Optical frequency measurements of 6s 2S1/2-6p 2P1/2 (D1) transitions in 133Cs and their impact on the fine-structure constant

High resolution laser spectroscopy of the 6s S-2(1/2)-> 6p P-2(1/2) transition (D-1 line) in neutral Cs-133 is performed 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 femtosecond 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 computerized 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 5x10(-6) rad. Optical frequencies for all four pairs of hyperfine components are measured independently, from which the D-1 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 D-1 line centroid to be f(D1)=335 116 048 748.1(2.4) kHz, and the 6p P-2(1/2) state hyperfine splitting to be 1 167 723.6(4.8) kHz. These results, in combination with the results of an atom interferometry experiment by Wicht [Phys. Scripta T 102, 82 (2002)], are used to calculate a new value for the fine-structure constant.