Sur la diffraction d'une onde plane electromagnetique par un reseau metallique

This paper deals with the diffraction of a plane electromagnetic wave by metallic gratings. Some calculations have already been achieved on an UNIVAC 1110 computer in both ultraviolet and visible regions.     

Etude de la diffraction par une fente pratiquee dans un ecran infiniment conducteur d'epaisseur quelconque

This paper deals with the rigorous electromagnetic theory of the diffraction of a plane wave by a split in a thick screen. The screen is perfectly conducting, the width of the slit is of the order of the wavelength, and the electric field is parallel to the slit. Particular attention has been paid to the numerical treatment in order to obtain a high accuracy with a short computer time.     

Reconstitution d'un profil d'eclairement echantillonne par une ligne de micro-photodiodes

We have studied experimentally the spatial photosensitivity distribution of a self-scanned photodiode array. Using this data, we have calculated a reconstruction function which restores exactly the profile of a light distribution sampled by an array infinite extent under the Nyquist conditions. We briefly discuss the noise limitations and the truncation effects introduced by the finite number of diodes of a real array.     

Transmission d'images et d'hologrammes par une nappe de fibres optiques au moyen d'un codage chromatique

A new method of addition of 2-D complex optical signals is proposed. The addition is performed by suitably matched spatial filtering. For that purpose the effect of compacting of periodic objects images is employed.     

Modulateur electrooptique pour l'extraction d'une impulsion lumineuse d'un train

The performance of a new light modulator are presented. It uses the Pockels effect in order to select a single pulse from a train. The repetition rate of commutation is greater than 100 Hz, the contrast ratio exceeds 10⁴ and the lifetime is about 5 x 10⁷ shots. The main advantage of this modulator is an easy working-up.     

Mise en evidence de la polarisation d'une onde de surface par absorption atomique

Particular polarisation of surface waves is demonstrated using hyperfine dipolar magnetic transitions. We compare the relative intensities of F = 2, M = 2 ? F = 1, M = 1 andF = 2, M =-2 ? F = 1, M = -1 lines in the fundamental state of ⁸⁷Rb     

Determination de la longueur de covariance d'une surface rugueuse par l'etude de la diffusion residuelle a l'infini

We propose a simple method for directly measuring the correlation length of a slightly rough surface by using the residual scattered light. As in a previous work about strong diffusors, we use the experimental results to discuss the validity of Beckmann's theory on surface scattering, now in the case of weak diffusors.     

Extraction d'invariants morphologiques d'une information par correlation optique et filtrage spatial

The set of characters on a page of text may be described as a two-dimensional random distribution of morphological constants (linguistic data and spread function of the writer) to which a gaussian noise is superimposed. Processing the whole page by using optical correlation and spatial filtering techniques allow one to extract these constants, thus supplying a recognition test of the writer. The general principles are explained, and the distinction of two type writer founts is experimentally verified.     

A la recherche d'un nouveau boson de spin un

The spin-1 partner of the goldstino (gravitino) under supersymmetry may be very light and very weakly coupled. We study how it could appear in e⁺e^? annihilations, K and ω decays and beam dump experiments. If its mass is very small it would be produced and interact like a pseudoscalar particle somewhat similar to an axion, but with different decay modes.     

Activation de l'hydrogène moléculaire par une petite particule de nickel

The electronic structure of an icosaedral cluster of 13 nickel atoms is studied using an Extended Hückel method. Near the highest occupied level at ?7.65 eV, h and t orbitals are accidentally degenerate so that the fundamental state possesses a high spin multiplicity (2S + 1 = 9) accounting for the magnetism of the particule. We have considered several geometries for a hydrogen molecule approaching the cluster at a minimum distance of 1.47 Å. In all cases the H-H bond is weakened because of electron transfer from the nickel towards the hydrogen atoms and partial filling of the antibonding orbital of h₂. During the increase of the H-H distance the weakening is easier than in the absence of a nickel catalyst leading to the dissociative adsorption of the molecule. The activation mechanism of H₂ requires participation of both the s and d orbitals of the metal and leads to a lowering of the spin multiplicity. Finally, we have shown that a dissociative adsorption is obtained when the H₂ molecule approaches a triangular face. After breaking of the H-H bond, the more stable system corresponds to atomic H adsorption on two opposite vertices.     

Étude par diffraction d'électrons lents d'une surface (100) de NaCl perturbée par l'irradiation électronique incidente

The results of a study of a cleaved NaCl surface by low energy electrons are reported. Intensity and angular spectra of the elastically diffracted electrons present anomalies depending on the sample temperature. The analysis of these spectra shows that surface perturbations are produced by the incident beam. The conclusion is that to avoid surface evolution during experience, cooling of the sample is necessary.     

Estimation du nombre d'echantillons et du domaine spectral necessaires pour determiner une forme moyenne par une technique de superposition

An average form can be extracted from a set of forms, assuming that they are all obtained by a common process. Considering this set as a representative sample of a statistic population allows the determination of the minimal required number of forms and also the useful spectral domain.     

Utilisation des franges d'interference en lumiere diffuse pour l'etude de l'etat de surface d'un diffuseur

A diffuser, illuminated successively by light of two neighbouring wavelengths, produces on a photographic plate placed in a parallel plane two intensity distributions which are homothetic except for certain decorrelation terms. This homothecy (scale variation) can be compensated by a longitudinal displacement of the photographic plate between the two exposures and consequently, the decorrelation depends only on the structure of the diffuser. Under these conditions, if the photographic plate is given an additional displacement in its own plane between the two exposures, the contrast of Young's fringes obtained in the Fourier plane characterises the roughness of the diffuser.     

Oxydation de la face (111) d'un monocristal de chrome

Interaction of oxygen with (111) oriented chromium single crystal surface was studied by electron diffraction (LEED and RHEED). From the clean surface, oxygen adsorption induces a $\text{Cr}\text{(111)(}\text{3}\text{×}\text{3}\text{)}\text{R}\text{30°}$ -O structure. No change in the geometry of the LEED pattern occurs with additional oxygen exposure or heating, although the RHEED study denotes the nucleation of rhombohedral oxide on the surface. The orientation relationships with the substrate are determined and compared to those found in the case of (110) chromium surface.     

Emission infra-rouge de la molecule co dans la zone de reaction d'une flamme oxyacetylenique

Rotational intensities in the first overtone band of the vibration-rotation spectrum of the CO molecule, emitted in the inner cone of an oxy-acetylene flame, have been measured using a grill spectrometer. The deduced vibrational and rotational temperatures are in agreement with adiabetic temperature. A Boltzman distribution among vibrators is obeserved.The measurements in the 6-4 band have allowed us to obtained the vibration-rotation inter-action factor.     

Procede experimental de restitution de la partie imaginaire de l'amplitude complexe d'un objet de phase

An experimental process is described, capable of restoring the imaginary part of the complex amplitude of a phase object. It is based on a combination of phase-modulation interferometry and scanning the object by a flying-spot method. This experiment proves that it is possible to obtain the sign of the phase, and so the direction of surface relief.     

Sur la contribution des differents porteurs à la zone de charge d'espace d'un semiconducteur multivallée

We study, in the case of the parabolic approximation, the space charge of a two-valley conduction band semiconductor; the electrons associated to these valleys are supposed to have different effective masses. By using a non-degenerate distribution we analyse the contribution of the different carriers to the space charge and the effect of surface band bending, temperature, effective masses, dope and type of material. We point out that the space charge can be governed by either type of carrier or both. By comparing the results with those given by a single valley model, we show the incidence of the initial hypothesis on the value and evolution of the space charge with temperature and surface band bending. As an exemple, we study briefly n-type GaSb.     

Utilisation de l'heterodynage optique pour des mesures locales de la vitesse de phase d'ondes elastiques

A new optical method for measuring the acoustical wavelengths with a great accuracy has been investigated. An acousto-optical interaction is performed and the acoustical frequency range is such that the diffraction take place in “Bragg regime”. The phase modulation of the scattered laser beam is visualized by means of an optical heterodyning. When the location of the acousto-optical interaction is linearly displaced, the detected beating signal supplies a local measurement (10 μ) of the acoustical wavelength.     

Tenseur de mobilite d'un super-reseau en presence de desordre d'interface

Mobility tensor, limited by island-like interface disorder, in type-I superlattices, is studied within a simple model (Kronig-Penney and plane waves). Computed results are presented, for GaAs-Ga_1?xAl_xAs superlattices, versus variations of temperature, superlattice period and defect parameters such as localisation, width and thickness. This interface scattering process has a significant influence on near equilibrium transport at low temperature.