The ubiquity of C-H bond breaking by MeV ion irradiation

Abstract

Hydrocarbons are inadvertently present on the surfaces of most materials, and hydrogen is a major contaminant in amorphous carbon. These C-H bonds are easily broken by the electronic excitation induced by the passage of an energetic (MeV) ion, as seen, for example, in the registration of ion damage tracks in many organic materials. In this session of the conference we shall see that, when the C-H bonds are broken, structural modification of the material occurs. These structural changes include, for example, not only the loss of hydrogen, but also involve graphite formation in amorphous carbon and hydrocarbons, and surface or interface atom rearrangement in other materials. This effect is important not only in interactions with “dirt” but also as a phenomenon that can be exploited for the modification of material properties. We speculate herein that the effects observed can be accounted for by a model of the process based on damage induced by secondary electrons arising along the path of the beam ion. The simultaneous breaking of several adjacent C-H bonds by a number of these electrons allows stronger C-C or C-substrate atom bonds to form, which in turn produce atomic rearrangement. The freed hydrogen atoms eventually combine to form H_2 which diffuses from the material.