Fine Structure of Singly Ionized Helium

The separation in energy ($S$) between the $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ and $2{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ states of singly ionized helium has been measured to high precision by a microwave method. Helium atoms in a resonant cavity are excited to the $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ state of the ion by bombardment with 300-volt electrons. Rf power is applied to induce transitions between the two states and the 40.8-ev photon which arises from the decay of the $2{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ state to the $1{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ ground state of the ion is detected by a photomultiplier tube. A method of synchronous detection is employed to improve the signal-to-noise ratio. Twenty-six determinations of ($S$) have been made under differing experimental conditions in an attempt to uncover sources of systematic error.