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In the intense field (long-wavelength) limit, the oscillating motion of an electron wave packet leads to multiple passes by the scattering Coulomb center. The influence of the Coulomb focusing, in combination with multiple returns, focuses parts of the electron wave function, increasing the efficiency of such intense field processes as multiphoton double ionization. Our calculations show enhancement by more than an order of magnitude for multiphoton double ionization of He at 0. 8.
Brabec et al. (1996) studied this question.