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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.

We investigate experimentally multi-orbital effects in high-order harmonic generation (HHG) from aligned CO2 and N2O molecules by intense femtosecond laser fields with linear and elliptical polarizations. For either of the aligned molecules, a minimum in the harmonic spectrum is observed, the position of which shifts to lower-order harmonics when decreasing the intensity or increasing the ellipticity of the driving laser. This indicates that the minimum originates from the dynamic interference of different channels, of which the tunneling ionization and recombination are contributed via different molecular orbitals. The results show that both the highest occupied molecular orbital (HOMO) and low-lying HOMO-2 in CO2 (or HOMO-1 in N2O) contribute to the molecular HHG in both linearly and elliptically polarized strong laser fields. Our study would pave a way for understanding multi-electron dynamics from polyatomic molecules irradiated by strong laser fields.

We perform an experimental study of the multi-orbital effect on the high-order harmonic generation (HHG) from aligned N2 molecules in both linearly and elliptically polarized intense laser fields. Measured by a home-built extreme ultraviolet (XUV) flat grating spectrometer with the pump–probe method, the angular distributions of different orders of HHG are obtained, which show distinctive behaviors for harmonics in the plateau and the cut-off regions. The ellipticity dependence of HHG is investigated by aligning the molecular axis parallel or perpendicular to the laser polarization. Our results indicate that both the highest occupied molecular orbital (HOMO) as well as the lower one (HOMO-1) contribute to the HHG of N2 molecules, in either linearly or elliptically polarized intense laser field. The study paves the way for understanding the ultrafast electron dynamics of molecules exposed to an intense laser field.