Collective multielectron tunneling ionization in strong fields

We investigate the quantum mechanical process of two-electron tunneling in strong external electric fields. Numerical solution of a two-electron s-wave model reveals: the existence of collective tunneling ionization in a mode where both electrons stay at equal distance from the nucleus. Otherwise the lagging electron is immediately recaptured. The corresponding double ionization rate fails to explain nonsequential multiple ionization in strong-field laser experiments. However, an empirically modified version of the analytical one-electron tunneling rate of Ammosov, Delone, and Krainov agrees with the experiments to a surprising accuracy. The reason for this agreement is presently unknown.