Molecular structural effects in below- and near-threshold harmonics in XUV-comb generation

The below- and near-threshold harmonics generated by ${\mathrm{O}}_{2}$ molecules and Xe atoms have been investigated experimentally by using an extreme-ultraviolet (XUV)-comb setup that is driven by an intense 1040-nm laser field. It is found that the yields of the below- and near-threshold harmonics generated by ${\mathrm{O}}_{2}$ molecules show a significant suppression when compared to Xe, although the two species have nearly identical ionization potentials. Our theoretical analysis shows that the harmonic yield suppression is related to the two-center interference effect of ${\mathrm{O}}_{2}$ and the tightly bounded orbital of the O atom. Our study provides useful information for the XUV-comb technique when extending the working media from atoms to complex systems.