A non-perturbative multi-photon picture of strong field ionization

We outline the non-perturbative multi-photon picture of strong-field ionization. Processes are described in terms of angular momentum selection rules and photon absorption pathways as in the perturbation theory. We apply the framework to separate out ionization pathways responsible for the enhanced ionization of specific initial p−states interacting with circularly polarized fields at wavelengths spanning from the single photon to the tunneling limits of ionization. The approach is unique and gives a unified description of ionization by both perturbative short-wavelength and non-perturbative long-wavelength radiation fields.