G. Tréglia

Surface core level spectroscopy of transition metals: A new tool for the determination of their surface structure

Abstract It has been recognized since 1964 that shifts in the binding energies of core electrons detected by high-resolution X-ray photoelectron spectroscopy (XPS), are very sensitive probe of the chemical environment of the atom which undergoes the X-ray transition. Recently this technique has been widely applied to surface studies. Actually the surface atoms have their structural environment modified and therefore their core levels are shifted from their bulk positions. Such an effect has been first observed on polycrystalline Au in 1978 and on W(110) in 1979. Since, a very large number of experiments have been performed on the 4f core level positions of elements of the 5d series for various orientations of the surface plane. Systematic trends have been put forward and explained by theoretical models. When adsorbed atoms are present on the surface, a given core level shift should correspond to a given chemical environment and therefore this technique can be used to check the validity of structural models. We review the experimental and theoretical work done in this field, critically compare the various theoretical models and discuss their limit of validity.

Segregation and Ordering at Surfaces of Transition Metal Alloys: The Tight-Binding Ising Model

From the electronic structure of the disordered alloy we derive an effective Ising Hamiltonian for segregation and ordering processes at transition metal alloy surfaces. In this tight-binding Ising model (TBIM), the Hamiltonian contains a linear term, quasi-concentration-independent, which proves to be very close to the difference in surface tensions between pure constituents and a quadratic one involving effective pair interactions larger at the surface than in the bulk. The former explains the success of the popular phenomenological approaches based on surface tension arguments and the latter could be of prime importance in surface ordering processes.

Is the segregation-dissolution kinetics driven by a surface local equilibrium ? An answer via the kinetic tight-binding Ising model

We study here the relation between the surface segregation at thermodynamical equilibrium in an alloy AcB1−c and the kinetics of either segregation in the system A-B or dissolution of AB or BA. This is possible within a new kinetic model based on the electronic structure: the kinetic tight-binding Ising model (KTBIM). In particular, the existence of a local equilibrium between the surface and the first underlayer is stressed. Some illustrations are given in the particular case of the W-Re system.

Surface segregation in PtRh alloys revisited in the framework of the tight-binding Ising model

Abstract The recently developed tight-binding Ising model (TBIM) is used to interpret the puzzling experimental results on surface segregation in PtRh alloys. More precisely, both the nature of the segregant element (Pt) and the anomalous increase with temperature (up to 1000 K) of the surface enrichment have required some authors to invoke a generally neglected effect: the vibrational entropy. We show here that this term is indeed negligible and that the TBIM, which is derived from electronic structure, accounts for the experimental observations (Pt surface enrichment, oscillating concentration profile). As for the apparent increase in surface segregation with temperature, it is attributed to kinetics limitations which prevent from reaching the equilibrium.

Surface segregation in CuNi and AgNi alloys formulated as an area-preserving map

Abstract The tight-binding Ising model (TBIM) coupled with a mean field approximation formulated as an area-preserving map (APM) is applied to surface segregation of CuNi and very dilute Ag(Ni) alloys. In the former case, it rules out the surface segregation crossover with bulk concentration suggested by some experiments and calculations, by concluding in a Cu-enrichment independent of the concentration and the temperature. In the latter case, it accounts for the spectacular surface sandwich observed experimentally in terms of Ag-segregation at the surface of the Ni-rich precipitated phase, slightly below the temperature of phase separation.

W(100) reconstruction studied by core level spectroscopy

Abstract We compare the hydrogen and thermally induced reconstructions of W(100) by means of surface core level spectroscopy and analyse the energy positions of the spectral lines in the framework of existing theoretical models. Surprisingly, our results for both reconstructions differ more than would be expected from previous admitted structural models.

Study of the W (Ta) core level shifts induced by the adsorption of oxygen on tungsten (tantalum) (110)

The authors present new experimental XPS spectra of 4f7/2 levels of tungsten in the W(110)-O system and a theoretical model in which the change of the binding energy of core levels of W atoms bound to O is due to a small charge transfer from these atoms towards oxygen. The same analysis is done for a Ta(110)-O system. Finally, using thermodynamical arguments, they link the displacements of core levels to the variation of the binding energy of O along the transition series and show that they always correspond to larger binding energy.

A 'quenched molecular dynamics' approach to the atomic stability of the (100) face of BCC transition metals

Up to now, all the models proposed to describe the reconstruction of W(100) assume that the atomic displacements are limited to the surface plane. The authors present here a quenched molecular dynamics approach in which they minimise the total energy, in a tight binding framework, with respect to alternate lateral displacements of atoms and interlayer spacings involving the first four layers. One finds that this multilayer relaxation favours the zigzag reconstruction for W(100) with lateral displacements decreasing rapidly with the distance to the surface. Finally, they check the influence of various parameters, especially that of the d band filling, paying some particular attention to Ta(100).

A theoretical inquiry into the question of W and Ta (100) atomic structures

In spite of the very large number of experiments (LEED, AES, UPS, MeV He+ scattering, work function, FIM) carried out on W (100), no structural model consistent with all the data has been proposed yet: in particular, the question of the reconstruction thermally induced when the sample is cooled below room temperature remains a puzzling problem. Furthermore, from a theoretical point of view, no definitive answer has been given. Actually, either the mechanism invoked for the reconstruction is too weak, or some contributions are omitted or calculated without sufficient care. The authors compute here the surface energy of W (100) taking into account the band term treated in the tight binding approximation, a pairwise repulsive potential of the Born-Mayer type and the electronic correlation contribution obtained from a perturbation treatment of the Hubbard model in the band limit. This energy is then fully minimised with respect to all coordinates of surface atoms, keeping all atoms neutral for any displacement. They find that the unreconstructed surface is the most stable at T=0K and discuss this unexpected result. A similar calculation for Ta (100) leads to opposite conclusions.

Multilayer relaxation and reconstruction in bcc and fcc transition and noble metals

Using a tight-binding scheme and a ‘quenched-molecular dynamics’ method, we calculate the multilayer relaxation of bcc transition metals for the (110), (100) and (111) faces (which present only perpendicular relaxations) and the (211), (310) and (210) faces (for which both parallel and perpendicular components are present). Moreover, we account for the multilayer reconstruction of W(100) and its non-occurrence for Ta(100). Then performing similar calculations in the case of all fcc transition and noble metals, we obtain the multilayer relaxation of the low index surfaces: (111), (100) and (110). Finally, we explain the material dependence of the (1 × 2) reconstruction of the (110) face, which is only observed for the 5d metals (Ir, Pt, Au). The model allows us to predict order-disorder surface phase transition for the (110) face of all these fcc metals.