HIGH-ORDER ABOVE-THRESHOLD IONIZATION SPECTRUM OF HYDROGEN AND PHOTOELECTRON ANGULAR-DISTRIBUTIONS

The multiphonon ionization of the hydrogen atom resulting from its int eraction with an intense and short laser pulse has been investigated t heoretically, by solving the time-dependent Schrodinger equation (TDSE ) via the state-specific expansion of the time-dependent wavefunction. We report above threshold ionization (ATI) spectra and photoelectron angular distributions (PADS) for photo energies ranging between 2 and 4 eV and for laser intensities up to 2 x 10(14) W cm(-2). In agreement with recent experimental observations, for h omega = 2 eV and 3 eV a plateau-like structure was found in the ATI spectra. Furthermore, for high-order ATI peaks the PADS show a variety of side-lobes, including emission at 90 degrees off the laser polarization axis. These results are interpreted in terms of tunnelling, multiphonon absorption and int erference phenomena.