ABSOLUTE-FREQUENCY MEASUREMENT OF THE NARROW-LINEWIDTH 24-THZ D-D TRANSITION OF A SINGLE LASER-COOLED BARIUM ION

A single Ba-138+ 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 a n optically pumped ammonia laser. The absorption lines corresponding t o two Zeeman ComPonents of the 5d 2D3/2 - 5d 2D5/2 transition, symmetr ically located about the line center, were scanned while the laser fre quency was being simultaneously measured relative to a Cs primary freq uency standard by the National Research Council phase-locked frequency chain. The frequencies of the Zeeman components were obtained, and th e transition line center was determined to be 24 012 048 319 +/- 1 kHz .