High-order harmonics generated from multiple orbitals of aligned O${}_2$ molecules are studied theoretically and we focus on the effect of two-center interference during photoexcitation. We find that the contribution of the harmonics generated from different molecular orbitals to the total harmonics varies distinctively with the laser intensity. The total harmonic spectra are dominated by the harmonics generated from a 1${\pi }_g$ valence shell at lower intensities, but are dominated by those generated from a 1${\pi }_u$ valence shell at higher intensities. This change is a sequence of the structure-induced two-center interference during photoexcitation of harmonic generation. This study indicates that by properly choosing the intensity of driving laser pulses, one can extract the orbital information of both the 1${\pi }_g$ and the 1${\pi }_u$ valence shells by the generated harmonics. This provides a potential way to tomographically reconstruct the low-lying 1${\pi }_u$ valence shell.

• Received 23 May 2013

DOI:https://doi.org/10.1103/PhysRevA.88.033826