From collectivity to the single-particle picture in the photoionization ofclusters

Photoionization of alkali-metal clusters is investigated theoretically for different photon energy regimes. At low energies the photo cross section is characterized by the well-known plasmon peak resulting from collective ele ctron dynamics. For high energies the ionization cross section exhibits an oscillatory behavior, which can be explained by single-particle effects. In this paper we use the random phase approximation (RPA) to calculate the ph oto cross section on an equal footing for both, the low- and the high-energ y regime, Thereby, we can show that the cross sections for photoionization calculated in the collective RPA and in the single-particle picture indeed merge for high photon energies. Moreover, we demonstrate that the oscillato ry behavior can already be identified at low photon energies where the cros s section is not yet exponentially small. Hence, it should be possible to i dentify the oscillations experimentally. [S1059-2947(99)07706-9].