Measurement of the strontium triplet Rydberg series by depletion spectroscopy of ultracold atoms

We report on the atom loss spectroscopy of strontium Rydberg atoms in a magneto-optical trap, using a two-photon excitation scheme through the intermediate state $5\mathrm{s}5\mathrm{p}{}^3{\mathrm{P}}_1$. Energies of the $5\mathrm{s}n\mathrm{s}{}^3{\mathrm{S}}_1$ and $5$\mathrm{s}$n$\mathrm{d}$ , ^3$\mathrm{D}$_{1,2}$Rydbergseriesof$^{88}$Srintherange$13 \leq n \leq 50$ are determined with an absolute accuracy of 10 MHz, including the perturbed region where the $5\mathrm{s}n\mathrm{d}{}^3{\mathrm{D}}_2$ series couples to the $5\mathrm{s}n\mathrm{d}{}^1{\mathrm{D}}_2$ series. This represents an improvement by more than two orders of magnitude compared to previously published data. The quantum defects for each series are determined using the extended Rydberg-Ritz formula in the range where there is no strong perturbation. A value of $1377012721(10)\mathrm{M}\mathrm{H}\mathrm{z}$ for the first ionization limit of ${}^{88}$Sr is extracted.