Intercombination line frequencies in ${}^{171}$Yb validated with the clock transition

We have carried absolute frequency measurements of the $(6s^2){}^1{S}_0$ $-$ $(6s6p){}^3{P}_1$ transition in ${}^{171}$Yb (the intercombination line), where the spin-1/2 isotope yields two hyperfine lines. The measurements rely on sub-Doppler spectroscopy to yield a discriminator to which a 556 nm laser is locked. The frequency reference for the optical frequency measurements is a high-quality quartz oscillator steered to the GNSS timescale that is bridged with a frequency comb. The reference is validated to $\sim 3\times {10}^{-12}$ by spectroscopy on the ${}^1{S}_0-{}^3{P}_0$ (clock) line in laser cooled and trapped ${}^{171}$Yb atoms. From the hyperfine separation between the $F=1\mathrm{/}2$ and $F=3\mathrm{/}2$ levels of ${}^3{P}_1$ we determine the hyperfine constant to be $A{(}^3{P}_1)=3957833(28)$ kHz.