Fine Structure of Singly Ionized Helium

Abstract

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 $2^{2}S_{\frac{1}{2}}\ensuremath{-}2^{2}P_{\frac{1}{2}}$ transition. This transition is followed immediately by spontaneous emission of a 40.8-ev photon associated with the $2^{2}P_{\frac{1}{2}}\ensuremath{-}1^{2}S_{\frac{1}{2}}$ 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.The present value of $\mathcal{S}$ for ${\mathrm{He}}^{+}$ is 14 043\ifmmode\pm\else\textpm\fi{}13 Mc/sec. The stated uncertainty is equal to three times the standard deviation plus an estimated 3 Mc/sec for the uncertainty in the corrections for systematic effects. This result, consistent with previous results, is in agreement with the best available theoretical value of 14 043.2\ifmmode\pm\else\textpm\fi{}3.0 Mc/sec.