OXYGEN-ADSORPTION ON CS-GRAPHITE INTERCALATION COMPOUNDS UV AND X-RAY PHOTOELECTRON STUDIES ON CSC8 AT 100-K - FORMATION OF OXIDE, PEROXIDE AND SUPEROXIDE ON THE SURFACE

The Cs-graphite intercalation compound (stage 1) surface has been studied after controlled oxygen exposures at 100 K. The CsC8 surface contains 0.5 monolayer alkali atoms and during the oxidation process, three oxygen species O2-, O-2(2-) and O-2(-) have been identified with the UP spectra as well as with O 1s core level XP spectra. At low exposures, the O2- oxide is almost solely present on the surface which has never been pointed out on other submonolayer Cs films. The peroxide O-2(2-) is then formed and above 0.25 L, Cs migration from the bulk to the surface occurs up to a mean coverage of about 1.2 monolayer of cesium. Between 0.25 and 0.6 L O-2, this migration induces a structural transformation of the layer, which corresponds with the onset of the 0(2)(-) species and the disappearance of the O2- species. For higher exposures, this superoxide becomes dominant but is never completely pure. At 50 L O-2, the oxidation reaches its limit for 100 K: no more diffusion of Cs onto the surface and no more oxygen sticking are noticed even if parts of the surface remain free from oxide coverage.