Diffraction effects in the photoionization of clusters

In many atomic or molecular clusters an electron cloud exists which is delocalized over a volume of well defined shape. It is shown that the total and partial photoionization cross sections of those clusters oscillate on a scale of energy typically reached by synchrotron radiation. The frequencies of the oscillations are rrlated to geometrical properties of the electron cloud, such as its thickness and the diameter of the cluster. These properties can in principle be extracted from the experimental photo cross section.. As specific examples we discuss an alkali-metal cluster (Na40) and the fullerene C60.