Invited Paper AS+BI+RA-MoM1
Improving Accuracy in Quantitation by XPS: Standards, Cross-sections, Satellite Structure

Monday, October 21, 2019, 8:20 am, Room A211

Determining elemental composition by XPS requires determining relative peak intensities from the elements concerned, and then normalizing using Relative Sensitivity Factors (RSF’s). Such RSF’s are usually generated from standard materials using the intensity from the “main” peak only and ignoring any associated satellite structure (shake-up, shake-off, and multiplet splitting components) intensity. In the seminal paper by Wagner, et al, (1) this resulted in a conclusion that the calculated Li(1s) photoionization cross-section, σ, (2) was too low by ~40%, relative to F(1s). This apparent discrepancy cast doubt on the claimed 5% accuracy of the calculated σ values for low Z elements (2). We show that this is incorrect. The discrepancy is due, primarily, to the fact that Li(1s) loses almost zero intensity from the main peak into associated satellites, whereas for F1s it is substantial, spreading over 100ev to lower KE. A calculated σ always refers to the total intensity of photoemission from the orbital concerned, including any intrinsic satellite structure. In addition, the experimental Li(1s) peak intensity in Wagner, et al. (1) was actually overestimated, owing to inclusion of overlapping satellite structure spreading from the nearby F(2s) peak. For the 1s intensities of the first row elements, a crude theoretical approximation predicts the total intensity lost from the “main” peak to satellites quite well. It involves an exponential dependence on the number of valence shell electrons present, which for Li+ of LiF, is zero, and for F- is 6 (the filled 2p shell), yielding values of zero loss for Li+ and 20-30% for F-. Full ab initio quantum calculations for these ions support the crude estimate (calculated Li+ losses are 1.4%; F- losses are 22.7%). In contrast to earlier claims to the contrary(3), satellite losses, for elements across the periodic table, vary widely from core-level to core level, element to element, and most critically with the bonding situation of an element (see, for example ref 4). Thus any “universal” element RSF’s, using main peak intensities only, though semi-quantitatively useful, are inherently limited in potential accuracy achievable, sometimes by up to a factor of 2. What is needed, for improved accuracy, are sets of RSF’s appropriate to different bonding situations. Such sets would be particularly useful for the current large inexperienced XPS user base. We attempt to give guidelines.

1. C.D. Wagner, et al, Surf. Interface Anal. 3, 211, 1981

2. J.H. Scofield, J. Elec. Spec. 8, 129, 1976

3. V.G. Yarzhemsky, et al, J. Elec. Spec. 123, 1, 2001

4. P. S. Bagus, E. Ilton, and C. J. Nelin, Catalysis Letters, 148, 1785, 2018