Calculation of Electron Inelastic Mean Free Paths (IMFPs). VII. Reliability of the TPP-2M IMFP Predictive Equation

We report comparisons of electron inelastic mean free paths (IMFPs) determined from our predictive IMFP equation TPP-2M and reference IMFPs calculated from optical data. These comparisons were made for values of the parameter Nv (the number of valence electrons per atom or molecule) that we have recommended and those that were recommended in a recent paper by Seah et al. [Surf. Interface Anal. 31, 778 (2001)]. The comparisons were made for eight elemental solids (K, Y, Gd, Tb, Dy, Hf, Ta, and Bi) and two compounds (KBr and Y2O3) for which there were appreciable differences in the recommended Nv values from the two sources and for which optical data were available for the IMFP calculations. The average of the root-mean-square (RMS) deviations for the ten materials between IMFPs from the TPP-2M equation with our Nv values and the reference IMFPs was 11.0 % while the corresponding average with the Seah et al. Nv values was 20.2 %. The larger average in the latter comparison was mainly due to large (> 20 %) RMS deviations for four materials (K, Hf, Ta, and KBr). For the other six materials, the RMS deviations with the Seah et al. values of Nv were similar to those with our values of Nv. Based on the comparisons for these ten materials, we believe it is preferable to use our values of Nv in the TPP-2M equation.