Double photoionization of helium at low photon energies

The cross sections for single and double photoionization of helium at energies from the threshold of double ionization to 280 eV are calculated by combining the previously developed hyperspherical close-coupling method with a discretization procedure for the continuum spectrum of ${\mathrm{He}}^{+}$. No pseudoresonances due to the discretization are found. Both the length and acceleration forms of the dipole operator are used, and the results hardly depend on these different gauges. The results are compared with previous theories and experiments.