Photoelectron spectroscopy and electronic structure of Ca@C60

The electronic structure of an isolated Ca@C60 molecule was probed by photoelectron spectroscopy (PES) of Ca@C 60 − , generated by laser desorption of a preformed Ca@C60/fullerene thin film. The PES spectrum of Ca@C60 was found to be similar to that of C 60 − , except that Ca@C60 has an electron affinity of 3.0 eV, about 0.3 eV higher than that of C60. The spectrum suggests that Ca atom donates its two 4s electrons to the C60t lu lowest unoccupied molecular orbital. Thus, the interaction between the central Ca atom and C60 is quite ionic, and Ca@C60 can essentially be expressed as Ca2+@C 60 2− .