Influence de la chimisorption de l'oxygéne et l'azote sur l'émission ionique secondaire d'échantillons monocristallins de nickel et d'alliage nickel-chrome

The influence of an active gas adsorption on the secondary ion emission yields of a mono-crystalline target bombarded with a primary argon beam has been investigated. On the one hand, the chemisorption changes the electronic structure on the surface metal atoms and increases the positive ion emission yields, on the other hand, channeling conditions of the primary ions through the target lattice are modified according to the relation of the chemisorbed layer with the metal lattice. Two samples were examinated: pure nickel and a nickel chromium alloy in either an oxygen or a nitrogen partial pressure. Important modifications of the coverage are due to the sputtering yield variations. The experiments described here suggest that secondary ion emission could be used for chemisorption studies.     

Mesure de l'impulsion echangee au cours de l'interaction onde evanescente-atome

In vacuo, during an interaction between a moving atom and a surface wave of frequency v, the exchanged momentum is greater than hv/c. First we show, using a semi-classical treatment, that this momentum is ?k_x in agreement with De Broglie's relation p = ?k, but unlike the usual notion of wave momentum attached to the Poynting vector. We present experimental methods to measure this momentum and we give results for two atom speeds.     

E´tude par la diffraction dese´lectrons lents et la spectroscopie auger de l'adsorption du soufre sur l'or

A study has been made of the adsorption of sulphur on the (111), (100) and (110) faces of gold using LEED and AUGER spectroscopy combined with a radioactive tracer technique employing ³⁵S. The content of the surface unit mesh can thus be determined. A preliminary adsorption state with localization of sulphur atoms at sites of maximum coordination of the metal is observed. This state is transformed by reconstruction into a surface compound, or adsorbed two-dimensional sulphide. The results are compared with previous results from HEED and experiments of reversible chemisorption. On the (110) face the equilibrium between different adsorbed phases of sulphur is studied under conditions for which sulphur dissolution or evaporation is negligible. A temperature- coverage diagram is determined which shows the regions where sulphur is present at equilibrium in one or two phases.     

Etude de l'autodiffusion dans la phase cubique centree de l'europium et du gadolinium

The self-diffusion coefficient follows a relation of the form : D = (1,0_?0.4^+0.7)exp (?shape=case>34400RT±700)

$\text{cm}{}^2\text{sec}$

for b.c.c. europium D = (1,0_?0.3^+0.5) × 10^?2 exp(?

$\text{32700 ±4000}\text{RT}\text{)}$

$\text{cm}{}^2\text{sec}$

for β-b.c.c. gadolinium.Whereas europium has normal self-diffusion parameters, β-b.c.c. gadolinium must be set in the class of the anomalous b.c.c. rare-earth metals.From these results we conclude that there exists no evident connexion between the instability of the 4f shell and the activation energies anomalously low in the b.c.c. phases of the rare-earth metals.     

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.     

Étude expérimentale de l'influence de l'angle d'incidence des électrons primaires sur le rendement de l'émission Auger

We have studied the variation of several components of the secondary emission from a (001) surface of aluminum as a function of the polar and azimuthal angles of incidence, the current being measured on the whole half-space of re-emission. We have observed an important modulation of the L₂₃VV Auger peak, that may amount to 30%, when the crystal is rotated in its own plane.     

Autodiffusion cationique dans l'oxyde de nickel monocristallin sous pression partielle d'oxygene voisine de la pression de dissociation de l'oxyde

⁶³Ni diffusion measurements on single crystalline nickel oxide have been performed at temperatures between 1073 and 1373 K and in partial pressures of oxygen equal to the NiO dissociation pressure. In such conditions, the intrinsic defect content is very low and the preponderant defect consists in doubly ionized nickel vacancies (V″).Diffusion coefficient values lead to the result

$\text{D}{}_{\mathrm{v}}\text{(}\text{cm}{}^2\text{s}{}^{\mathrm{?1}}\text{) = 5.2510}{}^{\mathrm{?6}}\text{exp}\text{( ?}\text{154}\text{kJ mol}{}^{\mathrm{?1}}\text{RT}\text{)}$

The activation energy thus determined agrees with the value of the migration enthalpy of the doubly ionized nickel vacancy. Thus, it appears that under such experimental conditions, the vacancy content is determined by the impurity content.     

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.     

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).     

Sur l'entropie de vibration des défauts dans l'approximation d'einstein

The vibration entropies of bulk or defect atoms of a crystal (vacancies, clean or rough surfaces) are given, in general, with a good precision in the Einstein approximation. In this approximation we compare the Montroll-Potts model to a model with central potentials between first nearest neighbours and to a model with central and second nearest neighbours and also angular interactions in order to show that, for sites having unsymmetrical neighborhood, one has in general to define different Einstein frequencies for different normal modes of vibration. The results in the last model show also the importance of angular forces and central forces between second neighbors. Finally the precision of the calculation of the surface entropy is i if one does an Einstein-like approximation on a dynamical matrix written for each k∥.     

Étude théorique par la méthode des orbitales moléculaires de l'adsorption du césium sur la face (110) de l'arséniure de gallium

The adsorption of one cesium atom on a (110) gallium arsenide surface is investigated by means of a molecular-orbital method. Several surface deformations are considered. The values obtained for the extraction energy of one electron, the net charge of cesium atom, and the adsorption energy, allow to infer that the cesium atom is placed above the surface in neighbourhood of a gallium atom.     

Étude de l'adsorption de l'oxygène sur ruban de rhénium par mesure des variations du travail d'extraction électronique

Adsorption and work function changes have been followed on a recrystallized rhenium ribbon [preferentially oriented with (0001) plane parallel to the surface] in an ultrahigh vacuum apparatus. Gas pressure was monitored by mass spectrometry and work function variations measured with the diode method in the retarding potential range, using an electron gun. Despite changes in saturation coverages, work function differences at saturation, and sticking coefficient with the adsorption temperature of the sample, curves giving the surface potential with coverage are characterized by a common part till a critical coverage ($\text{5 × 10}{}^{14}\text{atoms}\text{cm}{}^2$), in the 300–850 K range. Comparison with LEED, thermal desorption and electron induced desorption results of the literature is in agreement with an adsorption induced surface rearrangement, above this critical coverage.     

Experiences sur l'anisotropie de la resistance de surface des supraconducteurs pres de Hc3

In this paper we present some experimental results in the anisotropy of the surface resistance near H_c3. The materials used in this work have values of κ between 1.2 and 4.6. The results seem to indicate that the actual theories are incomplete due to the fact that the influence of the κ parameter is not sufficiently taken into account.     

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 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.     

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.     

Application de l'effet photoelectrique inverse a la photometrie

We derive a relation between the direct and converse photo-electric effects which takes into account the geometrical features of possible experiments. We describe a device in which both effects acting successively give an electronic signal proportional to the square of the photo-electric yield of the investigated material. Absolute photometric calibrations in the spectral 20–1000 eV may be achieved by this method.     

Etude de l'adsorption de monocouche a l'interface solide-solution diluee par spectroscopie de photoelectrons (XPS): Application a l'adsorption de phospholipides sur des fibres damiante

The adsorption of phospholipids on asbestos chrysotile in a solid-dilute-solution system has been studied by X-ray photoelectron spectroscopy (XPS). Two batches with different particle sizes were used: with thin and long fibres (SFA), and with short fibres (FCC). The fibres were analysed on a portion of filter after filtration. This method of sample preparation results in good reproducibility of the absolute intensities of the XPS peaks. Under these conditions, the surface coverage for monolayer adsorption was determined using an overlayer model. Comparison of the results deduced from the XPS data with those obtained from chemical analysis shows that for low initial concentrations of phospholipids, a different mode of adsorption occurs on the FCC fibres.     

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.