Absolute frequency measurement of the 6s²¹S₀ → 6s6p³P₁F = 3/2 → F′ = 5/2 ²⁰¹Hg transition with background-free saturation spectroscopy

We report the development of a method for eliminating background-induced systematic shifts affecting precise measurements of saturation absorption signals. With this technique, we measured the absolute frequency of the 6s21S0 → 6s6p3P1 transition in 201aHg (F = 3/2 → F' = 5/2) to be 1181541111051(83) kHz. The measurement was referenced with an optical frequency comb synchronized to the frequency of the local representation of the UTC. This specific atomic line is situated on the steep slope of the Doppler background at room temperature, which results in a frequency systematic shift. We determined the dependence of this shift on the properties of both the spectral line and the background of the measured signal.