We report in situ x-ray photoemission, ultraviolet photoemission (UPS), and electron-energy-loss photoelectron-spectroscopy studies of the electronic structure of nitrogen-containing amorphous hydrogenated carbon (a-CN:H). Films with different nitrogen concentrations are prepared by the ion-beamdeposition technique. The photoemission measurements do not reveal a shift in the Fermi level with respect to the top of the valence band, as would be expected from an n-type nitrogen-doping mechanism proceeding via substitution. The C 1s core-level shifts observed as a function of nitrogen content are identified as chemical in nature, and the variation in line shape and peak position is explained in terms of a two-phase superposition model. A simple model fit to the He I UPS valence-band spectra reveals the effects of nitrogen on the spectral shape. It illustrates, in particular, the sensitivity of the partial p-m density of states (DOS) to the presence of nitrogen, and confirms the role played by the m bonds in controlling the electronic structure of a-C:H. The observed increases in area and width of the p-~DOS with increasing nitrogen content are in line with an existing model which favors a defect density increase and a size increase of the graphitelike islands formed by the clustering of m. states in aromatic rings.

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Abstract.Highly oriented pyrolytic graphite (HOPG) as well as polycrystalline graphite (pcgraphite) were bombarded with 3.5keV H + ions by means of a Penning ion source. The implanted graphite was characterized by in situ electron spectroscopy techniques such as UPS, XPS and EELS. Our UPS valence band measurements of the hydrogen saturated graphite reveal it to be an insulating phase, and XPS measurements show a shift of the Cls core level to higher binding energy with respect to pristine graphite. This behavior is explained by a Fermi energy shift upon hydrogen bombardment of graphite. In addition, a close resemblance in the electronic structure of hydrogen bombarded graphite and amorphous hydrogenated carbon films (a-C :H) is shown which suggests the modification of pristine graphite to an amorphous network [1] of mostly tetrahedrally bonded carbon atoms by hydrogen implantation. 79.20-m, 79.60-i, 73.60-n Graphite is an interesting material in the field of plasma fusion research because of its outstanding physical properties as a first wall material for limiters in fusion reactors (Tokamak) [2][3][4][5]. In particular, one requires a low-Zmaterial with a low vapor pressure. Graphite meets these specifications. PACS:During a tokamak discharge, the first wall interacts with the plasma and is bombarded by hydrogen species which leads to a considerable modification of the surface. Due to the immobility of the implanted hydrogen ions at room temperature, an upper limit of the hydrogen content nn/nc --0.45 is fou/ad in the near-surface region