Effect of encapsulated atoms on the electronic structure of the fullerene cage:: A case study on La2@C78 and Ti2C2@C78 via ultraviolet photoelectron spectroscopy

Ultraviolet photoelectron spectra (UPS) of metallofullerene, La-2@C-78 were measured using a synchrotron-radiation light source. Its spectral onset energy was 0.70 eV below the Fermi level, indicating the semiconductive nature of this metallofullerene. The UPS consisted of numerous crests and troughs. Further, a change in intensity upon tuning the excitation energy was observed; however, the intensity of the change was not as large as those observed for other fullerenes. The UPS of La-2@C-78 differ considerably from those of Ti2C2@C-78, although they are thought to have the same cage structure. Herein, this difference is explored using density-functional theory, and the origin of this difference was determined to be the hybridization of the pi-electron wave functions.