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 N-2 versus Ar, O-2 versus Xe, F-2 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 theta(F) between the molecular axis and the field direction. The theoretical results agree quite well with experiment for N-2 and O-2 but give too low a value of the peak angle theta(F) for CO2. Our calculations give small values of the ionization rates for O-2 and CO2 at theta(F) = 0 and 90 degrees, in agreement with experiment. Other calculations, including the MO-ADK model and methods involving a numerical integration of the time-dependent Schrodinger equation, exhibit substantially weaker suppression at these angles.