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 $\left(54.4\mathrm{eV}\right)$, sequential ionization competes with nonsequential ionization. Remarkably, even below $54.4\mathrm{eV}$, 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.

• Received 26 April 2007

DOI:https://doi.org/10.1103/PhysRevA.76.030701