Behavior of molecular hydrogen exposed to strong dc, ac, or low-frequency laser fields.: II.: Comparison of ab initio and Ammosov-Delone-Krainov rates -: art. no. 063408

Ionization rates are calculated for molecular hydrogen oriented parallel to an external electric or low-frequency electromagnetic field using a three-dimensional, fully correlated ab initio method. The calculation includes also the range of field intensities where barrier suppression leads to over the barrier instead of tunneling ionization. The resulting ionization rates (considering also the influence of vibrational motion and excitation) are compared to those predicted by the (semiclassical) Ammosov-Delone-Krainov (ADK) model. In agreement with earlier findings for helium it is found that the ADK model predicts very accurately ionization rates in the tunneling regime, but overestimates the rate in the over-the-barrier regime. This conclusion is, however, only true, if the tunnel-ionization rate of H-2 is (at least to a good approximation) isotropic.