The hyperfine separation $\Delta \nu (2S; \mathrm{D})$ of the metastable $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ state of the deuterium atom has been measured by an atomic-beam magnetic-resonance method that has been described previously. We found that $\Delta \nu (2S; \mathrm{D})=40\mathrm{}924.439\pm 0.020$ kc/sec. From this result and the ground-state separation $\Delta \nu (1S; \mathrm{D})$ we obtain $R\left(\mathrm{D}\right)\equiv \frac{\Delta \nu (2S; \mathrm{D})}{\Delta \nu (1S; \mathrm{D})}=\frac{1}{8}(1.000\mathrm{}0342\pm 0.0000006)$. Comparison of this value with the corresponding quantity for hydrogen, $R\left(\mathrm{H}\right)=\frac{1}{8}(1.000\mathrm{}0346\pm 0.0000003)$, confirms within experimental error the theoretical prediction that $R\left(\mathrm{D}\right)=R\left(\mathrm{H}\right)$.

• Received 12 September 1956

DOI:https://doi.org/10.1103/PhysRev.104.1585