Possibility of breakdown of atomic stabilization in an intense high-frequency field

We have calculated the ionization yield for a hydrogen atom irradiated by a short (5-cycle) intense pulse, with the atom initially in the 1s or 2p (m{≠}0) state. We numerically solved the time-dependent Schr\"odinger equation, first making the dipole approximation, and then including the photon momentum. At sufficiently high intensities we find significant differences between making and not making the dipole approximation, suggesting that atomic stabilization may break down in superintense fields.