The role of tunneling in the ionization of atoms by ultrashort and intense laser pulses

Classically allowed transport is shown to compete with quantum tunneling during the ionization of atoms by ultrashort and intense laser pulses, despite Keldysh parameters smaller than unity. This is done by comparing exact probability densities with the ones obtained from purely classical propagation using the Truncated Wigner Approximation. Not only is classical transport capable of moving trajectories away from the core, but it can also furnish ionization probabilities of the same order as the quantum ones for intensities currently employed in experiments. Our results have implications ranging from a conceptual correction to semiclassical step models in strong-field physics to the ongoing debate about tunneling time measurements in attoclock experiments.
Comment: Small modifications to the text after feedback from readers. Layout of figures updated. Integration of Hamilton's equations updated to include automatic stiffness detection