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Influence of neighbouring atoms in Surface Core Level Shifts

We have shown that oxygen adsorption on Re(0001) induces the appearance of new core level components, whose binding energy is not simply determined by the coordination of the Re atoms, but, notably, is also affected by the presence of oxygen adsorbates at second and third nearest neighbour positions.


E. Miniussi et al., J. Phys. Chem. C 116, 23297 (2012)

Surface Core Level Shift measurements are a powerful probe of the local electronic structure of adsorbate/substrate system and of their evolution under reaction conditions.
Although, in fact, core level electrons are spatially confined to the region surrounding the atomic nucleus, they are sensitive to the charge redistribution induced, e.g., by chemical bond formation.
The interaction of oxygen with 4d and 5d transition metals has traditionally attracted a great deal of interest, both because it has long provided a general model of the adsorbate−substrate interaction mechanisms, and because it plays a key role in a number of industrially relevant catalytic reactions involving oxidation.
In our work, the morphology and electronic structure of clean and oxygen-covered Re(0001) were investigated by means of  high energy resolution core level photoemission spectroscopy and DFT calculations.
Our analysis of the Re 4f7/2 core level  spectra of the clean surface led to the identification of two components, which we assigned to photoemission from bulk and surface Re atoms. The experimentally estimated SCLS (−95 meV) is in good agreement with the theoretical predictions (−130 meV).
The effects of oxygen adsorption on Re(0001) were investigated by monitoring the evolution of the Re 4f7/2 core
level during oxygen exposure at 300 K. Similarly to what observed in other adsorbate/TM systems, oxygen chemisorption on Re(0001) leads to a suppression of the photoemission signal from the clean surface and to the appearance of new oxygen-induced SCL components.  As a general trend, the SCLS of  non-equivalent populations of Re atoms characterized by a different number of O−Re bonds are seen to move to higher BEs as the oxygen coverage increases. The evolution of the O-induced SCLSs as a function of the oxygen coverage was further addresses by DFT.

 

Our simulations, which reproduce fairly well the experimentally observed trends, show that the CL states of Re(0001) are not only affected by the formation of direct bonds with oxygen atoms, but they are also sensitive to the interaction with second and third NN adatoms,  thus indicating a substantial contribution of non-local effects to the SCLSs.
A linear relationship is indeed found between the theoretical Re 4f7/2 SCLSs and the DFT-calculated effective charge per Re atom, which receives a contribution also from non-local interactions.
We also addressed the link between the oxygen-induced SCLSs and the surface-projected d-band centre, which is a key parameter in determining the chemical reactivity of transition metal surfaces. In an initial state picture, SCLSs are related to a modification of the Re valence 5d bandwidth,  due either to the reduced coordination of first-layer atoms or to the interaction with the O 2p levels, which leads to the formation of bonding and anti-bonding states, and to a subsequent d-band broadening. Our results show a fairly lineardependence of the total SCLSs on the surface-projected d-band centre, thus indicating that initial state effects are largely predominant on possible core hole screening mechanisms occurring in the final state.



Retrieve article
Non Local Effects on Oxygen--Induced Surface Core Level Shifts of Re(0001);
E. Miniussi, E. R. Hernandez, M. Pozzo, A. Baraldi, E. Vesselli, G. Comelli, S. Lizzit,  D. Alfè
J. Phys. Chem. C 116, 23297 (2012).
Last Updated on Friday, 18 May 2018 08:55