Frequency comb measurements of Ca Rydberg states and the Ca ionization energy

A recent publication by Zelener [Jetp Lett. 110, 761 (2019)] reported transition frequency measurements in Ca from the 4s^{2 1}S₀ ground state to 4sns ¹S₀ states with n=40-120. The Ca atoms were trapped in a MOT. Excitation was detected using trap loss spectroscopy. While the Doppler effect was minimized in the MOT environment, systematic density-dependent shifts remained. The published lineshapes were asymmetric and far from Lorentzian. Furthermore, frequency measurements were referenced to a commercial wavelength meter. We report new frequency-comb-based measurements of these transitions using atoms in an atomic beam. A two-photon Doppler-reduced method is used to excite 4sns ¹S₀ states using cw lasers at 423 and 390 nm. The resulting lineshapes are symmetric and Lorentzian. Verification of both laser metrology and absolute accuracy are verified by reproducing measurements of well-known transitions in Cs and Rb, close to the fundamental wavelengths of our frequency-doubled ti:sapphire lasers. From the measured transition energies we derive the ionization energy of Ca.