Abstract
Two-photon ionization of helium and beryllium in the region of (2,n) and (3,n) autoionizing series is investigated in the perturbative regime. The problem involves above threshold ionization as well as multichannel coupling for series lying above the second ionization threshold. Our method is based on the Feshbach formalism in the context of L2-basis functions. It allows us to parametrize the ionization cross section in the region of resonances by generalizing the usual Fano and Starace forms to multiphoton processes. The resonance parameters are given for (2,n) and (3,n) series of helium and (3,n) series of beryllium lying below the Be+(3s) threshold. Symmetries 1Se and 1De are considered.
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