Abstract
We investigate two-photon double ionization of helium by intense ({approx_equal}10{sup 15} W/cm{sup 2}) ultrashort ({approx_equal}300 as) soft-x-ray pulses (E=91.6 eV). The time-dependent two-electron Schroedinger 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.
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