Accurate measurement of the 12.6 GHz "clock" transition in trapped /sup 171/Yb/sup +/ ions

We have measured the frequency of the /sup 171/Yb/sup +/ 12.6 GHz M/sub F/=0/spl rarr/0 ground state hyperfine "clock" transition in buffer gas-cooled ion clouds confined in two similar, but not identical, linear Paul traps. After correction for the known differences between the two ion traps, including significantly different second-order Doppler shifts, the frequencies agree within an uncertainty of less than 2 parts in 10/sup 13/. Our best value, based on an analytic model for the second-order Doppler shift, for the frequency of the clock transition of an isolated ion at zero temperature, velocity, electric field and magnetic field, is 12642812118.466+0.002 Hz.