Mechanisms of Photo Double Ionization of Helium by 530 eV Photons

We have measured fully differential cross sections for photo double ionization of helium $450\mathrm{eV}$ above the threshold. We have found an extremely asymmetric energy sharing between the photoelectrons and an angular asymmetry parameter $\beta \simeq 2$ and $\beta \simeq 0$ for the fast and slow electrons, respectively. The electron angular distributions show a dominance of the shakeoff for $2\mathrm{eV}$ electrons and clear evidence of an inelastic electron-electron scattering at an electron energy of $30\mathrm{eV}$. The data are in excellent agreement with convergent close-coupling calculations.