Unexpected behavior of the near-threshold double-photoionization cross section of beryllium

The threshold region of the double-photoionization cross section of atomic beryllium was investigated using monochromatized synchrotron radiation. The photon energy dependence of the double-photoionization cross section can be described by the Wannier power law up to 1.7 eV above threshold. However, we unexpectedly find oscillations in the cross section, which are in excellent agreement with a modulated threshold law based on the Coulomb-dipole theory Phys. Rev. Lett. 49, 365 (1982)]]. This new finding casts some doubts on the general applicability of the Wannier power law.