We theoretically investigate the frustrated double ionization (FDI) of molecules with different alignment-dependence using a three-dimensional classical ensemble method. The numerical results show that the FDI probability decreases with increasing wavelength, which is similar to the wavelength dependence of the FDI probability of atoms. Tracing the classical trajectories reveals that the contributions to molecular FDI from single-recollision and multiple-recollision mechanisms are equal in the short wavelength regime. In the long wavelength regime, the single-recollision FDI channel dominates in FDI. The nature in which molecular FDI occurs is identified and explained.

Circularly and elliptically polarized high-order harmonics have unique advantages when used in studying the chiral and magnetic features of matters. Here, we studied the polarization properties of high-order harmonic generated from alignment nitrogen molecules driven by cross-linearly-polarized two-color laser fields. Through adjusting various laser parameters and targets, such as the relative phase, the crossing angle, the intensity ratio of driving fields, and the molecular alignment angle, we obtained highly elliptically polarized high-order harmonics with the same helicity in a wide spectral range. It provides a possible and effective way to generate elliptically polarized attosecond pulses. Finally, we show the probability of controlling the spectral range of elliptically polarized harmonics.

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