New measurement of the relativistic Doppler shift in neon.

The Doppler free two photon transition from the metastable 1S{sub 5} to 4S{sub 1}{prime}{double_prime} in neon was measured for both a fast beam ({approx}120 keV) and a slow thermal beam. The transition frequencies for each beam were measured independently relative to a hyperfine component of a nearby I{sub 2} reference line. The absolute frequency shift between the two beams was obtained. This measurement of the relativistic Doppler shift verifies the time dilation effect to an accuracy of 2.3 ppm which represents a more than 10 fold improvement over previous measurements.