From collectivity to the single-particle picture in the photoionization of clusters

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 electron 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 photo cross section on an equal footing for both, the low- and the high-energy 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 oscillatory behavior can already be identified at low photon energies where the cross section is not yet exponentially small. Hence, it should be possible to identify the oscillations experimentally.