Absolute frequency measurements of the D1 lines in 39K, 85Rb, and 87Rb with ∼0.1 ppb uncertainty

We report values for the D-1-line frequencies with 0.13 ppb uncertainty in the alkali atoms K-39, Rb-85, and Rb-87. The frequencies are measured using an evacuated ring-cavity resonator whose length is calibrated against a reference laser locked to the D-2 line in Rb-87. The measured frequencies, along with sub-ppb measurement of the atomic masses reported previously, form important links in a possible high-precision determination of the fine-structure constant using the photon-recoil shift measured in an atom interferometer. We also obtain, with improved precision, the hyperfine-coupling constants in the P-1/2 state of these atoms, and the fine-structure interval in the 5P state of Rb.