Chemical state information from the near-peak region of the X-ray photoelectron background

Following the work of Tougaard [Appl. Surf. Sci. 100/101 (1996) 1] it is now generally accepted that the contribution to the energy loss background of a photoelectron peak arising from the transport of electrons through the solid is very small at the position of the peak. Nevertheless, the sharp rise in background at the peak, as originally defined by Shirley [Phys. Rev. B 5 (1972) 4709] is very real and is probably the most easily recognised feature of the X-ray photoelectron spectrum. To undertake quantification in XPS analysis it is normal to remove a background which is based on the original Shirley algorithm, i.e. an integration of the peak lying above background using an integration constant which allows the background to merge with the experimental data at some point beyond the peak. At this point in the spectrum the extrinsic loss, defined by Tougaard, has become significant and varies with the thickness of any overlayer which might be present, including any surface contamination. Thus the Shirley scattering parameter, as the constant of integration has been called, contains a contribution from extrinsic losses and a contribution from the rise in background at the peak, which for clarity we have referred to as the intrinsic loss. We have developed a method to obtain a value for the intrinsic part of the background, defining it by a parameter, kappa. The value of kappa varies systematically from one element to another and from one chemical state to another. This behaviour is shown to provide an additional basis for interpretation of XPS spectra (C) 2001 Elsevier Science B.V. All rights reserved.