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Dive into the research topics where C. Gaubert is active.

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Featured researches published by C. Gaubert.


Surface Science | 1976

Theory of angular dependence in electron emission from surfaces: Application to Auger emission from several metal surfaces

D. Aberdam; R. Baudoing; E. Blanc; C. Gaubert

Abstract An exact multiple scattering description of electron emission from inner sources is given using the same matrix formalism as applied in LEED theory, and some approximations are discussed. The model is derived for sources localized on atomic sites and represented in terms of spherical waves but it may be adapted for other types of inner sources. This model is used to study the anisotropy of the Auger emission from aluminum and some transition metals (Fe, Ni, Cu). The results obtained show that: (a) The intra-layer multiple scattering term dominates the calculated angular dependence. (b) The anisotropy due to multiple scattering is very important but too strong compared to experiment; this may be related to the over simplified model used for the source wave function, (c) The isotropic emitter does not account for experimental results (d) The intrinsic anisotropy of the emitter is of first importance in the observed angular dependence. Thus we conclude that not only a full multiple scattering description must be used, but also a realistic description of the Auger source is needed in particular to give account of the experimental behaviour of aluminum and of transition metals.


Surface Science | 1978

Angular resolved Auger emission from crystal surfaces: New experimental results and predictions from a simple model

D. Aberdam; R. Baudoing; E Blang; C. Gaubert

Abstract In a first part, new angular resolved CVV Auger emission results (including line shape after background substraction and anisotropy measurements) are reported concerning the Al L 23 VV transition on clean Al(001) and Al(110), the Ni M 23 VV transition on clean Ni(001), and the Ni M 23 VV and S L 23 VV on the Ni(001)-(11,11)-S system. In a second part we discuss a semi-quantitative analysis of the Auger emission anisotropy with a model which includes at the same time the final state effects due to multiple elastic scattering of the ejected Auger electron on its escape path and a description of the Auger source in solids as realistic as possible. The band aspect of the valence states id described as s, p, d, … like contributions at the emitters lattice site, and we use atomic results for the matrix element angular momentum dependence. In a manner consistent with this model we have analyzed the available anisotropy measurements on Al(001), Al(110), Cu(001), Cu(110), Cu(111), Fe(001) and Ni(001)-(11,11)-S. The results are consistent for various faces of the same material and correspond, moreover, to the model predictions, indicating that the most important aspects of the Auger emission anisotropy are included. In a third part, on the Ni(001)-(11,11)-S system, we investigate the sensitivity of the emission anisotropy of adsorbate Auger transitions to structural parameters such as the adsorption site coordinance and the adsorbate-substrate distance. We find a large sensitivity in both cases, comparable to LEED. Moreover, satisfactory agreement is found only when the structural parameters are given the values known from LEED studies and when the angular momentum predicted by the above model is used for the ejected Auger electron.


Surface Science | 1988

Low-energy electron reflection and transmission on Ar films condensed on polycrystalline platinum

M. Michaud; L. Sanche; C. Gaubert; R. Baudoing

Abstract Low-energy electron reflection (LEER) and transmission (LEET) spectra have been recorded in the range 1–16 eV for argon films grown on polycrystalline platinum. The data are compared with low-energy electron diffraction (LEED) calculations of the elastic reflection and transmission of an electron beam incident on the (111) surface of a semi-infinite Ar solid azimuthally disordered. Structures in LEER spectra reflect essentially the band structure characteristics of the Ar(111) surface, whereas in transmission, similar structures are strongly modified by scattering from phonons, defects and imperfections of the film as evidenced by changes in LEET spectral line shapes as a function of incidence angle and film thickness. These findings are discussed in relation with the ability of the LEED formalism to describe elastic or quasi-elastic scattering from partially disordered films at very low energies.


Surface Science | 1976

A LEED data reduction method: Description and application to the relaxation of aluminum (001) and (110) surfaces

D. Aberdam; R. Baudoing; C. Gaubert; E.G. McRae

Abstract A LEED data reduction method is described and applied to Al (001) surface. The method consists of averaging rotation diagrams (azimuthal intensity profiles) obtained at constant electron energy between 500 and 1000 eV in the whole range of the colatitude angle of incidence. The method avoids certain difficulties, stemming e.g. from the energy dependence of the mean optical potential, that are present in other methods in which data obtained at different energies are averaged. The averaged data are analyzed using the two-beam Darwin theory. Values for the surface normal relaxation and mean optical potential are obtained from the study of the position, width and profiles of the averaged peaks. The (100) surface of aluminum is found not relaxed while the (110) surface is contracted. The values found for the mean optical potential are similar to those obtained from other sources. The precision of the averaging method for surface structure is not better than that of direct calculation. Experimental factors limiting the precision of the method (surface roughness, background interference due to Kikuchi lines) are specified.


Surface Science | 1975

Medium energy electron diffraction on aluminum

D. Aberdam; R. Baudoing; C. Gaubert

Abstract Measurements of the specular beam intensity in the medium range of energy (645 eV and 980 eV) are presented for the aluminum (001) and (110) surfaces. They have been performed with a LEED goniometer allowing to achieve high accuracy in the determination of the angular parameters of the diffraction experiment. The experimental results are displayed in the form of iso-intensity maps in (θ, φ) coordinates at a constant primary beam energy. θ denotes the colatitude angle of incidence and is varied in the range 30° to 80°. φ denotes the azimuth and is varied between 0° and 360°. Sections of the maps at constant angle of incidence are known as rotation diagrams. A model calculation ignoring intra-layer multiple scattering, but incorporating inter-layer multiple scattering processes shows the prominent role of the latter, in many cases, and the non-negligible contribution of the former in other cases. Sections of the maps at constant azimuth are called incidence-profiles. They are very sensitive to the azimuth and very hard to compute. Using the preceding model calculation, average incidence profiles have been computed, using as input parameters the refractive and absorptive potentials and the surface normal relaxation parameter obtained by a pseudo-kinematic analysis of experimental average incidence profiles as explained in another paper. The peak position and width have been compared to those obtained from the pseudo-kinematic calculation. From the result, it is shown that if averaging is performed according to a specific set of restrictive rules, the pseudokinematic analysis of the data has a semi-quantitative meaning.


Surface Science | 1983

Angular resolved auger emission from clean and sulfur covered Ni(110) surface

R. Baudoing; E. Blanc; C. Gaubert; Y. Gauthier; N. Gnuchev

The angular dependence of the nickel M 23 VV and of the sulfur L 23 VV Auger transitions are studied in detail, on clean and sulfur covered Ni(110) surfaces. New experimental data are presented for the anisotropy of both transitions as a function of polar and azimuthal angles of emission. Our model, which incorporates at the same time the multiple scattering effects in the final state wave function and the intrinsic anisotropy of the Auger emitter, is found to give a satisfactory account of the observed auger anisotropy. We find a large sensitivity to the position of the sulfur adsorbed atoms. The best agreement is obtained for the hollow site, slightly less than 0.9 A above the top nickel layer. This conclusion is consistent with previous LEED and MEIS studies, but does not agree with the long bridge site obtained from quantum chemistry calculations. Moreover the sulfur emitter on this particular Ni(110) face appears to have an intrinsic anisotropy.


Applied Surface Science | 1986

Observation of LEED threshold effects on polycrystalline platinum

C. Gaubert; R. Baudoing; Y. Gauthier; M. Michaud; L. Sanche

Abstract We report the observation of LEED threshold interference structure on polycrystalline samples of platinum. These structures, observed at low energy by high-resolution electron reflection measurements, are interpreted in terms of a recently developed theoretical framework. Our analysis shows how observation and detection of threshold effects is possible even on polycrystalline samples under certain conditions. These conditions are described with emphasis on the use of threshold effects for studying polycrystalline samples.


Surface Science | 1977

Theory of angular dependence in electron emission from surfaces: Application to secondary emission from Al(001) and Al(110)☆

D. Aberdam; R. Baudoing; E. Blanc; C. Gaubert

The model of electron emission derived tor Auger emission in a previous paper, is extended to the study of angular resolved secondary emission and applied to the description of experimental results obtained on Al(001) and Al(110). We include exactly all the final state effects, and we give a simple description for the source wavefield which is taken as a set of plane waves created incoherently at a given depth and with any possible wavevector coupled to the direction of observation. Having no specific model for the cascade processes at the origin of the source wavefield, we have taken a uniform distribution of sources. The experimental data have been taken at a primary energy at 350 eV and at normal incidence, such that the anisotropy depends no longer on the incidence conditions. On one hand the polar angle plots are dominated by a cos2θ like dependence; on the other hand the modulations on polar or azimuthal plots are found less than 10%. Both effects are well accounted for by our model. As far as the correlation of experimental and calculated fine structures is concerned, a close examination of the individual contributions from deeper and deeper crystal slices suggests a non-uniform distribution of sources normal to the surface, the top and first slice having a small contribution.


Surface Science | 1983

Meed study of Al(001) surface

Nazma Masud; Robert Baudoing; D. Aberdam; C. Gaubert

Abstract Medium Energy Electron Diffraction (MEED) is expected to be a valuable tool for surface crystallography. In this paper, we check the technique, using approximations specific to the medium energy range, against experiments performed on aluminum (001) at 77 K. It is confirmed that nearly no surface relaxation occurs on that surface. The present extensive check thus proves that the method is efficient for the study of surface normal relaxations. This gives support to more sophisticated applications such as the structure of surfaces where chains of atoms have a particular significance.


Surface Science | 1971

Study of the temperature dependence of leed intensities from the (001) surface of BaTiO3 between 20°C and 650°C

D. Aberdam; C. Gaubert

Abstract A study of the temperature dependence of some intensity maxima in the intensity versus energy curves from the (001) surface of BaTiO 3 between 20°C and 650°C has shown the existence of a loop in the plots of the logarithm of intensity versus temperature. This loop extends from about 100°C to about 430°C and may be tentatively interpreted in terms of a surface layer, tetragonal up to 430°C and cubic down to 100°C. The slope of these plots, for the decreasing temperature part of the loop, is in reasonable agreement with the slope predicted from a kinematical calculation of the temperature effects, based on the temperature parameters computed from the bulk phonon spectrum.

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R. Baudoing

Centre national de la recherche scientifique

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D. Aberdam

Centre national de la recherche scientifique

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E. Blanc

Centre national de la recherche scientifique

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Y. Gauthier

Centre national de la recherche scientifique

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J.P. Ganachaud

École Normale Supérieure

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M. Cailler

École Normale Supérieure

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E Blang

Centre national de la recherche scientifique

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Robert Baudoing

Centre national de la recherche scientifique

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J. Rundgren

Royal Institute of Technology

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