Hydrogen atom in circularly polarized microwaves: Chaotic ionization via core scattering

The ionization of hydrogen atoms in a circularly polarized (CP) microwave field is studied using classical mechanics and simulations. We first focus on the ionization of circular orbits in a model two-dimensional atom for which there have been previous simulations and studies. It has been suggested that in the ionization of both circular and elliptical states collisions with the nucleus play only a minor role. However, the ionization of circular states, in particular, is known to be strongly dependent on the details of the microwave pulse shape. We explain this observation and show that, in some cases, essentially the entire initial ensemble of atoms is ionized during the rise time of the microwave field. Under these conditions, atoms are switched directly into unbound parts of phase space and promptly scatter to infinity without collisions with the nucleus-succinctly, the rise time of the pulse is responsible for the vast majority of ionization events. We identify the conditions leading to this situation based on analytical and numerical studies of the behavior of the Jacobi constant K (energy in the rotating frame) during the rise time of the field. A large number of classical simulations are presented not only for initially circular states but also for states of medium eccentricity. For orbits that survive the pulse rise time these computations support a model of ionization that involves collisions with the core. We argue that the complexity of the ionization of hydrogen atoms in CP microwaves can be ascribed to the fact that orbits of different initial eccentricity in the original ensemble are not only switched to different regions of phase space, but also to different values of the Jacobi constant, during the rise time of the pulse.