Ionization of Excited Atoms by Intense Single-Cycle THz Pulses

We have employed intense, single-cycle THz pulses to explore strong-field ionization of low-lying Na Rydberg states in the low-frequency limit. At the largest fields used, F similar or equal to 430 kV/cm, electrons with energies up to 60 eV are created. The field ionization threshold is greater than expected for adiabatic over-the-barrier ionization and is found to scale as n(-3). In addition, for a given field amplitude, higher energy electrons are produced during the ionization of the most tightly bound states. These observations can be attributed to the suppression of scattering from the nonhydrogenic ion core, the long times required for Rydberg electrons to escape over the barrier in the field-dressed Coulomb potential, and the failure, in the single-cycle limit, of the standard prediction for electron energy transfer in an oscillating field. The latter, in particular, holds important implications for future strong-field experiments involving the interaction of ground-state atoms and molecules with true single-cycle laser fields.