Analysis of electronic stopping cross sections of organic molecules

Abstract

The experimental values of electronic stopping cross sections of Li projectiles measured in many organic substances are analyzed with regard to the influence of bonding on the stopping cross section. The invalidity of Bragg's rule, especially for low-velocity projectiles, demands a concept for the analysis which goes beyond simply dividing molecular stopping cross sections into the atomic stopping cross sections of the constituents. Two different approaches are discussed. In the first one, the organic molecule is decomposed into characteristic molecular groups and their corresponding contributions to the molecular stopping cross sections are derived by using only symmetry arguments. The influence of bonding is already included in these values and the problematic atomic stopping cross sections are thus avoided. The second approach, which is more restricted, treats the various bonds in a molecule separately with respect to their specific contributions to the molecular stopping cross section. The results obtained with both procedures are tested by using these values to calculate stopping cross sections of more complex organic compounds. The comparison with measured values shows excellent agreement.