Repulsion et attraction lors de la mise en ordre de films bidimensionnels de métaux alcalins, le césium et le lithium sur la surface NI(111)

Abstract : The chemical nature of an alkali adatom adsorbed on a metallic surface is the likely oldest unsolved question of surface science. However a consensus does exist about the fact that, either polarized or ionized, the alkali adatom gives rise to an intense dipole moment which produces in particular the well known dramatic decrease of the electronic affinity of the surface due to alkali metal adsorption. Consequently, the general belief is that, within a two-dimensional alkali layer, individual dipoles of the alkali adatoms is governed by dipole-dipole repulive interactions, at least at low coverages, in the isolated adatom limit. The purpose of the present work was: (i) to produce evidence about those repulsive interactions in connection with the ordering of an alkali metal adlayer growing on a metallic surface; (ii) to determine wich kind of interaction is dominating the ordering of an alkali layer at high coverages when the concept of isolated dipole no larger holdes. We have choosen to study two systems Cs/Ni(111) and LilNi(111). The highly electropositive cesium atom produces a very strong dipole moment upon adsorption. Since Ni(111) has a small corrugation potential with respect to the alkali adatom adsorption, Cs1Ni(111) was a good system to test dipole-dipole interactions. At variance, the corrugation of the lateral potential was expected to play a bigger part in lithium&adsorption than in cesium adsorption. The study was performed under ultra-high & vacuum conditions by means of low energy electron diffraction, coverages Salcalin being calibrated and monitored using Auger electron spectroscopy. Two phase diagrams were thus obtained (temperature versus coverage). (i) the two dimensional layer is stable for coverage lower than 0,12 (atomic ratio - the full monolayer is at a coverage of '" 0,25); in this coverage, the stable two dimensional ordering is actually governed by repulsive interactions; ii) the layer is metastable above a coverage of 0,17 and for 0,17 <=0 CS <=0,25 , the structure is independant of the coverage, thus showing that is due to attractive interactions between cesium adatoms. As expected, the lithium two-dimensional ordering is strongly influenced by the corrugation of the substrate potential since the deposit of Li on Ni ( 111) produces a series of commensurate structures. The important observation made in the present work about CslNi(111) meets those made about KJAg(001) and KJCu(100) and leads us to wonder whether the transition repulsive interactions -4 attractive interactions, which is today considered as marginal, is general or not. The present work anyway questions the large amount of studies deroted to the bidimensionals ordering of alkali metals sin ce at coverages where interactions can be attractive (high covarages in the monolayers range), the stable arrangement can be hidden by metastable phases which are rarely tested by authors. Betwen 100 and 450 K, the Cs/Ni(111) diagram shows phases which parametrs are in good agreement with a model dominated by dipole-dipole repulsion : the coverage SCs is proportional to the inverse of the square parameter. Further annealings lead to the following conclusions
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Abdel Ilah Faké. Repulsion et attraction lors de la mise en ordre de films bidimensionnels de métaux alcalins, le césium et le lithium sur la surface NI(111). Autre. Université Henri Poincaré - Nancy 1, 1991. Français. ⟨NNT : 1991NAN10275⟩. ⟨tel-01748581⟩

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