Absolute-frequency measurement of the narrow-linewidth 24-THz D-D transition of a single laser-cooled barium ion.

A single (138)Ba(+) ion, trapped in a rf trap and cooled to 15 mK by 493- and 650-nm dye-laser radiation, was probed by 24-THz radiation from an optically pumped ammonia laser. The absorption lines corresponding to two Zeeman components of the 5d(2)D(3/2)-5d(2)D(5/2) transition, symmetrically located about the line center, were scanned while the laser frequency was being simultaneously measured relative to a Cs primary frequency standard by the National Research Council phase-locked frequency chain. The frequencies of the Zeeman components were obtained, and the transition line center was determined to be 24012 048319 +/- 1 kHz.