Sub-dekahertz spectroscopy of 199Hg+

Using a laser that is frequency-locked to a Fabry-Perot etalon of high finesse and stability, we probed the 5d(10)6s S-2(1/2) (F = 0, m(F) = 0) <----> 5d(9)6s(2) D-2(5/2) (F = 2, m(F) = 0) electric-quadrupole transition of a single laser-cooled Hg-199(+) ion stored in a cryogenic radio-frequency ion trap. We observed Fourier-transform limited linewidths as narrow as 6.7 Hz at 282 nm (1.06 x 10(15) Hz). The functional form and estimated values of some of the frequency shifts of the S-2(1/2) <----> D-2(5/2) "clock" transition (including the quadrupole shift), which have been calculated using a combination of measured atomic parameters and ab initio calculations, are given.