Recently, High-order-harmonic generation(HHG) from molecules has been receiving increasing attention as the phase and amplitude of high order harmonics contain structural and orbital information of molecules, which is employed to reconstruct molecular orbitals with attosecond temporal and sub-"A°" ngstrom spatial resolutions.Compared with HHG from atoms, the physical picture of HHG from molecules is generally more complex due to the additional degrees of freedom and complex molecular structures.Thus, understanding the fundamental difference between the ionization mechanisms of molecules and atoms in strong laser fields is important for obtaining precise molecular orbital information with the molecular HHG spectra.

Researchers at Shanghai Institute of Optics and Fines Mechanics (SIOM/China) experimentally investigated high-order-harmonic generation (HHG) of diatomic molecules (O₂, N₂) and their companion atoms (Xe, Ar, respectively, which are of nearly the same ionization potentials as the corresponding molecules) with midinfrared femtosecond laser pulses [PHYSICAL REVIEW A.2013,88,043401]. They found that the cutoff of O₂ extends to higher energy as compared to that of Xe, and the extension of cutoff energy of O₂ depends on the driver wavelength. Their theoretical analysis showed that the ionization suppression induced by two-center interference should be responsible for the harmonic cutoff extension of O₂.