Intercombination line frequencies in ¹⁷¹Yb validated with the clock transition

We have carried absolute frequency measurements of the (6s²)¹S₀ - (6s6 p)³P₁ transition in ¹⁷¹Yb (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 time scale that is bridged with a frequency comb. The reference is validated to ∼3 × 10^-12 by spectroscopy on the ¹S₀ - ³P₀ (clock) line in laser cooled and trapped ¹⁷¹Yb atoms. From the hyperfine separation between the F = 1/2 and F = 3/2 levels of ³P₁, we determine the hyperfine constant to be A(³P₁) = 3957833(28) kHz.