Mechanism of hydrocarbon formation upon interaction of energetic hydrogen ions with graphite

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.