Structure and dynamics of excited atoms

We have studied processes in excited atoms driven by strong microwave radiation fields to gain a quantitative understanding of what are usually thought of as highly nonlinear processes. We have discovered that circularly polarized microwave fields produce ionization only at fields above 1/16n4, the much higher field required for static field ionization. We have also observed above threshold ionization at microwave frequencies and shown that in the microwave regime it is easily described by a classical theory, which ties together above threshold ionization at microwave frequencies and shown that in the microwave regime it is easily described by a classical theory, which ties together above threshold ionization and ponderomotive energy shifts. Finally, we have made significant progress toward observing collisions which begin and end under control.