We report the first successful measurements by cascade radio-frequency spectroscopy of the hyperfine structures of excited $S$ and $D$ states of rubidium. We obtain the following magnetic dipole coupling constants (in MHz): for ${\mathrm{Rb}}^{87}$, $A\left(7\mathrm{}{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}\right)=312.5\mathrm{}\pm 3.5$, $A\left(5\mathrm{}{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}\right)=14.1\mathrm{}\pm 0.5$, $A\left(5\mathrm{}{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}\right)=-8.0\mathrm{}\pm 1.5$; for ${\mathrm{Rb}}^{85}$, $A\left(7\mathrm{}{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}\right)=92.6\mathrm{}\pm 1.0$, $A\left(5\mathrm{}{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}\right)=4.1\mathrm{}\pm 0.5$. The anomalous, negative value of $A$ for the $5{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}$ state may be related to the anomalously small fine-structure interval of the $5D$ state of rubidium.

• Received 10 July 1972

DOI:https://doi.org/10.1103/PhysRevLett.29.695