Evidence of two-center interference in high-order harmonic generation from CO₂

Two recent pump-probe experiments on the high-order harmonic generation (HHG) from partially aligned CO₂ molecules have shown that the HHG yields versus the ionization rates as functions of the pump-probe delay time exhibit inverted modulation. The inversion has been attributed to the quantum interference in the recombination process from the two oxygen centers. Using the Lewenstein model to calculate HHG from fixed CO₂ molecules and taking into account the partial alignment of the molecules versus the time delay, we interpret that the inversion is due to the degree of overlap between the partially aligned molecules and the angular dependence of the HHG yield. We also note that the harmonic order where inversion occurs may change with probe laser intensity due to the depletion of the ground state.