Ellipticity effects and the contributions of long orbits in nonsequential double ionization of atoms

Using a semiclassical model based on tunneling, electron kinematics in the laser field, and phase space volume, we evaluate the ion and electron momentum distributions in nonsequential double ionization of a model atom by an elliptically polarized laser field. For ellipticities exceeding approximately 0.3, we find that the shortest quantum orbit (having the shortest travel time) no longer dominates the double-ionization rate, in contrast to the case of linear polarization. Simultaneously we observe significant violations of symmetry patterns in the ion-momentum distributions and the electron-electron momentum correlations, compared with the case of linear polarization. These violations are very sensitive to the laser pulse duration because of the absence of the late returns for very short pulses. Some of these qualitative effects in the photoelectron distributions should be experimentally detectable. Observation would verify the significance of the late returns and the underlying quantum-orbit concept.