Doppler-free measurement of the 546 nm line of mercury

We have made Doppler-free observations of the 546 nm line of neutral mercury by using saturated absorption and frequency modulation spectroscopy. Absolute wave numbers were measured for 14 resolved components arising from six mercury isotopes. Combining these measurements with precisely known isotope shifts and hyperfine-structure splittings, we derive a value for the wave number in 198 Hg from each of the measured features. Our final value for the wave number of the 198 Hg line is in good agreement with a recently reported measurement using Fourier transform spectroscopy but has an uncertainty that is smaller by a factor of 12.