Optical frequency measurements of6sS1∕22−6pP

High resolution laser spectroscopy of the 6s 2S1/2→6p 2P1/2 transition D1 line in neutral 133Cs is per-formed in a highly collimated thermal atomic beam by use of a femtosecond laser frequency comb and narrow-linewidth diode laser. The diode laser is offset locked to a single frequency component of the femto-second laser frequency comb and probes the optical transitions between selected pairs of ground-state and excited-state hyperfine components. A photodiode detects the excited-state decay fluorescence, and a comput-erized data acquisition system records the signal. The Doppler shift is eliminated by orienting the laser beam in a direction perpendicular to the atomic beam to within a precision of 510−6 rad. Optical frequencies for all four pairs of hyperfine components are measured independently, from which the D1 line centroid and excited-state hyperfine splitting are obtained by least-squares minimization with the ground-state splitting as a fixed constraint. We find the D1 line centroid to be fD1 =335 116 048 748.12.4 kHz, and the 6p 2P1/2 state hyperfine splitting to be 1 167 723.64.8 kHz. These results, in combination with the results of an atom interferometry experiment by Wicht et al. Phys. Scripta T 102, 82 2002, are used to calculate a new value for the fine-structure constant. DOI: 10.1103/PhysRevA.73.032504 PACS numbers: 32.10.Fn, 42.62.Fi, 06.30.Ft, 06.20.Jr I