(Invited) Excited States of Atoms Encapsulated in the C₆₀ Cage: A Spin-Polarized Time-Dependent Density-Functional Study

Among various endohedral fullerenes, "X@C60", the icosahedral-symmetry C60 cage and an atom trapped inside the cage, has been studied for more than thirty years ever since the proposal of the C60 fullerene cage based on the carbon-cluster mass spectra because of its importance from the viewpoint of an artificial atom and also as the realization of an isolated singel atom trapped inside the chemically inert cage. The system is now also attracting interests in the field of neutrino mass spectroscopy for using the coherent isolated atoms to detect the radiative emission of neutrino pair. In the present work, we study the excited state properties of atoms trapped inside the C60 cage using the spin-ploarized time-dependent density-functional theory. In the Figure, the photoabsorption spectra of the empty C60 cage, the N atom, and the N@C60 system are shown. From the spin-polarization analysis, the peak at 8.5 eV of the N@C60 spectrum is found to correspond to the excitation of the N atom trapped inside the C60 cage. The presence of the outer C60 cage therefore slightly modifies the excitation energy of the N atom by +0.2 eV. We will also discuss the excitations of various atoms including Cu, Ag, and Au atoms which are trapped inside the C60 cage. Figure 1