Time-dependent treatment of two-photon resonant single and double ionization of helium by ultrashort laser pulses

We report the results of accurate time-dependent calculations of two-photon ionization of helium by ultrashort pulses. Ionization amplitudes and generalized cross sections are extracted from the wave function using exterior complex scaling. For photon energies above the first ionization threshold, two-photon single ionization is enhanced by core excited resonances, in processes visible with pulses as short as 2 fs, when the photon frequency is equal to a transition energy in He+. We explore the dependence of the total cross section in the vicinity of the threshold for sequential double ionization on pulse duration. A signature in the single differential cross section of two-photon sequential ionization with the ground state of the ion as the intermediate state is seen to be suppressed by sufficiently short pulses in favor of the nonsequential process, while the triple differential cross section shows that attosecond pulses can access different electron dynamics than those of longer duration. The peaks in the single differential cross section due to sequential ionization with the excited intermediate states of the ion are observed to occur at energies displaced by about 2 eV from the expected values by interference effects between continuum channels.