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Perturbative calculation of two-photon double electron ionization of helium

We report the total integrated cross-section (TICS) of two-photon double ionization of helium in the photon energy range from 40 to 54 eV. We compute the TICS in the lowest order perturbation theory (LOPT) using the length and Kramers–Henneberger gauges of the electromagnetic interaction. Our findings indicate that the LOPT gives results for the TICS in agreement with our earlier non-perturbative calculations.