(color online) Momentum distributions of the recoil ions: The black line is for Ar²⁺ ions produced in a laser field of wavelength λ = 795 nm and intensity , and other lines are for He²⁺ ions: (a) by laser fields of wavelength 403 nm and intensity (red), (green), and (blue), respectively; (b) by laser fields of wavelength λ/k and intensity for (red) and (green), respectively. For convenience of comparison, each line is normalized by its maximum, and the momentum is normalized by .