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

We report on the observation of the highly forbidden S-1(0)-P-3(0) optical clock transition in laser-cooled Hg-199 atoms. More than 95% depletion of cold Hg-199 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 mu K. We determine the S-1(0)-P-3(0) 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.