Recollision model for double ionization of atoms in strong laser fields

We present calculations on the ratio between double and single ionization for He atoms subjected to an intense laser pulse with a wavelength of 0.39 or 0.78 $\mu \mathrm{m}.$ Using a recollision model, we estimate that this ratio reaches a maximum of $5.6\times {10}^{-4},$ four times smaller than the experimental value. This theoretical value is nevertheless a factor of 12 larger than previous recollision values, due to the inclusion of spin factors and a smaller width of the returning wave packet due to the Coulomb field. The recollision mechanism is modified by taking the laser field at the recollision into account, which lowers the required energy for escape from the nucleus. This leads to a lower intensity threshold for the observation of double ionization, and to a factor of 2–3 difference in the probability for double ionization between recollision calculations and experiment. Since this is similar to the expected uncertainty within the recollision model, the calculations support the identification of recollision processes as the main mechanism for double ionization.