Minimum structure of high-harmonic spectra from aligned O2 and N2 molecules

We experimentally investigated the high-order harmonic generation (HHG) from aligned O2 and N2 molecules in a linearly polarized laser field,and presented the dependence of the harmonic spectrum on the driving laser intensity and molecular alignment angle.The minimum position of HHG of O2 varies with changing the laser intensity,which is caused by multi-orbital interference.However,the location of the observed minimum structure in N2 harmonic spectrum remained unchanged upon changing the laser intensity.The mechanism of the spectral minimum for N2 case is regarded as a Cooper-like minimum in HHG associated with the molecular electronic structure.This work indicates that harmonic spectroscopy can effectively uncover information about molecular structure and electron dynamics.