Long-orbit effects in nonsequential double ionization

Basic quantum processes with atoms and molecules subject to intense laser fields, including above-threshold ionization, generation of high-order harmonics and nonsequential double ionization (NSDI), can be described and understood within the concept of complex quantum trajectories (orbits). The quantum orbits approach not only allows us to evaluate transition amplitudes, which are inaccessible by other methods, but also provides a simple semiclassical picture capable of predicting new effects. Usually the quantum orbit shortest in time provides the major contribution to the probability amplitude, while the long orbits are of minor importance, due to the photoelectron wave packet spreading. In this work, we show that in the case of NSDI in elliptically polarized light long orbits corresponding to the second and the third returns of the photoelectron to its parent ion contribute more than the first one. We discuss a qualitative effect based on the left-right asymmetry in the electron-electron correlation spectra, which permits us to discriminate the return contributing most under certain conditions. We also propose an experimental scheme for the visualization of this effect