text: It typically has a strong maximum at the minimal number of photons to ionize the system, with successive peaks (known as ATI peaks) separated by the photon energy and thus corresponding to higher numbers of photons being absorbed.
  In the non-perturbative regime the bound states are dressed with the electric field, shifting the ionization energy. If the ponderomotive energy of the field is greater than the photon energy , then the first peak disappears.