Recoilless optical absorption and Doppler sidebands of a single trapped ion

Spectroscopic measurements of the electric-quadrupole-allowed 5d¹⁰6s ²{S}1/2$ to 5${d}⁹$6${s}²$ ${}²D5/2 transition near 282 nm on a single, laser-cooled Hg⁺ ion give a recoil-free absorption line (carrier) and well-resolved motional sidebands. From the intensity ratio of the sidebands to the carrier, the effective temperature of the Hg⁺ ion was determined to be near the theoretical minimum of 1.7 mK. A fractional resolution of better than 3×{}10^-11 for this ultraviolet transition is achieved.