Bound-level "multiplets" in photoelectron spectra of above-threshold ionization

Evidence has been found for peak-splitting "multiplets" in photoelectron spectra of above-threshold ionization. This evidence was obtained by analyzing numerically the wave functions computed for a model atom undergoing ionization by an intense laser beam. The multiplets can be explained as replicas of far-off resonance bound-state levels that are transiently and adiabatically populated by a laser pulse and then ionized. The heights of the multiplet peaks relative to the true multiphoton peaks depend on the laser pulse shape.