Precision measurement of the /sup 2/S/sub 1/2/ - /sup 2/D/sub 5/2/ 411 nm transition in laser cooled

Laser-cooled, trapped Yb+ is a versatile candidate for future frequency standards, with potential clock transitions in the microwave, infrared and optical regions. Research at NPL has concentrated on possible optical standards offered by narrow linewidth S-D and S-F transitions in the blue region of the spectrum.' The partial term scheme for Yb' is shown in Fig. 1. The %,12-2F,/1 octopole transition at 467 run is expected to have an ultranarrow natural linewidth (c<l &). Observation of this transition could lead to an extremely high Q-optical reference, with low magnetic field sensitivity in the odd isotope case. At NPL, trapped "Yb' has been laser-cooled using 369.5 nm radiation generated from a frequency-doubled dye laser operating at 738 nm? Buildup of ions in the zD5/2 metastable level and reduction of cooling efficiency is prevented by driving out of this level with 935 nm diode laser radiation, to the 4f1'5d6s 'D[3/ 2],/* level, which decays rapidly to the ground state. The 411 nm %1/2-2D5/r transition has been driven? which under certain conditions completely extinguishes the 369.5 nm fluorescence due to 2D5/. state decay into the very-long-lived 2F,,, state. This state has a measured lifetime in excess of 5 days.' Our ability to observe the weak octo'D,/,