Décomposition thermique du monoxide de carbone sur une surface de molybdène: Formation de couches superficielles de carbone et de carbure

We have investigated the decomposition of carbon monoxide on polycrystalline and (001), (110) monocrystalline molybdenum surfaces. This study was performed by massspectrometry, for thermal desorption studies, Auger electron spectrometry (AES), low energy electron diffraction (LEED) and photoelectron spectroscopy (ESCA). By heating the clean Mo surface in CO or by heating the Mo surface covered with CO, the dissociation of chemisorbed CO leads to a build-up of carbon layer which inhibits the subsequent adsorption. Two distinct types of fine structure are associated with the KLL line of carbon Auger spectra. If the Mo surface is heated at a temperature between 300 and 1500 K, the Auger peak is characteristic of a “graphite layer”. If the Mo surface is heated at a temperature up to 2000 K, the Auger peak is characteristic of a “carbure” layer. This “carbure layer” give rise to a surstructure which agrees with a Mo₂C surface layer and was also investigated by ESCA. Chemical shifts of (1s) C and (3d) Mo photoemission bands were observed and attributed to the bounding between Mo and C atoms in the Mo₂C layer.     

Étude comparée de la première couche d'adsorption de méthane sur nitrure de bore et graphite

The adsorption of methane on crystalline boron nitride has been studied between 77 and 90 K by the volumetric method, especially in the first monolayer domain. It is shown that the thermodynamical properties of the layer are similar to those previously obtained on graphite. It can be inferred from the results that below 77 K, the layer undergoes two first-order phase transitions: a 2D gas-2D liquid transition and a 2D liquid-2D solid transition. In its solid state the localization of the layer is certainly the same on both substrates.     

Calcul de l'emission infrarouge de milieux laser CO2

Vibrationally-excited CO₂ present with inverted population emits infrared radiation at 4·26 μm. For an emitter consisting of optically-thick isolated lines, calculations allow determination of the emissivity and radiancy of a homogeneous layer of gas. Infrared emission is a useful tool for measurement of the energy contained in the upper laser level. A very thin layer of cold gas strongly alters the spectral distribution of the intensity and causes the total radiancy to decrease.     

Etude par spectroscopie de photoelectrons et d'electrons auger des etapes initiales de l'oxydation du chrome polycristallin

The joint application of various techniques of surface analysis (XPS, UPS, AES and work function measurements) allowed us to pinpoint the initial stages in the oxidation of polycrystalline chromium in the temperature range 180–720 K. At weak exposures ( < 10 L) the oxygen was dissociatively adsorbed without providing any information on the amount of oxygen involved or the sites of adsorption (chemisorption vs. physical interaction due to the roughness of the surface). At stronger exposures the oxide Cr₂ O₃ is observed to form, and this is covered by a layer of chemisorbed oxygen at ordinary temperatures and below. The maximum thickness of the oxide layer depends on the temperature, as would be expected from the logarithmic oxidation kinetics. The change in shape of the Auger peaks of chromium during oxidation has been interpreted in terms of interatomic transitions made via the 2p levels of oxygen.     

Étude des interactions chimiques gaz-solide par la technique du faisceau moléculaire: V. réactions de l'oxygène et de l'oxyde de carbone avec des rubans polycristallins de tantale

We use the reflection of a noble gas (helium) molecular beam to study the superficial reactions of oxygen and carbon monoxide with polycrystalline tantalum. The fraction of the incident beam which is specularly reflected gives direct information on the formation of an oxygen chemisorbed layer and we observed only a single apparent binding state for oxygen chemisorbed on tantalum. The initial value of the sticking coefficient of oxygen on clean polycrystalline tantalum is 0.86, decreasing rapidly during the formation of a chemisorbed layer. The specularly reflected fraction of the incident beam is also modified by the chemisorption of carbon monoxide and this modification would confirm the dissociative character of the CO chemisorption on tantalum. The influence of the partial pressure of CO on the temperature at which the surface is completely covered by the products of the dissociative chemisorption of CO shows that the coverage becomes equal to unity at temperatures at which the solubility limit of CO is attained and tantalum carbide is formed.     

Influence de l'absorption d'atomes de plomb sur la résistance électrique de films d'or

Precise electrical resistance and thickness measurements are associated to study lead adsorption on gold films in ultra-high vacuum. Fuchs' theory is used to interpret the large size effects observed when the lead deposit is less than one monolayer. At low coverages the ratio of electrical resistivity variation to lead concentration is independent of temperature and equal to 1.8 ± 0.1 microOhms cm/at%. With temperatures ranging from ? 150 to + 150°C, the evolution of size effects is related to structural changes in the lead overlayer. A saturation in the diffuse scattering of conduction electrons by adsorbed adatoms is shown to occur when the lead deposit is about half a monolayer; this is used for comparing surface roughness of annealed and unannealed gold films. For lead coverage greater than a monolayer, an alloy identified by X-ray diffraction as AuPb₂ is formed. The electrical resistance variation is consistent with the thinning of the gold film by removing atoms which diffuse towards the free surface through the alloy layer.     

Critical behavior of the de gennes elastic constants near the nematic-smectic- A transition of TBBA

We have studied the behavior of elastic constants A, B, and C deduced from ultrasound velocity anisotropies in the vicinity of the nematic-smectic- A transition of terephthal-bis- p- p(')-butylaniline. A is associated with compressibility, B with layer compression, and C with the coupling between compressibility and layer compression. We show that the exponent of A is of the preasymptotic 3D- XY type, whereas those of B and C are in between the 3D- XY values and those associated with the anisotropic fixed point. This behavior is consistent with the extended crossover regime predicted by Patton and Andereck [Phys. Rev. Lett. 69, 1556 (1992)].     

Oscillations collectives dans les couches minces granulaires de rubidium

The optical study, under oblique incidence, of the rubidium taken in thin film, reveals two absorption bands the characteristics of which depend on the thickness of the layer, on the incidence angle and on the incident beam polarization. These experimental results can be interpreted simply and with a good enough precision by the surface plasma oscillations hypothesis.     

Effet de l'oxydation en surface et en volume sur les spectres de pertes caractéristiques d'énergie d'électrons dans les couches minces de titane

Transmission and reflection electron energy loss spectra, together with electronic diffraction patterns and Auger spectra have been registered on evaporated titanium thin films. The optical, crystallographic and chemical properties for the bulk and the surface of the films are compared. The 25 eV energy loss observed in the spectra of the low-energy electrons seems to give evidence for the formation of a superficial oxide layer. This assumption is confirmed by the transmission energy loss spectrum of pure rutile which is also presented.     

Sur la fiabilité du pic 1s du carbone de contamination comme étalon interne d'énergie dans les spectres de photoélectrons

The hydrocarbon contamination of an AEI ES 100 spectrometer has been studied in order to determine the reliability of the C 1s line as a standard in X-ray photoelectron spectra of non conducting solids. With alumina samples, it is shown that the contamination layer of this non ultra-high vacuum spectrometer is a suitable standard of energy for the correction of the charging effect and that it gives relative measurements with an accuracy of ± 0.2 eV when the sample temperature is varied from ?180°C to 200dgC or the sample holder is biased between ?10 and 10 V. About 75 spectra were recorded with powdered alumina compressed into a copper grid and the standard deviations of the corrected kinetic energies of the Al 2p and O 1s photoelectrons are respectively 0.09 and 0.06 eV.No effects of the X-ray tube current and the irradiation time are detected. The charging effect increases with the sample temperature (+0.6 eV from ?180 to 200°C).     

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.     

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).     

Fermi surface measurements of AuTe2I by the shubnikov—de haas effect

Fermi surface measurements of the layer-compound AuTe₂I have been performed by the Shubnikov—de Haas effect. One of the Fermi surfaces, which consists of a cylinder parallel to the b-axis or an ellipsoid with anisotropy of at least 10:1, reflects the layer structure of the material. We observe in addition two other Fermi surfaces of lower anisotropy     

Etude experimentale de la structure fine du spectre Raman de rotation de l'oxygene

It has been demonstrated that the output from a discharge pumped KrF laser (249 nm) is capable of ionizing a variety of molecules. The nature and yield of ions generated in this process, which have been identified by time of flight mass spectrometry, exhibit a striking intensity dependence.     

Disordering to order: de Vries behavior from a Landau theory for smectic phases

We show that Landau theory for the isotropic (I), nematic (N), smectic-A, and smectic-C phases generically, but not ubiquitously, implies "de Vries" behavior: i.e., a continuous A-C transition can occur with little layer contraction while the birefringence increases significantly once the system moves into the C phase. Our theory shows that 1st order A-C transitions are also possible. These transitions can be de Vries-like, but in general need not be. Generally, de Vries behavior occurs in models with unusually small orientational order and is preceded by a first order I-A transition. These results correspond well with experimental work to date.     

Etude par resonance paramagnetique electronique de sulfures de molybdene et de tungstene

Chalcogenides MeS_3−x (Me = Mo, W; 0 ⩽ x ⩽ 1) have been prepared by the thermal decomposition of tetrathiometallates. These compounds are largely amorphous; desulfurization as well as crystalline organization occur when the heat treatment temperature (TT) increases. At 1000°C a perfect hexagonal arrangement can be obtained (x = 1). ESR spectra of amorphous species having x in the range 0–1 have been recorded. A part of the ESR signals is ascribed to sulfur chains whilst the other part is attributed to Me (V) species. The intensity of both series of ESR lines is a decreasing function of TT. The spectra are characteristic of Me (V) in an axial symmetry environment. At TT ≅ 1000°C a narrow ESR signal remains, which is assigned to electron spin centers of impurities (g-value close to that of the free electron). The results are explained either in terms of continuous structural evolution of the solid during heat treatment or in terms of a biphasic mixture of amorphous MoS₃ and MoS₂. The variation of the spin concentration as a function of TT is reported and compared with the results previously obtained from a study of the transport properties of these solids. In all cases the values obtained for Me = W are smaller than those observed on Me = Mo.     

Comportement asymptotique de la fonction de distribution radiale dans les liquides

We study the influence of pair potential on the asymptotic behaviour of the radial distribution function in liquids. We use a perturbation expansion from a model of rigid spherical core inKirkwood's integral equation. It is pointed out that in metals as well as in insulators the asymptotic behavior is connected to that of the pair potential. However, the results are numerically different from those obtained from thePercus-Yevick approximation.     

Controle de l'enrichissement de l'uranium metallique a l'aide du nitrate de cellulose

Abstract

Le nitrate de cellulose permet d'enregistrer le passage de particules α done de mesurer l'activité spécifique d'une source radioactive épaisse. Nous utilisons cette propriété pour mesurer le taux d'eorichissement en uranium 235 d'un éhantillon d'uranium métallique. Afin de faciliter la mesure et de s'affranchir de la lecture du nombre de traces au microscope optique, les échantillons sont révélés dans un bain spécial et le nombre de traces α par unité de surface est évalué en fonction du pouvoir réflecteur de l'échantillon.

Les moyens et le temps nécessaires pour une mesure sont, ainsi que le prix de revient, très réduits.

Cellulose nitrate can be used for the registration of α-tracks and therefore for the measurement of the specific activity of thick radioactive sources. This characteristic is applied to the determination of ²³⁵U enrichment rate of metallic uranium samples. For the measurement of track density a method is developed which replaces the counting procedure under a microscope. The samples are etched in special solutions and track density is evaluated from the diffuse reflecting power of the samples.

The main advantages of this method are its rapidity, easiness and low cost.     

毫秒脉冲激光辐照硅基PIN的温度场应力场数值分析

为了研究毫秒脉冲激光辐照硅基PIN多层结构产生的温度场和应力场的特点,本文基于热传导理论和弹塑性力学理论,利用等效比热容法处理相变潜热,考虑多个热源,尤其是底层铝电极反射的影响,并考虑硅基PIN探测器每层材料参数的非线性影响,采用有限元模拟软件COMSOL Multiphysics,对毫秒脉冲激光辐照硅基PIN多层结构的过程进行了二维数值模拟,得到了材料表层及内部各层的瞬态温度场与应力场的时空分布和变化规律.结果表明,底层铝电极对激光的反射,使得在底层铝电极处及附近硅层的温度都略有升高.在此基础上,分析了毫秒脉冲激光辐照硅基PIN的硬破坏机理,即熔融前力学损伤导致硅基PIN探测器的功能失常.