Ionization of Rydberg atoms by subpicosecond half-cycle electromagnetic pulses

We have ionized Rydberg atoms using subpicosecond half-cycle electromagnetic pulses. The threshold electric field required to ionize a Rydberg state with effective quantum number n* is found to scale as n^*-2 for states with n*>13 in contradistinction to the n^*-4 threshold scaling for static field ionization and high order multiphoton ionization. This novel result is explained using a classical model.