Title: Ionization of Excited Atoms by Intense Single-Cycle THz Pulses

Sha Li and R.R. Jones

University of Virginia, Charlottesville, VA 22904, USA

Abstract:

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, $F \sim$ 500 kV/cm, electrons with energies up to 60 eV are detected. 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 long times required for Rydberg electrons to escape from the Coulomb potential, and to the breakdown of the slowly varying envelope approximation in the standard simpleman's prediction for energy transfer in an oscillating field.

Status: Published Physical Review Letters 112, 143006 (2014).