The fine structure of helium levels $\mathrm{}\left(1s3p\right)\mathrm{}{}_{}{}^3{}_{}{}^{}P_J^{}{}_{}{}^{}$, $J=1, 2$ is determined by the microwave-optical method outlined in the preceding paper. Helium atoms are excited to $3{}_{}{}^3{}_{}{}^{}P_J^{}{}_{}{}^{}$ by electron bombardment and the optical radiation $3{}_{}{}^3{}_{}{}^{}P_J^{}{}_{}{}^{}\to 2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\left(3889\mathrm{} \mathrm{A}\right)$ is observed with a photomultiplier tube. The intensity of this radiation changes when radio waves induce transitions between the fine structure sublevels. The separation $3{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}-3\mathrm{}{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ is found to be 658.0±1.0 Mc/sec, which is to be compared to the most recent optical estimates of 750±300 Mc/sec.

• Received 1 August 1956

DOI:https://doi.org/10.1103/PhysRev.105.573