AVS 49th International Symposium
    Surface Science Thursday Sessions
       Session SS-ThM

Paper SS-ThM2
Origin of the Negative Shift Observed in the XPS Spectra of Cu and Ag Cations having d@super 10@ Electronic Configuration

Thursday, November 7, 2002, 8:40 am, Room C-108

Session: Electronic Structure and Stimulated Processes
Presenter: D.A. Kukuruznyak, University of Washington
Authors: D.A. Kukuruznyak, University of Washington
J.G. Moyer, University of Washington
A.L. Ankudinov, University of Washington
J.J. Rehr, University of Washington
F.S. Ohuchi, University of Washington
Correspondent: Click to Email
We discuss the phenomenon of the negative chemical shift in the XPS spectra, where an oxidized ion appears to be chemically reduced. The effect is illustrated in the the Cu@super 1+@ cation within a spinel crystal structure and in Ag@super 1+@ in silver fluorides, AgF and AgF@sub 2@. We have found that the negative chemical shift of the core levels is caused by similar shifts of the ions 3-d and 4-d Valence Bands. We therefore modeled the experimental valence band photoemission spectra by theoretical DOS of the d-levels using an ab initio FEFF8 code. This code is based on a relativistic Greens function real space full multiple scattering formalism and allows accurate determination of the position of the levels with respect to the Fermi energy. We have determined that the negative shift of the d-levels is not caused by a long-range electrostatic interaction (Madelung potential). The amount of the total charge on the Cu@super 1+@ cation in the spinel structure was similar to that of copper in Cu@sub 2@O, thus charging is not the cause of negative shift either. It was also found that only Cu 4s and 4p electrons participate in the chemical bonding. Completely filled d@super 10@ shells of the copper in spinel structure do not form a band, but were localized, having an atomic-like character. The 3d-level therefore appeared as a false valence band edge in the spectrum. A similar effect was observed for the silver compounds. We claim that the negative chemical shift is not due to charging or splitting effects, but occurs when a completely occupied non-bonding d@super 10@ shell appears on the XPS spectra as a false valence band edge.