Resonant and non-resonant ionization of helium by XUV ultrashort and intense laser pulses

We study the multiphoton ionization of He by a strong ultrashort laser puls e. We solve the time-dependent Schrodinger equation numerically by means of a spectral method of configuration-interaction type (CI). Our method is ba sed on an expansion on B-spline functions, more precisely each two-electron configuration is built as an antisymmetrized B-spline product for the (two -dimensional) radial part and dipolar spherical harmonics for the (four-dim ensional) angular part. We focus on both the atomic structure calculations and the laser-atom dynamics. We show that our approach allows one to proper ly treat the electron-electron correlations so as to provide an accurate de scription of many eigenstates simultaneously The method is applied in two d ifferent situations: (a) the multiphoton excitation and ionization of heliu m with photon energies ranging from 5.4 to 32.6 eV and (b) the two-photon i onization of helium below the He+(2l) threshold and three-photon ionization above the He2+ threshold with photon energies ranging from 25.8 to 34 eV. The results are compared with other CI calculations (where a comparison is possible) and, in case (a), tb Hartree-Fock calculations. In case (b) we pa rticularly focus on the dynamics of the production of doubly-excited struct ure by laser pulses with a duration which is shorter than the autoionizatio n lifetime.