High-precision frequency measurement of the 423-nm Ca i line

Abstract

We have performed an accurate frequency calibration of the $4{s}^{2} {}^{1}{S}_{0}\ensuremath{\rightarrow}4s4p {}^{1}{P}_{1}$ principal resonance line of the neutral calcium atom at 423 nm. Doppler-free cw excitation on a Ca atomic beam was performed by utilizing a Sagnac geometry in the alignment of the excitation beams. From frequency calibrations against a frequency comb, stabilized to a global positioning system (GPS) disciplined Rb standard, the transition frequency is determined at 709 078 373.01(35) MHz for the main $^{40}\mathrm{Ca}$ isotope. Slightly lower accuracies are obtained for the transition frequencies of the less abundant isotopes. The achieved fractional uncertainty of $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ exceeds the requirements for including this transition in investigations that aim to probe a possible variation in the fine-structure constant $\ensuremath{\alpha}$ on cosmological time scales.