Non-sequential double ionization of an atom in an intense laser field is a prototypical problem in multi-body strong field interactions. To examine non-sequential double ionization, an often used approach is the semi-classical method, by which initial conditions are set according to quantum expressions and the ensuing dynamics modelled classically. Here we present a comparative study of quantum and semi-classical methods in the single active electron approximation. We show that semi-classical methods over-estimate the kinetic energy of rescattered electrons by up to 150%. We further show that the return time of the recolliding electrons deviates between quantum and semi-classical methods at low intensities, and that the deviation is attributable to the choice of the initial tunnelling position.
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