Molecular fragmentation and formation processes in PDMS are studied by measuring and discussing positive and negative ion spectra of pure carbon or hydrocarbon compounds. Calculations of the hydrodynamic adiabatic expansion of the material from the inner track result in the formation of carbon clusters in agreement with the negative ion spectra. A universal pattern in the negative ion spectra can thus be explained. Results of molecular breakup calculations are compared with the observed positive ions which are assumed to be from the ultratrack. From the comparision we conclude that the breakup of the carbon bonds is entropic and takes place during the desorption while the main part of the hydrogen separation occurs afterwards.