Double photoionization of helium from threshold to high energies

We present accurate calculations for the single and double photoionization of helium from the threshold to the high-energy limit. At low and intermediate energies, we use the ab initio hyperspherical close-coupling method, which is extended up to 1 keV by introducing an energy-dependent reaction region. At high energies we employ an accurate Hylleraas initial state and a 3$C$ correlated final-state wave function. The 3$C$ wave function is shown to be correct to leading order in $Z/k.\mathrm{}$ We find good agreement with recent experiments over the entire energy range. The two calculations merge at $\sim 1\mathrm{keV}$ to within 5%.