Hyperfine structure and three-channel quantum-defect theory of6sndD21Rydberg states of Ba

The hyperfine structure of $6snd{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ Rydberg states of ${}_{}{}^{135,137}{}_{}{}^{}\mathrm{Ba}$ is reported for principal quantum numbers ranging between $n=10\mathrm{}$ and $n=50\mathrm{}$. From our data, singlet-triplet mixing coefficients have been deduced. A three-channel quantum-defect model has been used to analyze the resonance in singlet-triplet mixing of $6snd{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{} \mathrm{and} {}_{}{}^3{}_{}{}^{}D_2^{}{}_{}{}^{}$ Rydberg states in the vicinity of the $5d7d{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ perturber. For the determination of the scattering matrix, hyperfine-structure data were found to be essential. Good agreement has been achieved between calculated and experimental singlet-triplet mixing coefficients.