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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.
Jones et al. (Mon,) studied this question.