Electron Inelastic Mean Free Paths in Organic Materials Especially for Polyethylene and Guanine.

Abstract

We have calculated the electron inelastic mean free paths (IMFPs) in the range of 50∼2000 eV for 14 different organic compounds using the Penn algorithm. We report here mainly the calculated values of IMFPs for polyethylene and guanine, because their difference in density is the largest in this group of organic compounds. All the compounds had similar electron energy loss functions, and the computed IMFPs were similar in their magnitude and in the dependence on electron energy. The ratio of each calculated IMFP was 1.4 at 2000 eV ; this ratio is much smaller than those of metallic elements and inorganic compounds. Comparison of the calculated IMFPs for the organic compounds with the values obtained from our predictive IMFP formula TPP-2, which provides parameters in a modified Bethe formula, showed systematic differences of about 40%. These differences are attributed to the application of extrapolation of TPP-2, proposed on the basis of the calculated results of IMFPs for mainly high-density materials, to the low-density materials such as the organic compounds. We have, therefore, developed a modified empirical expression for one parameter in TPP-2 based on the IMFP data for the groups of elements, inorganic and organic compounds. Thus obtained modified equation, denoted TPP-2 M, gave satisfactory values of IMFPs for all materials in which IMFPs were calculated from experimental optical data using the Penn algorithm.