Indices de refraction et birefringence spontanee de l'orthophosphate de plomb ferroelastique

Refraction indices at room temperature and the temperature dependence of the spontaneous birefringence have been measured in ferroelastic lead orthophosphate. The 180°C phase transition in this material has been determined as a thermodynamic first order transition.     

Etude de la structure du tellure liquide par diffraction de neutrons au voisinage de la temperature de fusion

The structure factor of liquid tellurium has been measured by neutron diffraction at the melting temperature and in the supercooled state. These results are a confirmation of the existence of an A₇ type stable configuration in a large range of temperature between 400 and 1100°C.     

Étude des réactions de protons de 400 MeV sur des noyaux de masse moyenne par une experience de coïncidences proton-gamma

A p-γ coincidence experiment has been performed with 400 MeV protons on medium nuclei. The angular and momentum analysis of the scattered protons demonstrates a nucleon primary quasi-free process. No indication of knockout reactions on clusters has been seen. The residual nuclei which have been identified from γ-ray spectra are mainly produced by evaporation processes. Intranuclear cascades followed by evaporation are sufficient to describe the multinucleon removal but residual nuclei near the target nucleus are sensitive to the nuclear structure.     

Comportement thermoelastique de monocristaux de Cu2O

The velocity of elastic waves in monocrystals of Cu₂O has been measured with good precision at 24° C as a contribution to turther development of lattice dynamics of this compound. There is no significant deviation of the velocity in connexion with the non-stoechiometric properties of various samples. Elastic waves behaviour has been investigated up to 700° C and will be connected to plastic properties.     

Spectroscopie de photoelectrons d''aldehydes et de cetones aliphatiques

The ultraviolet photoelectron spectra of 11 aldehydes and 19 aliphatic ketones have been recorded using He(I) radiation, and the two first ionization potentials have been measured. The effects of 12 saturated alkyl substituents at the group C=O were studied using two models: the Taft polar constants (σ*) and the INDO energies (E_n) of molecular orbitals. The first IP, corresponding to the removal of an electron of the oxygen lone-pair, depends primarily on the inductive effect of the substituent(s). The second IP corresponds to either a σ orbital located on the main alkyl substituent, if it contains more than two carbon atoms, or a π orbital, if this is not the case. We have separated the compounds into two sub-populations in order to obtain reasonable correlations between IP and σ* or IP and E_n. The first sub-group of compounds have substituents with no carbon atom beyond the β position, whilst the second sub-group have substituents with at least one carbon atom in the γ position.     

Elargissement et deplacement par effet de pression des raies d'absorption de quelques multiplets de l'atome neutre de manganese

The profiles of many absorption lines, belonging to the violet and near ultra-violet resonance multiplets and to two intercombination multiplets of the MnI atom, have been determined in the presence of foreign gases at various pressures for several temperatures.Values of the Lennard-Jones interatomic potential constants have been derived from measurements of the line collision broadening and displacement. These values are tabulated. The temperature dependence of the broadening and displacement parameters, for some perturbers of the violet resonance lines has been examined. The discrepancies between theoretical and experimental values of the van der Waals constants, included in the formula of the Lennard-Jones potential, have been discussed. It is shown that, in the case of complex atoms, terms of multipole-dipole interaction must be included in the interatomic potential expressions.     

Chaleur specifique du mononitrure de thorium de 7 a 300K

The specific heat of ThN has been measured between 7 and 300K by an adiabatic method with continuous heating, and has been compared to that of UN. The magnetic specific heat of UN between 50 and 300K is certainly lower than that of a U⁴⁺ ions gas submitted to an octahedral crystal field of about 500 cm⁻¹.     

Spectres de photoelectrons des alcynes

The ultraviolet photoelectron spectra (UPS) of 11 monosubstituted and 25 disubstituted aliphatic alkynes have been recorded using He(I) radiation. The inductive effects of 15 saturated alkyl substituents on the first ionisation potential (emission of a photoelectron from a π orbital of the triple bond) were studied, and correlated with Taft's polar constants (σ*). An attribution of the second ionisation potential has been proposed, which accords with previous experimental results on both alkenes and carbonyl compounds. Some vibrational frequencies of molecular ions have been determined for the least-branched 15 alkynes.     

Prediction lineaire de parametres optiques de l'ecriture

Laser emission in the wavelength region 14–18 μm has been observed in optically-pumped ¹⁵NH₃ using the P₂6 line of a pulsed HF laser. Pressure and temperature dependence, as well as various optical configurations for the cell, were investigated. Each of the stronger laser lines was observed to have a pressure-dependent structure and several of the lines are apparently related through a cascade mechanism.     

Obtention des bandes de cameron par photo excitation de co en matrices d'argon de krypton et de xenon

CO trapped in solid matrices of argon, krypton and xenon has been excited by 10 eV photons. Cameron bands v′=0, v″=0,1,2,3,4 are observed during the irradiation. If red shifts are observed in argon and krypton matrices, the transition in xenon shows no differences with gas phase data.     

Effet Hanle des niveaux de triplet de la configuration (4s, 4d) de l'atome de zinc

The Hanle effect has been used to measure the lifetimes of the three triplet levels of the (4s, 4d) configuration of zinc as well as the cross sections for destruction of alignment by collisions against rare gas atoms or nitrogen molecules.     

Etude de la structure magnetique et de la transition de typr “spin-flop” de Rb2FeF5

The nature of the magnetic interactions in the chain compound Rb₂FeF₅ has been investigated using neutron diffraction and magnetic measurements under high applied fields. Rb₂FeF₅ orders antiferromagnetically at T_N = 8.0 ± 0.5 K; the magnetic structure is of the A_Z + G_X mode and the moment of the Fe³⁺ ion extrapoled to 0K is 3.5 ± 0.2 μ_B, this low value being due to zero-point spin reduction. Within a chain the Fe³⁺ ions are antiferromagnetically coupled with an exchange constant of J/k = ?8.8 K. A spin-flop behavior has been observed and interpreted on the basis of the molecular field theory. The critical field was found to be H_C = 65 kOe at 1.7 K.     

Coadsorption de l'oxygène et du monoxyde de carbone sur des surfaces de rhénium

Temperature programmed desorption ($\text{2.65}\text{K}\text{sec}$) has been used to study carbon monoxide and mixed layers of carbon monoxide and oxygen on rhenium ribbons, strongly oriented parallel to the (0001) plane. Four binding states, populated in decreasing energy have been detected. Interpretation of the results on β states agrees qualitatively with King's model postulating dissociation of carbon monoxide molecules and a repulsive interaction energy between carbon and oxygen atoms. However, in the coadsorbed layers studies, it is shown that all the oxygen atoms do not play a part in the recombination process, during desorption, and that when oxygen is adsorbed after carbon monoxide, a displacement reaction occurs, due to apparent transfer from β states towards molecular α states. Optimization of the results on pure carbon monoxide layers leads to an interactional energy ω, equal to 3 $\text{kcal}\text{mole}$, and is only possible if is assumed that β states are formed on alternatively filled and empty rows.     

Structure magnetique du sulfure de fer FeS de type blende

Zinc blende type FeS, which is cubic at room temperature and becomes orthorhombic, pseudo-tetragonal ($\text{c}\text{a}\text{< 1}$) at the magnetic transition, has been studied by neutron diffraction. The magnetic space group is F_c 2 2' 2' : ferromagnetic (001) planes are coupled antiferromagnetically, and the moments lie along the a axis, in agreement with previous Mössbauer measurements. The exchange integral between nearest neighbours in the ab plane is thus weaker than those between nearest neighbours in the ac and bc planes.     

Utilisation de l'heterodynage optique pour des mesures locales de la vitesse de phase d'ondes elastiques

A new optical method for measuring the acoustical wavelengths with a great accuracy has been investigated. An acousto-optical interaction is performed and the acoustical frequency range is such that the diffraction take place in “Bragg regime”. The phase modulation of the scattered laser beam is visualized by means of an optical heterodyning. When the location of the acousto-optical interaction is linearly displaced, the detected beating signal supplies a local measurement (10 μ) of the acoustical wavelength.     

Effets de taille quantiques dans le pouvoir thermoelectrique de couches minces de Bi et de BixSb1−x

The existence of quantum size effects has been observed by thermoelectric power measurements in Bi and Bi_xSb_1-x thin films. The observed discontinuities at 90 K in the curves α vs film-thickness can be understood when the quantization of energy levels in the conduction band is taken into account.     

Etude par RMN de la relaxation spin réseau d'un syst`eme AB de noyaux en interconversion lente

Nuclear Overhauser effects allow us to determine all transition probabilities in a two-spin AB system, using a triple resonance apparatus we have constructed. Theoretical and experimental studies of the relaxation in the special case where the nuclei belong to a molecule interconverting slowly between two different conformations have been made, and the exchange ratio has been deduced.     

Etude par diffraction neutronique de la forme haute pression de FeOOH

A powder sample of iron oxyhydroxide (FeOOH_HP) synthesized under high pressure-high temperature conditions, has been studied by neutron diffraction. The magnetic structure has been determined. The magnetic and chemical unit cells are the same and the antiferromagnetic arrangement is collinear with the spins parallel to the c-axis. The crystallographic structure affinement by the Rietveld method has confirmed that FeOOH_HP is isostructural with InOOH (space group P2₁nm) and has established the positional atomic parameters. The magnetic moment of iron (4.7μ_B at 4.2 K) and the O-H distance in the hydrogen bond (1.11 Å) are discussed with regard to the values obtained for the other forms of iron oxyhydroxide.     

Influence des impuretes et de l'inhomogeneite d'un plasma sur la mesure de sa temperature electronique

The effect of impurities on soft X-ray continuum radiation emitted from a high-temperature deuterium plasma has been studied. The results of these calculations are useful in the determination of the electronic temperature of a laser-created plasma. Such a plasma is inhomogenous and contains impurities.     

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.