Atomic and molecular processes in external fields

Photoionization of hydrogen by periodic but strongly modulated anharmonic laser fields is investigated. Above-threshold ionization spectra are calculated by numerically solving the time-dependent Schrödinger equation in the Kramers-Henneberger frame by the split-operator method. Effects of amplitude and phase modulations of the laser field are examined by comparing the energy spectra of harmonic and anharmonic laser fields at the same cycle-averaged laser intensity. It is found that amplitude modulations with high peak intensities yield enhanced electron energies, and show substructures of the spectra due to atomic excitations. The energy spectra are found to be relatively insensitive to phase modulations at harmonic frequencies, but show an additional peak sequence for subharmonic modulations at half of the laser frequency. Possible applications to experiments with attosecond laser pulses are indicated.