Observation of the 1 S 0 – 3 P 0 optical clock transition in cold 199 Hg atoms

We report on the observation of the highly forbidden 1S0–3P0 optical clock transition in laser-cooled 199Hg atoms. More than 95% depletion of cold 199Hg atoms is detected in the magneto-optical trap. Using the free-of-field detection method, the AC Stark shift from the cooling laser is removed from the in-field spectroscopy. At low-power clock laser pumping, the linewidth of the clock spectroscopy is approximately 450 kHz (full width at half-maximum), which corresponds to a Doppler broadening at the atom temperature of 60 μK. We determine the 1S0–3P0 transition frequency to be 1,128,575,290.819(14) MHz by referencing with a hydrogen maser and measuring with a fiber optical frequency comb. Moreover, a weak Doppler-free signal is observed.