The partial photoionization cross sections of
exhibit strong modulations of the HOMO and HOMO-1 photoelectron line intensities over a wide range of photon energies. These modulations were interpreted as quantum oscillations resulting from the size of the short range potential in which the valence electrons are bound. More recent measurements revealed an additional modulation superimposed on the main oscillations. This low frequency modulation can be understood as a beating frequency reflecting the finite width of the spherical-shell like potential. First evidence for such a behavior is reported and discussed.