Strong-field approximation for diatomic molecules: Comparison between the length gauge and the velocity gauge

We investigate the ionization and high-order harmonic generation (HHG) of diatomic molecules using the strong-field approximation (SFA) in both the length gauge (LG) and the velocity gauge. We focus on the effect of two-center interference that has an important role in both molecular ionization and HHG. Our simulations reveal the significant disagreement between the two gauges for the description of this effect. For the behavior of molecular ionization rate with respect to the internuclear distance, which is closely related to this effect in ionization, the two gauges give qualitatively conflicting answers in some cases. For the striking hollow structure in the molecular HHG spectrum, where this effect dominates, the predictions of the two gauges also differ significantly. Further comparisons suggest the preference of the LG SFA in describing this interference effect in intense-field-molecule interaction.