Resonances and interferences in above-threshold ionization

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Abstract
Intermediate resonances are predicted to enhance or suppress the different electronic kinetic-energy peaks in above-threshold ionization (ATI) processes. We reach these conclusions on the basis of numerically exact three-dimensional computations of hydrogenic ATI proceedings via several field-broadened [3(s,p,d) or 4(s,p,d,f)] intermediate bound states. Two effects, one attributable to interferences between different resonances, operating in the circular polarization case, and one due to interferences between different ‘‘precursor’’ partial waves of the same resonance, operating in the linear polarization case, have been discovered. The latter effect is shown to give rise to the appearance of a doubling in each one of the partial-wave components of the ATI peaks, with each component experiencing an avoided-crossing-type process as the intensity is raised. We find that for near-resonance ATI, the breakdown of N-order perturbation theory occurs at much lower intensities than for off-resonance ATI.