Characterizing the perturbed 6snd series of Ytterbium using MW spectroscopy

The 6snd Rydberg series is perturbed weakly by a doubly excited state lying between the 6s26s and 6s27d states. We have used microwave transitions between 6sns-6s(n$+$1)s and 6sns-6s(n-1)d states to determine the energies of the 6snd Rydberg states of~28$\le $n$\le $40. We have analyzed the energies if the perturbed series using quantum defect theory, which allows the characterization of the perturbed series by only four parameters. The quantum defect theory model~predicts the energies to the accuracy of a few MHz. We have also made lifetime measurements of the 6snd states and recorded the forced autoionization spectra as consistency checks of our analysis.