Spectrum of Field-Ionized Triplet Gerade Rydberg States of H₂ and Comparison to Multichannel Quantum-Defect Theory

A spectrum of field-ionized triplet Rydberg states of gerade symmetry H₂ has been measured, excited from the υ″ = 0, N″ = 1-3 rovibrational levels of the metastable c³Π_u^-2pπ state in a 6 keV fast molecular beam. The field-ionized spectrum is kinetic energy labeled in order to separate it from the well-studied υ⁺ ≥ 1 autoionization spectrum. The spectrum consists of both ns and nd Rydberg series with n between 10 and 28 converging to the υ⁺ = 0, N⁺ = 1-3 levels of the X^+ 2Σ_g⁺ ground state of H₂⁺. Transitions with changes in vibration and/or rotation are also identified. The spectral positions of 59 transitions in the field ionization spectrum are identified and assigned quantum numbers using the predictions of multichannel quantum-defect theory (MQDT). The transition energies and subsequent effective quantum defects are compared between the experiment and theory. Most of the observed transitions, within the experimental uncertainty of 0.2 cm^-1, agree with the energies predicted by MQDT.