Photo-ionization of the helium atom close to the double ionization threshold: towards the Ericson regime

We calculate the photo-ionization cross-section from the ground state of the helium atom, using the complex rotation method and diagonalization of sparse matrices. This produces directly the positions and widths of the doubly excited 1Po resonances together with the photo-ionization cross-section. Our calculations up to the N=9 threshold are in perfect agreement with recent experimental data and show the transition from a regular structure at low energy to a chaotic one at high energy, where various resonances strongly overlap.