[en] In this paper, the principle of several methods of laser spectroscopy are reviewed in the perspective of application to thermal collision studies. Methods of linear-absorption, saturated-absorption and two-photon absorption are considered. It is shown how from the recording of line profiles, one can extract valuable data on various collisional processes that occur in vapors and discharges

[en] This series of lectures will be concerned with instabilities which arises in plasmas of high electrical conductivity confined by magnetic fields. The introduction of a finite electrical resistivity provides the plasma with an additional degree of freedom to reduce its potential energy. The discussion of resistive instabilities in MHD is approached by first considering the ideal gravitational interchange instability in an infinitely conducting plasma slab. The introduction of finite resistivity in situations which would be stable according to the Suydam criterion in ideal MHD, results in a more slowly growing instability known at the resistive G-mode. The consideration of this mode in the simple case of a uniform magnetic field will form the basis for the subsequent discussion of resistive instabilities in a plane current layer with a non-uniform magnetic field - the resistive G-mode, the ''rippling'' mode arising from an equilibrium resistivity gradient and the ''tearing'' mode driven by a non-uniform current density. An alternative discussion of the G-mode is given utilising the so-called ''quasi-mode'' formulation

[en] This is an introductory paper to theoretical and experimental gravitational wave research. In the first part, we give the precise mathematical description of a weak gravitational wave in the general framework of metric theories. Then we discuss the effect of these waves on material systems and the principle of the present detectors. The third part concerns the generation of gravitational radiations and reviews the possible astrophysical sources. Finally, we discuss the limitations and the state of the art of present detectors

[en] We study a one arm pendular Fabry-Perot interferometer with specifications corresponding to the two arms interferometers designed to detect gravitational radiation. We consider the non linearities originating from the radiation force and the effects of time delays due to the finite length of the arm. We derive the exact and the associated ''predictivised'' equations for the motion of the suspended mirror. We show that effects of time delays increase considerably the stability of the device when the optical relaxation time is of the order of the period of the pendulum, a case of relevance when light is recycled. However the thermal noise does not seem to be much modified when calculated within a simple approximation scheme

[en] A review is made on the results concerning the stellar populations in the central region of the galaxies

[en] There is to-day a general agreement to consider General Relativity as the most efficient theory to describe the observational data in cosmology. In particular, Friedmann's Models offer good interpretation of the Hubble's Law and of the 3⁰K cosmic radiation considered as the present residual energetic effect to the big bang. To-day cosmologists begin to study the energetic component of the universe, while, during the past years, only its material component was taken in account. The study of the early universe is now approaching the primeval explosion. At this knowledge level, the fondamental structures of our mind are reflected upon the object of their studies and there exists a parallel between the physical universe and the mathematical universe of the sets

[en] The basic ideas of the model microfield method are shortly reviewed. Recent applications to the broadening of the alkali metal resonance lines, of hydrogen lines and of overlapping helium lines are presented. Some approximations are discussed with special emphasis on the problem of the ion dynamics

[en] The manner in which laser spectroscopic techniques can be use to probe collisional processes in atomic vapors is reviewed. A discussion of the saturation spectroscopy of three-level systems, coherent transient spectroscopy, and a beam-laser spectroscopy system is presented. It is shown that such experiments can provide useful information on both the total and differential cross sections for atom-atom scattering

[en] A 21-cm line study of a sample of galaxies shows that in some of them, the gas extends far away from the optical limits. Some of these galaxies are isolated, as for example NGC 1744. On the contrary, NGC 520 and IC 5039 seem to suffer an interaction from a nearby companion

[en] During the past twenty years, considerable progress has been made in the production and in the use of synchrotron radiation. Nowadays, the advent of ultra-high vacuum toroidal grating monochromators coupled to modern storage rings has made possible a new class of sophisticated experiments whose development has gone hand in hand with the continuous improvements in the technology of synchrotron radiation. The simple example of photoionization processes in helium is taken to illustrate these parallel developments. The present status of experiment is summarized, with particular emphasis on recent measurements of very weak photoionization processes leaving the positive He⁺ ion in the n = 2 and n = 3 excited states. Further improvements are still expected from the use of ondulators mounted on new storage rings, which will offer spectacular possibilities to the experimental investigations, in particular in atomic physics