Fine Structure of Singly Ionized Helium

The energy difference ($\mathcal{S}$) between the $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ and $2{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ states of ionized helium has been measured by a pulsed microwave method. Helium atoms are excited to the metastable $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ state of the ion by a micro-second pulse of electrons of about 250-ev energy. After bombardment and after the atoms and ions excited to nonmetastable states have decayed to their respective ground states, a pulse of microwave power is applied to induce the $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}-2\mathrm{}{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ transition. This transition is followed immediately by spontaneous emission of a 40.8-ev photon associated with the $2{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}-1\mathrm{}{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ transition. The photons are observed by counting photoelectrons with high-speed counters gated on synchronously with the pulses of microwave power. Two different levels of rf power are used alternately to provide a means of normalizing the data for variations in the population of the $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ state.