Two-photon double ionization of helium above and below the threshold for sequential ionization

Abstract

We present accurate cross sections for two-photon double ionization of helium at photon energies above and below the threshold for sequential double ionization. Above this threshold $(54.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, sequential ionization competes with nonsequential ionization. Remarkably, even below $54.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, sequential ionization leaves a clear signature in the magnitude and shape of both the total and energy-sharing cross sections---even though at these energies it is only a virtual process.