Hyperfine structure in 6snd Rydberg configurations in barium

The hyperfine structure of the 6snd Rydberg configurations of barium has been investigated using a two-step excitation process and an atomic beam. Accurate hyperfine energies have been obtained on both the 1 D and 3 D levels, which are analysed in terms of singlet-triplet mixing. The mixing coefficients extracted for the 1 D 2 and 3 D 2 levels are shown to be internally consistent, to agree with known Lande factors and can well be predicted with existing multichannel quantum defect analyses. Finally the asymptotic behaviour of the singlet-triplet mixing coefficient is discussed in the framework of simple atomic theory.