Both the physical picture of the dynamics of atoms and molecules in intenseinfrared fields and its theoretical description use the concept of electrontrajectories. Here we address a key question which arises in this context: Aredistinctly quantum features of these trajectories, such as the complex-valuedcoordinates, physically relevant in the classically allowed region of phasespace, and what is their origin? First, we argue that solutions of classicalequations of motion can account for quantum effects. To this end, we constructan exact solution to the classical Hamilton-Jacobi equation which accounts fordynamics of the wave packet, and show that this solution is physically correctin the limit $\hbar \to 0$. Second, we show that imaginary components ofclassical trajectories are directly linked to the finite size of the initialwavepacket in momentum space. This way, if the electronic wavepacket producedby optical tunneling in strong infrared fiels is localised both in coordinateand momentum, its motion after tunneling {\em ipso facto\/} cannot be describedwith purely classical trajectories -- in contrast to popular models in theliterature.
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