Observation of TripletndAutoionizing Rydberg States inH2

Single-photon laser spectroscopy of ${\mathrm{H}}_2$ from the metastable $c{}_{}{}^3{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{}$ state has been used to study triplet $\mathrm{nd}$ autoionizing Rydberg states, $n\approx 10-20$. We observe a new and extensive spectrum which shows a well-resolved fine structure due to interaction of the Rydberg electron with the extended cord. An ab initio Hund's case-$d$ model is shown to predict the transition frequencies in isolated portions of the spectrum. In general, however, extensive channel interactions will necessitate a multichannel quantum-defect-theory analysis.