Paper SS-TuP17
Charge Transfer and Shake Satellites in XPS: A Unified View

Tuesday, October 19, 2010, 6:00 pm, Room Southwest Exhibit Hall

The satellites in X-Ray photoemission spectroscopy, XPS, can provide information about the electronic structure, especially for ionic materials. [1,2] We present a new analysis of the XPS satellites that, when applied to CeO₂ which is an important substrate for model studies of catalysis, enables a new understanding of this material. While the origin of intra-atomic effects in terms of atomic near-degeneracies is well understood, [1] there are two different ways of describing the inter-atomic effects that lead to satellites. The first is to ascribe the satellites to charge transfer, CT, from ligands, or other neighboring atoms, into unoccupied or partly occupied shells of the ionized atom. This interpretation is commonly applied to oxides and other ionic crystals. [2] In the CT model, the satellites gain intensity through the mixing of XPS allowed configurations with XPS forbidden configurations. However, changes in the orbitals between the initial and final states are normally neglected in the CT model. On the other hand, the second way of understanding satellites as shake satellites, [3,4]ascribes the driving force for the loss of intensity from the main peak into the satellites to the orbital relaxation, which screens the core-hole. This orbital relaxation allows valence excited, or shake, states to obtain intensity. [3] However, both mechanisms must be taken into account to correctly describe the inter-atomic contributions to XPS satellites and, in the present work, we reconcile these two different points of view. We show that the covalent mixing of metal and ligand orbitals in the wavefunctions, for both initial and final states, is a powerful way to understand the intensities of satellites due to inter-atomic effects. For the XPS of NiO, where both intra and inter-atomic effects must be taken into account, our analysis of the CI wavefunctions shows the underlying covalent character of the main and satellite peaks. We also stress the importance of covalent mixing in the initial state in order for there to be substantial satellite intensity. We show that different degrees of covalent character in the initial states of CeO₂ and LaAlO₃ lead to very different satellite energies and intensities for the isoelectronic metal cations. Our analysis provides direct and unambiguous evidence that the inter-atomic satellite features are closely related to the covalent binding in the material.

(1) PS Bagus, ES Ilton: Phys. Rev. B 73, 155110 (2006).

(2) FMF de Groot: J. Electron Spectrosc. Relat. Phenom. 67, 529 (1994).

(3) T Aberg: Phys. Rev. 156, 35 (1967).

(4) L Sangaletti, F Parmigiani, PS Bagus: Phys. Rev. B 66, 115106 (2002).