Charge Donation by Calcium into the t 1g Band of C ₆₀

Photoemission spectra of compounds prepared by the reaction of C(60) films with calcium show two distinct metallic phases, whereas alkali-doped C(60) films have only one. In the first phase the bulk t(1u) band, derived from the lowest unoccupied molecular orbital of C(60), is partially occupied. This is followed by an insulating phase that has the composition Ca(3)C(60) in which the t(1u) band is filled and has properties analogous to those of K(6)C(60). Continued exposure to calcium produces a second metallic phase in which electrons are donated into the t(1g) band. The superconductivity of Ca(5)C(60) is associated with the t(1g) band.