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

Abstract

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 c3Πu –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 + 2Σg + 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.