Doppler-free two-photon spectroscopy of neon. I. Fine structure and hyperfine constants for the 4d' subconfiguration

Using Doppler-free two-photon spectroscopy, we have measured the fine structure of the 4d' subconfiguration of neon and the hyperfine constants of the four levels of this subconfiguration for the 21 isotope. We describe the expérimental set-up and we point out the interest of putting the experimental cell inside a Fabry-Pérot cavity locked on the laser wavelength. We compare our expérimental results for the hyperfine constants with the theoretical predictions of Liberman. We show that there is a slight disagreement for the two J = 2 levels. This is explained by introducing a non zero spin orbit coupling ξ4d for the external electron.