Velocity distribution and second-order Doppler shift in an optically pumped cesium beam frequency standard

The velocity distribution and the second-order Doppler shift were obtained in an optically pumped cesium beam frequency standard at the Korea Research Institute of Standards and Science (KRISS) by using Fourier transforms of Ramsey lineshapes. The second-order Doppler shift changes as much as 3/spl times/10/sup -15/ for a variation 20 K at an oven temperature of 383 K. It also changes at a rate of -2.3/spl times/10/sup -14//dB around an optimum microwave power. We found that the velocity selective effect of a tilted laser beam can reduce the second-order Doppler shift. In particular, this effect becomes larger as the linewidth of the laser beam becomes narrower.