Optical frequency measurements of6sS1∕22−6pP1∕22(D1)transitions inCs133and their impact on the fine-structure constant

High resolution laser spectroscopy of the $6s{}^{2}S_{1∕2}\to 6p{}^{2}P_{1∕2}$ transition ($D_1$ line) in neutral ${}^{133}\mathrm{Cs}$ 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 $5\times {10}^{-6}\mathrm{rad}$. 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 $f_{D_1}=335116048748.1\left(2.4\right)\mathrm{kHz}$, and the $6p{}^{2}P_{1∕2}$ 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 et al. [Phys. Scripta T 102, 82 (2002)], are used to calculate a new value for the fine-structure constant.