Accurate measurement of the 12.6 GHz ''clock'' transition in trapped Yb-171(+) ions

We have measured the frequency of the Yb-171(+) 12.6 GHz M(F) = 0 --> 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(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.