Experimental determination of the single-photon transition rate between the2S13and1S01states of He i

The highly forbidden single-photon transition rate between the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ and $1{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ states of Hei has been measured in a radio-frequency He discharge. The population of metastables in the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ state was determined by Fabry-Perot interferometric profiles of absorption from the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ to the $4{}_{}{}^3{}_{}{}^{}P_{0,1,2}^{}{}_{}{}^{}$ states. High spectral resolution and precision ultraviolet radiometry were used to determine the brightness of the emission feature observed at 625.54 ± 0.05 Å, compared to the theoretical value of 625.56 Å. Because of the very low transition rate, this feature is weak, but it is shown that it is due to the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}-1\mathrm{}{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ transition. The value for the radiative transition rate obtained in these experiments was 1.10 × ${10}^{-4\mathrm{}}$/sec, with a $2\sigma$ uncertainty of ± 30%. A comparison to the theoretical value of 1.27 × ${10}^{-4\mathrm{}}$/sec is given. The relevance of this work to the beam-foil experiments which have measured the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}-1\mathrm{}{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ transition rate for high-$Z$ He-like ions is also discussed.