Measurements of the Rydberg transition energies for the $n^1S_0$ state and the ionization potential for $^{40}$Ca atoms

Two-photon spectroscopy has been applied to measure the Rydberg transition energies in the $n^1S_0$ state of $^{40}$Ca atoms at $n= 40$–$120$. The ionization potential determined from these experimental data is $49305.91966(4)$ cm$^{-1}$. Using the frequency-doubled radiation and fundamental radiation from a diode laser, the frequencies of two-photon transitions to Rydberg states with the principal quantum numbers of $75$ and $76$ have been measured simultaneously with the frequencies of resonances of saturated absorption in a rubidium vapor cell. The chosen transition frequencies for rubidium atoms are known with high accuracy. Such frequency references make it possible to improve the accuracy of determination of the Rydberg transition energies in $^{40}$Ca atoms.