Ab initio calculations of perturbed core-excited resonances in H 2 associated with H, n =3: analogies and differences with doubly-excited helium

We have carried out ab initio R-matrix calculations of highly excited diatomic hydrogen (R = 1.4 au, (1)Pi(u) symmetry) in the region near the n = 3 group of thresholds of H-2(+). Spheroidal coordinates were used and all channels with l = 0, 1 and 2 associated with the H-2(+) n = 2 and 3 thresholds were included in the calculations. Generalized multichannel quantum defect theory (GMQDT) has been employed to calculate the resulting resonance structures. Strong configuration interaction is predicted to occur between the various ionization and Rydberg channels, producing an irregular pattern of Rydberg resonances characterized by strongly varying electronic autoionization widths. Analogies with doubly excited resonances in He are uncovered despite the lowered symmetry of the molecule as compared to the atom.