On the invalidity of Bragg's rule in stopping cross sections of molecules for swift Li ions

Abstract

We discuss the invalidity of Bragg's rule for stopping cross sections of molecules for Li ions in the velocity range 1.5 · 108 cm/sec to 4.8 · 108 cm/sec. Here the influence of the chemical bonding in a molecule normally leads to strong deviations from Bragg's additivity rule. Only in a few of the investigated compounds the deviations seem to be small compared with the experimental error of a few percent. In our boron compounds the measured cross section of the molecule is smaller than the sum of the stopping cross sections of the single constituents. This can be explained in a first order description by the transfer of electrons in the bonding. With this description it is possible to determine from the measured molecular stopping cross sections the charge transfer in certain compounds.