ANGULAR MOMENTUM IN HARMONIC GENERATION AND ABOVE-THRESHOLD IONIZATION

In this paper, we present a discussion of the role that angular momentum plays in the interaction of an intense laser field with a single-electron atom. To do this, we have made a detailed analysis of how different partial waves, i.e., different angular momentum states, participate in producing the photoelectron energy, angular distributions, and harmonic generation spectra. We show that the two regimes---tunneling and multiphoton excitation---correspond to quite distinct evolution patterns across the partial wave decomposition. The natures of these patterns are particularly significant in the harmonic generation spectra and emphasize how harmonic generation is mainly produced in transitions back to the initial state. We also find that at high intensities, the photoelectron energy spectrum is composed of two different but well defined sets of above-threshold ionization peaks. These two sets are separated by a structureless region and the second set deviates substantially from the initial exponential decrease of the peak intensities versus energy. Similar features have been very recently reported experimentally.