2Shyperfine structure of atomic deuterium

We have measured the frequency splitting between the $(2S,F=1∕2)$ and $(2S,F=3∕2)$ hyperfine sublevels in atomic deuterium by an optical differential method based on two-photon Doppler-free spectroscopy on a cold atomic beam. The result $f_{\mathrm{HFS}}^{\left(D\right)}\left(2S\right)=40924454\left(7\right)\mathrm{Hz}$ is the most precise value for this interval to date. In comparison to previous radio-frequency measurements we have improved the accuracy by a factor of 3. The specific combination $D_{21}=8f_{\mathrm{HFS}}^{\left(D\right)}\left(2S\right)-f_{\mathrm{HFS}}^{\left(D\right)}\left(1S\right)$ of metastable and ground state hyperfine frequency intervals in deuterium derived from our measurement agrees well with the value for $D_{21}$ calculated from quantum electrodynamics.