Spectrum of the Autoionizing Triplet Gerade Rydberg States of H₂ and its Analysis Using Multichannel Quantum-Defect Theory

A new spectrum of the autoionizing triplet states of gerade symmetry of H₂ has been recorded from the υ″ = 1–4, N″ = 1–3 rovibrational levels of the metastable c ³Π_u^– state in a supersonic beam. The spectrum consists of overlapping ns and nd Rydberg series with n in the range between 4 and 45 converging to the υ⁺ = 1–4, N⁺ = 0–5 levels of the X^{+ 2}Σ_g⁺ ground state of H₂⁺. Numerous perturbations caused by s–d and rovibrational channel interactions are revealed in the spectrum and were fully assigned by combining double-resonance experiments and ab initio multichannel quantum-defect theory (MQDT). The energy- and internuclear-distance-dependent eigenquantum-defect parameters of MQDT were derived from available ab initio calculations of the low-lying electronic states of H₂ and the ground state of H₂⁺ and were subsequently refined in a global fit to experimental data. The positions of 552 triplet ns and nd Rydberg levels of H₂ (361 of which were measured in the present study) could be reproduced with a root-mean-square deviation of 0.2 cm^–1.