Methane production from ATJ graphite by slow atomic and molecular D ions: Evidence for projectile molecule-size-dependent yields at low energies

We present experimental results for methane production from ATJ graphite impacted by atomic and molecular D ions in the energy range 5–60 eV/D. A systematic trend of the methane yields for the different molecular species compared at the same impact energy/D is observed: while all three species lead to methane yields that coincide within the experimental uncertainty at the high energy end of the investigated range, at lower energies the yields diverge by progressively larger amounts, with the incident triatomic molecular ion leading to the largest yields per atom, and the atomic ion to the smallest. The difference at the lowest investigated energy (10 eV/D) is about a factor of two. Total chemical sputtering yields obtained by classical molecular dynamic simulations also indicate that molecular projectiles lead to larger yields per atom than atomic projectiles. The energy dependence of the total yield increase obtained by the simulations, however, is different than that observed experimentally for methane production.