Sub-dekahertz Ultraviolet Spectroscopy of ¹⁹⁹Hg⁺

Using a laser that is frequency locked to a Fabry-Pérot étalon of high finesse and stability, we probe the 5d¹⁰6s²S_1/2(F = 0)↔5d⁹6s²²D_5/2(F = 2) Δm_F = 0 electric-quadrupole transition of a single laser-cooled ¹⁹⁹Hg⁺ ion stored in a cryogenic radio-frequency ion trap. We observe Fourier-transform limited linewidths as narrow as 6.7 Hz at 282 nm ( 1.06×10¹⁵ Hz), yielding a line Q≈1.6×10¹⁴. We perform a preliminary measurement of the 5d⁹6s²²D_5/2 electric-quadrupole shift due to interaction with the static fields of the trap, and discuss the implications for future trapped-ion optical frequency standards.