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.     

Explication des anomalies observees dans une etude quantitative du rendement photoelectrique de l'argent par la non-localite de la reponse dielectrique

Discrepancies had been obtained for p polarization in the quantitative analysis of the photoemission yield of silver. They had been accounted in a semi-phenomenological way by introducing a “surface term”. It is shown here, that a non-local theory of the dielectric response introduces a term which is of the same form as the “surface term”. Therefore, the analysis of photoemission yields is an experimental test to prove the non-local character of the dielectric response.     

Extension des methodes de moires a l'etude de la microstructure d'un objet diffusant: Moires aleatoires

The statistical analysis of a diffusing object (i.e. with random microstructure) by optical means is necessarily performed by studying the wavefront radiated under suitable illumination. Two approaches exist: either “in situ” analysis of this wavefront (i.e. at the object itself), or farfield analysis. We briefly recall the drawbacks and limitations of those approaches. We then demonstrate the possibility of a third one, based upon a generalization of moirés to random structures. This last approach has fewer theoretical and practical limitations than the two first ones. A particular case is treated theoretically and preliminary experimental results are given.     

Application effective de l'interferometrie holographique par double exposition a l'etude des deformations de ceramiques,dues a l'impact d'un projectile

This paper reports upon experiments made in the field of high speed holography by double exposure. These experiments allowed to establish at different moments, the map of deformations of the surface of a ceramic plate undergoing the action of a projectile impact.     

Contribution a l'etude des mecanismes de relaxation dielectrique des amines primaires dans le domaine ultra-hertzien

The authors have studied the mechanism of dielectric relaxation of primary amines from the point of view of the following characteristics : relaxation time distribution, activation energy, resultant behaviour of autocorrelation function of the orientational polarisation, spectral density of energy as a function of temperature and chain length of the aliphatic type molecules.The analysis of the relaxation mechanism is made on the basis of semi-empirical macroscopic model of Davidson-Cole. The relaxation mechanism can be analysed as a function of the combined effects of intermolecular and intramolecular relations which are related to the dipolar behaviour of amine group as well as the effect of hydrocarbonic aliphatic chain.     

Determination de rugosite par correlation des speckles dans l'image de la surface diffusante

Two speckle patterns are recorded on a photographic plate which lies in the image plane of a rough surface. The angle of incidence of the parallel beam illuminating the object is slightly varied between the two exposures. It is possible to localize and measure different roughness of the surface by observing suitably the photographic plate. We discuss the experimental setup and the effects of the optical system.     

Contribution a l'etude de la transition magnetique dans Cr0,75V0,25N

Magnetic susceptibility measurements of a Cr_0.75V_0.25N sample have shown three anomalies at 30, 90 and 227 K respectively. On the other hand, neutron-, X-ray-diffraction and specific heat measurements confirm the presence of an antiferromagnetic transition at 227 K. The two other anomalies (at 30 and 90 K) may be due to other phenomena for instance to the existence of unidentified impurities present in the sample.     

Contribution a l'etude de l'effet de charge sur echantillon isolant en spectroscopie de photoelectrons (XPS)

The accurate measurement of sample current for several non-conducting materials studied by XPS has proved that this effect is negligible and does not contribute to the surface potential determination. The influence of parameters of the sample (surface state, geometry, emission yield), those of the spectrometer (X-ray intensity, retarding potential), and the consequence of the use of a flood gun have been studied on simulated insulating samples starting from electrical characteristics and an exact value of the surface potential. Mechanisms are proposed to explain the different effects observed. The results show that electrical compensation of charge effect is difficult and that the energy scale linearity is affected on a retarding field spectrometer.     

Etude par conductivite electrique de la cinetique d'oxydation des magnetites finement divisees substituees par le zinc

The oxidation kinetics by electrical conductivity of Zn-substituted magnetites into the lacunar γ phase of same spinel structure was investigated. From the α = ?(t) curves which are identified with those obtained in microbalance it was established that the kinetics is governed by a diffusion law under variable working conditions, the chemical diffusion coefficient being only constant for high zinc substitution extents. When the temperature was raised the log $\text{σ = ?(}\text{1}\text{T}\text{)}$ curves show discontinuities attributed to the precipitation of a rhombohedral phase.     

L'adsorption du bismuth sur des films minces d'or etudiée par variation de résistance electrique

Adsorption of bismuth on gold thin films is studied by electrical resistance variations. At low coverage the increase of gold resistivity due to Bi adatoms is 2.3 ± 0.1 μΩ cm/at% and is independent of temperature and thickness of the Au layers. At 20° C and ?150°C, the shape of the resistance variation curve for increasing Bi coverage indicates that the adatoms form a first monolayer having a higher density at lower temperature. At 85°C two Bi monolayers are made successively: the first is adsorbed on the free surface of the gold film; the second grows on the other face, between the gold and the glass substrate, after migration of the Bi atoms through the grain boundaries or other defects. These results are verified by Auger electron spectroscopy. They are used to show that the initial reflectivities for conduction electrons on the two surfaces are similar in recrystallized gold films; the specularity coefficients P and Q have been evaluated using the Fuchs theory. For clean surface, the electron reflection is large specular (P ? Q ? 0.75 ± 0.05) and becomes entirely diffuse when a monolayer of bismuth is adsorbed on each face of the gold film.     

Evolution de la caracteristique courant-tension des varistances a base d'oxyde de zinc avec la pression partielle d'oxygene de l'atmosphere de frittage

In order to decrease the resistivity of zinc oxide grains which is responsible for the intensity limitation observed at high current densities, the dependence of current-voltage characteristics of zinc oxide based varistors on oxygen partial pressure has been investigated. From these studies it appears that the conductivity increases with decreasing oxygen partial pressure, this phenomenon being more significant at low voltages than at higher ones. These results can be related to a slight increase of the donor density, while the superficial trap density decreases strongly, involving a collapse of the barrier height and of the non-linearity exponent.     

Etude de l'absorption optique du vanadium en couches minces interpretation par la theorie des bandes d'energie

Optical absorption of vanadium thin films has been determinated from 0.32 to 5.5 eV from reflectance and transmittance data. The films have been deposited in ultra high vacuum and the measurements have been realised in situ. Between 1 to 5.5 eV, we note a large absorption band which is independant of the photons angle of incidence and polarisation; with an important maximum and a shoulder respectively at 3.1 and 2 eV. We explain this absorption band by direct interband transitions deduced from theorical bands calculated by Yasui et al. From 0.32 to 1 eV, the observed absorption is principally due to intraband transitions.     

L'etincelle sous vide,source de continu dans la region au dessus de 100 eV

A giant pulse current discharge (150 kA) has been used to feed a vacuum spark in order to obtain a continuum in the extreme ultraviolet range. Emission was observed in the region 200-50 Å. The short light pulse delivered (300 ns) is suitable for absorption spectroscopy measurements of fast transient media.     

Étude par spectroscopie auger,diffraction d'électrons lents et rapides des premiers stades de la sulfuration de la face (100) de NiO par H2S

A structural study of the different stages during NiO(100) sulphurization by H₂S was carried out by RHEED, LEED and AES. On exposure to H₂S (PH₂S < 10^?5 Torr) The “clean” surface, obtained by UHV cleavage, was found to react with H₂S to produce islands of Ni(100) covered with an ordered c(2 × 2) S structure up to 300°C. Growth of Ni₃S₂ islands occurs on increasing the temperature and the exposure to H₂S.     

Mesure de la temperature et des coefficients d'absorption de certaines raies de l'argon neutre dans un arc stabilise par parois et optiquement epais

Temperatures have been determined from relative line intensities in an arc stabilized in argon at atmospheric pressure. Proper corrections were made for self-absorption. Absorption coefficients have been measured for argon lines and were compared with calculated values. Good agreement was observed in regions with LTE.     

Étude de l'adsorption électrochimique de l'oxygène sur des lames minces d'or par la mesure de leur résistance électrique

The electrical resistance changes of thin gold film electrodes of preferential orientation [111] with film thickness and potential have been studied. The applicability of the Fuchs-Sondheimer (FS) relation to the decrease of resistance observed at the first negative polarization and the first few potential sweeps for different thicknesses have shown that this phenomenon is due to a surface process, interpreted as a cleaning of the electrodes. The resistance changes observed during the electrochemical adsorption and desorption of oxygen also obey the FS relation. The analysis of the resistance variation with the charge exchanged during these reactions has allowed us to show that the electrochemical adsorption of oxygen, on these gold films occurs by a two-dimensional island mechanism with formation of different structures of the surface layer. The values of the resistivity change caused by the adsorption of 1% oxygen atoms (with respect to the total number of metal atoms in the films) at low coverage have been compared with those observed in other systems (metal-gas, metal-metal).     

Étude par la diffraction des électrons lents et la spectroscopie auger de l'adsorption du soufre sur le platine

The adsorption of sulphur on the (111), (100) and (110) faces of platinum has been studied by LEED and Auger spectroscopy. By the combined use of radioactive S³⁵ the contents of the surface unit meshes were determined “in situ” for each state of adsorption. On the (111) face one observes a preliminary adsorption with localization of sulphur atoms at sites of maximum coordination of the metal. At higher pressures and temperatures, S-S distance contracts and a compact hexagonal arrangement, with sulphur atoms outside sites, is obtained. The thermal stability of the various structures was studied as a function of sulphur coverage. During adsorption on the (100) and (110) faces, two steps may be distinguished: the first one seems to correspond to an adsorption in sites, the second corresponds to the formation of a two-dimensional compound whose square mesh is slightly different on the two faces.     

Contribution des moments octupolaire et hexadecapolaire a l'elargissement des raies spectrales de molecules linearaires

Expressions for linewidths due to hexadecapole-dipole, hexadecapole-quadrupole, hexadecapole-octupole, and hexadecapole-hexadecapole interactions have been determined from the Anderson-Tsao-Curnutte theory. We have shown—for linewidths of CO₂ with self-broadening and broadened by N₂, as well as for linewidths of CO with self-broadening and broadened by N₂, O₂, H₂, and CO₂—that allowance for octupolar and hexadecapolar mements yields improved agreement with experimental data for high |m|-values.