Model for the chemical erosion of graphite due to low-energy H+ and D+ impact

A methane erosion yield model has been developed using the principal atomistic reactions outlined by Kuppers and co-workers [eg., A. Horn, A. Schenk, J. Biener, B. Winter, C. Lutterloh, M. Wittmann, and J. Kuppers, Chem. Phys. Lett. 231, 193 (1994)] with additional terms to account for the energy of the incident particles, namely, kinetic ejection and damage deposition. Furthermore, modifications were made to the previous models by using distributed activation energies for methyl and hydrogen release as well as an activated Eley-Rideal abstraction process. Fitting of this model to experimentally measured methane yield data shows excellent agreement, except for low energy (less than or equal to 25 eV) impact at temperatures above similar to 800 K. We have provided a sound physical basis for the behavior of the free fitting parameters and conclude that most of the processes associated with low-energy impact on pyrolytic graphite leading to methane production have been incorporated. Possible extensions of the model to include heavy hydrocarbons and total chemical erosion yields are also discussed. Due to the lack of comprehensive experimental data on the flux dependence of hydrocarbon erosion, the flux dependence of the fitting parameters could not be explored and so the effect of flux density requires further modeling considerations. (C) 1998 American Institute of Physics.