Background Width – How Wide should a Background be?
This example shows a Shirley Background that includes only the Two (2) main peaks. This Background does not include the satellites that are due to shake-up or multiple splitting . Is that OK? Usually not. But sometimes the data-analyst does not need that information. Be careful if you limit the background range to be small. You might miss important chemistry information.
This example shows a Shirley Background that includes the high BE satellite structure. This wider background requires the data-analyst to think more and to assign a chemical state or electronic state to the structures at higher BE.
This example shows two iterated Shirley background widths (ranges) for the Ca (2p) peaks in CaCO3. The shorter lower background produces an obvious endpoint that intercepts real spectral data at ~352 eV. Is this a better background than the longer Shirley background that stops at ~364 eV?
Perhaps a Linear background would allow all peaks above the background to be included. A Tougaard Background (not shown) might be the better background. Perhaps we should use two (2s) separate Shirley backgrounds. This example raises many questions about which Background Shape (TYPE) to use and which Background Width (range) to use. Later in this webpage we will provide more guidance on this questions, and also describe how the backgrounds are different in the way they correct for XPS signal.
This example shows two iterated Shirley background widths (ranges) for the Cr (2p) peaks in Cr2O3 (exposed bulk of a single crystal). The shorter higher background produces an obvious endpoint that intercepts real spectral data at ~592 eV. Is this a better background than the longer Shirley background that stops at ~620 eV? These difference choices affect the information that can be derived from this spectrum.
The following two (2) survey spectra use two different Background Widths to integrate the peak area of the Cu (2p3) signal because the Cu (2+) chemical state produces a Shake-up Structure that should be included with peak-fitting the Cu (2p3) signal and also when generating Atom % values for the amount of Copper present in this sample of pure CuO.
Tougaard Background Introduced
This spectrum has a very wide Iterated Shirley Background Width and a Tougaard Background width. Tougaard Background widths are often very wide because they take into account more electronic interactions that are not directly considered when using an Iterated Shirley Background.
This example shows 3 distinct Shirley Backgrounds. Which one is correct? Which was is best for generating Atom% quantitation results? Which one is best if we are peak-fitting? These questions are not easy to answer.
This set of spectra shows the basic anatomy of a chemical state spectrum for a pure graphite (HOPG) type of carbon. Please note that most chemical state spectra have spectral windows that are between 20 and 30 eV in width. This one has a 45 eV spectral window width. This wider than normal width allows the data-analyst to learn more about the “extrinsic” energy loss plasmon that starts at ~298 eV. The very high S/N of this spectrum allows the data-analyst to vertically zoom the spectrum to get a clear view of the pi-pi* bands that exist for carbon having conjugated sp2 pi-bonds.