Sequential versus non-sequential double ionization in strong laser fields

The recollision model has been applied to separate the probability for doub le ionization into contributions from electron-impact ionization and electr on-impact excitation for intensities at which the dielectronic interaction is important for generating double ionization. For a wavelength of 780 am, electron-impact excitation dominates just above the threshold intensity for double ionization, approximate to 1.2 x 10(14) W cm(-2), with electron-imp act ionization becoming more important for higher intensities. For a wavele ngth of 390 nm, the ratio between electron-impact ionization and electron-i mpact excitation remains fairly constant for all intensities above the thre shold intensity for double ionization, approximate to 6 x 10(14) W cm(-2). The results point to an explanation of the experimental results, but more d etailed calculations on the behaviour of excited He+ ions are required.