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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 (54.40.3em0exeV), sequential ionization competes with nonsequential ionization. Remarkably, even below 54.40.3em0exeV, 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.
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Horner et al. (2007) studied this question.
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