Making non-adiabatic photoionization adiabatic

We consider the process of tunnel ionization of atoms driven by circularly-polarized (CP) pulses. For a slowly varying electric field of the laser the intuitive semiclassical picture of adiabatic tunnel ionization pictures the electron passing through the potential barrier induced by a static electric field with constant energy.  In reality, however, the laser field alternates in time and the energy of the electron is not conserved in this nonadiabatic tunneling ionization, as the distribution of electron energies at the tunnel exit reveals, with a width of the order of the ponderomotive energy of the laser. Extensive theoretical and experimental studies have been performed to probe and understand these nonadiabatic effects in photoionization.

Our goal is to understand nonadiabatic processes in CP pulses using electron trajectories in the combined laser and Coulomb fields in a reference frame rotating with the laser field.  We show that in the the rotating frame, counter-intuitively, the energy of the electron is constant during tunnel ionization, and as a consequence follows the much simpler picture of adiabatic ionization. This change of perspective allows us to understand and predict, for instance, the role played by ring currents in atoms, the role of the shape of the laser envelope, and in general, to shed light on classical-quantum correspondence.