Angular Distribution In Two-Photon Double Ionization Of Helium By Intense Attosecond Soft-X-Ray Pulses

We investigate two-photon double ionization of helium by intense (approximate to 10(15) W/cm(2)) ultrashort (approximate to 300 as) soft-x-ray pulses (E=91.6 eV). The time-dependent two-electron Schrodinger equation is solved using a coupled channel method. We show that for ultrashort pulses the angular distribution of ejected electrons depends on the pulse duration and provides insight into the role of electron correlations in the two-electron photoemission process. The angular distribution at energies near the "independent-electron" peaks is close to dipolar while it acquires in the "valley" of correlated emission a significant quadrupolar component within a few hundred attoseconds.