Collisions of low-energy ions Ar+ and N2+ with room-temperature and heated surfaces of tungsten, beryllium, and a mixed beryllium-tungsten thin film

Mass spectra of product ions arising from collisions of the seeding gas ions Ar+ and N2+ with room temperature and heated (150 °C and 300 °C) surfaces of the prospective fusion-device materials, tungsten, beryllium, and a thin mixed film of beryllium and tungsten on a silicon substrate were measured over the incident energy range of the projectile ions 15-70 eV. A fraction of the product ions were in all cases sputtered hydrocarbon ions from the hydrocarbon surface layer which covers the surfaces at room temperature and gradually diminishes with increasing surface temperature. Sputtering of the of the sample substrate was observed only with the beryllium and mixed beryllium-tungsten film surfaces (ions of Be+, BeOH+, BeH+, BeO+) while sample impurities yielded Na+, K+ and, in the case of the beryllium surface also Li+, Mg+, Ca+, CaOH+. With the tungsten surface, no sputtered ions of tungsten or its compounds could be detected. The differences between the impact of Ar+ and N2+ in product ion composition were usually small. With the projectile ion N2+, heterogeneous chemical reactions between the projectile ion and surface adsorbed material were inferred from the increase of the respective ion yields, not observed with Ar+: formation of HCN+ in a reaction with surface hydrocarbons, and formation of NO+ in a reaction with surface oxides (probably beryllium oxides) on the thin Be-W film surface.