Experimental investigation of rubidium atoms above the field-ionization limit using a time-resolved wave packet approach

We have performed a time-resolved study of the behavior of Rb atoms in electric fields (0.1-3 kV/cm) at energies close to and above the field-ionization threshold. This is done using a pair of picosecond laser pulses that creates an electronic wave packet and monitors its evolution in terms of the time-autocorrelation function. In the discussion of the results, emphasis is put on the relation with the classical picture of field ionization, viz. the escape of an electron over the saddle point in the potential energy. This picture is found to be capable of qualitatively explaining the observed dependences on excitation energy and light polarization. Furthermore, the observed dynamics as a function of field strength is shown to be in agreement with the general scaling properties of the Hamiltonian of hydrogen, which also accounts for the measured time periods quantitatively. The relation between time- and frequency-domain measurements is shortly addressed, as is the question of why a hydrogenic approximation explains the observed dominant features adequately.