The S matrix for processes induced by intense lasers in atoms can be expanded in terms of the contributions of quantum orbits which are the trajectories of electrons that enter the continuum through ionization and subsequently propagate in the laser field, in order finally to recombine with the atom or to rescatter. The quantum orbits are complex as a consequence of their origin by tunnelling. Their superposition in the S -matrix element results in various eiects such as constructive and destructive interference and may simulate resonant behaviour. Processes considered include above-threshold ionization, high-order harmonic generation and non-sequential double ionization.
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Kopold et al. (2002) studied this question.