Selection rules in the few-photon double ionization of the helium atom

We analyse selection rules for the emission of two electrons from the helium atom following the absorption of a few photons in an intense laser field. The rules arise, as generalization of the well-studied one-photon case, due to the symmetries of the accessible final states in the two-electron continuum. We show, in particular, that an increase in the number of absorbed photons leads to alternating suppression and non-suppression of the back-to-back emission of the two electrons. Results of numerical simulations using a model of the helium atom are in agreement with the theoretical predictions. (Some figures in this article are in colour only in the electronic version)