Accurate frequency calibration of the Ca I principal resonance line

Abstract

We have performed an accurate frequency measurement of the 4s² ¹S₀ → 4s4p ¹P₁ principal resonance of the neutral calcium atom at 423 nm. From frequency calibrations against a frequency comb stabilized to a GPS-disciplined Rb standard, the transition frequency is determined at 709,078,373.01(35) MHz for the main ⁴⁰Ca isotope. The transition frequencies of the less abundant isotopes were also measured with slightly lower accuracies. The achieved fractional uncertainty of 5×10^-10 exceeds the requirements for including this transition in investigations that aim to probe a possible variation in the fine structure constant α on cosmological time scales.