Strong field atomic ionization: origin of high-energy structures in photoelectron spectra.

Two distinct interpretations have been proposed to account for conspicuous enhancements of the ionization peaks in the high energy part of above-threshold ionization spectra. One of them ascribes the enhancement to a multiphoton resonance involving an excited state, while other analysis performed for zero-range model potential link it to "channel closings, " i.e., to the change in the number of photons needed to ionize the atom when the laser intensity increases. We report the results of model calculations that confirm the existence of a resonant process in atoms and shed light on why short-range potential models can mimic the experimental observations.