Symmetrized amplitudes of the helium-atom double photoionization

The symmetrized gerade and ungerade amplitudes of the helium atom double photoionization are extracted from the convergent close-coupling calculations at fixed excess energies of 9, 20, 40, and 60 eV above the double-ionization threshold, and varying energy-sharing ratios. The amplitudes are fitted with a simple Gaussian ansatz. Although some deviations from this ansatz are clearly visible, especially at small mutual angles, the fully resolved triple differential cross sections generated from the calculated and fitted amplitudes are very close. This observation lends support to the practical parametrization suggested by Cvejanovic and Reddish [J. Phys. B 33, 4691 (2000)]. A more thorough testing of the ionization amplitudes can be achieved by performing a complete double-photoionization experiment and by making a comparison with other highly accurate and computationally intensive theories.