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 ${}^1{\bf P}^o$ 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