We report calculations of electron inelastic mean free paths (IMFPs) of 50-2000 eV electrons for a group of 14 organic compounds: 26-n-parafin, adenine, ficarotene, bovine plasma albumin, deowyribonucleic acid, diphenylhexatriene, guanine, kapton, polyacetylene, poly(butene-1-sulfone), polyethylene, polymethylmethacrylate, polystyrene and poly(Zvioy1pyridme). The computed IMFPs for these compounds showed greater similarities in magnitude and in the dependences on electron energy than was found in our previow calculations for groups of elements and inorganic compounds (Papers II and 111 in this series). Comparison of the IMFPs for the organic compounds with values obtained from our predictive IMFP formula TPP-2 showed systematic differences of -40%. These differences are due to the extrapolation of TPP-2 from the regime of mainly high-density elements (from which it had been developed and tested) to the low-density materials such as the organic compounds. We analyzed the IMFP data for the groups of elements and organic compounds together and derived a modified empirical expression for one of the parameters in our predictive IMFP equation. The modified equation, denoted TPP-2M, is believed to be satisfactory for estimating IMFPs in elements, inorganic compounds and organic compounds.

We have calculated inelastic mean free paths (IMFPs) for 41 elemental solids (Li, Au and Bi) for electron energies from 50 eV to 30 keV. The IMFPs were calculated from experimental optical data using the full Penn algorithm for energies up to 300 eV and the simpler single-pole approximation for higher energies. The calculated IMFPs could be fitted to a modified form of the Bethe equation for inelastic scattering of electrons in matter for energies from 50 eV to 30 keV. The average root-mean-square (RMS) deviation in these fits was 0.48%. The new IMFPs were also compared with IMFPs from the predictive TPP-2M equation; in these comparisons, the average RMS deviation was 12.3% for energies between 50 eV and 30 keV. This RMS deviation is almost the same as that found previously in a similar comparison for the 50 eV-2 keV range. Relatively large RMS deviations were found for diamond, graphite and cesium. If these three elements were excluded in the comparison, the average RMS deviation was 9.2% between 50 eV and 30 keV. We found satisfactory agreement of our calculated IMFPs with IMFPs from recent calculations and from elastic-peak electron-spectroscopy experiments.

This work extends our previous calculations (Surf. Znterfme Anal. 11, 577 (1988)) for the 200-2000 eV range. Substantial variations were found in the shapes of the IMFP versus energy curves from element to element over the 50-200 eV range and we attribute these variations to the different inelastic scattering properties of each material. Our calculated IMFPs were fitted to a modified form of the Bethe equation for inelastic electron scattering in matter; this equation has four parameters. These four parameters could be empirically related to several material parameters for our group of elements (atomic weight, bulk density and number of valence electron per atom). IMFPs were calculated from these empirical expressions and we found that the root mean square difference between these IMFPs and those initially calculated was 13%. The m d i e d Bethe equation and our expressions for the four parameters can therefore be used to estimate IMFPs in other materials. The uncertainties in the algorithm used for our IMFP calculation are difficult to estimate but are believed to be largely systematic. Since the same algorithm has been used for calculating IMFPs, our predictive IMFP formula is considered to be particularly useful for predicting the IMFP dependence on energy in the 50-2000 eV range and the material dependence for a given energy.

An analysis is given of the consistency of calculated and measured electron inelastic mean free paths ͑IMFPs͒ near solid surfaces for electron energies between 50 and 10 4 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 ͑EPES͒. 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 Au͒ 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 Au͒ 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 element͒ 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 Pt͒ and of calculated IMFPs for six compounds ͑Al 2 O 3 , SiO 2 , KCl, poly͑butene-1-sulfone͒, polyethylene, and polystyrene͒.

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