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Multiphoton ionization of hydrogen by an intense 248-nm linearly polarized field.

We study multiphoton ionization of hydrogen by a linearly polarized 248-nm field (the photons have an energy of 5.0 eV) up to an intensity of 10{sup 15} W/cm{sup 2}. We compare two very different computational methods for the calculation of the total ionization rate: on the one hand, the full numerical solution of the time-dependent Schroedinger equation; on the other hand, results obtained within the Floquet formalism, which assumes a quasiperiodic time dependence. We obtain very good agreement between the two sets of calculations throughout the whole intensity range. The calculated rates also agree well with third-order perturbation theory up to 10{sup 13} W/cm{sup 2}, but are orders of magnitude lower at 10{sup 15} W/cm{sup 2}. The shortfall at high intensities is attributed to the change from a nonresonant third-order process to a resonant fourth-order process caused by ac Stark-induced shifts in the energy-level structure of hydrogen. In fact, the ionization rate between 2 and 6{times}10{sup 14} W/cm{sup 2} is almost constant, giving rise to a plateau feature in the rate versus intensity curve.