The intrinsic asymmetry of photoelectron peaks: dependence on chemical state and role in curve fitting.

Correct estimation of peak and background in XP-spectra is necessary for quantitative analysis. The problem is made difficult because neither the peak nor the background have a known shape and their separation requires the photoemission process to be treated as a sequence of well separated events. Background removal can be undertaken using the Tougaard method which is based on a careful analysis of the electron transport taking place in a solid bur this procedure is designed to remove only the extrinsic component. There is a further component of the background which is intrinsic to the peak and associated with the photoemission event at the atomic/molecular level. In this paper we build on previous papers, which were concerned with metallic elements, to assess the importance of oxidation on the intrinsic asymmetric shape of the peak and thus make this available for curve fitting. This was done, using a programme, Tryfit, based on the Shirley type algorithm modified by Proctor to quote separately the peak and background intensity in XP-spectra. The programme allows the shape parameter, kappa, to be extracted which helps in defining the 'intrinsic' shape of a photoelectron peak. The shape parameter, kappa, is found to be independent of instrumental effects and intrinsically related to atomic number and, as now shown in this paper, to a certain extent to chemical state. It is demonstrated in this paper that its use can help in quantifying individual peak contributions to a multicomponent XPS spectrum. (C) 1998 Elsevier Science B.V. All rights reserved.