Above-threshold ionization in K via a Cooper minimum in the continuum

We have measured the energy spectra of electrons resulting from the ionization of K using high-intensity (2x10¹² W/cm²) 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 (ATI) 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 2x10¹⁴ W/cm².