Influence des ondes de “galerie a echo” sur la diffusion d'une onde ultrasonore plane par un cylindre

An experimental method which allows measurement of the frequency (ka) of circumferential wave resonances which propagate in aluminium cylinders is described. It is also possible to deduce the number of wavelenghts on the circumference or the vibration mode of circumferential waves. This study allows a better understanding of the complex structures of the form function in the far field. It specifies the theoretical results which show the effect of each circumferential wave in the angular diagram.     

Determination de la temperature d'une flamme de diffusion methane-fluor au moyen du spectre de vibration-rotation de la molecule HF

Taking into account self-absorption and a thermal gradient, the thermal gradient alone, and none of these effects, in rotational-temperature determinations involving the 1–0 and 3–0 bands of the vibration-rotation system of HF and the 0–1 band of the ²δ⁺-²δ⁺ system of CN, we observed good agreement between three measured rotational temperatures.     

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.     

Étude d'une surface de NaCl sous bombardement électronique de basse énergie: Influence de l'irradiation sur la charge de la surface

The preliminary results of angular low energy electron spectroscopy studies on cleaved NaCl surfaces are reported. The first problem to solve was to prevent the sample from charging, that is, to prevent the secondary emission coefficient from getting lower than unit. The case δ<1 is shown to be due to the dissociation of NaCl under bombardment. The dissociation was stopped when cooling the sample to about 125 °K. Metallic Na particles are formed on the surface under irradiation. The energy loss spectrum shows absorption bands at 1.5, 2.7, and 3.8eV. These bands are characteristic of the dissociation rate, dependent on sample temperature and on time of irradiation.     

Un nouveau theoreme d'echantillonnage pour le degre de coherence complexe en vue de la reconstitution d'une source lumineuse isotrope

We present some results obtained by using the standard technique of light scattering on so-called “microemulsions”. From the data of single-photon counting of light scattered from 0° to 90° the particle scattering factor P(θ) is obtained. Fitting with the theoretical curves gained for various kinds of particles enables us to deduce in a rather direct and simple way the shape and the size of particles.     

Influence des brandons sur la propagation d'un feu de forêt

A two-dimensional weighed-site small-world network is proposed to study the action of firebrands (lofted flaming or glowing debris) on fire spread through homogeneous or heterogeneous systems. The firebrand emission distance obeys an exponentially-decreasing distribution law. For homogeneous systems, the effect of firebrands is strengthened when the fire impact length decreases and the characteristic firebrand emission distance increases. As a result, jumps in the rate of spread appear and time oscillations in the burning area can occur. For heterogeneous systems, this effect becomes weaker as the degree of disorder and the distance of firebrand emission increase. The influence of characteristic lengths of radiation, firebrand emission, and medium heterogeneity on fire spread is discussed. To cite this article: B. Porterie et al., C. R. Physique 6 (2005).     

Etude de la pression de radiation de l'onde evanescente par absorption de lumiere resonnante

In this paper we show how the technique of measuring the deflection of atomic beams by absorption of resonant light could be applied to study the density momentum of the evanescent wave.     

Absorption de lumiere par des atomes dans une onde evanescente

We have measured the absorption of the sodium D₂ line when the atoms are in the evanescent part of an optical wave undergoing total reflection. The discrepancy observed between theoretical and experimental results may be due to a surface effect.     

Influence des parametres experimentaux sur la restitution du sinus de la phase en interferometrie

Combination of phase-modulation interferometry and scanning of the object is capable of restoring the sine of the phase everywhere in the object field. So, information on the direction of the surface relief is obtained. The influences of the analysing surface characteristics and of the process of scanning are studied.     

Etude par RMN et rayons X de sesquicarbure d'uranium

We have measured the position and linewidth of the ¹³C NMR signal in U₂ C₃ between 4 and 300 K. This study confirms the existence of an electronic transition around 50 K which brings possibly a non magnetic fundamental state. This transition involves no crystallographic distorsion and no lattice parameter anomaly.     

Action d'oxydations et de réductions électrochimiques successives sur la structure superficielle d'électrodes d'or polycristallines: Étude optique et életrochimique

Flat, mechanically polished gold electrodes have been subjected to several hundred cycles of electrochemical oxidation and reduction (0.4 to 1.9 V NHE) in normal sulfuric acid. The evolution of these electrodes has been followed by electrochemical measurements (capacity and potentiokinetic curves) and optical (ellipsometry) measurements, and the surfaces have been examined by metallographic microscopy. After a number of cycles the electrochemical treatment transforms the originally disturbed surface into a structured surface, with appearance of grain boundaries and faceting analogous to thermal faceting. Considering the work of Vetter and Schultze, we interpret the restructuring of the electrode surface as being due to two facts: mobility of metal atoms, because of their delocalization during the oxygen desorption phase and the anisotropy of the surface energy, owing to the presence of adsorbed oxygen. The complex evolution of the optical properties of the metal cannot be accounted for simply by facetting, and thus we have been led to postulate an accumulation in the metal of traces of undesorbed oxygen.     

Perspectives sur la théorie des pseudogroupes de transformations analytiques de type infini

This paper deals with the local and abstract structure of Lie pseudogroups of infinite type. The formal aspect is considered in several cannonical ways, one of which is compatible with the known pathological behaviour of the exponential function. This defines a very precise program for the development of a convergent abstract theory of analytic Lie pseudogroups of infinite type.     

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     

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.     

Etude des effets spectraux et spatiaux liés à la propagation d'un faisceau laser resonnant dans une vapeur de baryum

We present some characteristic features of the interaction between a resonant laser pulse and a dense atomic medium (barium vapor). A spectral analysis of the coherent transmitted light displays side bands related to the Rabi frequency. A study of the spatial behaviour of the laser beam shows that light filaments are generated and we specify in what experimental situation (light intensity, atomic concentration) this phenomenon takes place. Starting from Maxwell-Bloch equations for a resonantly driven two-level atomic system we built a computation code whose numerical results are in good agreement with our experiments.     

Mesure de la duree de vie de niveaux d'energie d'ions par transfert de coherence dans une collision de penning: Application aux niveaux 4f 2F72 et 2F52 de Mg+

Metastable 2³S₁ He atoms are optically oriented and their orientation is transferred to metallic ions produced by Penning collisions $\text{[He (2}{}^3\text{S}{}_1\text{)}\text{+ M →}\text{He (}{}^1\text{S}{}_0\text{)}\text{+ M}{}^{\mathrm{+}}\text{+}\text{e}$. If r.f. transitions are induced in the 2³S₁ He level, the circularly polarized light emitted by the ions in a direction perpendicular to the static magnetic field is modulated at the r.f. frequency. The variation of the modulation amplitude with frequency depends on the lifetime of the level of the excited ion.We describe an experimental apparatus with which we measure the modulation amplitude from 1 MHz to 110 MHz. In this way the lifetime of the 4f²F_{$\text{7}\text{2}$} and ²F_{$\text{5}\text{2}$} Mg⁺ levels has been determined.     

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.     

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.