IONIZATION OF RYDBERG ATOMS BY HALF-CYCLE PULSES

We study the ionization of Rydberg atoms by half-cycle electromagnetic pulses as a function of the duration of the pulses covering a broad range of time intervals connecting the adiabatic and sudden regimes. We show that the threshold fields for ionization lie on a universal classical scaling invariant curve. We present quantum mechanical results which agree well with the universal curve. The n-2 threshold field scaling recently found experimentally is a linear approximation to the universal curve by the tangent in the region where the pulse duration is of the order of the orbital period of the atom. We predict the existence of an ultrashort pulse limit in which the threshold field scales as n-1. Effects due to non-hydrogenic core are shown to be negligible.