Distributions energetiques experimentales des electrons secondaires emis par des cibles evaporees d'aluminium,de cuivre,d'or et d'argent

Secondary electron energy spectra of Al, Cu, Au and Ag evaporated layers are obtained with a two-grid retarding field spectrometer for primary beam energy between 150 eV and 2000 eV. The accuracy of the full width at half maximum and the most probable energy, and their variation with primary beam energy, are given. Comparison with recent results obtained from polycrystalline or simple-crystal massive targets suggests lower work functions for evaporated layers.     

Autodiffusion en volume dans la phase plastique de l'acide pivalique (acide 2.2 dimethyl propanoique)—I: Mesure des coefficients de diffusion et de l'effet isotopique

Self-diffusion studies have been performed in the orientationally disordered or the so-called plastic phase of pivalic acid. Single crystals of high purity (99.9999%) containing 10⁹?10¹⁰ dislocations m^?2 have been used. Thin layers of pivalic acid labelled with ¹⁴C or tritium were deposited on sample surfaces. Concentrationpenetration curves were established by serial sectioning. Lattice self-diffusion coefficients D, were measured from 281 to 304.75K. At 281K, the value of D is independent of time. From 281 to 301K, D is given by: D(m²S^?1) = (4.9 ± 0.3)10^?4 exp [? (59± 1) kJ mole^?1/RT].The activation enthalpy of the lattice self-diffusion is roughly equal to the heat of sublimation (L_s = 57 kJ mole^?i) and in good agreement with values obtained from NMR. The mass factorf ΔK, where f is the correlation factor and ΔK is a correction factor, has been measured using isotope effect studies. Between 281 and 301K the value obtained is fΔK* 0.1^+0.2_?0.1     

Diffusion en surface de l'argent sur les plans (001), (111), (110) et des surfaces vicinales du cuivre

The diffusion coefficients of silver on singular and vicinal surfaces of copper have been measured using an oxidation method to determine the concentration profiles. The diffusion was carried out under ultra high vacuum at low temperature (250 to 500 °C) with a low concentration of the diffusing species (less than 0.5 monolayers). Comparison of the results with the terrace-ledge-kink model shows that the diffusion occurs along the ledges and also explains the diffusion on the vicinal surfaces. The differences between this model and the diffusion observed on the singular surfaces are explained by the presence of separated ledge loops on these surfaces.     

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.     

Proprietes electroniques et electrogalvaniques dutellurure d'argent β domaine d'existence

A study of the electronic and electrogalvanic properties of β silver telluride has been performed on samples whose non-stoichiometry has been determined by coulometric titration with the cell Ag/RbAg₄I₅/Ag₂Te/Pt. The data lead to the adoption of a model of Frenkel defects on the silver sublattice that are fully ionised. Their formation enthalpy is H_F′ = 0,70 eV. On the tellurium side this model is completed by the association V_AgV_Ag^x. Th homogeneity range is reported from ?10°C to 132°C: it extends essentially on the tellurium side of the stoichiometric composition.     

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.     

Adsorption de l'éthyléne et de l'acétylène sur des films de rhénium evaporés sous ultra-vide; Hydrogénation de l'éthylène: II. Discussion des résultats expérimentaux et interprétation

A qualitative model is proposed in order to explain our experimental results on ethylene chemisorption on evaporated rhenium films and hydrogenation of ethylene (part I). The surface must present at least two kinds of surface sites (A and B). The second type (B), either preexists on the surface, or is induced by the adsorption phenomenon itself. On the most energetic ones (A), dissociation of ethylene and hydrogen is complete. Adsorption of ethylene is characterized by a sticking coefficient value of 0.1 if they are free and 1 if they are hydrogen covered. On sites B, ethylene is adsorbed without full dissociation (sticking coefficients equal to 0.015). independent on adsorption temperature. Hydrogen desorption is due to full dissociation of ethylene on the surface and a displacement reaction while ethane is produced by reaction between non-dissociated adsorbed ethylene and hydrogen in the gas phase. The same Rideal-Eley mechanism applies for hydrogenation of ethylene in quasi-stationary conditions, along with a self-poisoning mechanism involving dehydrogenation leading to C₂H₂ non-hydrogenable adsorbed species.     

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.     

Adsorption de l'oxygène sur le graphite: Nouvelle détermination graphique de l'entropie différentielle d'adsorption

Physical adsorption of oxygen on graphite has been investigated at various temperatures from 59 to 71 K. A new graph for the direct determination of the differential change in entropy of adsorption give us phase changes in adsorbed layers and the two-dimensional critical temperature, about 66 K in this case.     

Proprietes electriques du chlorure d'argent monocristallin et charge d'espace

Very high dielectric permittivity and polarisation in AgCl crystals are observed in d.c. and very low frequencies (from 10^?4 Hz to 100 KHz) with different electrodes (silver painting and solid chrome). The potential distribution is also studied with vibrating wire method.Near 100°C and for frequencies below 1 Hz, non-linear phenomenon appear for a.c. voltages greater than 0,1 V. When a.d.c. voltage is superposed, we observe three kinds of behaviour: for low voltages and low frequencies, adsorption predominates; for some volts, space charge appears and beyond surface states produce high permittivity.Shape of potential distribution and dielectric permittivity can be explained by space charge polarisation, adsorption of Ag⁺ ions at the electrodes and filling of surface states.     

Thermode´sorption et photode´sorption du potassium adsorbe´sur du cuivre

La thermode´sorption et la photode´sorption onte´te´e´tudie´es pour des dépo?ts de potassium sur une face (100) du cuivre. Pour des dépo?ts voisins de 1 monocouche ou plus denses, la thermode´sorption est conformea`un mode`le de processus active´et la photode´sorption a un rendement quantique voisin de 10^?7 atome/photon. Pour des dépo?ts moins denses les phénome`nes sont plus complexes et il apparai?t des effets qui pourraiente?tre liésa`des changements de la couche de potassium adsorbe´.     

Solution de l'equation de Boltzmann pour des batonnets durs unidimensionnels

We present the exact solution of the linear Boltzmann equation for one-dimensional hard rods. In particular, we compute the velocity correlation function and the Van Hove self-correlation function.     

Fluorescence resolue dans le temps des niveax de singulet et d du zinc

The time resolved spectroscopy technique is applied to investigate the (4s, nd) ¹D₂ series of Zn I using a selective stepwise excitation via the resonant 4¹P₁ state. Radiative lifetimes of n¹S₀ (n = 6?8) and n¹D₂ (n = 4,5) levels are measured. Upper limits for transition probabilities 4¹P₁?n¹S₀ (n = 9?12) and 4¹P₁?n¹ (n = 6?10 ) are obtained. Strong configuration interaction effects are observed in the ¹D₂ series and compared with those in Mg I and Cd I.     

Inhibition par le cuivre de l'adsorption du monoxyde de carbone sur les alliages nickel-cuivre

The evolution of the relative concentration of CO bridged species with the NiCu alloys composition allows to estimate the copper efficiency to inhibit this kind of chemisorption. This efficiency decreases as the copper content increases. One copper atom inhibits almost nine nickel atoms for a copper concentration of 5%; this number decreases below two nickel atoms for a copper content greater than 40%.     

Proprietes electroniques et electrogalvaniques du sulfure d'argent α domaine d'existence

The study of electronic and electrogalvanic properties of α silver sulphide has been performed on samples whose non-stoichiometry is controlled by annealing under sulphur pressure or by coulometric titration with the cell Ag/AgI/Ag_2+εS/Pt.The data obtained lead to the adoption of an electronic model according to which the Frenkel defects are completly ionised. The homogeneity range has been drawn between 193° and 550°C. It extends to both sides of the stoichiometric composition.     

Proprietes electroniques et electrogalvaniques du tellurure d'argent α domaine d'existence

A study of the electronic and electrogalvanic properties of α silver telluride has been performed on samples whose non-stoiehiometry has been determined by coulometric titration with the cell Ag/AgI/Ag₂Te/Pt. The data lead to the adoption of a model of Frenkel defects on the silver sublattice that are fully ionised. The homogeneity range is reported from 162 to 508°C: it extends essentially on the tellurium side of the stoichiometric composition. The free energies of formation of the α and β forms of the tellurides whose compositions are near Ag_1,6Te, Ag_1,9Te and Ag_2,0Te have been determined between 23°C and 360°C.     

Transport du cuivre à travers un film mince d'or protecteur

Thin gold films were deposited by “sputtering” onto thick copper substrate. The crystalline structure and topography of the deposits were investigated by scanning and transmission electron microscopy. A scanning Auger microanalysis was performed on the surface and in depth on the gold deposit before and after heating. The observations reveal that the copper arrives at the gold surface especially via holes. The transport rate of the copper via theses holes is very fast compared to the diffusion via grain boundaries. The origin of the holes is discussed: they result of strains due to the gold epitaxial growth on copper grains.