Laser-induced recollision phenomena: interference resonances at channel closings.

An analytical theory of the resonancelike phenomena in high-order above-threshold ionization is presented that explains details of the experimental spectra and theoretical simulations. It traces the observed features to the constructive interference of "quantum orbits" with long travel times at laser intensities where the N-photon ionization channels close. Characteristic differences show up between even and odd N. The effects are generic to all laser-induced recollision phenomena. For nonsequential double ionization, their signature in the momentum distribution of the final electrons is identified.