Hydrogen atoms in strong laser fields: Quantum-mechanical calculations of ionization and scattering

The time-dependent Schrödinger equation (TDSE) for a hydrogen atom in strong laser fields is solved numerically by finite-difference methods. These methods are applied to photoionization of hydrogen in quasi-static and alternating harmonic and anharmonic laser fields. In addition a model of laser included electron-atom collisions is presented, which allows one to calculate time-dependent collisions in the single-electron approximation. For quasi-static fields, Stark shifts and time-dependent ionization rates are discussed and compared with previous theoretical results. For harmonic fields, departures from the quasi-static behaviour are explained by scattering, leading to interference between the waves ionized in successive halfcycles. For anharmonic fields, arising e.g., from the reflection of intense pulses at solid surfaces, it is found that the enhanced peak fields present in such waves can cause atomic excitations and corresponding substructures in the photoelectron energy spectra.