No abstract

Although an atom is a manifestly quantum mechanical system, the electron in an atom can be made to move in a classical orbit almost indefinitely if it is exposed to a weak microwave field oscillating at its orbital frequency. The field effectively tethers the electron, phase-locking its motion to the oscillating microwave field. By exploiting this phase-locking, we have sped up or slowed down the orbital motion of the electron in excited lithium atoms by increasing or decreasing the microwave frequency between 13 and 19 gigahertz; the binding energy and orbital size change concurrently.

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.