Photo-double-ionization of He: Fully differential and absolute electronic and ionic momentum distributions

We measure fully differential cross sections for photo-double-ionization of helium at energies 1, 6, and 20 eV above threshold. The data have been obtained by measuring in coincidence the momentum vector of the He${}^{2+}$ ion and one of the electrons. Using time-of-flight and imaging techniques, we cover a solid angle of 25–100 % 4$\pi$ of the final-state continuum of all particles. Therefore the experiment is not confined to any particular set of angles or energy sharing, and allows for a reliable absolute calibration. We present momentum distributions of the ions and a comprehensive set of differential cross sections for electron emission. The latter are on an absolute scale and cover both equal and unequal energy sharing—for both the fast and the slow electron fixed—and a wide range of polar angles. We also present the first data for noncoplanar geometry. For all energies the cross section is sharply peaked around the coplanar emission, i.e., both electrons are preferentially emitted in the plane of the recoiling ion and the photon polarization direction. For most of the geometries the shape of the cross sections is well described by fourth-order Wannier theory calculations.