Electron bombardment enhancement of the reactivity of graphite with atomic hydrogen: threshold-like effects

The chemical reactivity of graphite surfaces exposed to atomic hydrogen is enhanced under electron bombardment by an enhancement factor ε′ which depends on the incident electron energy. A sharp threshold-like increase in ε′ is observed for crystalline graphite at an energy Eth which depends on the angle of incidence θ of the electron beam with respect to the basal-plane surface normal. Measurements of the dependence of Eth on θ are reported here. Model calculations indicate that the effect is due either to a direct excitation of the electronic transition responsible for the enhancement or to plasmon production which subsequently drives the transition. Further experiments are suggested to distinguish between these mechanisms.