizes are controlled by sodium concentration and by the condensation cell effective length. The signal intensities observed for homogeneous and heterogeneous clusters are proportional to the homogeneous and heterogeneous nucleation center numbers respectively. We can measure the efficiency for the homogeneous nucleation center production versus sodium concentration. This process is the onset of the condensation phase transition.