Fully integrated double photoionization cross sections for He are obtained on a fine energy mesh in the threshold region from ab initio calculations using the hyperspherical -matrix with a semiclassical outgoing waves method. These data are shown to support the Wannier threshold law σ ++ = CE m and provide the constant of proportionality C = 1.175 ± 0.025, the exponent m = 1.043 ± 0.022, as well as the energy range of validity, which is found to be about 1.3 eV.
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Bouri et al. (2007) studied this question.