Universal features in sequential and nonsequential two-photon double ionization of helium

We analyze two-photon double ionization of helium in both the nonsequential ((h) over bar omega < I-2 approximate to 54.4 eV) and sequential (<(h)over bar>omega > I-2) regime. We show that the energy spacing Delta E = E-1 - E-2 between the two emitted electrons provides the key parameter that controls both the energy and the angular distribution and reveals the universal features present in both the nonsequential and sequential regime. This universality, i.e., independence of (h) over bar omega, is a manifestation of the continuity across the threshold for sequential double ionization. For all photon energies considered, the energy distribution can be described by a universal shape function that contains only the spectral and temporal information entering second-order time-dependent perturbation theory. Angular correlations and distributions are found to be more sensitive to the value of (h) over bar omega. In particular, shake-up interferences have a large effect on the angular distribution. Energy spectra, angular distributions parametrized by the anisotropy parameters beta(j), and total cross sections presented in this paper are obtained by fully correlated time-dependent ab initio calculations.