Double photoionization of doubly exited helium

The R-matrix-Floquet technique for atoms in strong laser fields is extended to describe double photoionization by combining it with B-spline basis sets. One advantage of this extended R-matrix-Floquet approach is that the effects of the intense laser field on double photoionization can be taken into account nonperturbatively. The results agree with existing experimental and theoretical results for double photoionization of the ground state of He within 5% above a photon energy of 4.2 a.u. We further apply this approach to study double photoionization of the autoionizing 2s(2) S-1(e) state of He to demonstrate its capability in dealing with autoionization of the initial state and photoionization simultaneously.