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-wav e model reveals: the existence of collective tunneling ionization in a mode where both electrons stay at equal distance from the nucleus. Otherwise th e lagging electron is immediately recaptured. The corresponding double ioni zation rate fails to explain nonsequential multiple ionization in strong-fi eld laser experiments. However, an empirically modified version of the anal ytical one-electron tunneling rate of Ammosov, Delone, and Krainov agrees w ith the experiments to a surprising accuracy. The reason for this agreement is presently unknown.