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new opportunities for the interaction with other species. The current study is related to the application of carbon materials in fusion research devices which has long been dominated by graphite due to its favourable thermal and other material properties. To improve energy confinement in fusion plasmas, it is desirable to minimize vacuum contaminations which may cause radiative energy losses. The main concern is on oxygen impurity control and light oxygen-gettering elements such as boron, beryllium, and lithium have been tested as constituents of the first wall (B, Be) or as wall conditioning additive (Li) [3]. Although the next stage fusion research reactor may not be all-carbon based [4], there will still be certain applications which rely on a more thorough knowledge of the interaction of carbon, light reactive elements, and plasma impurities. In our study, experiments are performed with a-C because this material's structural properties can be controlled by the substrate temperature during deposition. It is hoped that this will contribute to the understanding of the influence of carbon structure on the interaction with reactive species. EXPERIMENT Amorphous carbon samples were deposited on silicon(100) substrates by electron beam evaporation of graphite. Depositions were carried out between room temperature and 800'C. Metal atoms were delivered by commercial alkali-metal dispensers (SAES Getters) through a thermally activated chemical reaction. Performance of all preparation and analysis steps in an ultrahigh vacuum setup with a base pressure of less than 2.10-1° mbar enabled us to keep oxygen contaminations at this stage at about 1 to 2 at%. The alkali-metal content was increased stepwise 167 Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society

and all stages of sample processing were monitored by subsequent in-situ analysis of the films by ultraviolet and x-ray photoelectron spectroscopy (UPS/XPS) in a Leybold-SPECS EA11/100MCD spectrometer. XPS was performed with MgK0 -radiation of 1253.6 eV with a resolution of about 0.9 eV. UPS were excited with the Hel and Hell lines with energies of 21.22 eV and 40.82 eV, respectively. Resolution in UPS was 0.1 to 0.2 eV. Electron energies are referred to the Au 4f 7/2signal of a clean gold sample at 84.0 eV, and its Fermi edge, respectively. Spectra are presented as measured with neither background nor satellite subtraction. While XPS mainly yields information about the elemental composition within the escape depth of photoelectrons, UPS probes the electronic structure which in the case of carbon materials is closely related to the geometric structure. In addition, our experimental setup allows the application of a bias voltage during measurements and thus the determination of the sample's work-function from the spectral onset. Controlled oxidation of the clean samples was carried out in-situ by exposure to molecular oxygen in doses of only several Langmuir to some hundred Langmuir followed by analysis. While some samples were annealed in-situ at up to