Numerical test of the Wannier threshold law for double photoionization of helium

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 R-matrix with a semiclassical outgoing waves method. These data are shown to support the Wannier threshold law σ++ = CEm 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. (fast track communication)