Many-electron effects inBaC60: Collective response and molecular effects in optical conductivity and photoionization

The optical conductivity of ${\mathrm{BaC}}_{60}$ is calculated using a model of an atom embedded inside a finite shell of jellium. The investigations are carried out within the framework of time-dependent local-density theory. The optical conductivity of ${\mathrm{C}}_{60}$ shows a strong collective resonance (plasmon) at 17.6 eV, which is nearly degenerate with the Ba 5p-d giant dipole resonance, leading to strong interference effects in ${\mathrm{BaC}}_{60}$. Molecular multiple-scattering effects appear as prominent oscillations in the Ba 4d photoionization spectrum of ${\mathrm{BaC}}_{60}$, without any reduction of the overall intensity of the collective 4d-f giant dipole resonance. The oscillations also show up in the various partial photoionization cross sections of the central Ba atom and the ${\mathrm{C}}_{60}$ shell.