Doppler-free two-photon spectroscopy of neon

Doppler-free two-photon transition between the $3s$, $J=2\mathrm{}$ metastable state and the $4d^{\prime }\left(\frac{5}{2}\right)$, $J=3\mathrm{}$ excited state of neon has been performed with a monomode cw dye laser. We have measured the hyperfine constants of the $4d^{\prime }\left(\frac{5}{2}\right)$, $J=3\mathrm{}$ state of ${}_{}{}^{21}{}_{}{}^{}\mathrm{Ne}$ to be $A=-235.9\mathrm{}\pm 0.6$ MHz and $B=11\mathrm{}\pm 9$ MHz. The isotopic shifts for this two-photon transition are $E\left({}_{}{}^{22}{}_{}{}^{}\mathrm{Ne}\right)-E\left({}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}\right)=2780.0\mathrm{}\pm 2.5$ MHz and $E\left({}_{}{}^{21}{}_{}{}^{}\mathrm{Ne}\right)-E\left({}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}\right)=1452\mathrm{}\pm 6$ MHz. From these results, the possibility of a slight departure from the mass formula cannot be rejected.