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1

Monte Carlo Simulation of Partially Ordered Atomic Layers Adsorbed on the Monocristalline Surfaces

100%

Wiatrowski G., Le Bossé J., Lopez J., Rousseau J., Zasada I.

Acta Physica Polonica A

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1993

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vol. 83

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issue 5

587-596

EN

The calculations of the correlations functions and structure factors are performed by means of the Monte Carlo simulation. The results for O/Ni(001) system are presented and compared with those obtained using self-consistent molecular field approximation and cumulant expansion method. The simulation for more complex system CO/Pt(111) is also performed. The calculations of the topological correlations allow us to find the coverage dependence of the incoherent scattering amplitude which can be experimentally detected.

2

Apport des techniques EXAFS et apparentées a la determination de l'ordre local: application a la tribologie

100%

Lopez J., Le Bossé J., Hansali G., Belin M., Martin J.

Acta Physica Polonica A

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1993

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vol. 83

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issue 5

551-566

EN

CONTRIBUTION OF EXAFS-LIKE TECHNIQUES TO THE DETERMINATION OF LOCAL ORDER: APPLICATION TO TRIBOLOGY: The determination of local order at atomic scale becomes more and more fundamental for understanding superficial phenomena such as tribology and catalysis. Among a few techniques available in this field, we are only concerned with the EXAFS-like ones (EXAFS, SEXAFS, EXELFS, SEELFS, etc.). The difficulties appearing all along the numerical treatment of such spectra are carefully analysed and a methodology is proposed in view to obtain reliable radial distribution functions. Some applications of these techniques are presented in two fields connected with tribology: friction induced amorphisation in high vacuum and characterization of interfacial solid bodies created during a wear process including an antiwear additive.

3

Caracterisation de l'ordre par diffraction d'electrons

100%

Le Bosse J., Lopez J., Hansali G., Wiatrowski G., Rousseau J.

Acta Physica Polonica A

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1993

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vol. 83

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issue 5

567-580

EN

CHARACTERIZATION OF THE ORDER BY ELECTRON DIFFRACTION: In the case of a solid with a local atomic order, we can consider only a statistical description of atomic positions. For a homogeneous phase, this description is given by the two-site occupancy correlation function, the three-site occupancy correlation function, ... etc. The local atomic order is investigated by experiments in which the solid is bombarded with a well-collimated monoenergetic beam of particles (here electrons). Starting from a model describing the force law between an electron and an atomic scatterer and from a model of the solid describing the distribution of atoms in space, a computation of elastically backscattered intensities can be carried out. The latter depends on geometrical parameters which are determined in such a way that one gets the best agreement between the results of calculation and the measurements. This work aims to provide the way of proceeding to calculate the diffuse intensity. The main difficulty in this task appears when we undertake to treat the problem of multiple scattering. We provide here a detailed description of a calculation corresponding to the case of the backscattering of electrons at a single crystal surface covered with a partially disordered atomic layer. Assuming that the coverage in adatoms is small and these adatoms are weak scatterers, we can neglect the multiple scattering events inside the layer. This simplifying assumption leads to an expression of the diffuse intensity in terms of: the Fourier transform on the 2D surface lattice of the pair correlation function; the renormalized transition matrices of the different kinds of adatoms.

4

Two-Site Correlations in a Surface Layer Adsorbed on the Crystal

100%

Wojtczak L., Zasada I., Le Bossé J., Lopez J., Rousseau J.

Acta Physica Polonica A

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1991

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vol. 80

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issue 5

631-648

EN

The incohorent scattering of electrons by layer adsorbed at a single crystal surface is determined by the Fourier transform of correlation functions of elements forming this layer. The model of the description of atoms or molecules adsorbed on the surface is formulated by the occupation operators which are represented by the pseudospin operators. The calculations of the correlation functions are performed by means of methods which consider a given pair of elements embedded in the effective field of remaining elements of the system while the interaction between the elements of the pair are taken in its exact form. Two approaches are presented here, i.e. the cumulant average and constant coupling approximations, and the case of binary chemisorption is considered in detail. The problem of the correlation symmetry is also discussed.

5

Topological Correlations in a Layer Adsorbed on a Crystal Surface

100%

Wojtczak L., Zasada I., Bosse Le J. C., Lopez J., Rousseau J., Institute of Physics U. o. Ł., Laboratoire de Physique des Interfaces B.

Acta Physicae Superficierum

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1990

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vol. 1

EN

The incoherent scattering of electrons by a layer adsorbed at a single crystal surface is determined by the topological correlations of elements forming the adsorbed layer. The model for the description of atoms or molecules adsorbed on the surface is formulated in terms of occupation operators which are expressed in terms of pseudospin operators with a given spin value. The correlations can be determined by the fluctuation dissipation theorem in connection with the susceptibility or given directly by means of the Green functions properly chosen. An example of the topological or chemical disorder of two components is considered in detail. The calculations of the topological correlations allow us to find the incoherent scattering amplitude as a function of the surface coverage which can be experimentally detected.

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4 Acta Physica Polonica A

1 Acta Physicae Superficierum

3 1993

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1 1990

5 Lopez J.

4 Rousseau J.

3 Le Bossé J.

3 Zasada I.

2 Hansali G.

2 Wiatrowski G.

2 Wojtczak L.

1 Belin M.

1 Bosse Le J. C.

1 Institute of Physics U. o. Ł.

1 Laboratoire de Physique des Interfaces B.

1 Le Bosse J.

1 Martin J.

Monte Carlo Simulation of Partially Ordered Atomic Layers Adsorbed on the Monocristalline Surfaces

Apport des techniques EXAFS et apparentées a la determination de l'ordre local: application a la tribologie

Caracterisation de l'ordre par diffraction d'electrons

Two-Site Correlations in a Surface Layer Adsorbed on the Crystal

Topological Correlations in a Layer Adsorbed on a Crystal Surface