Effect of the ground-state correlations on the helium double photoionization and ionization with excitation

We calculate the helium double photoionization and ionization with excitation cross sections using the convergent close-coupling formalism. We investigate the effect of the ground-state correlations by employing various highly correlated ground-state wave functions: 6-, 10-, and 14-parameter Hylleraas and 7-, 15-, and 18-term multiconfiguration Hartree Fock. As a test of the quality of our calculation, we compare the double to single photoionization cross-section ratio calculated in the three different gauges: length, velocity, and acceleration. We also calculate the partial photoionization cross sections up to $n<~6$ ion excited states and compare them with the latest experimental data. Using the 14-parameter Hylleraas ground state, we obtain almost identical results for the three gauges from the double ionization threshold to 1 keV, and have good agreement with the recent measurements.