Beyond (1D+time) dynamics in the microwave ionization of excited H atoms: surprises from experiments with collinear static and linearly polarized electric fields

We begin with a brief review of the ionization of 3D hydrogen atoms with la rge principal quantum number n(0), first by a static electric field F-s and then by linearly polarized (LP) electric field. Near its onset, LP ionizat ion can be understood with (1D + time) theory. Various kinds of resonant ph enomena are important. We continue with a brief review of the polarization dependence. When the dynamics is dominated by the main pendulum-like resona nce zone, a separation of timescales leads to ionization near onset that is independent of polarization. Ln other parameter ranges, polarizarion-depen dent effects occur that can be understood only in higher dimensions, minima lly (2D + time). Finally, we present preliminary results from new experimen ts using collinear LP and F-s fields, both of which can be strong. The data show the importance of (a) multiphoton resonances driven between Stark sub states of the initial no manifold and (b) striking, regular oscillations re corded for fixed microwave parameters as a function of F-s. The mechanisms responsible for (a) are understood. Those responsible for (b) are not, but the oscillations exhibit empirical scaling behavior that will help to unrav el the wave packet dynamics. (C) 1999 Published by Elsevier Science B.V. Al l rights reserved.