A gas of rubidium atoms has been excited by a combination of ultraviolet and far-infrared excitation to a superposition of Stark states lying just above the classical saddle point. Using an atomic streak camera, we have demonstrated that the atom ejects an ultrafast train of electron subpulses nearly equally spaced in time, with a repetition rate of approximately 50 GHz. The frequency characteristics of this pulse train are seen to be extremely sensitive to small changes in the static electric field. These measurements imply that, by variation of the electric field during the electron emission, it is possible to create shaped ultrafast electron pulses analogous to shaped optical pulses.
We have measured the energy spectra of electrons resulting from the ionization of K using high-intensity (2ϫ10 12 W/cm 2 ) nanosecond laser pulses at 266 nm. At this wavelength, the energy of one photon is enough to ionize the atom, but the single-photon excitation is suppressed by a Cooper minimum in the ionization cross section. Little direct ionization is observed but, interestingly, a substantial amount of above-threshold ionization ͑ATI͒ is detected. The experimental results show that the atom can absorb up to three photons even though the one-photon channel is directly in the continuum. We have calculated the perturbed wave function of the intermediate state at the one-photon level during the laser pulse and discuss what the radial dependence of the wave function implies for the relative strength of the direct and ATI ionization channels. We have also repeated the experiment using femtosecond laser pulses having focused peak intensities near 2ϫ10 14 W/cm 2 .
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