Abstract
The current model for hydrocarbon formation upon the interaction of energetic hydrogen ions with graphite assumes a reaction of adsorbed hydrogen with carbon surface atoms requiring a temperature of 800–900 K. This model is strongly questioned by results of hydrocarbon formation from a layered 13C/12C sample and by desorption measurements after room-temperature implantation. The formation of 13CD4 and 12CD4 is monitored during deuterium bombardment of a pyrolytic graphite sample covered with a layer of 200 Å of 13C. Only for ion energies corresponding to ranges smaller than 200 Å 13CD4 dominates, while at higher energies only 12CD4 is found. The methane is thus formed at the end of ion range rather than in a surface reaction of back-diffusing ions. Desorption experiments using a slow temperature range after room-temperature implantation show the release of CD4 prior to D2, thus indicating a diffusion-limited release of CD4 already formed during the implantation.
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