AVS 52nd International Symposium
    Applied Surface Science Wednesday Sessions
       Session AS-WeM

Paper AS-WeM2
Quantitative XPS: Quadrupole Terms, Shake-Up, Shirley Background and Relative Sensitivity Factors

Wednesday, November 2, 2005, 8:40 am, Room 206

Session: Essential Tools for Surface Analysis
Presenter: M.P. Seah, National Physical Laboratory, UK
Authors: M.P. Seah, National Physical Laboratory, UK
I.S. Gilmore, National Physical Laboratory, UK
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An analysis is provided of the XPS intensities measured in the NPL XPS database for 46 solid elements. This present analysis does not change the previous conclusions concerning the excellent correlation between experimental intensities following deconvolution using the angle-averaged REELS data and the theoretical intensities involving the dipole approximation and Scofield's cross sections. Here, we test the more recent calculations for cross sections by Trzhaskovskaya et al with quadrupole terms and find that their cross sections diverge from the database by up to a factor of 2. The quadrupole angular terms lead to small corrections also evaluated in the present analysis. Measurements of the extent of shake-up for the 46 elements broadly agree with theoretical calculations but details of the observed element-to-element variation are not reproduced in those calculations. The predicted theoretical constancy in the shake-up contribution implies that the use of the Shirley background will lead to a peak area that is a constant fraction of the true peak area including the shake-up intensities. If this were the case, the peak areas using the Shirley background would be a valid measure of intensity. We would therefore expect that the ratio of the peak area using the Shirley background to the intensity calculated using the cross sections, etc, would be a constant value, less than unity, with a reasonably small scatter. Unfortunately, the variability of the shake-up contribution, and the variability with which the Shirley background reproduces the angle-averaged background under the peak, itself, both combine to leave a 38% relative standard deviation in this ratio. The Shirley background thus appears to be a poor method for general quantitative analysis. Its use may need to be limited to specific situations where the sensitivity factors have been obtained from reference samples similar to those being analysed.