Enhanced double ionization rate from O molecules driven by counter-rotating circularly polarized two-color laser fields.

We report that the nonsequential double ionization (NSDI) probability of an O target can be enhanced greatly in a counter-rotating circularly polarized two-color driving field. The field is composed of a fundamental frequency and its third harmonic, and the combined electric field traces out a four-leaf-clover pattern. The electron ionized by such a field has more chances to collide with the valence electrons in the O molecule, which significantly enhances the NSDI probability. This effect is more evident in low-intensity fields. We also find that the enhancement appears in a broad range of the field ratio of two colors and that both the NSDI yield and the underlying electronic behavior varies notably with the field ratio.