Singlet gerade Rydberg states of molecular hydrogen

We investigate the singlet gerade Rydberg series with low total angular momentum J=0,1,2 of molecular hydrogen near the ionization threshold. They are excited by starting at selected rotational levels of the B ¹Σ⁺_u (v'=0) state. The spectra show Rydberg series converging at the different rovibrational levels of the X ²Σ⁺_g electronic ground state of H⁺₂ with the Rydberg electron in an s or d-electronic orbital. An identification of the final states is done with the help of a model multichannel quantum defect theory (MQDT) calculation, which is also used to calculate quantum defects from a small set of parameters. Nearly n-independent quantum defects are derived from the experiment for the different Rydberg series with the Rydberg electron in the s-orbital. In the d-Rydberg series perturbations through l-uncoupling are small. An electronic s-d interaction between Rydberg series is present, which is partly induced by the doubly excited gerade configuration (1σ_u)². Decay processes of the Rydberg states through autoionization, predissociation, or electric field ionization are found in the excitation spectra.