Paper AS-TuM4
The Active-Background Method in XPS Data Peak-fitting

Tuesday, October 29, 2013, 9:00 am, Room 204

Quantifying the intensity of peaks in x-ray photoelectron spectroscopy (XPS) data requires the proper modeling of the background signal. This is traditionally accomplished by first assessing the background intensity and then peak-fitting the backgroundless spectrum. One of the main disadvantages of the traditional or static method is that it usually requires forcing the background to pass through two user-defined points, one at each side of the spectrum. In contrast, with the active-background method the intensity and shape of the background are defined during the peak-fitting process. Other advantages of the active method are the following:

• The area of the peaks is frequently underestimated when the static method is employed. This is because the operator typically cannot guarantee that there is no appreciable contribution to the signal at the two chosen points; instead, the operator has to assume it.[1].

• The relative contributions of the Gaussian and Lorentzian widths to the FWHM of the peaks are dependent on the fitting energy range when the static method is employed.

• The traditional (static) iterative Shirley-Sherwood background algorithm might fail for some types of spectra. This is not the case for the Shirley Sherwood background when the active method is employed. Another issue is that it takes longer to converge, that is, more iterations are required with the static than with the active method.

• The active treatment allows for simultaneously applying a combination of background types.

• When the background is subtracted before peak-fitting, the Poisson-character of the data is no longer applicable, so the uncertainty of the peak parameters cannot be calculated employing the covariant matrix method.

• The operator might stop thinking about about the background once it has been subtracted, which can lead to errors in the static method. In the active method, the changing role of the background is accounted for as the operator considers other options during peak-fitting.

• Last, but not least, the active method always provides better fits than the static method.

The importance of implementing the active background had been discussed in other reports.[2,3] In this talk some practical examples of the application of the active-background method are fully discussed.

[1] J. Muñoz-Flores, A. Herrera-Gomez. “Underestimation of the peak areas for the static and active background methods in XPS data peak-fitting.” To be submitted.

[2] A.M. Salvi and J.E. Castle. J. Elec. Spec. Rel. Phen. 95 (1988) 45.

[3] J. Vegh. J. Elec. Spec. Rel. Phen. 46 (1988) 411.