Improved ionization potential of calcium using frequency-comb based Rydberg spectroscopy

We report new frequency-comb-based measurements of Ca Rydberg energy levels. Counter-propagating laser beams at 390 nm and 423 nm excite Ca atoms from the $4s^2~^1\mbox{S}_0$ ground state to $4sns~^1\mbox{S}_0$ Rydberg levels with $n$ ranging from 40 to 110. Near-resonant two-photon two-color excitation of atoms in a thermal beam makes it possible to eliminate the first-order Doppler shift. The resulting lineshapes are symmetric and Gaussian. We verify laser metrology and absolute accuracy by reproducing measurements of well-known transitions in Cs, close to the fundamental wavelengths of our frequency-doubled ti:sapphire lasers. From the measured transition energies we derive the ionization potential of Ca, $E_{\rm IP} = 1, 478, 154, 283.42 \pm 0.08 \mbox{(statistical)} \pm 0.07 \mbox{(systematic)}$ MHz, improving the previous best determination by a factor of 11.