Resonant hot-electron production in above-threshold ionization

We have observed surprising detailed structure in the photoelectron kinetic-energy spectrum of xenon under high-intensity short pulse conditions. We show that most of the photoelectrons with kinetic energies from 0-50 eV result from resonant processes at intensities up to 1.9x10(14) W/cm(2). In particular, we find that the high-energy photoelectron structure in above-threshold ionization is actually composed of narrow individual peaks whose energy positions do not shift with intensity. The amplitudes of the structures change rapidly with intensity and turn on at different specific intensities. While those structures appear to be due to resonances, they cannot be attributed to traditional Rydberg transient resonances.