| AVS 64th International Symposium & Exhibition | |
| Applied Surface Science Division | Tuesday Sessions |
| Session AS+MI+SS-TuM |
| Session: | Quantitative Surface Analysis: Effective Quantitation Strategies |
| Presenter: | Alberto Herrera-Gomez, CINVESTAV-Unidad Queretaro, Mexico |
| Authors: | A. Herrera-Gomez, CINVESTAV-Unidad Queretaro, Mexico D. Cabrera-German, Universidad de Sonora F.-S. Aguirre-Tostado, CIMAV-Monterrey A. Dutoi, University of the Pacific M.-O. Vazquez-Lepe, Universidad de Guadalajara P. Pianetta, Stanford University D. Nordlund, Stanford Synchrotron Radiation Lightsource O. Cortazar-Martínez, CINVESTAV-Unidad Queretaro, Mexico L. Gomez-Muñoz, Cinvestav-Unidad Queretaro, Mexico A. Torrea-Ochoa, CINVESTAV-Unidad Queretaro |
| Correspondent: | Click to Email |
By describing the photoelectric phenomenon as the absorption of a photon by a core electron and its subsequent emission, it is possible to understand many of the features found in XPS data ... as long as the photon energy is far away from the threshold of deeper core levels. For these cases, “the independent particle approximation, which had been thought to be applicable to atomic photoionization cross sections well above threshold, is clearly not applicable.”[1,2] Even at energies away from deeper thresholds, quantification of the composition can be done only if we are also willing to employ empirical background subtraction methods such as the Shirley function to assess peak intensities. Through this method, employed in conjunction with others methods accounting for the background due to inelastic scattering (i.e., Tougaard background), it is possible to properly reproduce the experimental background [3] and assess the composition of surfaces. Of course, the intensity related to the background is ignored for composition calculations. Ignoring the background due to inelastic scattering is perfectly self-consistent because these electrons are effectively accounted by the Debye-Waller attenuation factor calculated from the inelastic mean free path. However, there is not any attenuation factor to account for the ignored Shirley electrons.
Through the study of the peak intensity of the Cr 3p peak with photon energies around the Cr 2s threshold, we found that these two issues, 1) channeling effects on the peak intensity and 2) the need to ignore the Shirley electrons for composition calculations, are, in fact, related. The behavior of the Cr 3p background strongly suggest coupling between the 3p and the valence states. These coupling most exist for a brief time after photon absorption, and the observed photoelectrons should actually come from one of these coupled states. That is, “describing the photoelectric phenomenon as the absorption of a photon by a core electron and its subsequent emission” is not the complete paradigm for photoemission.
[1] E.W.B. Dias, H.S. Chakraborty, P.C. Deshmukh, S.T. Manson, Breakdown of the Independent Particle Approximation in High-Energy Photoionization, (1997) 4553–4556.
[2] W. Drube, T.M. Grehk, S. Thieß, G.B. Pradhan, H.R. Varma, P.C. Deshmukh, S.T. Manson, Pronounced effects of interchannel coupling in high-energy photoionization, 46 (2013). doi:10.1088/0953-4075/46/24/245006.
[3] 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.