Stable classical configurations in strongly driven helium

We present a detailed analysis of the classical dynamics of the frozen-plan et configuration of helium in an external, linearly polarized electromagnet ic field. It is shown that the phase space of the collinear frozen planet i n the held is mixed regular-chaotic and exhibits regular islands correspond ing to nonlinear resonances between the periodic external force and the fro zen-planet dynamics. An analysis of the transverse stability properties of the configuration shows that - except for regular regions corresponding to high-order atom-field resonances - the configuration is generally unstable with respect to deviations from collinearity. We find that stability of the driven frozen planet in all phase space dimensions can be enforced by the application of an additional, static electric field. The minimum atomic exc itation necessary fur the localization of a quantum state on the thereby st abilized atom-field resonances is estimated. (C) 1999 Elsevier Science B.V. All rights reserved.