Dramatic Distortion of the $4d$ Giant Resonance by the $C_{60}$ Fullerene Shell*

Giant resonances are universal features of the excitation of finite many-fermion systems: nuclei, atoms, fullerenes, and clusters. They represent collective, coherent oscillations of many particles and, most prominently manifest themselves in photon absorption cross sections or in the so-called "zero" sound in Fermi-liquids. The photoionization cross section for the endohedral Xe$@C_{60} $ atom is investigated within the framework of representing the $C_{60} $ by a delta-type potential. Results demonstrate that in Xe$@C_{60}$ the $4d^{10}$ giant resonance is distorted significantly when compared with that of the isolated Xe atom. The reflection of the photoelectron waves by the $C_{60}$ causes strong oscillations in the photoionization cross section resulting in the replacement of the Xe $4d$ giant resonance by four prominent peaks. The approximation of the $C_{60}$ by an infinitely thin real potential preserves reasonably well the sum rule for the $4d$ electrons but modifies the dipole polarizability of the $4d$ shell.