Décomposition thermique du monoxide de carbone sur une surface de molybdène: Formation de couches superficielles de carbone et de carbure

We have investigated the decomposition of carbon monoxide on polycrystalline and (001), (110) monocrystalline molybdenum surfaces. This study was performed by massspectrometry, for thermal desorption studies, Auger electron spectrometry (AES), low energy electron diffraction (LEED) and photoelectron spectroscopy (ESCA). By heating the clean Mo surface in CO or by heating the Mo surface covered with CO, the dissociation of chemisorbed CO leads to a build-up of carbon layer which inhibits the subsequent adsorption. Two distinct types of fine structure are associated with the KLL line of carbon Auger spectra. If the Mo surface is heated at a temperature between 300 and 1500 K, the Auger peak is characteristic of a “graphite layer”. If the Mo surface is heated at a temperature up to 2000 K, the Auger peak is characteristic of a “carbure” layer. This “carbure layer” give rise to a surstructure which agrees with a Mo₂C surface layer and was also investigated by ESCA. Chemical shifts of (1s) C and (3d) Mo photoemission bands were observed and attributed to the bounding between Mo and C atoms in the Mo₂C layer.     

Temporal Non-locality

In this article I investigate several possibilities to define the concept of “temporal non-locality” within the standard framework of quantum theory. In particular, I analyze the notions of “temporally non-local states”, “temporally non-local events” and “temporally non-local observables”. The idea of temporally non-local events is already inherent in the standard formalism of quantum mechanics, and Basil Hiley recently defined an operator in order to measure the degree of such a temporal non-locality. The concept of temporally non-local states enters as soon as “clock-representing states” are introduced in the context of special and general relativity. It is discussed in which way temporally non-local measurements may find an interesting application for experiments which test temporal versions of Bell inequalities.     

États de surface du sodium dans l'approximation des électrons quasi-libres

Electron states located near a (110) surface of a Na crystal are investigated by the method of matching the crystal wave function to the outside solution at the surface. The lower energetic states are in the fundamental state. An “anormal” optical band situated about 1.8 eV is discussed when the imaginary part of the dielectric function ?₂ has been determinated considering transitions with the k-vector component parallel to the surface.     

An anomalous transformation in string field theory

Conformal field theory techniques are used to analyze a recently proposed transformation of Witten's interaction vertex from center-of-mass to mid-point variables. As presented in the literature the transformed vertex has the surprising property of being non-local in the mid-point coordinates unless an additional non-unitary transformation is performed. This renders the kinetic term non-local and poses additional problems. Here we show that, besides being non-local, the transformed vertex does not satisfy the overlap equations and fails to be BRST invariant. These bad properties are the result of an associativity anomaly of the type considered by Horowitz and Strominger. However, this anomaly is “almost trivial” in the sense that most of its bad effects can be avoided by choosing a proper regularization.     

Non-local corrections to the electronic structure of metal surfaces

A non-local exchange-correlation term, derived from the static dielectric function of Vashishta and Singwi (VS), is introduced into the density functional for an interacting electron gas. The coefficient is chosen to give the same surface energy for two adjacent jellium metals of nearly equal density as is obtained from the VS dielectric function. This density functional is then used to calculate surface energies and work functions for metal-vacuum interfaces. The results for both surface energies and work functions found in this way are significantly higher than the experimental values. Possible explanations for the discrepancies are discussed.     

Microscopic theory of surface dielectric response in a local representation

We give a formulation of surface dielectric response in a local LCAO or Wannier representation. It is shown that this representation allows for a practical solution of the response integral equation and thus makes possible an explicit and self-consistent calculation of the nonlocal RPA response function ?^-1. The formulation takes into account lattice potential effects and is therefore particularly suited for investigations of surface dielectric response and screening in transition metals, semiconductors and insulators. We present model calculations of charge densities induced in a metal thin film by localized perturbations in the surface region. It is demonstrated that “surface effects”, resulting from differences in the effective atomic potential for different layers, must be included in calculations of surface response in systems with tightly bound electrons.     

Photoemission spectroscopy investigation of the electronic structure of carbidic and graphitic carbon on Ni (111)

Photoemission measurements have been carried out on different forms of carbon deposits of catalytic importance on the (111) face of nickel, and give a picture of the electronic structure of “carbidic” and “graphitic” carbon.The photoemission spectra of graphitic overlayers strongly resemble spectra taken on graphite bulk samples but show an (almost) rigid shift of the levels with respect to the Fermi level.The results we obtain for carbidic carbon can be understood on the basis of recent surface electronic structure calculations. In particular a p-like state has been observed very near the Fermi level, which is probably responsible for the high chemical reactivity of carbidic carbon.     

Hybrid bulk-surface polaritons for solid plasma slabs in the ωS to ωP frequency range

The existence of a particular surface mode in the ω_S-to-ω_P frequency range is established, for thick plasma slabs, independently of the dielectric function damping term. The studied mode presents an hybrid behaviour characterized by a standing surface charge oscillation and the propagation in the bulk of partially longitudinal radiations normal to the slab surface. A new interpretation of the plasmon dispersion curve back-bending is deduced. Experimental procedures for putting the hybrid mode in evidence are proposed and applications in photoemission, wave guiding etc. anticipated.     

Etude de la cinetique de la reaction de polymerisation de resines epoxy et polyester par analyse du comportement dielectrique

In this piece of research, we have chosen to study the dielectric behaviour of different resins while in process of polymerization and determine the dielectric parameters most relevant to the monitoring of the kinetic process taking place; our aim being to design electromagnetic sensors which will enable us to monitor the polymerization process thoroughout.Consequently we have confined ourselves to carrying out radiofrequency measurements by using only a few set frequencies, without attempting to monitor, as many researchers have tried to, the evolution of dielectric relaxation [1, 5].We have studied two types of resins, one of them being epoxy-based and the other polyester-based. We have succeeded in demonstrating that the perception of the real terms of the complex dielectric constant as well as of the evolution of the loss tangent angle, both complementary elements, are essential for a thorough analysis of the kinetic process.Whereas ε is most suitable for the monitoring of the concluding phase of the polymerization, tan δ is more appropriate to the study of the opening stage of the process, while allowing us to define with great accuracy the exothermicity point reached after gel time [6, 7, 8].     

Etude de la propagation d'une onde du type Franz sur des cylindres

A “Franz-type” wave was generated on the outside of solid cylinders immersed in water with “ka” values ranging from 25 to 800. In this investigation, the velocity and the attenuation of this wave have been obtained for different materials. Good agreement is obtained with the theory and other experiments.     

Observation of the surface photoelectric effect of metals: Surface photoyield spectra of aluminum

Surface photoemission spectra of metals have been discussed theoretically since Sommerfeld developed his model of a metal in 1928, the so-called jellium model. The subject is still highly controversial among theorists. A spectroscopic technique — surface photoemission spectroscopy on jellium (SPJ) — which fully exploits the properties of synchrotron radiation, made it possible to separate unambiguously the photoyield caused by the surface photoelectric effect of metals optically excited byp-polarized light from other contributions to the yield. The technique was applied to aluminum and surface photoyield spectra were obtained for the first time. The extreme surface sensitivity, which is implicit in the transition matrix element of the surface photoexcitation process of metals, was experimentally confirmed. The new technique, therefore, provides one of the most powerfull tools for the investigation of the charge distribution and the dielectric response at metal surfaces.     

Spin-resolved photoemission form ferromagnetic Ni(110)c(2 × 2)-S: Theory and comparison with experiment

Changes in spin-resolved photoemission spectra, which had been observed experimentally upon chemisorption of a c(2 × 2)-S layer on ferromagnetic Ni(110), are investigated theoretically by means of dynamical one-step-model photoemission calculations. Two large peaks produced about 1 and 2 eV below the clean Ni peak can, with the aid of the bulk band structure along ΓX and along the “surface-Umklapp” direction LW, be explained as due to Umklapp coupling in the initial state and in the final state, respectively.     

The Van der Waals interaction between an atom and a solid

Measurements of the scattering of neutral atoms by metal cylinders have indicated a serious discrepancy between theory and experiment for the strength of the long-range Van der Waals interaction. To clarify the content of the theory, we present here a formal analysis of the Van der Waals interaction between an atom and a solid, based on the approach of McLachlan. A key quantity in the theory is the response of the solid to an external, time-dependent dipole. This response is amenable to classical analysis, and we calculate it for a variety of model systems. When the solid may be treated as a continuum with a flat surface, the concept of surface impedance allows a convenient parameterization and we may incorporate thereby magnetic effects, non-local dielectric response, and the influence of finite layers in the solid. We also study the modifications induced by a rough surface, finding the average change in the coupling strength in the presence of weak roughness.     

Electronic structure and polarizability of quantum metallic wires

The electronic structure and the linear response to an external electric field of simple metal wires with a quantum-size cross-section have been studied within the density-functional theory and the “jellium” model. It is found that an increase in the wire radius leads to a nonmonotonic change in the work function and static polarizability of the wire. The photoabsorption spectra of Na wires with different cross-sections are obtained. The effect of a dielectric environment on the properties of metallic wires is investigated. An increase in the permittivity of a medium brings about a decrease in the static polarizability of metallic wires. It is demonstrated that the surface plasma resonance in the photoabsorption cross-section for Na wires placed in a dielectric matrix is shifted from the continuous spectrum toward the range somewhat below the ionization threshold.     

用于高速摄影变象管的软X射线光电阴极

光阴极由衬底(包括介质阴极的导电基底)和光电发射膜构成。采用了聚丙烯、Formvar和Paylene三种有机薄膜作阴极衬底。建立了这些薄膜的制备技术。用一台自制的软X射线单色仪在277—7469ev光子能量范围内测量了这些薄膜的透过率。 研究了CsI、CsBr、Au和MgF₂四种光电阴极的光电发射特性和光电发射与阴极厚度的关系,找出了最佳阴极厚度。用软X射线单色仪在277—7469ev光子能量范围内测量了最佳厚度阴极的绝对量子效率,四种阴极最大值分别为4.50、2.90、0.25和0.12。我们还在同一阴极衬底上分区制备了四种阴极,在变象管荧光屏上比较其亮度,结果和测量的一致。 用LAB5型表面分析仪对CsI和Au阴极的光电子初能量分布作了测量,CsI阴极光电子初能量分布半高宽远小于Au。因此CsI是适用于高速摄影变象管比较理想的软X射线光电阴极。     

Electronic properties of alkali metal/silicon interfaces: A new picture

Adsorption of Na and Cs on the Si(100)2 × 1 surface in the monolayer range is investigated by core level and valence band photoemission spectroscopy using synchrotron radiation. The alkali metals are found to induce an electronic interface state near the Fermi level while hybridization between alkali adsorbate “s” and silicon substrate “3p” valence electrons occurs. These results provide evidence that the alkali metal/silicon bonding is covalent. This covalent bond is weak and polarized while plasmon at the alkali metal core level indicates adsorbate rather than substrate metallization.     

Spin-resolved photoemission by circularly polarized light from Au on Ag(111): experiment and theory

The electronic structure of 1 ML and 2 ML Au on Ag(111) has been studied experimentally and theoretically by spin-, angle- and energy-resolved photoemission in the highly symmetrical set-up of normally incident circularly polarized VUV radiation and normal electron emission. The measured spectra agree rather well with their relativistically calculated counterparts and can be consistently interpreted with the aid of spin- and layer-resolved densities of states. The spin polarization of spectral peaks directly reflects the double group symmetry type of the the corresponding initial states. For 1 ML (2 ML) Au on Ag(111) there are two (four) Au-derived states, which are confined to the adlayers (“film states”). For 2 ML Au, the “film-state” photoemission intensity depends strongly on the photon energy, which is understood as an interference effect. Since the agreement with the experimental photoemission spectra is improved by calculations assuming an additional Ag layer on top, and since the Au-derived experimental features are substantially broadened, an intermixing of Au and Ag is very likely.     

Sur la determination de la permittivite complexe des materiaux a tres fortes pertes

The characterization of materials having a high dielectric constant and a large loss angle presents great difficulties, notably due to the very slight phase shift to be determined. From the analysis of the propogation for dielectric materials (in rectangular wave guides and coaxial lines) a new experimental technique is proposed for coaxial lines using the properties due to the impedance shifts which allows the worthwhile increase in the accuracy of the measurements.     

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.     

An intuitive form of non-localism for quantum mechanics

The two-slit interference experiment provides a vivid demonstration of quantum mechanics. It is clear that one can describe the effects using standard probability theory if one makes a non-localism assumption. But many scientists dismiss non-localism as implying “action-at-a-distance”. This paper shows that it is possible to construct a non-local model which does not assume “action-at-a-distance”.