Ultraviolet photoelectron spectra of metallofullerenes, two Ca @ C₈₂ isomers

Ultraviolet photoelectron spectra (UPS) of two Ca@C₈₂ isomers (III and IV) were measured with a synchrotron radiation light source. The photoelectron onset energies of isomers III and IV were 0.7 and 0.8 eV below the Fermi level, respectively, which indicates their semiconductive nature. When the excitation energy is tuned, spectral intensity changes as other fullerenes have shown. Their upper valence band (0-5 eV) spectra are different from those of other metallofullerenes such as La@C₈₂,Sc@C₈₂ and Gd@C₈₂ as well as those of empty C₈₂ but their lower valence band spectra (below 5 eV) are almost identical. Comparison between the UPS and ab initio calculation assuming transfer of two electrons from encapsulated calcium atom to the cage (i.e. Ca⁺²@C₈₂^2-) suggests C₂(c) geometry for isomer III and C_s for isomer IV.