F. L. Hutson

Auger Lineshapes of Solid Surfaces - Atomic, Bandlike or Something Else?

A simple model is presented to predict the level of localization (i.e., localization onto an atomic, bonding lobe, molecular, or band orbital) of the two‐ and three‐hole final states in Auger lineshapes. In the spirit of the Hubbard model, the extent of localization is predicted from the one‐ and two‐center Coulomb interaction potentials and the relevant valence bandwidths. Results from the model are compared with experiment for the N KVV lineshape from NaNO3, and the S and Si L23VV lineshapes from Li2SO4 and SiO2. These lineshapes exhibit a wide range of localization consistent with the model.

Accurate determination of Auger line shape binding energies

Abstract A quantitative comparison between an experimental and a theoretical description of any Auger spectrum requires a common energy reference point such as the Fermi level. With non-conductors this is problematic, because the Fermi level is not well defined and charging may shift the Fermi level. A common approach used to overcome this problem employs spectra containing the Auger peak, core, and valence band (VB) photoemission peaks. It is then necessary only to locate a common reference point in the Auger and VB spectra relative to the core level peak. We have developed a procedure in which a comparison is made between model theoretical and experimental spectra so that a large energy range (about 20 eV) of each spectrum can be utilized to locate the common reference points. We have found that this method enables accurate reference points to be determined even with severe sample charging. The method is precise enough to observe a differential charging effect in the polyethylene (PE) spectra. Although much smaller than that previously reported, a breakdown in the Cini-Sawatzky expression as modified for covalent systems still is suggested for PE. This breakdown can now be attributed to the limited one-dimensional delocalization required in long chain polymers.

Summary Abstract: Understanding localized behavior in the Auger spectra of covalent systems such as graphite

Abstract : Localized behavior in the CVV Auger lineshapes of atomic and metallic systems are easily understood within the Cini-Sawatsky theory, which assumes the final state holes are either localized in a single atomic orbital or not at all. In covalently bonded systems, intermediate levels of localization may occur such as onto bond or group orbitals. A set criteria to determine the level of localization has been published, however, a general procedure for determining the distortion effects due to intermediate localization has not been reported. This work describes and justifies the use of the Cini expression on various sub-bands of the self fold of the DOS. Application is made to the Auger lineshape of graphite. Although complete delocalization has been indicated for graphite, a reinvestigation of the experimental lineshape, and comparison with a more complete theoretical lineshape, indicates partial localization of the holes in the group orbitals and none in the pi orbitals. This interpretation is consistent with results for ethylene and benzene.