Sequential versus non-sequential double ionization in strong laser fields

The recollision model has been applied to separate the probability for double ionization into contributions from electron-impact ionization and electron-impact excitation for intensities at which the dielectronic interaction is important for generating double ionization. For a wavelength of 780 nm, electron-impact excitation dominates just above the threshold intensity for double ionization, ≈1.2×10 14 W cm -2 , with electron-impact ionization becoming more important for higher intensities. For a wavelength of 390 nm, the ratio between electron-impact ionization and electron-impact excitation remains fairly constant for all intensities above the threshold intensity for double ionization, ≈6×10 14 W cm -2 . The results point to an explanation of the experimental results, but more detailed calculations on the behaviour of excited He + ions are required.