Half-cycle microwave stabilization of rubidium Stark atoms

We report on the observation of stabilization of laser-excited rubidium Stark atoms above the classical ionization limit with the help of short (9 ns) unipolar electric-field pulses. Dipping the field down from 40 to 0 V/cm and back to 40 V/cm, 30 ns after photoexcitation into the continuum, yields a large bound-state population. The results suggest an additional mechanism of the recombination as seen in the THz experiment by Bensky, Campbell, and Jones, [Phys. Rev. Lett. 81, 3112 (1998)].