Semiclassical complex-time method for tunneling ionization: Molecular suppression and orientational dependence

We apply a previously developed semiclassical complex time method to the calculation of tunneling ionization of several diatomic molecules and CO₂. We investigate the presence or absence of the molecular suppression effect by calculating ionization rates of N₂ versus Ar, O₂ versus Xe, F₂ versus Ar, and CO versus Kr. Comparisons with other theories, including the molecular-orbital-Ammosov-Delone-Krainov (MO-ADK) model and the strong-field approximation, are given. We also analyze the dependence of the ionization rate on the angle θF between the molecular axis and the field direction. The theoretical results agree quite well with experiment for N₂ and O₂ but give too low a value of the peak angle θF for CO₂. Our calculations give small values of the ionization rates for O₂ and CO₂ at θF=0 and 90^^∘, in agreement with experiment. Other calculations, including the MO-ADK model and methods involving a numerical integration of the time-dependent Schr\"odinger equation, exhibit substantially weaker suppression at these angles.