Chemical Sputtering and Surface Damage of Graphite by Low Energy Atomic and Molecular Hydrogen and Deuterium Projectiles...

We present experimental methane production yields for H⁺, H₂⁺, and H₃⁺ ions incident on ATJ graphite in the energy range 10-250 eV/H. Below about 60 eV/H, the molecular H species give higher methane yields/H when compared with isovelocity H⁺. The results are interpreted by considering the differences of the maximum binary collision energy transfer in the ejection of chemical sputtering products associated with undissociated molecules and incident atomic ions, using the same analysis as developed by Yao et al. (PRL 81, 550(1998)) in comparing sputtering of Au by isovelocity N⁺ and N₂⁺ ions. For both D and H atomic and molecular projectiles, the yields/atom coalesce onto a single curve below projectile energies of approximately 60 eV/atom, when plotted as function of maximum energy transfer, under the assumption that the incident molecular species are undissociated when ejecting the hydrocarbon chemical sputtering product. Raman spectroscopy of a graphite sample exposed to high fluences of D⁺ and D₃⁺ beams at high and low energies, confirmed the expectation that, according to this argument, there should also be more surface damage by incident molecular species than by isovelocity atomic ions. The two high-energy beam-exposed spots showed similar damage, while the low-energy molecular-beam- exposed spot showed slightly more damage than the corresponding D⁺ beam exposed spot.