Physics colloquium-“Experimental Clues About the Origin of the Shirley Background”
February 5 @ 4:00 pm
Center for Research & Advanced Studies of the National Polytechnic Institute, CINVESTAV (Queretaro, Mexico)
Abstract: In quantitative chemical studies with X-ray photoelectron spectroscopy, the total background signal is well described as the sum of two terms, one originated from inelastic electron-energy losses and another that is described by the empirical Shirley method [1,2]. The inelastic part is very well quantified by the Tougaard theory proposed in 1982 ; the Shirley part, in the near-peak region, can be quantified by the method illustrated by Proctor and Sherwood . Since the Tougaard background meets the experimental signal at binding energies ~ 50 or 100 eV above the peak, the Shirley contribution must banish at those energies. Therefore, the Shirley signal begins as a step-like function at the peak position to then vanish at higher binding energies (examples of the Shirley-signal in the entire range will be presented). We showed that the Shirley part of the background is related to the capture of photons by deeper levels . The physical process can be described as interchannel coupling  but adding a strong involvement of the valence band, i.e., a process resembling off-resonant participator photoemission with the generation of a shake-off type signal. Based on this model, an approximate method to estimate the Shirley signal in the near-peak region will be described.
 A. Herrera-Gomez, M. Bravo-Sanchez, F.-S. Aguirre-Tostado, M.-O. Vazquez-Lepe, The slope-background for the near-peak regimen of photoemission spectra, J. Electron Spectros. Relat. Phenomena. 189 (2013) 76–80. doi:10.1016/j.elspec.2013.07.006.
 A. Herrera-Gomez, M. Bravo-Sanchez, O. Ceballos-Sanchez, M.O.O. Vazquez-Lepe, Practical methods for background subtraction in photoemission spectra, Surf. Interface Anal. 46 (2014) 897–905. doi:10.1002/sia.5453.
 S. Tougaard, P. Sigmund, Influence of elastic and inelastic scattering on energy spectra of electrons emitted from solids, Phys. Rev. B. 25 (1982) 4452–4466. doi:10.1103/PhysRevB.25.4452.
 A. Proctor, P. Sherwood, Data analysis techniques in x-ray photoelectron spectroscopy, Anal. Chem. (1982) 13–19.
 A. Herrera-Gomez, D. Cabrera-German, A.D.A.D. Dutoi, M. Vazquez-Lepe, S. Aguirre-Tostado, P. Pianetta, D. Nordlund, O. Cortazar-Martinez, A. Torres-Ochoa, O. Ceballos-Sanchez, L. Gomez-Muñoz, Intensity modulation of the Shirley background of the Cr 3p spectra with photon energies around the Cr 2p edge, Surf. Interface Anal. 50 (2018) 246–252. doi:10.1002/sia.6364.
 E.W.B. Dias, H.S. Chakraborty, P.C. Deshmukh, S.T. Manson, Breakdown of the Independent Particle Approximation in High-Energy Photoionization, Phys. Rev. B. 78 (1997) 4553–4556.
Berthoud Hall 241