Multiphoton double ionization of helium by ultrashort XUV pulses: Probing the role of electron correlations

We investigate multiphoton double ionization of helium by ultrashort XUV pulses involving up to 5 XUV photons by numerically solving the time-dependent Schrodinger equation for helium in its full dimensionality. We explore the crucial role of dynamical electron correlations by systematically varying the pulse duration and the number of absorbed photons. With increasing photon number, a multitude of ionization pathways opens up. The relative importance of correlations varies with pulse duration. Even in the spectrally sequential regime, correlation effects become increasingly important as the pulse duration becomes shorter and as the number of absorbed photons is increased. We identify a pathway to double ionization (DI) involving correlated excitation ionization as an intermediate step which can provide a contribution to the total DI yield comparable to the conventional direct sequential double ionization. Supported by concurrent classical trajectory Monte Carlo simulations, we identify strong electron-electron correlations in double ionization even at large distances from the nucleus.