Relativistic laser-field-drift suppression of nonsequential multiple ionization

We have studied the multiple ionization of Xe with intense laser pulses. For the highest intensities, and, concomitantly, the highest ionic charge states, there is only evidence of sequential and not of simultaneous multiple ionization. The multiple-ionization rate below saturation is greatly reduced, compared to the theoretical predictions based on nonrelativistic multiple-ionization models. Since simultaneous multiple ionization requires the return of the first ionized electron to its parent atom, our results show suppression of this return. This suppression is expected to occur due to the relativistic photon momentum imparted by the laser field, as seen in simulations of relativistic classical trajectories.