Parametres thermodynamiques des defauts ponctuels dans le chlorure de strontium pur ou dope par des cations monovalents ou trivalents

The electrical conductivity of a great number of SrCl₂ crystals, either purified or doped with Na⁺, K⁺, Rb⁺, Gd³⁺ and Ce³⁺, has been studied. The microscopic models used for the interpretation of the results have been justified by dielectric loss measurements, ITC and EPR (for Gd³⁺). The thermodynamic parameters of point defects have been derived, over the temperature range 420–860 K, from the fitting of experimental data to the equations of the Lidiard-Debye-Hückel theory. The values of the enthalpies of Cl vacancy mobility (0.30 eV) and Cl interstitial mobility (0.76eV), association of Cl vacancy with Na⁺ (0.53 eV), K⁺ (0.42 eV), Rb⁺ (0.40 eV) and ionic Frenkel defects formation (2.20 ± 0.11 eV) have been used in the interpretation of the experimental results over a large concentration range. The solubility enthalpy of Na ⁺ is 1.2 ± 0.1 eV; the solubility of the rare earths is very small at low temperature. The thennodynamic parameters which have been found have been compared with other known experimental values and with theoretical results.     

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.     

Etude de la stabilite des defauts ponctuels dans PbF2-β par simulation

A computer simulation technique is used to investigate the defect properties of PbF₂-β. Rigid ion calculations of formation and binding energies of defect pairs are carried out. New shell-model and short-range potentials parameters are fitted to the elastic and dielectric properties of the material; they reproduce its phonon dispersion relations, but the use of the derived parameters to simulate the anion vacancy fails. A destabilisation of the lattice in the vicinity of the defect occurs due to an excessive separation between core and shell of the lead ion. Two methods are successively proposed to solve the problem: (a) the use of effective charges on the ions, taking into account the covalency: (b) the study of a quasi-harmonic shape of the potential for the core-shell interaction in lead.     

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.     

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.     

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.     

Etude de la reorientation des complexes lacune-impurete dans KCl dope par Mn2+

Depolarization thermocurrent and EPR measurements have been performed on Mn²⁺ doped KCl crystals. From these, we derived a model for the impurity-vacancy complexes reorientation.From EPR measurements we derived the binding energies of the first three associated states and we concluded that the second associated state was the deepest one; such a conclusion has not been previously drawn. The first two binding energy values are confirmed by conductivity measurements.Thermocurrent curves show two peaks: one of them being ascribed to impurity-vacancy complexes (next nearest neighbour—nearest neighbour vacancy jump), the other to aggregates: differents concentrations are used.     

Etude des interactions quadrupolaires dans quelques tungstates par resonance Mössbauer de 182W

The tungstates having a double-perovskite, trirutile, scheelite or a wolframite type structure have been studied by means of Mössbauer ¹⁸²W spectroscopy. It is shown that the electric field gradient at the W nucleus is essentially due to a lattice term in the scheelite structure while the valence term seems to be preponderant in the case of the wolframite structure.     

Étude de l'interdiffusion entre l'argent et le palladium dans des couches minces par spectrométrie auger

We investigate the interdiffusion in epitaxial thin films of Ag(111)-Pd(111) heated at 500°C during 15 min. The interdiffusion coefficient D is determined by two methods: (1) the concentration profile is established using the Auger technique associated with sputtering; (2) we follow the surface concentration during the diffusion process. This last technique avoids problems associated with ion sputtering. The two techniques give interdiffusion coefficients D in good agreement with a mean value of 7 × 10^?17 cm² s^?1.     

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.     

Effet Poole-Frenkel dans le monoxyde de silicium dope

The influence of electrical field on the electrical properties of pure or doped silicon monoxyde films has been examined. After analysis and confrontation of the experimental results obtained by direct and alternative currents and by thermally stimulated currents, it has been shown that the same Poole-Frenkel effect appears in these different circumstances. The explanation is based on the heterogeneities in amorphous solids.     

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.     

Photodégradation des colorants fluorescents dans le PMMA appliquée à l'étude de déformationa par moiré

Photobleaching of rhodamine 6G in PMMA illuminated by two interfering laser beams gives rise to a fluorescence grating which can be used for strain analysis by moiré technique in the illuminated slice of the model. According to the fluorescence decay curves, the maximum visibility of the moiré fringes is 0.24. Application to a disk under compression is reported.     

Mesures de durees de vie dans le cas de faibles intensites lumineuses

A simple method concerning the possibility of lifetime's measurement in the case of low light level, 20 photons/s, is described. Application to some n¹S₀ and ¹D₂ excited states of the Hg atom is given.     

Etude par spectrometrie Raman des complexes de l'iode dans les alcools et les ethers

Laser Raman spectra of iodine-alcohol and iodine-ether solutions have been studied. It was found a new line at 167 cm^?1 for alcohols and at 170 cm^?1 for ethers which is tentatively attributed to an intermediate Hassel complex.     

Conversion de frequences optiques dans le seleniure de gallium (GaSe)

Two experimental arrangements of an optical frequency up-converter using GaSe are described. Two wavelength regions have been covered by series of monochromatique rays (one ranging from 3.25 μ to 3.85 μ, the other one from 0.954 μ to 0.968 μ).     

Couplage Jahn-Teller dans le niveau excite des centres F dans CaO: Effet des contraintes uniaxiales

The zero phonon line associated with the emission band of the F centers in CaO is studied under stress. The absence of any splitting under [111] stress shows that the excited level is mainly coupled to E_g modes of vibration. The effect of the temperature on the intensities of the two components observed under [001] stress shows that the tunneling time is short with respect to the lifetime.     

Facteurs de lande des niveaux des configurations 5s5p et 5s5d de cd I

A plane Fabry-Pérot is used to measure, with a precision of about 10^-4, the Landé factor of the levels issued from the 5s5p and 5s5d configurations of cadmium I. Calculations taking into account intermediate coupling effects and the anomalous gyromagnetic ratio of the free electron are insufficient to explain some experimental data; in such cases it is necessary to perform a more detailed analysis of the corrections due to the relativistic effects.     

Étude par rmn de la localisation et des mouvements des protons dans les hydrures MgH2 et Mg2NiH4

Wide line and pulsed NMR studies are reported for MgH₂; and Mg₂NiH₄ hydrides at 79 and 30 MHz in the temperature range between 100 and 500 K. Line shape data confirm a rutile type structure for MgH₂, and lead to the evaluation that 97% of the hydrogen is present in this phase, the remainder being in solid solution MgH_x, (x<0.04).Mg₂NiH₄ shows a Gaussian line, whose peak-to-peak width decreases from 6.16 to 4.28 G in the range 320–365 K. From 365 to 480 K the spectrum shows a second, narrower, line (0.85 G), implying that approximately 7% of the protons have migrated from their initial position to energetically less stable sites. The thermal behaviour of the T₁ and T₂ relaxation times shows a dramatic variation in the 320–370 K temperature range in connection with the change of the proton localization. Relaxation mechanisms can be attributed mainly to conduction electron-nucleus interactions. With rising temperature, diffusion mechanisms are also involved. A diffusion activation energy of about 0.35 eV has been determined, with a diffusion coefficient 3.45 × 10^?8 < D < 4.6 × 10^?8 cm²/s.