Evaluation of Calculated and Measured Electron Inelastic Mean Free Paths Near Solid Surfaces

An analysis is given of the consistency of calculated and measured electron inelastic mean free paths (IMFPs) near solid surfaces for electron energies between 50 and 104 eV, the energy range of relevance for surface analysis by Auger electron spectroscopy and x-ray photoelectron spectroscopy. This evaluation is based on IMFPs calculated from experimental optical data and on IMFPs measured by elastic-peak electron spectroscopy (EPES). We describe the methods used for the calculations and measurements, and we identify the various sources of uncertainty. Most of our evaluation is based on IMFPs for seven elemental solids (Al, Si, Ni, Cu, Ge, Ag, and Au) for which there were at least two sources of IMFP calculations and at least two sources of IMFP measurements for each solid. Our comparison of the calculated IMFPs showed a high degree of consistency for Al, Ni, Cu, Ag, and Au. The comparison of measured IMFPs showed greater scatter than for the calculated IMFPs, but reasonable consistency was found for the measured IMFPs of Cu and Ag. The measured IMFPs for four elements (Ni, Cu, Ag, and Au) showed good consistency with the corresponding calculated IMFPs. It is recommended that IMFPs for these four elements (determined from fits of a simple analytic expression to the calculated IMFPs for each element) be used as reference values in future EPES experiments. More limited comparisons have been made of calculated and measured IMFPs for four additional elements (Fe, Mo, W, and Pt) and of calculated IMFPs for six compounds (Al2O3, SiO2, KCl, poly(butene-1-sulfone), polyethylene, and polystyrene).