Laser-assisted inelastic rescattering during above-threshold ionization

As an electron is freed from an atom by an intense laser it is accelerated by the oscillating field. Before escaping entirely, it may recollide with its parent ion. During this rescattering, the remaining electrons of the ion might be excited or ionized. The influence of the laser field on these impact excitation and ionization processes is investigated. Model calculations for helium at 780 nm and intensities near 10¹⁵ W/cm² show that the effect of the field on the total probability of promoting an electron from the ionic ground state is small. However, the laser alters the final excited-state populations, rapidly ionizing any excited bound states. We discuss the importance of these results in the search for the mechanism responsible for direct double ionization in the tunneling regime.