Frequency measurement of a single barium ion transition with an ammonia laser

The 5d /sup 2/D/sub 3/2/-5d /sup 2/D/sub 5/2/ clock transition at 24 THz (12- mu m wavelength) of a single, trapped, and laser cooled /sup 138/Ba/sup +/ ion was probed by an optically pumped ammonia laser. The clock transition frequency is low enough to be measured against the Cs standard frequency using NRC's existing phase locked frequency chain. Preliminary results, which are considered promising, are presented.<>