Frequency dependence of non-sequential double ionization

The frequency dependence of non-sequential double ionization (NSDI) of Ne is studied using a semiclassical rescattering model. It is found that the ratio (Ne2+/Ne+), calculated for a laser intensity of 1.0 x 10(15) W cm(-2), increases while the wavelength decreases down to about 250 nm, the limit of the applied model. Careful investigations of the trajectories show that there are two categories of double ionization: 'instant ionization' and 'delayed ionization'. By limiting the time interval between the re-collision time and ionization time, the 'delayed ionization' is identified to give rise to the high probability of NSDI in the short wavelength region.