The HgRb team reported the most precise absolute frequency measurement of rubidium 5S–7S two-photon transitions You can see the paper here: https://doi.org/10.1364/OL.38.004581

We reported the absolute frequency measurements of rubidium 5S–7S two-photon transitions with a cw laser digitally locked to an atomic transition and referenced to an optical frequency comb. The narrow, two-photon transition,
5S–7S (760 nm), insensitive to first-order in a magnetic field, is a promising candidate for frequency reference. The performed tests yielded more accurate transition frequencies than previously reported.