Methane production by deuterium impact at carbon surfaces

Molecular dynamics simulations have been performed of the sputtering of both deuterated amorphous carbon and graphite surfaces by D2 molecules at impact energies ranging from 7.5 to 30 eV/D. These simulations were done under conditions that replicate, as closely as possible, prior experimental investigations of D2+ impacts on ATJ graphite. The substrate structure is heavily modified by cumulative impacts, and the sputtering yields change with increasing fluence. For the graphite sample, the surface continues to evolve up to the highest fluences studied, 3.8 × 1020 D/m2. For the deuterated amorphous carbon surface, however, the sample reaches a steady state at fluences of less than 1 × 1020 D/m2, at which point the structure and sputtering yields change slowly, aside from statistical fluctuations. The structure of this ensemble of steady-state surfaces is examined in detail, and is highly supersaturated with deuterium, with an enrichment in sp3 carbon. The yields of the hydrocarbons sputtered from this set of surfaces show good agreement with experiment, and are significantly larger than those from the unmodified, bulk-like amorphous carbon surface, where atomic and radical species dominate. These simulations indicate that it is both feasible and necessary to perform sputtering simulations on surfaces that have been dynamically created by impacts, if meaningful comparison with experiment is desired.