  We have performed for the first time direct laser spectroscopy of the 1S0-3P0
optical clock transition at 265.6 nm in fermionic isotopes of neutral mercury
laser-cooled in a magneto-optical trap. Spectroscopy is performed by measuring
the depletion of the magneto-optical trap induced by the excitation of the
long-lived 3P0 state by a probe at 265.6 nm. Measurements resolve the
Doppler-free recoil doublet allowing for a determination of the transition
frequency to an uncer- tainty well below the Doppler-broadened linewidth. We
have performed absolute measurement of the frequency with respect to an
ultra-stable reference monitored by LNE-SYRTE fountain pri- mary frequency
standards using a femtosecond laser frequency comb. The measured frequency is
1128575290808 +/- 5.6 kHz in 199Hg and 1128569561140 +/- 5.3 kHz in 201Hg, more
than 4 orders of magnitude better than previous indirect determinations. Owing
to a low sensitivity to blackbody radiation, mercury is a promising candidate
for reaching the ultimate performance of optical lattice clocks.
