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 ${\mathit{n}}^{\mathrm{*}}$ is found to scale as ${\mathit{n}}^{\mathrm{*}\mathrm{-}2}$ for states with ${\mathit{n}}^{\mathrm{*}}$>13 in contradistinction to the ${\mathit{n}}^{\mathrm{*}\mathrm{-}4}$ threshold scaling for static field ionization and high order multiphoton ionization. This novel result is explained using a classical model.

• Received 17 November 1992

DOI:https://doi.org/10.1103/PhysRevLett.70.1236