Optical frequency measurements of 6s ²S_1∕2–6p ²P_3∕2 transition in a ¹³³Cs atomic beam using a femtosecond laser frequency comb

Optical frequencies of the hyperfine components of the D₂ line in ¹³³Cs are determined using high-resolution spectroscopy and a femtosecond laser frequency comb. A narrow-linewidth probe laser excites the 6s ²S_1∕2(F=3,4)→6p ²P_3∕2(F=2,3,4,5) transition in a highly collimated atomic beam. Fluorescence spectra are taken by scanning the laser frequency over the excited-state hyperfine structure. The laser optical frequency is referenced to a Cs fountain clock via a reference laser and a femtosecond laser frequency comb. A retroreflected laser beam is used to estimate and minimize the Doppler shift due to misalignment between the probe laser and the atomic beam. We achieve an angular resolution on the order of 5×10⁻⁶ rad. The final uncertainties (∼±5 kHz) in the frequencies of the optical transitions are a factor of 20 better than previous results [ T. Udem et al. Phys. Rev. A 62 031801 (2000)]. We find the centroid of the 6s ²S_1∕2→6p ²P_3∕2 transition to be f_D2=351 725 718.4744(51) MHz.