The hydrogen inventory in plasma exposed graphite surfaces

The trapping of hydrogen in graphite was investigated during exposure to an RF-discharge. The target placed in the wall of the RF-reactor was investigated in-situ by ion beam analysis. The hydrogen inventory was found to be twice as high as expected from ion beam implantation experiments in the similar energy range ≤200 eV. Also an enhanced ion induced desorption by the energetic analyzing beam was observed. This leads to the conclusion that a buildup of a hydrogen rich C:H-layer is formed by the simultaneous impact of thermal and energetic (≤200 eV) hydrogen. A model is presented which explains the formation of the C:H-layer by a stitching process of methyl radicals forming on the surface. Time resolved trapping measurements were performed to test for a transient uptake of hydrogen during plasma exposure. No such phenomenon could be detected below 600 K. Also the supposition that metallic impurities are responsible for a dynamic inventory at room temperature could be disproved. (orig.)