Experimental Studies of the Electronic Structure of Fullerenes

Materials which are based upon sp2-hybridized carbon networks have attracted a lot of attention in the last decades. They have in common that their electronic properties are predominantly determined by π electrons which occupy the bonding or anti-bonding C2p-π orbitals. Prominent members of this material class are, for example, the conducting polymers or intercalated graphite. The fullerenes, the third allotrope of carbon discovered in 1985 [1], represent another new class of materials with sp2-hybridized carbons showing many unexpected and fascinating properties. In this article we review high energy spectroscopic studies of the electronic structure of fullerene solids including pristine fullerenes, intercalated fullerenes, hetero-fullerenes and endohedrally doped fullerenes. We mainly restrict ourselves to data that have been obtained in our laboratories using electron energy-loss spectroscopy in transmission (EELS) and photoemission spectroscopy (PES). We emphasize, however, that many groups have beeh working in this field worldwide and that our data are in some cases representative of results first reported by other research groups.