The separated-oscillatory-field technique of subnatural-linewidth rf spectroscopy has been used to completely resolve the hyperfine structure of the $2{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ state of hydrogen. The three-component $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}-2\mathrm{}{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ resonance observed in zero magnetic field was fitted by a superposition of numerically obtained line profiles to determine the hyperfine spliting of the $2{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ state. The result is 59.22(14) MHz.

• Received 18 December 1974

DOI:https://doi.org/10.1103/PhysRevLett.34.377