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 (1σu)2. Decay processes of the Rydberg states through autoionization, predissociation, or electric field ionization are found in the excitation spectra.