Progress Toward a Stored Ion Frequency Standard at the National Bureau of Standards*

Two fundamental problems with the development of a primary frequency standard based on stored ions have long been apparent-the second-order Doppler shift and the low signal-to-noise ratio. Both problems have been addressed in experiments at the National Bureau of Standards (NBSbthe first by the development of the laser cooling technique and the second by the development of laser-optical-pumping techniques with high detection efficiency. Also, a hyperfine transition in 25Mf+ has been observed by rf-optical double resonance with a linewidth of 0.012 Hz and a Q of 2.4 x 10 '. A possible microwave frequency and time standard based on a ground-state h erfine transition in 201Hg+ and a %Hg+ or 201Hg+ are described. Key words: atomic frequency standards; atomic hyperfine structure; double resonance; Hg'; laser cool-ing; Mg'; optical pumping; stored ions. possible optical frequency standard based on the two-photon 5d9 6s 8 Dm ++ 5dl0 6s 2 S l ~ transition in 1