THEORY OF 2-PHOTON SPECTROSCOPY OF AUTOIONIZING STATES IN HELIUM AND BERYLLIUM

Two-photon ionization of helium and beryllium in the region of (2, n) and (3, n) autoionizing series is investigated in the perturbative reg ime. The problem involves above threshold ionization as well as multic hannel coupling for series lying above the second ionization threshold . Our method is based on the Feshbach formalism in the context of L(2) -basis functions. It allows us to parametrize the ionization cross sec tion in the region of resonances by generalizing the usual Fano and St arace forms to multiphoton processes. The resonance parameters are giv en for (2, n) and (3, n) series of helium and (3, n) series of berylli um lying below the Be+(3s) threshold. Symmetries S-1(e) and D-1(e) are considered.