Étude par esca du cobalt ou du molybdène déposé sur alumine: Analyse quantitative du recouvrement et mise en évidence d'états d'adsorption spécifique des cations

X-ray photoelectron spectra of the Co 2p_{$\text{1}\text{2}$}, and Mo 3d_{$\text{5}\text{2}$} levels for catalysts containing cobalt and molybdenum supported on alumina have been studied. Binding energies and relative peak areas have been investigated in detail in order to establish the formal metal oxidation states and environment of cobalt and molybdenum cations supported on alumina. Before a monolayer has been formed, only tetrahedric sites of alumina are occupied by Co²⁺ cations. After the monolayer has been formed, heterogeneous oxide Co₃O₄ appears. Both tetrahedral and octahedral sites of alumina are occupied by the molybdenum cation in the 6+ formal oxidation state.     

Emission and laser-excitation spectra of the ≈A2Π ⇐ ≈X2Π transition of rotationally cooled bromochloroacetylene cation in the gas phase

Rotationally cooled bromochloroacetylene cations have been studied in the ≈X²Π_ω and ≈A²Π_ω (ω = $\text{3}\text{2}$, $\text{1}\text{2}$) states in the gas phase by two complementary spectroscopic techniques. The ≈A²Π ? ≈X²π band system was excited in emission by electron impact of a seeded supersonic helium free jet and as fluorescence by laser excitation of cations formed by Penning ionisation in a cooled environment. Comparison of the two sets of data locates the spin—orbit component bands and the origins of the systems, and allows the main spectral features to be vibronically assigned. The vibrational frequencies of most of the fundamentals are thus obtained to within ±2 cm^?1 for this cation in both the states.     

Formation energies of point defects in rutile (TiO2)

The formation energies for the point defect species Ti⁴⁺ interstitials, Ti_i⁴⁺, and oxygen vacancies with an effective charge of + 2e, Vo²⁺, have been calculated using the polarisable point ion shell model. The formation energy of Ti_i⁴⁺ was found to be a 10.8 eV for occupation of the (0, $\text{1}\text{2}$, $\text{1}\text{2}$) site. The energy of Ti_i⁴⁺ at other sites is somewhat higher than this figure, and allowed the migration energy of Ti_i⁴⁺ parallel to the c axis to be estimated as 2.9 eV. The formation energy of Vo²⁺ was found to be 10.1eV. The variation of these formation energies with dielectric constant was also calculated. It was found that the energy of formation of Ti_i⁴⁺ varied considerably with dielectric constant, while the formation energy of Vo²⁺ was almost independent of dielectric constant. The relevance of these results to the presence of crystallographic shear planes in reduced rutile is considered.     

Evidence of mobile correlation between conduction layers in β-alumina

The ordering of the mobile ions in the conducting layers to form an $\text{a}\text{√3 ×}\text{a}\text{√3}$ superlattice has been investigated in K⁺, Ag⁺ and Rb⁺ β-aluminas by electron diffraction. In addition to the correlation of mobile ions within the conduction layer, the correlation between conduction layers has also been detected in Rb⁺ β-alumina with periods of 68.4 Å and 22.8 Å, or three and one multiple of the $\text{c}$ dimension. A possible mechanism for such long distance correlation through spinel blocks is discussed.     

Ellipsometry of tetragonal lead oxide single crystals

The refractive index of PbO tetragonal single crystals ($\text{n}\text{?}{}_0\text{= 2.73±0.05}$) and parameters of thin dielectric film on their real surfaces (n_f = 1.56±0.01, $\text{d = 10?85}\text{A}\text{?}$) have been measured by the method of ellipsometry at $\text{λ = 6328}\text{A}\text{?}$. For the first time the photo-induced changes of optical properties of PbO^(t) surface film have been observed.     

Cation self-diffusion in disordered VOx

The diffusion of ⁴⁸V in disordered VO_x, crystals has been measured by a serial-sectioning technique as a function of temperature (1100–1500°C) over the homogeneity range 0.78 < x < 1.28. The temperature dependence of the cation tracer diffusivity at each fixed composition is characterized by an Arrhenius behavior in the temperature range 1100–1500°C. The Arrhenius parameters decrease rather sharply near the stoichiometric composition as the composition increases from the metal-rich to the metal-deficient regime; the activation energy for diffusion decreases from ~71 $\text{kcal}\text{mole}$ to ~48 $\text{kcal}\text{mole}$, and the frequency factor decreases by nearly two orders of magnitude (from ~5 $\text{cm}{}^2\text{s}$ to ~0.05 $\text{cm}{}^2\text{s)}$. It is concluded that the significant difference in the cation self-diffusion behavior between the metal-rich and the metal-deficient VO_x may be attributed to the significant differences in the defect structures of the two regimes. The various possible diffusion mechanisms are explored, and comparisons of the cation diffusion behavior in VO_x, with that of the related transition-metal monoxides TiO_x and Fe_1?δO are made. It is concluded that the experimental results for the entire composition range are consistent with the process of diffusion occurring by the migration of monovacancies in equilibrium with defect clusters, the nature of the clusters being different for x < 1 and x #62;; 1.     

Cell dimensions and antiferromagnetism of lunar and terrestrial ilmenite single crystals

X-Ray diffraction and anisotropic magnetic measurements have been made on single crystals of lunar ilmenite and on terrestrial ilmenite from Bancroft, Ontario, Canada and the Ilmen Mountains, U.S.S.R. The elongated $\text{c}$-axis of lunar ilmenite, previously reported, is confirmed by new measurements. The shorter $\text{c}$-axis found in terrestrial specimens is ascribed to Fe³⁺ substitution for Ti⁴⁺ in the titanium layer. Magnetic measurements on the same specimens show that, in agreement with the Ishikawa-Shirane et al. model, the initial shortening of the $\text{c}$-axis by the above substitution of small amounts of Fe³⁺ (<8%) causes an increase in Fe²⁺?Fe²⁺ exchange coupling through Fe³⁺ in the titanium layer that lowers the Néel transition temperature. The Weiss temperatures and other magnetic parameters confirm this model proposed by Ishikawa and Shirane et al.Additional transitions found in one of the terrestrial specimens (Bancroft) have been ascribed to a small amount of an exsolved spinel phase, possibly a solid solution phase of magnetite-ülvospinel. The spinel phase is localized in hematite-rich blebs which exsolved from the host ilmenite-rich phase.     

Surface segregation of PdAg alloys induced by ion bombardment

A Pd-rich phase has been observed to segregate on the surface of PdAg alloys under the influence of 500 eV Ar⁺ and N₂⁺ bombardment. By examining the X-ray photoemission spectra of these surfaces, we report changes in the structure of the Pd 3d peaks which are specifically linked to this phase. Studies of the spectral changes with Ar⁺ dose show that one $\text{Pd 3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ peak arises from electrons originating from the altered overlayer and a second $\text{Pd 3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ peak arises from the underlying alloy with a binding energy dependent on the underlying concentration. The altered layer is found to vary in thickness between one and five layers, depending on the bulk alloy concentration. The thickness of this layer is also dependent on the nature of the primary ion since a different altered layer thickness is found using nitrogen ion bombardment.     

Transport processes in NaCl single crystals doped with a trivalent cation; solubility,association and migration of Y3+

The self-diffusion coefficient of ²²Na⁺ and the ionic conductivity are measured as a function of temperature in NaCl single crystals doped with labelled yttrium ions, as well as the diffusion coefficient of ⁸⁸Y³⁺. We give the theoretical equations for the transport processes in the alkali halides doped with a trivalent cation; These are derived from the theory of the crystals doped with divalent cations. Solubility of YCl₃ in NaCl is accurately determined through a radioactive technique. The free enthalpy of solubility of free Y³⁺ is: (2.91?13 kT) eV. A good fit of the whole set of data is obtained by treating the association between Y³⁺ ions and vacancies by the single equation $\text{K}{}_2\text{= 12}\text{exp}\text{(-9.5)}\text{exp}\text{(}\text{1.17}\text{kT}\text{)}$. The jump frequency of Y³⁺ is $\text{w}{}_2\text{= 1.37 × 10}{}^{13}\text{exp}\text{(-}\text{0.98}\text{kT}\text{)}\text{sec}{}^{-1}$. The results are compared to those characterising the divalent foreign cations.     

Evidence for inelastic processes in the 24Mg(13C, 12C)25Mg reaction at elab = 30 MeV

The ²⁴Mg(¹³C, ¹²C)²⁵Mg reaction has been studied at 30 MeV using a magnetic spectrometer. Differential cross sections for transitions to several final states in ²⁵Mg have been measured and analysed using an exact finite range DWBA code. The DWBA predictions have fitted the bell-shaped distributions satisfactorily, yielding spectroscopic factors which are in reasonable agreement with those obtained using (d, p) reactions. The exceptions are the $\text{3}\text{2}$⁺ state at 0.97 MeV which displays a marked departure from the bell-shaped angular distribution obtained for the other $\text{3}\text{2}$⁺ state at 2.80 MeV, and the $\text{7}\text{2}$⁺ state at 1.61 MeV whose angular distribution has an unusual shape, displaying a deep minimum located at the grazing angle. A semiquantitative model has been used to suggest that the angular distribution for the 0.97 MeV state is evidence for the coupling of inelastic processes in this transition. In the case of the 1.61 MeV state it is suggested that the angular distribution shows the influence of indirect Coulomb excitation on the transfer cross sections.     

Calculation of motional activation energies for interstitial ions in the rutile structure using a minimum energy path model

Motional activation enthalpies have been calculated for interstitial cations in the rutile structure, using a minimum energy path approach. Coulombic, overlap repulsion and polarization energy terms were included in the potential energy expression. Preexponential factors in the overlap repulsion terms were evaluated by minimizing of the total electrostatic energy, and permanent dipole moments of the anion by an iterative method. The most favorable diffusion path was found to be parallel to the c-axis in the $\text{1}\text{2}$, 0, z direction in both TiO₂ and MgF₂.Motional activation enthalpies increased with increasing interstitial cation radius and were larger in TiO₂ than in MgF₂. The path was independent of radius and material. Strongly anisotropic conductivity was predicted for both compounds.     

Isomers and high-spin states in 205At

The properties of high-spin states in ²⁰⁵At have been investigated to $\text{J ?}\text{37}\text{2}$ and an excitation energy of > 4.0 MeV. The properties of the $\text{29}\text{2}$⁺ isomer at 2340 keV have been established and those of the $\text{25}\text{2}$⁺ isomer at 2063 keV have been further investigated. In the present study the mean lives of these isomers have been determined as 11.2±0.2μs and 98±2 ns respectively. The M2 branch of the $\text{13}\text{2}$⁺ state at 970 keV to the $\text{9}\text{2}$⁺ ground state has been measured. This decay is attributed to a proton single-particle $\text{i}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}\text{→}\text{h}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ transition. The trend of excitation energies of a number of proton excitations in the odd-mass nuclei from ²¹¹At to ²⁰¹At has been compared to the corepolarization model and the average $\text{ν}\text{f}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}{}^{\mathrm{?1}}\text{π}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ interaction energy has been deduced. Comments are made upon probable configurations for many of the levels.     

Effects of effective potential on g-factor in Si inversion layers

Taking into consideration the effective potential and the magnetic field dependent dielectric function, the effective g-factor and the magnetic susceptibility have been calculated. Results show that both quantities oscillate with magnetic field.The amplitude of the effective g-factor found in this way has the value between 3.26 and 2.49 for surface electron densities n ranging from 1.0×10¹² to 7.0×10¹² cm^-2. The enhancement of g-factor is divided into two parts, noninteracting and interacting part. The interacting part is dominant and shows $\text{n}{}^{\mathrm{<mtext>-3</mtext><mtext>2</mtext>}}$ behavior.     

Spin and lifetime of the ω− hyperon

A total of 101 ω^? decays have been found in K^?p interactions at 10 and $\text{16 GeV}\text{c}$. The decay angular distribution has been fitted under the assumptions that the ω^? has spin either $\text{1}\text{2}\text{or}\text{3}\text{2}$. It has been found that the probability of isotropy (spin$\text{1}\text{2}\text{)}$ is less than one in a thousand, whereas the probability for spin $\text{3}\text{2}$ is about 70%. Thus, we exclude the spin $\text{1}\text{2}$ assignment. The lifetime of the ω^? is found to be (1.41_?0.24^+0.15) × 10^?10s, in agreement with our earlier result based on about $\text{1}\text{5}$ of the present sample.     

Hole mobility anisotropy and crystal structure of β-9,10 dichloroanthracene

Hole drift mobilities have been measured in β-9,10 dichloroanthracene by the transient photoconductivity method. Mobilities were found to be isotropic in the ac plane, 0.6 cm²/V sec, and 0.18 cm²/V sec normal to this plane, b¹, at room temperature. At lower temperatures the mobilities were thermally activated, with an activation energy of ～0.25eV. The crystal structure of β-9,10 dichloroanthracene has been determined; it is triclinic. $\text{a = 8.582}\text{A}\text{?}$, $\text{b = 16.825}\text{A}\text{?}$, $\text{c = 3.869}\text{A}\text{?}$, $\text{α = 97.90}\text{,}\text{?}\text{β = 97.90°, γ = 75.99°}$. The influence of the crystal structures of the α- and β- forms on the intermolecular electronic overlap and hence carrier mobilities are discussed.     

The decay scheme of 127Ba and half-lives of some low-lying excited states in 123, 125, 127, 129Cs

Radioactive sources of ^{123, 125, 127, 129}Ba obtained in the spallation reaction have been studied with Ge(Li), Si(Li), NaI(Tl) and plastic detectors. On the basis of γ-ray energies, intensities, energy sums and coincidence information, the decay scheme has been proposed for ¹²⁷Ba. No high-spin isomer of ¹²⁷Ba has been observed. The lifetimes of $\text{5}\text{2}$⁺ → $\text{1}\text{2}$⁺ transitions in ^{123, 125, 127, 129}Cs have been measured. No evidence for the shape isomerism in ¹²⁷Cs has been found.     

Collective excitation of 103Ag following the (12C,p2nγ) reaction

Excited states of ¹⁰³Ag have been investigated in the ⁹⁴Mo(¹²C, p2nγ)¹⁰³Ag reaction and observed up to 6414 keV excitation energy. A perturbed band based on the 27.6 keV $\text{9}\text{2}$⁺ intrinsic state has been strongly populated up to spin $\text{27}\text{2}$⁺. In addition, two negative-parity bands with spins ranging from $\text{19}\text{2}$^? to at least $\text{29}\text{2}$^? were also observed. An interpretation of the experimental results is proposed in the framework of the axial-symmetric rotor-plus-particle model using deformed self-consistent quasi-particle states.     

A proposed experiment to measure surface roughness in Si/SiO2 system

The adsorption of oxygen on clean cleaved (111) silicon surfaces has been investigated by high resolution electron spectroscopy (HRES), Auger electron spectroscopy (AES) and ellipsometry. Localized vibrations ($\text{h}\text{?}\text{ω = 94}$, 130 and 175 meV) which are related to the binding state band of oxygen are identified with HRES. AES measures the concentration of adsorbed atoms basically independent of their binding state while ellipsometry refers additionally to the optical properties of the adsorbed layer. The same adsorption kinetics was found with the three methods. Oxygen therefore adsorbs in a single likely molecular state. The sticking coefficient S increases exponentially with the surface step concentration. S is also enhanced by the presence of nude ion gauges. Depending on these parameters sticking coefficients between 2 × 10^?4 and 10^?1 have been obtained. This result might contribute to an explanation of the large differences in earlier works.     

Multiphoton excitation of visible emission in Y2O2S: Er,Yb phosphors by 810 nm excitation

During investigations of anti-Stokes effects in erbium—ytterbium doped yttrium oxysulphide phosphors we have found an excitation of the erbium green emission due to 810 nm radiation (⁴I$\text{15}\text{2}$ → ⁴I$\text{9}\text{2}$ transition), the effect being of a two-photon type. There is also a two-photon excited blue emission (²H$\text{9}\text{2}$ → ⁴I$\text{15}\text{2}$ transition) due to the same radiation. Introduction of ytterbium impurity has a deleterious effect on the above 810 nm excitation, in contrast to its sensitization of the 1000 nm anti-Stokes excitation. To distinguish between various excitation schemes a two-beam excitation technique has been used. Results show that ytterbium inclusions lead to loss of energy from the 810 nm excitation by increasing the probability of downward transitions from the ⁴F$\text{9}\text{2}$ level to ⁴I$\text{11}\text{2}$ and ⁴I$\text{13}\text{2}$ states, reached after onephoton absorption. This effect is indicated by an increase in 1500 nm emission (⁴I_{$\text{13}\text{2}$} → ⁴I_{$\text{15}\text{2}$} with ytterbium concentration.     

Study of the 176Lu(n, γ)177Lu reaction using a gamma band-filter spectrometer

The γ-ray spectrum from the ¹⁷⁶Lu(n, γ)¹⁷⁷Lu reaction has been investigated in the energy range 450 to 1420 keV using a newly developed gamma band-filter spectrometer. A total of 153 transitions have been observed. Improvements to the ¹⁷⁷Lu scheme are suggested, and in particular the γ-decay of several levels of the $\text{1}\text{2}$^?[530↑] and $\text{3}\text{2}$^?[532↓] rotational bands is proposed. Special attention has been given to the Coriolis coupling between the two latter bands and the $\text{1}\text{2}$^?[541↓] band. Results on a complementary investigation of the ¹⁷⁵Lu(n, γ)¹⁷⁶Lu reaction are presented.