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 pulse. We solve the time-dependent Schrödinger equation numerically by means of a spectral method of configuration-interaction type (CI). Our method is based 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-dimensional) angular part. We focus on both the atomic structure calculations and the laser-atom dynamics. We show that our approach allows one to properly treat the electron-electron correlations so as to provide an accurate description of many eigenstates simultaneously. The method is applied in two different situations: (a) the multiphoton excitation and ionization of helium with photon energies ranging from 5.4 to 32.6 eV and (b) the two-photon ionization of helium below the He + (2 l ) threshold and three-photon ionization above the He 2+ 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), to Hartree-Fock calculations. In case (b) we particularly focus on the dynamics of the production of doubly-excited structure by laser pulses with a duration which is shorter than the autoionization lifetime.