| AVS 57th International Symposium & Exhibition | |
| Surface Science | Thursday Sessions |
| Session SS-ThM |
| Session: | Adsorption and Reactivity on Oxide Surfaces |
| Presenter: | P.S. Bagus, University of North Texas |
| Authors: | P.S. Bagus, University of North Texas C.J. Nelin, Maury's Trail E.S. Ilton, Pacific Northwest National Laboratory |
| Correspondent: | Click to Email |
We present an analysis of the width of X-Ray photoelectron spectroscopy, XPS, features that relates the broadening of these features to the chemical activity and reactivity of oxide surfaces. The use of synchrotron radiation allows high resolution measurements of core-level XPS with instrumental resolutions of ~250 meV, or even higher. However, the XPS spectra of many materials, especially oxides and other ionic systems, have features that are much broader than would be expected from instrumental and lifetime broadenings. For example, there are four levels that contribute to the main Mn 2p3/2 XPS peak of MnO that are not fully resolved with high resolution synchrotron measurements [1] even though the levels are separated from each other by ~1.5 eV. [2] In another example for the XPS of CeO2, [3] broadenings of the theoretical results of ~2.5 eV FWHM are required to have the theory match experiment. While surface effects or other inhomogenieties might, in principle, account for large broadenings, we investigate here the contribution to XPS broadening due to vibrational excitations in the final ionic states. Large Franck-Condon broadenings are known for photoemission from molecules [4] but have not been studied in connection with the broad features in the XPS of ionic crystals. We investigate changes in the metal-oxygen bond length for ionic states of MnO and CeO2, where a metal core-level has been ionized, using wavefunctions and energies for embedded cluster models of these materials. Initial and final state potential energy surfaces are determined for breathing motions of the nearest oxygen neighbors of the ionized metal cation. These surfaces allow us to determine the Franck-Condon envelope for transition from the ν =0 initial, unionized state to vibrationally excited levels of the final, ionized state and show the broadenings that arise from these excitations. Furthermore, we make correlations between the changes in the metal-oxygen bond distances in the ionic states with changes, increases, in the final state covalent metal-oxygen bonding over the initial state covalent character. This correlation may provide a new and novel way to use high resolution photoemission to obtain information about the nature and strength of the bonding in ionic materials.
(1) V Bayer, R Podloucky, C Franchini, F Allegretti, X Bo, G Parteder, MG Ramsey, S Surnev, FP Netzer: Phys. Rev. B 76, 165428 (2007).
(2) PS Bagus, ES Ilton: Phys. Rev. B 73, 155110 (2006).
(3) PS Bagus, CJ Nelin, ES Ilton, M Baron, H Abbott, E Primorac, H Kuhlenbeck, S Shaikhutdinov, HJ Freund: Chem. Phys. Lett. 487, 237 (2010).
(4) PS Bagus, EK Viinikka: Phys. Rev. A 15, 1486 (1977).