Recoilless optical absorption and Doppler sidebands of a single trapped ion

Spectroscopic measurements of the electric-quadrupole-allowed 5d^106s ^2S₁/₂ to 5d^96s^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 310^-11 for this ultraviolet transition is achieved.