SURFACE-PLASMON EXCITATIONS IN C-60, C-60K AND C-60H CLUSTERS

The optical photoabsorption cross section of the C60 cluster and of the endohedral complexes C60K and C60H has been calculated using the spherically averaged pseudopotential (SAPS) model combined with the density functional formalism for the ground state and the time-dependent density functional theory (TDDFT) for the response function. We have analyzed the results for the electronic structure comparing them with more refined calculations and we conclude that the main ingredients needed to describe the collective excitations are included in our model. Describing carbon as a monovalent atom we get a surface plasmon (pi plasmon) slightly below 8 eV, which is in the experimental energy range. The higher-energy surface plasmon seen in the experiments can be qualitatively described taking into account all the active electrons of the cluster, that is, considering carbon as a tetravalent atom (pi and sigma electrons together). The influence of the endohedral impurities is discussed, their main effect being to increase the Landau damping of the collective resonance, more for H than for K. We have also found the highest-order multipolar plasmon the fullerenes can support to be l almost-equal-to 7-8. A simpler jellium-shell model for pure C60 is also presented that gives for the plasmon frequencies results similar to those of the SAPS model.