Ion-track reaction-rate model for hydrogen production from polymers

A theoretical model has been constructed to calculate the early time evolution of hydrogen radicals that form molecular hydrogen in a single track during the ion irradiation of polymers. The calculated G(H2)-values (number of H2 molecules formed per 100 eV of absorbed energy) were compared to those recently measured during various ion irradiations of polyethylene (PE), polystyrene (PS), and polycarbonate (PC). In this model the LET (Linear Energy Transfer) of the energetic ion was distributed into two regions: (a) a Gaussian track-core of high energy-density which is caused primarily by atomic excitation, and (b) a long range, low energy-density distribution caused by electron scattering. Following the energy absorption, the interaction of a time dependent distribution of free hydrogen radicals, H., originating within the initial track begins. This process is described by a set of chemical rate equations. The effect of radical diffusion out of the original track is also included. Using H. diffusion rates based upon an extrapolation of known values for H2 and He in polymers, the hydrogen ion reaction rates were estimated. The effective C-H bond (or radical pair) energy was determined to be approximately 10 eV, but varied somewhat with polymer and ion species. The model calculates the residual carbon density profile along the ion track. The implications of this profile for other properties of irradiated polymers are also discussed. (orig.)