XPS analysis of single crystal RbC60 surface

The surface of single crystal RbC 60 was characterised by means of X-ray photoelectron spectroscopy (XPS). The atomic ratio of Rb to C up to 15 A depth proved to be about two or three times higher than the stoichiometry (1/60) from the composition analysis. The top surface was covered by the oxidized hydrocarbon material and was fast removed by short argon ion sputtering. The oxidized Rb component appeared from ca 5 A depth by the peak analysis of Rb3p spectrum, which shifted ca I eV towards the higher binding energy, while the Cls peak kept the same position upon sputtering. From the atomic ratio of Rb to O, the existence of a passivation layer made up of Rb 2 CO 3 (carbonate salt) was suggested. The stability of bulk RbC 60 under ambient atmosphere is explained by the existence of this kind of passivative Rb carbonate layer in the nano-surface region.